P32A-01
The Structure of the Mars Ionosphere
The Mars Express Radio Science Experiment MaRS sounds the ionosphere of Mars at microwavelengths and covers altitudes from the base of the ionosphere at 80 km to the ionopause at altitudes between 300 km and 800 km. The Mars ionosphere consists of a lower secondary layer M1 at about 110 km, and the main layer M2 at about 135 km altitude, both formed by solar radiation at X-ray and EUV, respectively. The precise and detailed observations of the Mars Express radio science experiment indicates the presence of another layer M3 in the topside above the main layer M2 with a shape of a Chapman function as the transition region between the photochemically induced and Chapman-like M1 and M2 layers and the transport dominated highly dynamical topside region above 200 km altitude. Sporadically, a region of enhanced ionisation below the M1 layer can be observed which is caused by the infall of meteors in the atmosphere.
P32A-02
Climatology of Mars Thermosphere and Exospheric Temperatures
The accelerometer experiment facility aboard Mars Reconnaissance Orbiter (MRO) obtained neutral density measurements and inferred temperature measurements 100km to 170km over the entire Southern Hemisphere of Mars in 2006. When combined with our previous accelerometer experiments on Mars Global Surveyor and Mars Odyssey, global measurements are obtained showing variations with latitude, seasons, day/night, aphelion/perihelion, and solar activity. The measurements also show a stability of climatology when compared with MEX Stellar Occultation measurements. Temperatures are found to be fairly constant within 40° latitude of the equator rising with altitude due to solar euv radiation absorption at higher altitudes. Exospheric temperatures are discovered to be near 130 K on the nightside (3am) and near 200K on the dayside (3pm). MTGCM models are about the same as observed at night, but somewhat warmer than observed on the dayside. The cooler dayside observations may indicate somewhat less atmospheric escape than the MTGCM temperatures would predict. We have discovered winter polar warming in the lower thermosphere from the accelerometer measurements. Apparently, meridional flow from the summer to winter hemisphere results in strong adiabatic heating near the winter pole causing the warming. This effect is stronger at winter North Pole than winter South Pole apparently due to the planet being near perihelion at North Pole winter.
P32A-03
MRO Accelerometer Experiment: Thermospheric Features Interpreted Using GCM Simulations
The MRO Accelerometer Experiment has recently measured upper atmospheric density and inferred temperatures in the southern hemisphere of Mars from 95 to 170 km during solar minimum conditions [e.g. Keating et al., 2006]. MRO began aerobraking in early martian southern Fall (Ls = 35) and continued for approximately 5-months with 450 aeropasses, ending in early southern winter (Ls = 109) near aphelion. This seasonal, latitude and solar local time sampling partially overlaps Mars Global Surveyor (MGS) sampling during Phase 2 aerobraking, when solar moderate conditions prevailed [Bougher et al., 2006]. Overall, data/model comparisons are very useful for understanding these MRO density and temperature datasets and placing them in the context of previous MGS Accelerometer measurements. Measured MRO latitudinal gradients in densities and inferred temperatures (at a constant altitude) will be compared with coupled MGCM-MTGCM model simulations in order to illustrate the role of the inter-hemispheric circulation in maintaining the observed density and temperature structure. Weak winter polar warming features for both MRO and MGS observing periods will be compared with each other (and with MGCM-MTGCM simulations), resulting in an initial characterization of weak inter-annual variations in this warming near aphelion. Finally, inferred MRO and MGS temperature profiles (including exospheric temperatures) will be compared with corresponding MGCM-MTGCM simulations. This comparison will contribute to an estimate of solar cycle variations of thermospheric temperatures near 70°S latitude for similar seasonal and solar local time conditions. Underlying thermal balances giving rise to these temperature variations will be extracted from MGCM-MTGCM simulations.
P32A-04
Martian Atmospheric Density Near 250km Altitude From MRO Radio Tracking Data
We present the atmospheric density results obtained from more than one year of Mars Reconnaissance Orbiter spacecraft (MRO) radio tracking data. The stronger signature of atmospheric drag in the MRO Doppler data, compared to the Mars Global Surveyor and Mars Odyssey missions, enables density estimation at higher temporal frequency, up to once every 2 orbits (4h). We processed the high quality, 1-second Doppler tracking data in short arcs using the GEODYN program, starting after the October 2006 solar conjunction. We first used long-arc durations (desaturation maneuvers are separated by 2 to 3 days), but the arc convergence started to deteriorate in April-May 2007. This increase in the Doppler residuals is linked to the seasonal density enhancement. The a priori atmospheric models do not completely capture the density structure, and modeling errors accumulate. Thus, we shortened the arcs (0.7day), with no data gap greater than 5 hours and no desaturation maneuver. The resulting Doppler residual RMS is low (0.7-0.8mm/s) and stable from arc to arc. To assess the robustness of the measurements and evaluate the temporal variability of the density, we processed the tracking data using different sets of a priori models and adjustable physical parameters. Densities calculated from two independent a priori atmospheric models (Stewart 1987 and the recent MCD4.2) show reasonable agreement, given the large prediction discrepancy. We also used MCD4.2 with a high dust opacity setting starting June 2007 to account for the dust storm observed by various remote sensing instruments. We use the obtained density results to evaluate those models. Taking into account the self-shadowing effect on the spacecraft cross-section in the force modeling has an influence on the drag and solar radiation estimation, but surprisingly not on the arc convergence. Of course, the density estimation frequency strongly influences the results. The scatter and formal uncertainty of the measurements increase when we decrease the estimation periods. We calculate the density variability at different timescales from various time series (2, 3, 4 and 6 orbits). The current seasonal trend of increasing atmospheric density is clearly visible in our results, from 10- 13kg.m-3 in November 2006 to 8.10-12kg.m-3 in June 2007 (at 250km altitude above the South Pole). However, contrary to previous MGS and Odyssey results, we do not detect the solar rotation periodicity in the density time series, because of low solar activity and more efficient CO2 cooling processes at lower altitude.