Variation of the Martian ionospheric electron density from Mars Express radar soundings

The Mars Advanced Radar for Subsurface and Ionospheric Sounding aboard Mars Express has been in operation for over 2 years. Between 14 August 2005 and 31 July 2007, we obtain 34,492 ionospheric traces, of which 14,060 yield electron density profiles and 12,291 yield acceptable fits to the Chapman ionospheric model. These results are used to study the Martian ionosphere under changing conditions: the presence or absence of solar energetic particles, solar EUV flux, season, solar zenith angle, and latitude. The 2‐year average subsolar maximum electron density n0 is 1.62 × 105 cm−3, the average subsolar electron density altitude h0 is 128.2 km, and the average neutral scale height H is 12.9 km. Solar energetic particle events are associated with a 6% increase in n0, a 3 km decrease in h0, and a 0–7 km decrease in H. The value of n0 varies smoothly between 1.4 × 105 and 1.8 × 105 cm−3, yielding d ln n0/d ln F10.7 = 0.30 ± 0.4; h0 varies between 115 and 135 km, while H remains relatively constant with EUV flux and season, in contrast with previous work. The value of h0 decreases toward the terminator at low latitude but increases poleward during summer; H varies from 11 km, for solar zenith angle less than 40°, to between 14 and 17 km near the terminator, depending on season. Near‐peak temperatures vary between 220 K and 300 K, less variation than indicated by modeling, probably due to sampling near solar minimum.