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D. L. Mitchell, R. P. Lin (UCB-SSL), H. Rème (CESR, Toulouse, FR), P. A. Cloutier (Rice Univ.), M. H. Acuña (NASA-GSFC)
The Electron Reflectometer (ER) onboard Mars Global Surveyor (MGS) measures the energy and angular distributions of 10 eV - 20 keV electrons. The ER can distinguish between solar wind electrons and ionospheric photoelectrons on the basis of their energy spectra. During the elliptical aerobraking orbits, a boundary separating solar wind and ionospheric plasmas was consistently observed at altitudes ranging from 200 to 800 km and solar zenith angles >75 degrees in the northern hemisphere (Mitchell et al., GRL 27, 1871, 2000). This boundary is likely coincident with the ionopause.
The MGS mapping altitude of ~400 km places the spacecraft close to the 380-km median ionopause altitude, resulting in numerous boundary crossings. We have categorized more than two million electron energy spectra from the first 15 months of the mapping orbit (February 1999 through April 2000) to investigate systematic variations of the ionosphere in response to crustal magnetic fields and solar ionizing radiation.
The distribution of crustal magnetic fields is the dominant factor influencing the probability that the ionosphere will reach the spacecraft altitude of 400 km or higher. Over strong-field regions of the southern hemisphere, this probability exceeds 50%, whereas over weak-field regions in the northern hemisphere (e.g., Tharsis and Elysium) it is less than 10%. The ionosphere over weak-field regions also shows a measureable response to variations in solar EUV, with the probability increasing from ~1% to ~20% as the F10.7 flux (scaled and time-shifted to account for solar rotation and the different orbital positions of Earth and Mars) increases from 30 to 80 sfu's. The probability over strong-field regions shows little response to variations in solar EUV, probably because of the dominating influence of the crustal fields.