Electron pitch angle distributions as indicators of magnetic field topology near Mars

We have analyzed electron angular distributions recorded near Mars over a period of 7 years in order to constrain the topology of magnetic field lines near Mars. We used 63 million pitch angle distributions of 115 eV electrons measured at ∼400 km altitudes by the Mars Global Surveyor (MGS) spacecraft and classified them according to their shape. Closed magnetic field lines are associated with the Martian crustal magnetic fields and are identified on the nightside by the presence of plasma voids or two‐sided loss cones (trapped distributions). Trapped distributions on the nightside are most often observed in regions surrounding moderate or strong crustal fields, indicating a source process such as reconnection populates the outer layers of closed magnetic field regions. Open magnetic field lines are identified in regions of strong crustal magnetic field by the absence of field‐aligned electrons returning from the planet (loss cones). In regions far from crustal sources many field lines intersect the collisional atmosphere. On the Martian dayside, closed field lines are identified by the presence of trapped or fully isotropic distributions, and they occur at times when ionospheric photoelectron features are evident in MGS electron energy spectra. Variability of the dominant pitch angle distribution shape in certain regions suggests that Martian magnetic field topology is dynamic and is controlled by external conditions.