The URL of this page is http://ham.space.umn.edu/spacephys/IMEXwebsci.html contact: John Wygant, IMEX Principal Investigator (firstname.lastname@example.org)
The most dramatic intervals of particle energization within the Earth's magnetosphere occur in the inner magnetosphere during major geomagnetic storms. This region is especially dynamic and unpredictable during and immediately following solar maximum, a period when major magnetic storms are most likely to occur. The most startling discovery about this region was made during the last solar maximum by the CRRES satellite. Data and subsequent modeling showed that a new and very energetic radiation belt can be formed in seconds deep in the inner magnetosphere by the interaction of an interplanetary shock wave with the magnetosphere. Such radiation belts can last for years after their formation and have important implications for spacecraft design. However, the existence of the interplanetary shock wave that created the new radiation belt was only inferred because there were no direct measurements of the solar wind. The complete causal chain from solar wind disturbance to particle acceleration could not be verified and tested. Few of the possible mechanisms that may accelerate charged particles during major storms have been tested by in-situ measurements at times when solar wind data were available. The key scientific objective of IMEX is to explain the processes involved in major geomagnetic storms as manifested in the inner portion of geospace:
- Interplanetary shock interaction with the magnetosphere
Storm main phase development and ring current injection
- Substorm particle injections and their relation to major storm development
- Ring current decay and storm recovery
- Relativistic electron acceleration
belt growth, evolution, and decay.
The physics of particle transport and energization in the inner magnetosphere is still not understood because there has never been a satellite with:
·the necessary sensitive electric and magnetic fields and particles instruments;
· an orbit period of <~12 hours so that phenomena occurring on the time scales of the phases of major geomagnetic storms may be resolved over radial distances from ~1-6 Re;
· an upstream monitor providing continuous solar wind information; and
· measurements during and just after solar maximum when the largest magnetic storms occur.
We propose a University-class explorer with the above properties, the Inner Magnetosphere Explorer (IMEX), designed to investigate these physical processes. IMEX will reveal the mechanisms responsible for the generation of intense electric and magnetic fields and waves during strong geomagnetic storms (Dst <-100) and the effects of such fields and waves on the energization and transport of charged particles from energies from several eV to 20 MeV will be thoroughly examined. IMEX is scheduled for launch at a time when large storms occur most frequently, during the peak and declining phase of the Sun's eleven year cycle. IMEX will measure fields and particles at a time when continuous monitoring of the solar wind is available from NASA's ACE mission. In addition, observations of the large scale structure of the ring current will be available from NASA's IMAGE and TWINS missions to provide a global picture which is complementary to IMEX's in situ measurements. The launch of this mission during this solar cycle is of critical importance because of the availability of solar wind monitoring by ACE, global imaging of the ring current by IMAGE and TWINS. IMEX will provide the data necessary to obtain the quantitative understanding which is critical to achieving a predictive capability. Such a mission has recently been ranked as the highest priority by the National Space Weather Initiative Implementation Plan (Wolf et al., 1998).
IMEX will be equipped with an especially sensitive electric field detector (from DC to 40 kHz), fluxgate and search coil magnetometers, a Langmuir probe, and particle detectors for measuring charged particles in the energy range of 10 eV to 20 MeV. Simultaneous measurements of waves and quasi-static electric fields together with plasmaspheric density, ring current and energetic radiation belt particles will allow us to assess the efficiency of various energization and transport mechanisms. Geosynchronous transfer orbit provides radial passes from ~1 Re to ~7 Re on time scales comparable to the 5-20 hour duration of major geomagnetic storms and will insure that IMEX spends sufficient time in regions where 100 eV to 50 MeV particles are injected by strong transient electric and magnetic fields.
A nominal two year mission is planned with launch in June, 2001 to insure that IMEX explores the inner magnetosphere at the maximum and the period immediately following the next solar maximum. This phase of the solar cycle has the strongest and most frequent coronal mass ejections and, consequently, the strongest interplanetary shocks and geomagnetic storms. Based on the statistics of the past solar cycle we expect 15-20 major geomagnetic storms (Dst<-100) and an equivalent number of interplanetary shocks during the two year mission.
IMEX science payload and spacecraft rely heavily on recent heritage to
ensure that key scientific goals are met at minimum cost and risk. IMEX
instruments are based on designs that have flown or will fly on the CRRES,
FAST, Polar, Wind, and Cluster satellites, as well as on sounding rockets.
The spacecraft will be provided by LASP at the University of Colorado and
is based on their STEDI satellite, SNOE. This heritage insures that IMEX
has no new technological risks and can be built within the cost and time
constraints of a UNEX. IMEX will be launched as a secondary payload on
a military Titan 4 launch. The IMEX team consists of scientists with extensive
experience in design and fabrication of particles and fields instruments,
and in the analysis of data and collaboration with theorists and simulators
to provide closure on the important physical processes. The spacecraft
engineers have built a successful UNEX class satellite, SNOE. The education
and outreach activities, as well as the collaboration with tribal colleges,
build on successful, existing programs developed by the Minnesota Space
Grant office, the Space Science Institute, the University of Colorado,
and the University of California.
Background: The most dramatic particle energization in the magnetosphere occurs during major geomagnetic storms associated with the peak and declining phase of the solar cycle. Most of these storms are driven by Coronal Mass Ejections (CME's) of high density plasma from the solar corona which can propagate through space at velocities of 600 km/s to 1500 km/s. Preceding the faster of these CMEs are collisionless shocks. Regions of strong magnetic field (10-30 nT) are formed through the shock compression of the interplanetary magnetic fields and also through the draping of magnetic flux around the CME cloud (Gosling et al., 1991; Kahler, 1988).
When the magnetic field orientation of these regions has a strong southward component, the power generated by the solar wind-magnetospheric dynamo can be enhanced by orders of magnitude to 1012 watts resulting in geomagnetic storms (Akasofu, 1981). The large pressure of the cloud can result in the compression of the magnetosphere to half its normal size (Studmann et al., 1986). Buffeting by turbulent fluctuations in the plasma density, velocity, and pressure of the solar wind may excite MHD eigenmodes within the magnetosphere. During the main phase of geomagnetic storms, a large fraction of the energy from the solar wind magnetospheric dynamo is deposited in particles in the inner magnetosphere. The dynamo directly drives the large scale convection electric field associated with the steady state ExB/B2 circulation of plasma through the magnetosphere. In addition, energy may be stored in the geomagnetic tail and explosively released during episodic flows and particle acceleration events associated with magnetospheric substorms (Akasofu, 1977; Baker et al., 1996). About half of the energy generated by the dynamo is dissipated in the earthward injection and energization of ring current ions, H+ and O+ with energies between 30 keV and 300 keV. The visual manifestation is an auroral display which intensifies and migrates from the Polar regions, where it is usually observed, to the mid-latitudes characteristic of the continental United States (Allen et al., 1989). The precipitation of electrons in the aurora is a major sink of energy as is the ionospheric heating from the enhanced electric fields and currents in the ionosphere. Particle acceleration during geomagnetic storms results in the energization of MeV electrons inside geosynchronous orbit (Baker et al., 1994). Energy transfer from the solar wind, and its storage, propagation and release during magnetic storms is a critical link in understanding of the effects of solar variability on the Earth, which is a major goal of NASA's OSS (see `Space Science for the 21st Century').
The inner terrestrial magnetosphere contains the Van Allen radiation belts - the stably trapped energetic ions and electrons - and the ring current. The inner magnetosphere is truly the "heart" of geospace. The radiation belts are the site of the most energetic particle acceleration in geospace and represent the greatest known risk to human technological systems in space. The ring current is the reservoir that absorbs and stores geomagnetic storm energy and then releases it slowly over subsequent days and weeks. We cannot understand the most energetic and dramatic events in geospace - geomagnetic storms and magnetospheric substorms - until we understand the inner magnetosphere. This is the goal of IMEX: to understand how particles are accelerated and convected in the inner magnetosphere. To do this, IMEX will measure the particles and the fields which govern their acceleration and convection
It might seem odd that much is yet unknown about the inner magnetosphere. After all, this region was first probed over 40 years ago by the first satellites. Following the discovery of the Van Allen belts by Explorer I in 1958, numerous satellites have flown through the inner magnetosphere. Many `operational' spacecraft in low-Earth orbit or in highly elliptical orbits have made magnetic field and/or energetic particle measurements. Other spacecraft such as National Oceanic and Atmospheric Administration and U.S. Air Force/Department of Energy satellites have skirted the inner magnetosphere at geostationary orbit. The space environmental sensors onboard many of these operational satellites have returned useful data. However, a comprehensive package of sensors to measure particles and fields (particularly electric fields) has been missing on nearly all of these satellites .Three scientific satellites deserve special mention. The European Space Agency's (ESA) first Geostationary Environmental Observing Satellite (GEOS-1) operated during 1977-1979 in a 12-hour (1.3 x 6.9 RE) orbit [Knott, 1982]. GEOS-1 yielded useful data at distances beyond ~ 5 RE, but not below ~ 5 Re because its sensors were designed for its initially-planned geosynchronous orbit. Another scientific mission that probed the inner magnetosphere was the Active Magnetospheric Particle Tracer Experiment (AMPTE)/Charge Composition Explorer (CCE), which operated from 1984 to 1989 [Krimigis et al., 1991]. This mission revolutionized our understanding of ring current composition [Hamilton et al. 1988]. However, with an apogee of 8.8 RE, AMPTE/CCE spent most of its time outside of 6 RE; in addition, it had limited instruments for wave measurements and high-energy particles and had no quasi-static electric field detector. The most recent and relevant mission in the inner magnetosphere was the Combined Release and Radiation Effects Satellite (CRRES) which operated from July 1990 to October 1991 [Johnson and Kierein, 1992]. CRRES was in an ideal orbit (350 km X 6.6 RE, 18° inclination) to measure radiation belts, the ring current, and fields. During its 15 month lifetime, CRRES witnessed intense geomagnetic storms and a most remarkable SSC event. CRRES provided a tantalizing glimpse of acceleration, transport, and loss processes in the inner magnetosphere. However, the CRRES mission was cut short by a failure of its power system before it completed a local time survey of the inner magnetosphere. Moreover, there were only limited solar wind measurements during the CRRES mission.
is important to realize that energization processes operate with the greatest
intensity for the 10-30 hour periods of main phase of large geomagnetic
storms. They occur deep in the inner magnetosphere where they are largely
hidden from the view of the majority of scientific spacecraft. Many of
the spacecraft designed to study magnetospheric dynamics have apogees at
10 -30 Re and have orbital periods ranging from 20 hours to several days.
Such spacecraft spend only a small fraction of their time within 6 Re.
They may encounter the aftermath of the storm and see that energetic particle
fluxes have been enhanced. However, unless they are lucky, they typically
miss the crucial intervals of particle acceleration. Even spacecraft at
geosynchronous orbit may be on the periphery of the regions of important
acceleration. These considerations emphasize the importance, for a mission
that investigates major geomagnetic storms, of an orbit which continually
samples the region between 6.6 and 1-2Re in the equatorial plane
Dynamics of the Inner Magnetosphere: There are few places in the magnetosphere where our experimental knowledge of fundamental processes is as limited as it is in the inner magnetosphere during major geomagnetic storms. In contrast to other regions of the magnetosphere, no studies of the dynamics of plasma flow or of particle acceleration occurring during major geomagnetic storms have been presented which include a full complement of in-situ measurements, as well as the necessary solar wind monitoring. Convective electric fields, ring current plasma pressures and densities, and the spectra of energized particles along with the characteristics of the solar wind which power these processes have not been measured simultaneously. Thus it is arguable that there has not been a real confrontation between observations and the ideas underpinning our understanding of the magnetosphere during major geomagnetic storms.
Measurements of electric fields are important in understanding the dynamics of particle convection and energization. The electric field relates directly to the global circulation of cold plasma through the magnetosphere through the ExB/B2 drift. In a collisionless plasma it is ultimately the action of the electric field on charged particles that is directly responsible for their energization. The rate of energization of a charged particle is given by dW/dt= qv·E. Depending on the particle species, its energy, and the magnetic field configuration, the dynamically significant electric field may be the quasi-steady electric field( varying over periods of hours) or substorm related electric fields (lasting for minutes and re-occurring episodically every 1-3 hours) or the large amplitude electric fields driven by especially strong interplanetary shocks preceding CMEs. The electric fields may be associated with large scale global mode oscillations of the magnetosphere after the impact of a shock which causes the magnetosphere to ring like a bell. Magnetic field line resonances in concert with the large scale convection electric field may be important in the loss of ring current particles (Li et al., 1993). Field line resonances in which magnetic field lines vibrate like guitar strings may be driven by fluctuations in the solar wind or magnetosheath or internally generated by the free energy in ion particle distributions. Recently it has been suggested (Roth et al, 1998; Thorne, 1998) that waves may play a primary role in relativistic electron energization.
It should be noted that in the inner magnetosphere below L=4, it has been difficult for particle detectors to measure the convective flow of plasma due to the confounding effects of photoelectron contamination, low Mach number flows (large thermal velocity spread of particle compared to smaller convective flows), and spacecraft charging. Thus electric field measurements are the only way to address many important issues.
Goals and Approach: Examples of science goals which IMEX will address
are discussed in more detail below, and illustrated with data from CRRES.
The examples will show the importance of measuring both particles and electric
and magnetic fields in the inner magnetosphere, as well as solar wind parameters,
in advancing our understanding of the physics of major storms. The motivation
for the science objectives and the approach towards addressing them with
the propsed instruments are described below.
1. Interplanetary Shocks and Other Mechanisms for Rapid Acceleration of Energetic Particles
The importance of direct in-situ measurements of particle acceleration in the inner magnetosphere and our comparative ignorance of this region is illustrated by the fact that we learned only recently that interplanetary shocks can play a dominant role in the prompt (~60 second) earthward injection and energization of ultra-relativistic (10-50 MeV) electrons and high energy protons (20-40 MeV) (Blake et al., 1993; Vampola and Korth, 1993). The effects of a single particle acceleration event can dominate the inner magnetosphere energetic particle environment for years. This rapid energization contrasts with the days-to-weeks time scales associated with diffusive processes which had previously been considered (Schulz and Lanzerotti, 1974).
1. (left,top) Observations of injection of >10 MeV electron drift echoes;
(left,bottom) Simultaneous measurements of the dawn-dusk electric field
and magnitude of the magnetic field (right,top). Simulated particle data
in the same format as "observed" at the position of the spacecraft with
"particle detectors" and (right,bottom) the imposed wave electric field
and magnetic fields calculated for the position of the spacecraft. electric
and magnetic fields.
The first clearly documented observation of such prompt acceleration is shown in Figure 1 which presents measurements of electric fields, energetic electrons (6-50 MeV), and energetic protons (20-110 MeV). A strong shock presumably impacted the magnetosphere at about 3:40 UT. One minute later, at L=2.5 and 3 MLT, a five order of magnitude enhancement in the energetic electron flux coinciding with the appearance of the electric and magnetic fields of a strong magnetosonic wave was observed. The data suggest a scenario that has been verified using a test particle simulation (Li et al., 1993). In this scenario, extraordinarily strong electric fields (240 mV/m) are induced in conjunction with the compression of the dayside magnetosphere by the shock (Wygant et al., 1992; 1994). The compression produces a magnetosonic wave which propagates from the dayside to the nightside in one minute. Electrons with energies of several MeV were preferentially injected earthward by the ExB drift due to the large azimuthal component of the wave electric field. The final spectrum of energized electrons at 2.5 Re was strongly peaked at about 15 MeV because the gradient B drift of these more energetic electrons matched the phase velocity of the magnetosonic wave as it propagated around the earth allowing the more energetic electrons to be injected further inward. The extremely rapid injection of these electrons occurred over a fraction of an orbital drift period and resulted in the creation of a spatially coherent `bunch' of ~15 MeV electrons. After the initial acceleration, the electrons orbited the earth with a drift period of about 90 seconds and repeatedly encountered the spacecraft, producing the equally spaced enhancements characteristic of electron drift echoes. The results of a test particle simulation, used to check this scenario, are presented on the right hand side of Figure 1 (Li et al., 1993). The simulation tracks the trajectories of 300,000 equatorially mirroring electrons in the presence of an analytic model for the wave electric and magnetic fields. These electrons formed a new radiation belt that persisted for the 8 remaining months of CRRES and was observed for several years by SAMPEX. Ion drift echoes, energized to 50 MeV, were also observed and were simulated using the same wave fields. The interplay between the observations and theoretical analysis for this event serves as a model for the scientific analysis of data from the IMEX mission.
Figure 2 presents CRRES measurements of energetic electrons for the entire 15 month mission as the spacecraft orbited between ~1 and 6.6 Re with a period of 10 hours. The color coded particle counts are plotted versus L-value on the vertical axis and time (orbit number) on the horizontal axis. The effects of the 3/24/91 strong shock acceleration event ( on orbit 587) which populated the region near L=2.4 with a new radiation belt lasting to the end of the mission are clearly visible (This event is described in detail in the next section). This figure shows that there were numerous other energization events involving particles of varying energies which are injected to different L values and last over time scales ranging from a day to months. A number of energization events were observed in the >6 MeV particles. Electrons were sometimes injected to below L=3 where they were trapped and slowly lost over periods of weeks to months. During a larger number of events, the electrons were injected to L=4 or 5 where they were lost over periods of days. The > 0.876 MeV electrons will be discussed more completely in a later section, but these electrons appear and disappear quasi-periodically with the 27 day rotation period of the sun. These events are typically due to fast
2. CRRES measurements of energetic electrons with energies above .876 MeV
(upper panel), > 6 MeV (middle panel), and >8.5 MeV) lower panel over the
14 month mission.
solar wind streams (Paulikas and Blake, 1979, Baker et al., 1994). In addition there are very strong energization events in the .876 MeV channel which are non-periodic which are almost certainly associated with CMEs. The physical processes in the inner magnetosphere directly responsible for the appearance and disappearance of these relativistic electron fluxes and their relation to solar wind structures are not understood. Thus, as discussed previously, there is a very rich phenomenology of acceleration events occurring over a variety temporal and spatial scales driven by different physical processes which will be investigated by IMEX . The acceleration mechanisms investigated will include those due to shocks, impacts by other solar wind density perturbations and discontinuities, intense substorm electric fields which are observed below L=4, large scale oscillations of the magnetosphere in association with global cavity modes, and mechanisms we do not yet know about. Similar observations of protons in the 1 MeV energy range and also in conjunction with solar protons over the energy range from 20 MeV to 100 MeV will be investigated with this spacecraft to determine the mechanisms responsible for injection and trapping.
The probability of observing an event of the magnitude of March 24, 1991 shock may be larger than once thought. It has recently been speculated (Shea and Smart, 1993;1996) that several other equally intense shock-induced prompt acceleration events have occurred since the 1960's but were not recognized because of inadequate monitoring. The evidence for these shocks is the sudden appearance of high energy charged particles in the inner magnetosphere on time scales rapid compared to those expected for radial diffusion. The surprising observation of these powerful energization events highlights the largely unexplored nature of the inner magnetosphere during major geomagnetic storms and suggests that we have yet to discover the full complement of particle acceleration mechanisms which can occur.
is important to realize that no solar wind data were available for this
event and for many similar shock encounters during the CRRES mission. Even
when the data were available, the resolution was often limited to a single
point per hour. Thus we have no direct information on the strength of the
shock, how hard it hit the magnetosphere, or what the unique properties
of the CMEs that drove the shock and the subsequent storm were. The scenario
developed through the test particle simulation made some very specific
predictions that can not be verified. For example, analysis indicates the
shock must have hit the magnetosphere a glancing blow, impacting first
at 15 MLT. It has been suggested that a second shock (or perhaps a density
enhancement) ~1 hour after the first shock may have resulted in a `double
hammer blow' on the magnetosphere to produce additional features in the
particle acceleration during this powerful event. At a later stage of the
analysis, an important tool used to reproduce the observed particle acceleration
for both ions and electrons was a 3-D MHD simulation of shock propagation
and the induced time dependent electric and magnetic fields. These fields
were recorded, inserted in the test particle simulation, and used to drive
the particle motions. The lack of solar wind data removed important constraints
on the MHD simulation boundary conditions. These issues are a re-occurring
theme in this proposal. A major scientific goal of the IMEX mission is
to analyze particle acceleration in the inner magnetosphere using the necessary
particle and fields data in the inner magnetosphere but to also use measurements
obtained in the upstream solar wind by the ACE (or Wind or Cluster) spacecraft.
The magnetic field measurements from ACE are sampled several times per
second and the plasma velocity and density are sampled at least once per
minute, sufficient to resolve solar wind structures for this investigation.
Electric Fields during the Main Phase of Major Geomagnetic Storms
2a. Time Scales for Onset of the Electric Field associated with Enhanced Reconnection: To understand the dynamics of reconnection driven planetary magnetospheres, we must determine how convection is established throughout the magnetosphere. The dayside inner magnetosphere is an excellent vantage point from which to observe because it is close to the reconnection region, but far enough away that the large-scale steady-state signature is not masked by local effects associated with boundary distortions. Coroniti and Kennel (1973) and Kennel (1995) have suggested that the communication of enhanced flow associated with frontside reconnection occurs via a rarefractive fast mode wave. Such a wave is most easily observed when there is a strong, rapid increase of reconnection due to the abrupt appearance of a southward IMF at the dayside magnetopause, as often occurs during major storms. Figure 3 presents anobservation of an interplanetary shock which coincides with a strong increase in a parameter (e*) predicting enhanced reconnection at the magnetopause. This is one of the several events for which interplanetary magnetic field data was available (but no solar wind velocity except one hour time averages). The panel labeled e* is a proxy of the solar wind electric field, e*= vBsin4(q/2), where q =0 for northward IMF and B is the magnitude of the magnetic field. and v is the solar wind velocity, which, in the absence of measurements with the necessary time resolution, is normalized to 1. e* is one of a number of parameters devised to understand coupling(see Wygant et al., 1983 for references). The initial perturbation in the duskward component of the electric field is a negative pulse lasting about one minute (due to the antisunward propagation of the magnetic perturbation of the compressional wave). Immediately after the passage of the wave, the average electric field jumped to 0.5-1.0 mV/m in the duskward direction consistent with sunward convection and this value persisted for hours as observed by CRRES as it traversed the dayside magnetosphere. This jump is evidence for the reafractive wave. Subsequent to this jump, the dawnward electric field increased slowly over the next 0.05 days to 2 mV/m.
Using IMEX and ACE data, we can investigate whether the whether electric field jumps in this type of event are associated simply with the passage of ashock, or whether an enhanced southward component of the IMF is also required. In addition, we will look at the response of the duskward electric field to increases in the southward component of the IMF which are not accompanied by shocks. Since rarefractive waves have been suggested as a trigger for reconnection in the near earth plasma sheet, it would be interesting to know how strong they are at different points along their propagation path in the inner magnetosphere on their way to the reconnection line. The longer time scale (~1 hour) increase in the duskward electric field, apparent in Figure 3, will be measured at different positions in the inner magnetosphere. In this way, the relative contributions of dayside processes to tail phenomena can be tested by looking at many such events at different local time and radial positions. In addition, the high time resolution measurements of ions and electrons available on IMEX will resolve both the parallel and perpendicular pressure variations (over periods of 30 seconds) associated with these waves. This is quite important because it is the redistribution of pressure caused by this wave which sets up the large scale flows.
3. (upper panels) Electric and magnetic field and spacecraft potential
measurements on the dayside magnetosphere from CRRES. Lower panels are
IMP-8 solar wind magnetic field data showing the shock, and e*,
a proxy for the reconnection electric field.
that the waves observed for ~2 hours after the shock are probably due to
ringing of the magnetosphere. Similar waves appear deeper in the magnetosphere
and may be particularly effective in driving radial diffusion (see section
2b. Convection Electric Field and Energization of the Ring Current: The defining process of major geomagnetic storms is the formation of the ring current from the earthward injection of ions (Williams and Sugiura, 1985; Lui et al., 1987). The recovery phase of major storms is associated with the loss of ring current ions through charge exchange with exospheric neutrals, scattering due to ion cyclotron waves, or perhaps through ExB convection to the front side magnetopause. Two pivotal interrelated questions concerning ring current dynamics are: (1) What controls convection ( i.e. the electric field) in the inner magnetosphere during the main and recovery phases of major geomagnetic storms? and (2) How does convection contribute to the formation of the ring current? There has been only one experimental study of electric fields and ring current particles during the main phase of a major geomagnetic storm in the equatorial plane of the inner magnetosphere (Wygant et al., 1997) and none with simultaneous measurements of the upstream solar wind. Thus, our knowledge of major storms is based on indirect evidence, theoretical scenarios, and extrapolation from quiet magnetic periods. A problem that most hinders our understanding of major geomagnetic storms is a lack of knowledge of the global structure of the convection electric field throughout the inner magnetosphere, its dependence on the solar wind, and its relation to particle boundaries such as the inner edge of the plasma sheet.
Case studies of CRRES electric field data during the main phase of major geomagnetic storms demonstrate that electric fields differ dramatically from previous expectations. As magnetic activity increases, electric fields become larger deeper in the inner magnetosphere than they are at larger radial distances. It had generally been believed that the convection electric field penetrates from the outer magnetosphere inward and should be shielded from the inner magnetosphere.These new results have important implications for our understanding of how convection couples from the near earth plasma sheet into the inner magnetosphere and how major geomagnetic storms expand from high to low latitudes. The electric fields and the plasma sheet source population together determine how rapidly and how far plasma is injected into the inner magnetosphere and determine the final energy spectrum of the particles (Chen et al., 1997). The electric field determines the location of the injection boundaries of electrons and ions and the dynamics of the plasmasphere during major geomagnetic storms. The large scale electric field in the equatorial plane maps along magnetic field lines into the ionosphere and, along with ionospheric conductivities, determines the ionospheric joule heating rate which in turn serves as an important boundary condition on the thermospheric response to storms (Fuller-Rowell et al., 1997).
Figure 4 (Rowland and Wygant, 1998) illustrates the change in the radial profile as a function of magnetic activity, as indicated by Kp. The right panel shows the average dawn-dusk component of the electric field from eight months of CRRES observations, binned by Kp, and plotted versus radial distance (L). The family of curves in the lower panel are determined from the Volland-Stern model, binned in the same way as the experimental data. The empirical Volland-Stern model is designed to reproduce the electric field in the inner magnetosphere as a function of location and Kp. Because the model was thought to display the qualitative features governing convection in the inner magnetosphere, it has often used to model the boundaries and trajectories of particles in this region. During periods of low Kp, the electric field decreases at radial positions closer to the earth, consistent with the concept of `ring current shielding'- the idea that electric fields are smaller closer to the earth because the large scale flow avoids regions where it must perform work by adiabaticly compressing and energizing the ring current plasma. As magnetic activity increases, deviations from the Volland-Stern model become dramatic. Data from CRRES has been obtainedfrom only 6 major geomagnetic storms at local times from 12 MLT to 22 MLT. For none of these storms was the solar wind monitored throughout the storm. No measurements of the large scale electric field from 22 to 12 MLT were made.
Fig. 4 (right) CRRES measured average dawn dusk electric field versus radial position (L value) for different Kp (left) The average dawn- dusk electric field as predicted by the Volland-Stern model.
Analytic calculations and simulations of the inner magnetosphere (Jaggi and Wolf 1973, Southwood 1977; Southwood and Wolf 1978, Harel et al., 1981) indicate that two of the most important mechanisms controlling the temporal evolution and spatial configuration of convection in the inner magnetosphere include: (1) the existence of strong plasma pressure gradients associated with adiabatic energization (de-energization) of ring current plasma as it convects closer ( farther) from the earth; and (2) the field aligned current system which is driven by the distribution of hot plasmas and which couples to the conductive ionosphere. The non-uniformity of conductivity due to electron precipitation and day-night differences is a complicating factor. The most important boundary condition is the solar wind driving. Transient substorm processes may also be effective in the earthward acceleration of plasma over comparatively short time periods. The IMEX mission will provide measurements of electric fields, magnetic fields, ring current plasma pressure, and precipitating plasma sheet electrons during 15-25 major geomagnetic storms, at local times (the dawnside and post midnight sectors) never previously been explored with these measurements. The upstream solar wind conditions will be monitored continuously. by the ACE spacecraft. IMEX will provide the first information on the interplay between the solar wind driving forces and the internal forces associated the distribution of hot plasma and coupling to the ionosphere. Simple comparisons from individual orbits of the topology of strong flow regions to ring current pressure distributions and structures like the plasma sheet at different local times and different storm phase, under different solar wind driving conditions, will be made. These measurements will be also analyzed statistically to produce global maps of convection electric field and pressure vs. L value and magnetic local time during major geomagnetic storms. The observations from IMEX will be compared to large scale simulations of the inner magnetosphere, including the recently developed GEM model.
It has been observed that the plasmasphere may vary in size by a factor of 2 during major storms. The measurements of the large-scale electric fields, statistically and for individual passes, will be compared to the plasmasphere density profiles to understand the role of field in eroding the plasmasphere and producing regions of detached plasma.
IMAGE and TWINS spacecraft will be providing maps of ring current pressure
which can resolve dawn dusk and noon midnight asymmetries in the ring current
plasma as well as radial profiles. These ring current images can provide
a global context for the IMEX observations. The Polar spacecraft using
an instrument not specifically designed has observed order of magnitude
changes in plasma ring current pressure over periods of 5 minutes in conjunction
with particle injection events. IMEX will complement similar observations
of the large scale dynamics of the ring current by providing information
on the processes driving the injection.
2c. Relation to Structure of Plasma sheet: The relation of the convection electric field to the plasma sheet boundary does not correspond to what we have learned to expect from less active periods. Measurements from low altitude spacecraft during less active times suggest that the region of strong electric fields coincides with the auroral zone which in turn maps along magnetic field lines to the plasma sheet. Figure 5 presents a pass near 18 MLT during the main phase of a geomagnetic storm. The upper panel displays ion and electron fluxes and the lower two panels show the dawn-dusk and the northward components of the electric field. The region of strong electron fluxes delineates the plasma sheet. Thus during this pass the average electric field is strongest earthward of the plasma sheet and is nearly zero inside of the plasma sheet. These measurements indicate that electric fields are preferentially enhanced deep in the magnetosphere where ring currents ions are energized. An important question that must be addressed is whether the rapid change in electric field is due to a sudden compression of the magnetosphere which results in the plasma sheet crossing the spacecraft and which moves the region of large electric field earthward of the spacecraft, or whether some other change in solar wind is responsible for the observed effects. This event provides an example of the spatial structure observed in Figure 4 for high Kp conditions. Individual orbits obtained during the main phase of the large storms often have dawn dusk electric field components of 4-7 mV/m and total field magnitudes approaching 8 mV/m which is consistent with potential drops, between L=2 and L=6, of 70 kilovolts. These values are about an order of magnitude larger than expected. This observation and others like it suggest we do not understand the processes which result in the earthward convection of the plasma sheet and the formation of its inner edge. CRRES electric field measurements during other major geomagnetic storms show a similar behavior near the dusk local time sector with electric fields consistently larger between L=2-5 than at larger distances.
IMEX would allow us to determine the structure of the electric field and its relation to the plasma sheet over those portions of the inner magnetosphere that were not measured by CRRES and in conjunction with solar wind measurements. In addition, since the IMEX low energy plasma analyzers provide 3d electron and ion distributions over the energy range for 10 eV to 30 keV, we will be able to study how the plasma sheet is populated by processes operating in the auroral acceleration region, as well as by earthward convection of particles from the tail. The six second time resolution for the distributions in conjunction with IMEX fields measurements will allow us to study phenomena as `bouncing ion clusters' (Quinn and McIlwain, 1979; Quinn and Southwood,1982; Mauk and Meng, 1983), and velocity dispersed ions (Williams, 1981; Peroomian et al., 1996) which are related to the acceleration processes populating the plasma sheet.
Fig. 5. Top 2 panels: H+ and electron fluxes (from the Lockheed IMS-LO) delineating the position of the plasmasheet.; Lower 2 panels: Dawn-dusk and northward components of the electric field.
Impulsive Substorm Related Fields: Energization of 10 - 300 keV Ions and
is known that substorm energy release is associated with the reconfiguration
of the geomagnetic tail from a tail-like to a more dipolar geometry. This
reconfiguration generally coincides with the appearance of particles with
energies of 10-300 keV which were presumably accelerated by the electric
fields generated in concert with the magnetic reconfiguration. The CRRES
data show that strong electric fields in association with dipolarization
occur in the inner magnetosphere (often during major geomagnetic storms)
just as they do during substorms in the outer magnetosphere. The processes
deep in the inner magnetosphere (L<5) that create impulsive electric
field associated with `dipolarization events' and the role of such fields
in the energization of electrons have not been assessed. IMEX will investigate
the relative contribution to the acceleration of particles by the large
scale convection electric fields lasting several hours versus the contribution
from transient fields associated with substorms.
3.Main PhaseAcceleration of Relativistic
and Energetic Ions
3a.Energization of ~1 Mev Electrons:Mechanisms for the energization of MeV electrons remain unknown. Figure 2 shows the variations in flux which occur episodically in these fluxes. Paulikas and Blake (1979) showed that MeV electrons appeared with a delay of about 2 days after the passage of fast solar wind streams. Recent observations from CRRES and SAMPEX also show that the electron flux decreases during the main phase of storms and recovers during the recovery phase of the storm (Baker et al., 1994). The flux decrease is attributed, in part, to the diamagnetic effect of the ring current which decreases the local magnetic field, producing a decrease in the energetic electron flux in concert with the conservation of the first adiabatic invariant. The electron flux increases to pre-storm and often higher values when the ring current decays. The energization mechanism resulting in the enhancement of energetic electron fluxes above the pre-storm values is not understood. A correlation with southward IMF, especially deeper in the magnetosphere, suggests that a necessary condition is the enhancement in convection or radial diffusion. If radial diffusion operates, there is the further question of what in the magnetosphere or in the solar wind generates the fluctuations which give rise to radial diffusion. In-situ measurements of electric fields and particles in the inner magnetosphere together with upstream solar wind conditions will enable an assessment of the efficacy of radial diffusion and convection.
Radial transport mechanisms may not be the only ones operating. Measurements suggest that the phase space density of >2 MeV electrons at geosynchronous orbit is often greater than the phase space density of corresponding electrons with the same first adiabatic invariant at greater distances contrary to the expectations from radial diffusion models (Li et al., 1997). A possible explanation is that resonant interactions with whistler waves can energize these electrons. In this scenario, electrons are accelerated from several hundred keV to several MeV over periods of hours through multiple interactions with the whistlers (Roth et al., 1998; Thorne, 1997; Horne and Thorne, 1998). Large amplitude waves must exist over spatially extensive regions and interact with electrons over several drift periods. Test particle simulations indicate that such waves can energize electrons by >1 MeV in several hours. This energization mechanism depends strongly on the amplitude, phase velocity, and the direction of propagation of the whistlers. IMEX will be able to provide this information during major geomagnetic storms.
3b.Radial diffusion:Radial diffusion has long been invoked as a mechanism for energizing electrons and ions. Radial diffusion rates have been determined by a number of indirect methods, but not from in-situ measurements of electric field fluctuations during major geomagnetic storms. The results from CRRES show that the power in the electric field as observed in the inner magnetosphere during quiet times is consistent with that calculated from indirect methods. However, electric field fluctuations increase by several orders of magnitude during the main phase of major geomagnetic storms so that radial diffusion rates may be several orders of magnitude larger than usually assumed.
evidence for large scale global compressional oscillations of the magnetosphere
have been obtained deep in the magnetosphere. These oscillations have been
observed with electric and magnetic field data between L=1.2 and L=3 on
the duskside immediately following a storm sudden commencement and during
the early period of the main phase (see Fig. 3). In order to identify a
fluctuation as a global compressional mode and assess its contribution
to radial diffusion, it is important to establish the azimuthal wave number.
One way to approach this is to use the magnetometers on the GOES spacecraft
which are widely separated in magnetic local time along with IMEX to assess
phase coherence over widely separated positions. At lower L, the high time
resolution Japanese magnetometer station at Kakioka may be compared to
the spacecraft magnetometer signal as it sweeps rapidly in magnetic local
time and radial position. In this way, we have been able to show that observed
fluctuations were coherent over the entire dayside of the magnetosphere
(Wygant et al., 1994). Global oscillations are of interest because the
radial displacement of the magnetospheric boundary is associated with oscillations
in the azimuthal component of the electric field which can accelerate particles
earthward through strong radial diffusion. Since solar wind data were not
available during the observations of these global oscillations, it is not
known whether these oscillations are the result of a continuous wave train
of pressure pulses in the solar wind or of a single or episodic sequence
of `blows'. IMEX will measure the characteristics of the global oscillations
while ACE obtains the upstream solar wind conditions.
Loss of particles from the ring current
There are also important unresolved questions associated with the loss of particles from the inner magnetosphere and its relaxation back to a quiescent state during the storm recovery phase. For example, since large scale convection electric fields have not been measured in most regions of the inner magnetosphere during major geomagnetic storms, it has not been possible to determine when and if convection to the frontside magnetopause is a significant loss mechanism for the ring current. This is important because recent analysis indicates that during the largest geomagnetic storms, the decay of the ring current is too rapid to be explained by pitch angle scattering by ion cyclotron waves, by Coloumb scattering, or by charge exchange (Kozyra et al., 1997; Jordanova et al., 1997).
of the role of ion cyclotron waves in the loss of ring current particles
at L=3 to 4 requires a characterization of these waves in the inner magnetosphere.
An analysis of the ability of ion cyclotron waves to interact with particles
has been hampered by the poor sensitivity of previous electric and magnetic
field detectors. While it has been possible to characterize the phase velocity
of these waves, their propagation direction has not been determined, and
thus their ability to pitch angle scatter particles has not been rigorously
5. Collaborative Observations
5a. Upstream Solar Wind Monitors ACE: Knowledge of the solar wind is important to any study of major geomagnetic storms. Lack of such knowledge results in a fragmentary understanding of the causal chain governing magnetospheric dynamics. CRRES, which operated during the last solar maximum between July of 1990 and October of 1991, was plagued by this problem. The only solar wind monitor at that time was IMP-8, which is in a circular geocentric orbit with a radius of about 30 RE. Not only was the solar wind coverage interrupted when IMP-8 was inside of the Earth's magnetosheath and magnetosphere, but solar wind coverage was often not available because a number of tracking stations were not operational. These tracking stations were in place before CRRES and after CRRES (in conjunction with ISTP) but not during CRRES. The result of this limited coverage is that data are only available about 30% of the time and the data obtained from the solar wind are interrupted by gaps ranging from minutes to many hours. Often the finest resolution is one hour. Complete solar wind coverage including magnetic field and plasma measurements is available for only one of the seven major storms encountered by CRRES during 1991 This is the only geomagnetic storm for which such data exists in the history of spacecraft measurements- a rather remarkable fact for a process of such importance. During the IMEX mission ACE will be at the Lagrangian point upstream of the earth, providing continuous solar wind measurements. ACE has the necessary spacecraft resources to last its two year prime mission phase and a three year extended mission. Goerge Withbroe of NASA's Office of Space Sciences states that funding for the extended ACE mission is assured under all conceivable funding scenarios (see Appendix C).
5b. Remote Sensing of Ring Current Evolution by IMAGE and TWINS: Direct measurements of large scale electric fields and ring current ion composition by IMEX can enhance the scientific return from IMAGE. One of the primary scientific goals of IMAGE is to understand the ring current. This objective is addressed through Energetic Neutral Atom (ENA) imaging. ENA imaging is the detection of neutral atoms created when ring current ions charge exchange with neutral exospheric atoms and then proceed to the detector in a straight trajectory. The spatial resolution of such images is about 0.8 Re and the temporal resolution is about 2 minutes. Direct in-situ measurements of ring current density, composition, and energy along the IMEX trajectory will provide information to constrain IMAGE inversion calculations of the global ring current structure. Conversely IMAGE data will allow us to place radial profiles of the electric field and ring current/plasma sheet particles obtained by IMEX in a global context. Similarly, when IMEX is near local midnight, in-situ measurements of time varying convection electric fields will provide crucial information on the physical processes producing the evolution of the ring current as observed by IMAGE. IMAGE is scheduled for launch in 2000 and should be operational, through 2002. Data from IMAGE will be available on the Web and in the public domain within 24 hours. The IMAGE team is interested in collaborations with IMEX (see letter from PI J. Burch in Appendix C). Similar observations and comparisons can be made with the ENA instrument on the first TWINS mission which be launched in early 2002. One of the IMEX Co-Is is also a TWINS Co-I which will facilitate comparisons.
Although we are aware of the possibility of the comparison of auroral UV imaging with IMEX in situ data, we do not call this out as a science goal due to the short (~6 month overlap) of the projects. Since ENA imaging is available on each of the TWINS spacecraft, this comparison is unlikely to disappear due to slippage of a single mission.
Other missions will also provide important complementary information. The
collaboration between IMEX and other ISTP spacecraft is facilitated by
the fact that this proposal includes scientists who are Co-I's on Polar,
Cluster, and Wind. The four ESA/NASA Cluster/Phoenix spacecraft are scheduled
to be operational during the IMEX timeframe. An example of a science
topic which could be addressed using Cluster and IMEX is the the causal
relation of energy release (dipolarization) events deep in the inner magnetosphere
to those in the more distant tail. Although this and other interesting
comparisons are possible with Cluster, we do not call them out as science
goals due to uncertainties about relative phasing of apogees. Other spacecraft
which will provide complementary measurements include the GPS spacecraft
in circular 12 hour, 4.2 Re orbits, the four LANL spacecraft in geosynchronous
orbit, and the GOES spacecraft.
D.2.a.Instrumentation: Table 1 summarizes a carefully focussed instrument package designed to meet the science objectives described above. It is based on pre-existing designs which have flown on CRRES, FAST, Wind, Polar, Ulysses, and sounding rockets. The instrument package consists of a fields instrument, a 3d plasma analyzer , and a high energy particle experiment which are controlled by a single Data Processing Unit (DPU). By relying heavily on heritage, we are confident that the highly capable instruments neccessary to answer the critical science questions can be built within the time and cost constraints of a UNEX. Specific cost saving which will be implemented on IMEX include: (1) use of a single DPU for all instruments with simplified interfaces; (2) one power converter for all instruments; and (3) simplified instrument modes which reduce hardware, software and operations costs.
CRRES was the only spacecraft to study the inner magnetosphere during solar
maximum with a sensitive electric field detector, particle composition
experiments in the ring current energy range, and radiation belt particle
detectors, it is interesting to compare CRRES with the IMEX.: (1) The time
necessary to construct a particle distribution is controlled by the spin
rate of the spacecraft and by detector design. The spin rate of IMEX will
be six seconds while that of CRRES was 30 seconds. Because of the faster
spin period and improvements in detector design IMEX can measure 2-D distributions
of ring current ions in 3 seconds, an order of magnitude faster than CRRES.
3d plasmasheet electron and proton distributions are measured every 6 seconds
by the UCB ESAs.(2) The most accurate measurement of the electric field
is determined from a fit of data obtained from one complete spin. This
measurement is obtained with a factor of five greater time resolution.(3)
Due to increases in analog to digital conversion capabilities, the electric
and magnetic fields are a factor of 16 more accurate. This means that small
electric and magnetic signals superimposed on top of large amplitude fields
can be detected. This allows us to detect wave modes such as ion cyclotron
waves deeper in the inner magnetosphere. (4) IMEX will measure the 3d magnetic
field perturbation using a search coil magnetometer sampled through burst
memory. This is the first such 3d measurements for a spacecraft in the
inner magnetosphere. (5) The instrument modes on IMEX will be simpler.(6)
IMEX does not have the multiplicity of particle detectors with overlapping
D.2.a.1: Fields Instrument: The IMEX fields instrument is based on the electric field instruments on the ISEE-1 (Mozer et al., 1978), CRRES (Wygant et al., 1992), Polar (Harvey et al., 1996), FAST, and Cluster spacecraft, on the WAVES instruments flown on Wind and Ulysses, and on the magnetometers on Wind, as well as on the experience gained by the IMEX PI and science team through their particiaption in these projects.
Electric fields instrument: The electric field instrument will measure the potential difference between cylindrical sensors at the ends of two pairs of orthogonal booms which are centripetally deployed in the spin plane of the spacecraft. Each boom consists of 45 meters of insulated wire with a 5 meter un-insulated wire sensor at the end. Cylindrical booms were selected primarily because the deployment units are both lower mass and simpler than those for spherical booms. The spacecraft spin axis will be oriented within +/-15 degrees of the sun. Both theoretical analysis and CRRES experience indicate that, with this geometry, the solar illumination of booms and spacecraft will be independent of rotation angle, and the absolute magnitude of the electric field will be measured with an order of magnitude more precision than when the spin axis is orthogonal to the earth-sun line. The sensors will be current biased to control their floating potential
and to minimize their sheath impedance. Minimizing the sheath impedance reduces the errors associated with spurious photoelectron currents flowing between the booms and the sensors and between the spacecraft and the sensors. Theoretical calculations and comparison of measurements from biased and unbiased probes on ISEE-1, CRRES, and Polar show that current biasing can reduce these errors by three orders of magnitude. In a manner similar to CRRES, Polar and Cluster, diagnostic sweeps in voltage will be used to determine the optimum value of the bias current. Using the above techniques and geometry, the CRRES electric field measurement was able to measure electric fields in both high and low density plasmas in the inner magnetosphere with an unprecedented absolute accuracy of 0.1 mV/m, as was shown in Figure 4.
The sheath impedance of the electric field sensors will be about 107 ohms. The capacitance of the sensors to a cylinder one Debye length ways will be about 1000 pF. Near apogee under most conditions the sensors will be capacitively coupled to the medium above about 100 Hz and resistively below this frequency. This means that at frequencies above about 100 Hz the effective antenna length is 1/2 of the length of the booms while for frequencies much lower the booms length is nearly 1.0 times the booms length.
The boom deployment units will be designed, constructed, and tested by Kaleva Design Inc. Members of the boom engineering team have been responsible for the booms on the Air Force S3-3, ESA GEOS 1 and 2, NASA/ESA ISEE-1, Swedish Viking, Swedish Freja, NASA/USAF CRRES, NASA Polar, FAST, and ESA/NASA Cluster spacecraft, as well as boom systems on dozens of sounding rockets. They have also performed analysis of the dynamic stability of booms during deployment, shadow entry and exit, and attitude maneuvers for the above spacecraft. Analysis indicates that IMEX booms have nostability problems.
The pre-amplifier system will be a high input impedance, low leakage current design similar to taht on CRRES. Shielding will be used where necessary to insure radiation survivability. Signals from opposing sensors will be differenced in analog circuitry to remove the contribution from the spacecraft potential and will be converted to a 16 bit word through the A-D system. They are sampled at 40 samples/s to provide the DC electric field measurement. The range of electric field values to be measured will be .03 mV/m to 500 mV/m. The potential difference between the spacecraft and the probe will also be telemetered (at 2 samples/s) providing important information on the electron thermal flux in low density plasmas. The telemetered values will range between +/-100 volts with a resolution of 3 mV. The spacecraft will also implement an electrostatic cleanliness specification to limit charging during energetic electron events. In addition, the power supply systems for the preamplifier on IMEX will incorporate floating power supplies with a dynamic range of 300 volts, as compared to the 100 volt systems on the CRRES spherical sensors and the 30 volts on the CRRES cylindrical sensors. This new feature of the electric field experiment on IMEX should dramatically reduce the incidence of pre-amplifier saturation in comparison to previous spacecraft operating in this region.
The electric field preamplifiers can be reconfigured on command to measure the current gathered by from the sphere during a constant voltage bias. This mode was used only intermittently on CRRES and only once per orbit on the Polar spacecraft but on IMEX it will be used to provide Langmuir probe sweeps once every 5 minutes. These twenty step logarithmic voltage sweeps between +/-30 volts will provide measurements of electron temperature and density over the range from 1 cm-3 from 103 cm and 1 eV to 30 eV. Each sweep will last 1.0 seconds at a specific phase of the boom angle relative to the sun. Experience on CRRES indicates that the Langmuir probe will provide measurements sufficient to track the evolution of large scale density variations during major geomagnetic storms.
The electric field instrument has a burst memory that can operate in two modes. The electric field signal is AC coupled with a roll-on frequency of 10 Hz and a dynamic range of .01 -100 mV/m. The burst mode will be capable of sampling two components of the electric field, three components of the search coil magnetic field at rate of 40,000 samples a second for periods of 1 second an playing back at about 1 kbit/s for a period of about one hour. The burst can be triggered to take place once per hour. Alternatively, the burst can be triggered when the electric or magnetic field sensors produce a signal exceeding a programmable threshold. A second mode consists of sampling at 2000 samples per second for 20 seconds. The high rate is designed to survey the properties of whistler waves to understand their direction pr propagation, phase velocity and amplitude. The second mode is designed to study the lower frequency waves responsible for scattering ions.
Magnetic field instrument: The fluxgate magnetometer sensors will be provided by Dr. Mario Acuna of the Goddard Space Flight Center The GSFC triaxial fluxgate magnetometer measures the three components of the ambient magnetic field in a range from +/-0.001 nT to +/-65536 nT. This 156 dB of dynamic range is obtained via an auto ranging system that steps through 8 different ranges with an intrinsic 12-bit resolution with each range. A similar magnetometer was successfully flown as part of the Wind spacecraft payload [Lepping et al., 1995]. The intrinsic bandpass of each sensor is 0-10 Hz. The sampling rate will be many tens of three axis samples per second. The mass of the sensors is <500 g and the mass of the associated electronics is < 2 kg. The power consumption is <3 W. Like the magnetometer on the Wind spacecraft, a digital spectrum analyzer will be flown. This system consists of a digital signal processor fast Fourier transforming the measurements. ULF radio spectrograms can be directly produced from these measurements. Besides the Wind spacecraft, the GSFC magnetometer is also part of the Lunar Prospector, Mars Global Surveyor, Mars Observer, NEAR, Voyager, and other programs where large dynamic range and sensitivity are required in the measurement of the magnetic field.
Search Coil: The fields instrument will use a triaxial search coil package which includes threeorthogonal sensors on a short extended boom. They are capable of measuringAC magnetic fields between 0.1-20 kHz with a sensitivity below 10-9 nT2 /Hz near 5kHz. The search coil cores will be made with a material called metglas,which makes the sensors very lightweight compared to their ferrite corepredecessors. The sensor mass is on the order of 750g for all three sensors,with power consumption on the order of 200 mW.
D.2.a.2: High Energy Particle EXperiment (HEPEX): HEPEX will make comprehensive measurements of ring current ions and energetic electrons from a few tens of keV to several tens of MeV. HEPEX consists of three sensors with different energy, angle and mass resolving capabilities as summarized in Table H-1. All three instrument are based upon flight proven designs with heritage in the Polar and CRRES missions. Although cost constraints within the UNEX program prevent an ideal instrument with comprehensive energy-angle-mass capabilities, the selected experiments provide the required measurements to complete the IMEX mission. Ring current ion composition, energy, and pitch-angle information are provided by a magnetic-spectrograph that uses an array of ion-implanted silicon detectors and satellite spin to make a 2-D measurement of the ions. A conceptually similar magnetic spectrograph measures energy and pitch-angle distributions of electrons up to ~ 1.5 MeV. Similar magnetic spectrographs have been used by Aerospace and others since the 1960s aboard many space missions (Vampola et al., 1992; Voss et al., 1992). In addition, the energy spectra of relativistic electrons are measured by a 5-element threshold sensor. The spacecraft DPU controls the three sensors, and processes the data for the telemetry system. HEPEX exploits proven technologies to make the crucial measurements at a minimum cost. The individual sensors are discussed below.
Ring-Current Ion Sensor (RIS): The RIS sensor is shown schematically in Figure 6. Ions are dispersed and analyzed by a magnetic spectrograph according to momentum per charge, and then detected by an eighty element array of silicon detectors. The magnetic circuit is completely passive, requiring no external power. There are no high voltages, and no need to vary the biasing voltages on the detectors. The result is an instrument that is sensitive, robust, reliable, and economical to construct. These ion-implanted arrays were used successfully in the Polar energetic particle (Ceppad) investigation (Blake et al. 1995). The 80 pixels allow much better mass characterization through the middle of the energy range and also helps to minimize background due to penetrating radiation. Penetrating background, bremsstrahlung, and Galactic cosmic rays will be approximately uniform along the array whereas the true ion signal of course will not. The detectors are DC coupled to an integrated-circuit chip developed by the Rutherford Appleton Laboratory. This chip also was used in Polar Ceppad (Blake et al. 1995). Five chips are used in RIS, each servicing 16 pixels. The signal amplitudes from the chips are routed to fast ADCs to give the energy of the incident ion; momentum dispersion and energy analysis then yield mass. The large amount of data thus generated is processed onboard before being telemetered.
The momentum dispersion of the major ion species is quite large at low energies, but becomes less at higher energies. Figure 7 shows the energy dispersion as a function of energy for H+, He+, and O+; finite geometric factor effects are included. It shows that for energies of up to ~ 120 keV, these species are well separated at the detector array, and above ~200 keV it is not possible to distinguish between He+ and O+ directly by energy of arrival at a given pixel. However, because the detector window degrades the energy of the O+ more than the He+, some separation can be achieved to higher energies. A two detector stack permits use of the dE/dx, E technique for the higher energy ions that are only slightly deflected. In order to reduce detector/amplifier noise to about 2-3 keV and permit dc coupling, the detector array and preamp/PHA chips are passively cooled by mounting the entire magnet assembly on the side of the spacecraft away from the Sun. Temperatures of ~ -50 degrees Celsius are expected for RIS. Passive cooling has been used very successfully in the Polar Ceppad investigation.
Recent advances in magnetic materials allow the use of higher fields than in the past. The RIS will employ magnets made of neodymium-iron-boron, which has nearly twice the maximum energy product (~44 megagauss-oersted) of the samarium-cobalt used a few years ago and the magnet yoke will be constructed of vanadium permendur. The spectrometer yoke provides excellent radiation shielding for the detector array and a broom magnet in the collimator sweeps away most electrons. An instrument fabricated using these materials was recently flown by the members of the IMEX team from the University of Maryland and performed well.
In summary, the RIS will provide 2-D distributions of H+, He+, and O+ twice per spin; the geometry factors of the instrument across the energy range is well-suited for observing the ring-current ion population. The instrument is a true spectrograph, so it is sensitive to all particle energies at all times.
Energetic Electron Spectrograph (EES): The EES is very similar in configuration to the RIS. The EES will also use the same detector and amplifier technology as RIS. Although in principle the combination of momentum dispersion and energy analysis provides redundant information for electrons, the two-parameter measurement gives an excellent rejection of background events. The magnetic field for the EES spectrometer is much lower than that used for the RIS, and therefore it is lighter and more compact; the energy range of the EES will be 10 keV to 1.5 MeV, giving good overlap with the energy range of the plasma spectrometer.
Relativistic Electron Detectors (RED): Electrons with energies from 2 to 15 MeV are measured with a set of five threshold sensors in a single box. These sensors consist of small cylindrical silicon detectors mounted behind beryllium shells selected to measure electrons with energies from 2 to 15 MeV. The shell and detector are mounted on a relatively thick plate that shields the rear hemisphere. Detectors of this configuration have been used successfully by Aerospace since the early 1960s, CRRES carried 2-shell sensors of this configuration (Blake and Imamoto 1992) but designed for protons only. Energetic protons are measured with energies from 20 to 100 MeV using the sensors at the cost of a single discriminator theshold.
IMEX Low Energy 3-D Plasma Analyzers
The IMEX spacecraft will include ion and electron electrostatic analyzers for low energy plasma measurements provided by the Space Sciences Laboratory at the University of California, Berkeley. The analyzer design is virtually identical to the FAST satellite instruments (Carlson and McFadden, 1998), with the exception of increased radiation shielding. The basic FAST top-hat design has a long spaceflight heritage including AMPTE, Giotto, Mars Observer, Wind, and Cluster. The analyzers consist of 90 degree hemispherical deflection systems that image particle angular distributions over a 180 degree x 5 degree field-of-view onto a microchannel plate detector. After entering the analyzer through an entrance hole in the outer hemisphere, particles are selected in energy by a deflection voltage on the inner hemisphere. Hemispheres are scalloped and blackened to minimize UV contamination. Figure 8 shows the instrument design with electron and ion analyzers mounted together, along with the digital and high voltage electronics. The packaging includes an aperture closing mechanism to prevent detector contamination prior to launch, and plumbing for nitrogen purge during storage. The electronics consist of a preamplifier board with 16 AMPTEK A121 charge sensitive amplifiers, test pulse generators, counters, and shift registors which are read by the interface board. The interface board contains circuits to control the preamplifier threshold, microchannel plate high voltage, sweep high voltage, command decoding and data averaging. Multi-spin averages of the data will be accumulated in RAM on the interface board before inclusion in the mass memory. The electronics makes use of field programmable gate arrays (FPGAs) with most of the design relying on FAST heritage.
The ion and electron analyzers have geometric factors of ~0.005 cm2-sr-eV/eV, dE/E ~ 15%, and contain 8 anodes with 22.5 degree Polar angle resolution. The analyzers will sweep through 64 energy steps, covering ~3 eV to ~30 keV, 64 times per spin. This high sweep rate ensures that narrow field aligned beams are not missed as the analyzers' fan-like field-of-view rotates with the spacecraft. The 64 energy x 8 Polar angle x 64 azimuthal angle measurement is averaged over an integer number of spins into a 128 bin 4pi solid angle x 16 energy array for transmission to the ground. The resolution of this array can be adjusted to provide better energy, angular, or time resolution depending upon the science requirements. Higher time resolution measurements of energy spectra averaged over angle or angular distributions averaged over energy are also available. This instrument provides continuous measurements of the 3d distributions once per spin.
order to reduce background due to penetrating radiation, the analyzer design
will include radiation shielding around the detectors with an effective
thickness of Al greater than that used for the CRRES low energy electron
sensors. Since the IMEX 3-D plasma analyzers have ~100 times geometric
factor of a CRRES low energy electron sensor, and only ~20 times the effective
sensor area, we expect a factor of ~5 improvement in signal to noise over
the CRRES sensors. In addition, incorporation of plastic anti-coincidence
scintillators around the detectors is being considered, if possible without
major design changes which would impact the UNEX budget. Similar analyzers
with anti-coincidence scintillators were developed by the lead scientist
for the Wind 3-D Plasma Analyzer (Lin et al., 1995).
Data Processing Unit
The suite of IMEX detectors, MAG, ESA, HEPEX and FIELDS will be controlled by a single data processing unit (DPU). The IMEX DPU is to be built at Minnesota. Connected to the various instruments with simple serial and/or parallel interfaces which will be defined in detail in phase A, the DPU will control the various sensor configurations. The DPU will collect data from the sensors and format it into a stream of CCSDS packets, a different subpacket type for each of the instruments. The DPU will time-tag the data as it is collected in order to allow unambiguous reconstruction of events on the ground. The DPU will also calculate a checksum for insertion into each packet to ensure data integrity on the ground. The packets will the be transferred over a serial interface to the on-board data handling (OBDH) computer where they will be stored in a solid state recorder (SSR) for later playback to the ground. It may be advantageous to attach the SSR directly to the science DPU allowing the DPU direct access to the 1.5Gbit memory and relieving the OBDH of the responsibility for the SSR control. The optimal location of the SSR will be determined in phase A.
The DPU will also receive and decode telecommands from the OBDH. These telecommands may be real-time commands received by the OBDH and passed immediately to the DPU for execution or they may come from the OBDH stored command table (SCT) which will allow commands to be delivered to the DPU at specific times. The commands may also compose a microprocessor memory load intended to provide new functionality for the science DPU. In any case, the DPU will decode the commands and act upon them accordingly. A limited number of commands could be hazardous to the operation of the spacecraft or the instruments (e.g. high voltage enabling or antenna deployment commands). Safeguards will be built into the DPU and the spacecraft to prevent execution of hazardous commands without complete validation. While the DPU will, as much as possible, be built to withstand the IMEX environment, it will also protect itself from dangerous conditions which could arise as a result of radiation. A watch dog timer will provide a hardware reset if the processor becomes hung-up. After a hard reset, the processor will configure its subsystems into ìsafeî modes and continue with normal operations as much as is possible.
DPU will be based on flight hardware and software built for the WIND WAVES
instrument. That DPU controlled five separate receivers using a variety
of interfaces similar to those proposed here. The IMEX DPU will be based
on the same rad-hard SA3300 microprocessor. Flight software will be based
on the existing multi-tasking real-time operating system with new code
as required to support the new instruments. Much of the existing flight
software (e.g. real-time kernel, interrupt handling, command reception
and decoding, timing, packet collection and formatting) will be reused
Many of the issues described in this section are discussed in more detail in the science and technical sections. The IMEX spacecraft will be launched into a low inclination geosynchronous transfer orbit with a period of about ten hours which gives two radial passes through the region of interest. This will allow sampling on a time scale commensurate major geomagnetic storms, the physics of intense electric fields and particle acceleration in the inner magnetosphere. IMEX will be spin-stabilized with a period of six seconds. Four 50-meter wire electric field antennas will be deployed in the spin plane. The spin axis will point toward the sun (within about 15 degrees) keeping the illuminated area of the wire antennas constant throughout a rotation (rather than varying by orders of magnitude). This insures that their photo-emission which results in variations in the sensor floating potential is relatively constant - a necessity for an accurate electric field measurement. Spinning at 10 rpm also allows particle sensors to measure particles with different pitch angles. The pointing requirements are not stringent and deviation from sun pointing is acceptable to allow horizon and sun sensors to provide accurate attitude and spin phase information. The spacecraft attitude will be maintained with a pair of body mounted magnetic torquers. Apogee will precess over about 17 months providing complete coverage of the inner magnetosphere over the two-year mission. The launch will take place in the middle of 2001 at the peak of solar activity. IMEX will ride along as a secondary payload on a military Titan IV launch (please find the letter of endorsement from Dr. D. Gorney of Aerospace Corp. in Appendix C).
will collect data during the maximum of the solar cycle when geomagnetic
storms and shocks due to Coronal Mass Ejections are the most powerful and
frequent. C). The spacecraft will have a science data rate of 17 kbits/s
or about 1.5 Gbit per day. The data will be stored on a solid state recorder
on the spacecraft. A single one-hour contact each day with an 8m TOTS tracking
antenna will allow all telemetry data to be collected. Since the instruments
and the spacecraft are designed to operate with little intervention from
the ground, operations costs will be held to a minimum. IMEX will require
only minimal operations activities each day. Operations will be provided
by the University of Colorado (LASP) using an existing student operated
operations center. After collection and low level processing, the data
will be distributed electronically to the various Co-I facilities where
data analysis will be done.
Data Analysis and Archiving
We propose a client-server based data distribution model with a WEB-based interface, which allows for substantial platform independence so that each of the scientists and engineers can use the analysis platform he prefers. Software is managed centrally, and each participating institution has its own copy of the data on-line. with the same level of easy access. The annual yield of 80 GB is easily managed on magnetic disks. This system is currently being used with the Wind/WAVES experiment. We are able to keep our data on-line and we can read bench, pre-launch, post-launch, near-real-time, and real-time data with identical software, as well as ephemerides, tables of frequencies, calibration constants, and data in CDF form from other spacecraft. Unlike most standard formats, this system describes the data as it is in the level 0 file. Calibration computations are made on the fly from functions within the system. The GSE for both the spacecraft and the experiments will use this system, redusing costs. We will have a telemetry access server at each of our hardware institutions (UM, CU, UCB, GSFC and AC). The server will usually be attached to the internet so that team members will have access to real-time data collected, for example, during spacecraft thermal vacuum testing.
Data received from the spacecraft (or during integration and test)will be sent directly to the Mission Operations Center (MOC) for level-zeroprocessing and for monitoring the health and safety of the spacecraft. The MOCsends the level 0 data electronically to the SOC at the UM which records the data, produces daily science summary plots which are placed on line for access by the scientific community, electronically sends level 0 data to Co-Investigators, and records data to CDs for long term archive. In addition the SOC will confirm data quality and generate key parameter datafiles which will be electronically sent to NSSDC.
data analysis software will be based upon programs developed for Geotail,
CRRES, Polar, Wind, and FAST. This software is currently used by most of
the IMEX scientists and includes a sophisticated program (SDT)that reads
level 0 data and can rapidly display particle and field data in a variety
formats. The main software development needed for IMEX is the level 0 access
software which will written at the University of Minnesota, and which is
callable in Fortran, C and IDL In addition,an extensive package of IDL
routines already exists for detailed scientific analysisof both particles
and fields data. The analysis software will be made available to any interested
scientist, along with level 0 data, via the World Wide Web, allowing the
space physics communityunlimited access to the full resolution data.
D.2.d. Science Team Roles
The Principal Investigator, Professor John Wygant (UMn) will have overall responsibility for the project and coordination of the efforts of the science team members. Dr. Wygant is a co-investigator on the Polar, Cluster and CRRES electric field instruments, and his roles was as the instrument scientist involved in the design, prototyping, fabrication and testing of the instruments. He also was the scientist in charge of over 30 balloon electric field payloads launched in 4 different campaigns. He is the PI on an ionospheric rocket with fields instrumentation very similar to that planned for IMEX. Co-I Professor Dan Baker (LASP, Univ. of Colorado) will have lead responsibility for the construction of the spacecraft. Dr. Baker is the Director of LASP, and, therefore, oversaw the development of the STEDI spacecraft, SNOE. He is a Co-investigator on numerous energetic particle instruments, including Polar and SAMPEX. Co-I Dr. Bernie Blake (Aerospace Corporation) will coordinate the accommodation with the launch vehicle. He is the Director of the Space Sciences Department of Aerspace, and has been a Co-investigator on more than 20spacecraft missions including the energetic particle instruments on Ulysses, SAMPEX and Polar.
Dr. Wygant will also have prime responsibility for development of the fields instrumention and software, with the assistance of Co-I Prof. Paul Kellogg. Dr. Kellogg has been a PI on many sounding rockets, and a Co-I on numerous NASA and ESA missions. The Plasma Scientist will be Co-I Dr. James McFadden (UCB). He will lead the design, construction, testing and software development for the 3d plasma instrument. Dr. McFadden is a Co-investigator on FAST mission, and on the Cluster, Wind and Mars Observer plasma instruments. He has also been a Co-I on more than 5 auroral sounding rockets. The hardware effort on the High Energy ParticleExperiment will be lead by Dr. Joe Fennell and Dr. James Clemmons (Aerospace Corporation). Dr. Fennell has been a Co-I on many NASA, ESA and Swedish spacecraft particle instruments, and a PI on several Air Force spacecraft.. Dr. Clemmons will have prime responsibility for the composition instrument, as well as the software development for the energetic particles.Dr. Clemmons is the PI on a NASA sounding rocket and has participated in construction of many particle detectors and analysis of data from several spacecraft. Dr. Manuel Grande (RAL) will provide the RAL chip for the ion composition experiment, and Dr. Michael Coplan will provide the magnets for HEPEX. Co-I Dr. Mario Acuna (GSFC) will provide the fluxgate magnetometer. Keith Goetz will have responsibility for the development of the DPU and instrument interfaces. He is a Co-I and waves instrument manager on numerous ESA and NASA missions including WIND and Ulysses, and has often been asked to participate in reviews of both spacecraft contractors andinstrument and mission development. Prof. Cynthia Cattell (UMn) will lead the data analysis software development, including the level zero access software. She is a Co-I on FAST, Polar (electric field, magnetic field and theory) and Cluster, and has a long involvement with multi-instrument satellite data analysis. She was a member of the Mercury Orbiter and Grand Tour Cluster Science Study Teams, and is the PI of the AMPS mission concept study.
Mary Hudson (Dartmouth College) will lead the theory and modeling studies.
She will work with Dr. XinLin Li (LASP/ CU) and Dr. Michael Temerin (UCB).
Drs. Li and Hudson will have prime responsibility for the simulations and
associated data analysis related to ring current evolution, shock acceleration
and substorm injection. Dr. Temerin will focus on analysis of data and
theory of electron energization via whistlers. Prof. C. Cattell will have
responsibility for the overall coordination of the Education and Outreach
program between the Co-I institutions and other related NASA mission outreach
programs, such as those for IMAGE and HESSI. She will also be the primary
IMEX contact for the HBC/OMU program (if it is funded). Dr. Randi Quanbeck
(UMn) will direct the Minnesota-based K-12/public program and the outreach
to tribal colleges. Dr. Cheri Morrow (SSI) will guide the Colorado K-12
and informal science program. Prof. Baker (CU) will coordinate the LASP
programs. Dr. Nahide Craig, education scientist for the HESSI project and
member of SSL's Center for Science Education, will guide the UCB Education
and Outreach effort and coordinate it with the HESSI EO plan. All Co-Is
will participate in analysis of the data. The experience and capabilities
of the scientists are described in the Appendix B.