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Michael T. Johnson
Researcher

General Information:

Polar Publications

• Johnson, M. T., J. R. Wygant, C. Cattell, F. S. Mozer, M. Temerin, J. Scudder, Observations of the seasonal dependence of the thermal plasma density in the Southern Hemisphere auroral zone and polar cap at 1RE , J. Geophys. Res., 106(A9), 19023-19034, 10.1029/2000JA900147, 2001.

  Abstract: Synoptic maps of the thermal plasma density in the auroral zone and polar cap around 1 RE altitude are produced from measurements of Polar spacecraft floating potential during 1 year of perigee passes. The densities are accurate to around a factor of 2 in this region of space. These measurements, which provide the first comprehensive maps of the thermal plasma density over the polar cap and auroral oval, show clear variations in plasma density due to solar illumination of the ionosphere. Number density increases by a factor of 5 for illuminated verses nonilluminated ionospheric conditions over the entire auroral oval and polar cap. The maps delineate the global extent of the large-scale auroral density cavity near 70 +/- 5 invariant latitude. These maps further show that the depth of the density cavity is strongly influenced by solar illumination. The minimum average density (~0.1 cm^-3) occurs in darkness near 1900 magnetic local time.

• Johnson, M. T., J. R. Wygant, C. A. Cattell, and F. S. Mozer, Seasonal variations along auroral field lines: Measurements from the Polar spacecraft , Geophys. Res. Lett. , 30 (6), 1344, doi:10.1029/2002GL015866, 2003.

  Abstract: Measurements from the Polar electric field instrument are used to study large electric fields and the ambient plasma density as a function of altitude (1.8 to 6.0 RE geocentric). Results from the premidnight sector (1800 to 2400 MLT) along auroral field lines show roughly a fourfold increase in the occurrence of small scale size, large amplitude electric fields (>100 mV/m) at altitudes from 1.9 to 2.5 RE for dark compared to sunlit ionospheric conditions. Density values inferred from spacecraft potential measurements show these electric fields to be correlated with low plasma densities (0.2 to 3.0 cm^-3). An increase in the average plasma density from 10 to 60 cm^-3 is also observed for sunlit compared to dark conditions for the same altitude range. In addition, the distribution of density measurements from 1.9 to 2.2 RE also show evidence for an increase in cold ionospheric plasma (from 30 to 60 cm^--3) for sunlit compared to dark ionospheric conditions.

• Johnson M. T., J. R. Wygant, The correlation of plasma density distributions over 5000 km with solar illumination of the ionosphere: Solar cycle and zenith angle observations , Geophys. Res. Lett., 30 (24), 2260, doi:10.1029/2003GL018175, 2003.

  Abstract: Measurements from the Polar electric field instrument are used to infer the thermal electron density of the polar cap for invariant latitudes (ILAT) > 80 deg as function of solar zenith angle (SZA) over a solar cycle. Results for altitudes from 1.75 to 2.0 RE geocentric show an increase in the plasma density for daytime ionospheric measurements as the solar cycle activity increases. Densities are found to increase from 75 cm^-3 at solar minimum by over a factor of 2 at solar maximum. The density measurements are also shown to increase with increasing solar F10.7 flux for SZA < 70 deg and remain relatively constant for SZA > 110 deg. These measurements are consistent with solar extreme ultraviolet (EUV) control of plasma density at these altitudes over the polar cap. The influence of solar EUV on plasma density is shown to extend to altitudes up to 4.5 RE geocentric.

Thesis

Here is my thesis:

• "Thermal Plasma Structure of the Magnetosphere: Floating Potential Measurements from the Polar Spacecraft"

  Abstract: The physical structure of the different regions in the magnetosphere is determined by the interaction of electric fields, magnetic fields, and charged plasma particles. In particular, the density of particles is important because it influences the propagation of plasma waves, affects the ability of the plasma to carry currents, and provides a mechanism for storing kinetic energy. The Polar spacecraft, owing to the large spatial and temporal coverage of its orbit, is ideal for studying the seasonal and solar cycle variations of the plasma density at geocentric distances less than 9 RE. In the analysis presented here, the spacecraft floating potential is measured using the Polar electric field instrument (EFI) and employed to infer the electron plasma density. These density measurements allow the examination of the characteristics of plasma density in different regions of the magnetosphere, the correlation of the plasma density with processes associated with the visible aurora, and the variation of the ionosphere as a source of magnetospheric plasma. The results presented here show strong seasonal plasma density variations at altitudes far above the ionosphere. These variations are directly correlated to changes in the illumination of the ionosphere below and are seen in both the polar cap and auroral oval. Altitude distributions of plasma in the auroral acceleration region show these density changes are correlated with the occurrence of large electric fields, and thus, with the generation of auroral displays. In the polar cap, these measurements show a dependence of the plasma density at 2 geocentric on both solar zenith angle and the solar cycle. These measurements provide useful boundary conditions for modeling different acceleration mechanisms responsible for the polar wind and can be compared to simulation results to determine the relative importance of physical processes on polar wind outflow.