Bipolar Wave Packets Parallel to the Magnetic Field on the Plasma Sheet Boundary

C. A. Cattell

At altitudes of 30000 to 40000 km, the POLAR satellite traverses the boundary between a region of near vacuum (the tail lobes) and a region of higher density, flowing plasma (the plasma sheet). At this boundary there are many sources of energy, including currents which flow along the geomagnetic field. These currents have been observed by the Polar magnetometer. Observations from the POLAR electric field instrument have shown that these energy sources very frequently cause waves to grow to such large amplitudes that the shape of the waves is distorted. Such processes are called non-linear and several types of nonlinear electric fields are described in other nuggets. An example of two different types of nonlinear waves, which occur in a region of field-aligned current, is shown in Figure 1. In the top (middle) panel the electric field parallel (perpendicular) to the geomagnetic field is plotted. This separation into components parallel and perpendicular to the geomagnetic field is done because, in the magnetic bubble surrounding the earth, the physical processes which occur in the direction of the earth's magnetic field are different from those which occur perpendicular to the magnetic field because particles can be thought of as being attached to a magnetic field line like beads on a string. A wave parallel to the magnetic field at a frequency of 20 Hz in a packet lasting ~0.2s can be seen in the top panel. This parallel wave does not have any associated fluctuations in the magnetic field, implying that it is a kind of wave called electrostatic. The waves most people are familiar with, for example radio waves and light waves, are electromagnetic and have both electric and magnetic field variations. The observed wave frequency is close to the frequency of ion acoustic waves, which are electrostatic and are analogous to sound waves in a fluid. The parallel, electrostatic packet is immersed in a region of large amplitude, higher frequency waves (~100 Hz) perpendicular to the magnetic field. The amplitude of these waves is modulated at a lower frequency which is similar to the frequency at which ions gyrate about the geomagnetic field (the ion cyclotron frequency). This type of modulation is often a clue that nonlinear processes are occurring. The perpendicular waves have associated magnetic fluctuations, implying that they are electromagnetic.

At lower altitudes in the auroral acceleration region, similar bipolar wave packets parallel to the magnetic field have been observed. At that altitude they occur at the ion cyclotron frequency and in association with several other types of electric field structures which are also at that frequency (see nugget 1). The high altitude waves may be due to the propagation of the low altitude ones and the relationship between the two is currently under study.

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Figure 1: Example of Nonlinear Waves Occurring in a Region of Field-aligned Current

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