Asymmetry of the Mars Ionosphere Boundary Altitude during a Solar Energetic Particle Event

The Electron Spectrometer (ELS) and the Ion Mass Spectrometer (IMA) from the Analyzer of Space Plasmas and Energetic Atoms (ASPERA-3) experiment on the ESA's Mars Express (MEx) spacecraft have been used to study a Solar Energetic Particle (SEP) event associated with a Class X solar flare on on January 27, 2012. Arrival of the SEP at Mars about 46 minutes later is observed as an increase in the background of these plasma instruments. The background counts were observed to increase sharply, followed by a gradual decrease that lasted for about 4 days. During this time, ELS and IMA also recorded passages across the Martian ionospheric boundary on the dusk side of the planet, twice during each MEx orbit. The altitude of the ionospheric boundary was thereby found to have behaved differently in the northern and southern hemispheres. The boundary increased in altitude in each hemisphere with a time delay as the flare pumped energy into the Mars system. After reaching peak altitude, the ionospheric boundary returned to its original configuration faster in the northern than in the southern hemisphere. This suggests that the main difference between the northern and southern hemispheres at Mars, namely the presence in the south of crustal magnetic fields, is responsible for the dissipation of the energy input at a slower rate in the southern than in the northern hemisphere.