Observation of conical electron distributions over Martian crustal magnetic fields

摘要

Electron angular distributions similar to bidirectional electron conics (BECs) near Earth's auroral regions have been previously reported at Mars. They are almost always symmetric about 90° pitch angle, having peaks between 35°-70° and 110°-145°. Signatures of Martian BECs are clearly observable from –.90 eV to –640 eV and they are mainly observed in darkness (60% of 150,000 conic events identified globally). Statistical analysis shows that BECs mostly occur on horizontal magnetic field lines over moderate crustal fields (-15 nT). They are surrounded by regions containing electrons with trapped/mirroring pitch angle distributions, which suggests that BECs form on closed field lines. The energy spectra of the conics exhibit substantial decreases in all energy levels in relation to neighboring regions, which mostly have access to the Martian magnetotail or magnetosheath. Upstream conditions (draped IMF direction, solar wind pressure, and EUV flux) do not affect the observation of the events. Therefore stability inferred from observations of similar BECs over the same geographical locations suggests that the driving conditions resulting in their formation operate on the crustal fields. We propose that conical electron distributions may be generated by merging of neighboring open crustal magnetic field lines resulting in the trapping of the incident plasma they carry initially. Electrons at –90° pitch angle may subsequently be pushed into the loss cone by wave-particle interactions, static, or time varying electric fields, resulting in the conics. BECs may also be generated by mirroring of the particles that are streamed to lower altitudes on nearby open field lines, which then diffuse and/or scatter onto inner closed field lines.