HR: 1330h
AN: P32A-1069    [PDF]
TI: Characterizing Primary Photoelectrons in Mars' Ionosphere
AU: * Chou, S H
EM: sherry@ssl.berkeley.edu
AF: Space Sciences Laboratory, University of California, Berkeley, CA 94720
AU: Mitchell, D L
EM: mitchell@ssl.berkeley
AF: Space Sciences Laboratory, University of California, Berkeley, CA 94720
AU: Lin, R P
EM: boblin@ssl.berkeley.edu
AF: Space Sciences Laboratory, University of California, Berkeley, CA 94720
AB: The Electron Reflectometer on Mars Global Surveyor measured the energy/angle distributions of 10 eV to 20 keV electrons at altitudes from 170 km to $>$ 17,000 km during aerobraking. Two low-altitude plasma regimes were identified, separated by a highly variable but persistent boundary called the photoelectron boundary, or PEB. Below the PEB, the energy spectra are dominated by ionospheric photoelectrons, and we observe the shapes of these spectra to change systematically with altitude and solar zenith angle (SZA). For $85\,^{\circ}$ $<$ SZA $<$ $95\,^{\circ}$, the 10 eV - 100 eV spectrum changes significantly from 170 km to 230 km, above which there are no further changes. At lower solar zenith angles ($65\,^{\circ}$ - $75\,^{\circ}$), these changes occur over a larger altitude range (170 km - 260 km). The 100 eV - 1000 eV spectrum changes significantly from 170 to 210 km, independent of solar zenith angle. The altitude range over which spectral changes are observed is close to the exobase, where collisions with atmospheric neutrals become important. Spectral changes likely result as the spacecraft approaches the exobase and the volume of ionosphere sampled by a given electron spectrum extends to lower altitudes, where collisions and compositional variations can modify photoelectron energies. When the spacecraft is well above the exobase, the volume of ionosphere sampled is independent of altitude, and the photoelectron spectrum is constant.
DE: 2459 Planetary ionospheres (5435, 5729, 6026, 6027, 6028)
DE: 2481 Topside ionosphere
DE: 2499 General or miscellaneous
DE: 5435 Ionospheres (2459)
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: 2003 Fall Meeting