HR: 1340h
AN: SA13A-1116    [Abstracts]
TI: Measuring Mars' Upper Atmospheric Density from 170-260km with Electron Reflectometry: Seasonal Response & South Polar Winter Warming
AU: * Engel, J H
EM: Jengel@ssl.berkeley.edu
AF: UCB - SSL, Space Sciences Lab, University of California, Berkeley, CA 94720 United States
AU: Lillis, R
EM: rlillis@ssl.berkeley.edu
AF: UCB - SSL, Space Sciences Lab, University of California, Berkeley, CA 94720 United States
AU: Mitchell, D
EM: mitchell@ssl.berkeley.edu
AF: UCB - SSL, Space Sciences Lab, University of California, Berkeley, CA 94720 United States
AU: Lin, R
EM: rlin@ssl.berkeley.edu
AF: UCB - SSL, Space Sciences Lab, University of California, Berkeley, CA 94720 United States
AU: Acuna, M
EM: Mario.H.Acuna@gsfc.nasa.gov
AF: NASA - GSFC, Goddard Space Flight Center , Greenbelt, MD 20771 United States
AB: Using MGS MAG/ER data and the technique of electron reflectometry, we present density measurements of Mars' nighttime upper atmosphere in the southern hemisphere between 1999 and 2004. For electron motion on open magnetic field lines connecting the solar wind to remanent crustal magnetization, reflection from magnetic gradients and absorption due to collisions with atmospheric neutrals results in a characteristic pitch angle-dependent attenuation in the electron flux, known as a loss cone. We develop a kinetic model of this interaction, assuming the validity of spherical harmonic expansions of the crustal field in regions of intense magnetization (such as Terra Sirenum), and assuming an adjustable, 2-species, isothermal atmosphere above 170km. Loss cones in the data are fitted to this model to constrain densities and temperatures in the upper atmosphere. Our range of sensitivity in altitude is based on the absorption probability of incident solar wind electrons and is ~170-260km. ~50,000 reliable measurements were made and binned by time and latitude. Major findings include 1) expected seasonal expansion/contraction of the upper atmosphere, 2) latitude dependence of densities and temperatures Showing southern winter polar warming, 3) observation of transition Altitude from CO2-dominance to O-dominance at ~200km. We present these findings and compare to the Mars Thermosphere Global Circulation Model (MTGCM) These measurements are important for improved understanding of the processes affecting Mars' upper atmosphere and of the orbital stability of future low-altitude spacecraft such as the 2005 Mars Reconnaissance Orbiter.
DE: 5409 Atmospheres--structure and dynamics
DE: 6225 Mars
DE: 0343 Planetary atmospheres (5405, 5407, 5409, 5704, 5705, 5707)
DE: 0394 Instruments and techniques
SC: SPA-Aeronomy [SA]
MN: 2004 AGU Fall Meeting