MGS nighttime observations of bi-directional electron conics over the moderate crustal magnetic sources on Mars

Conical distributions similar to bi-directional electron conics located near the auroral regions at Earth have been previously reported in the MGS MAG/ER data at Mars [Brain et al., 2007]. These bi-directional flux distributions are almost always symmetric about pitch angles of 90 degrees, with peaks at pitch angles from 30-70 and 110-150 degrees. About 300,000 of these events are detected over the nighttime hemisphere at 400 km altitude using electron angular distributions at 115 eV, but similar signatures are also detectable at energies from a few tens to hundreds of eV. Statistical analysis of the observations shows that these conics frequently occur over moderate horizontal crustal fields (~15 nT). The energy spectrum of the conics exhibit substantial decrease in all energy levels in relation to the spectra in the neighboring regions that don’t have conics. Typically these events are surrounded by either trapped or extended trapped radiation which implies that these conical distributions may be due to trapping of neighboring downward electrons on inner closed field lines. The absence of electrons at pitch angles around 90 degrees may be due to the fact that the only source of electrons for inner closed field lines is electrons scattered near the lower atmosphere. Or, similar to observations at Earth, diffusion in the pitch angles of trapped particles due to wave-particle interactions or time-varying electric fields may also be responsible for the loss of electrons around 90 degree pitch angles, which is also one of the main loss mechanisms of magnetically confined plasma at Earth. In this study we will report the characteristics of these events in detail and discuss their possible generation mechanisms.