Ion Heating in the Martian Ionosphere

Abstract

Energetic O+ and O 2+ ions with energies of up to a few hundred eV are observed in the Martian ionosphere. Corresponding ion velocity distributions show ion conics, suggesting that the observed ion populations have been heated perpendicular to the local magnetic field before experiencing a magnetic mirror force. Magnetic field observations support these interpretations: wave power at the local O+ and O 2+ gyrofrequencies in the spacecraft frame is observed coincident with the energetic ions, within an apparent magnetic field bottle‐like topology. Analysis of the observed ion conics leads to estimates of ion temperatures of 10–30 eV. We suggest that the ion populations are initially heated perpendicular to the local magnetic field by wave power propagating inward from the Mars‐solar wind interaction. The local magnetic field “balloons out” in response to these enhanced ion temperatures and pressures. The resultant magnetic field topology is bottle like; the transversely heated ions would subsequently experience a magnetic mirror force in the converging field regions, agreeing with the reported observations. Such strong heating events that significantly increase the ion temperature and pressure, thereby decreasing the net magnetic field, are rare and seem to occur under specific interplanetary magnetic field orientations. Events were observed to span the upper exobase region and just above, a region characterized by significant ion densities in an increasingly collisionless domain. Ion heating in this region has the potential to drive significant ion outflows, thus contributing to atmospheric loss from the planet.

Martian Electron Temperatures in the Subsolar Region: MAVEN Observations Compared to a One‐Dimensional Model

“…Ergun et al () suggested that ionospheric electrons could be directly heated by electric field power generated in the solar wind that propagates into the ionosphere. Fowler et al () show that below about 200 km, heating by this mechanism is negligible. At higher altitudes, wave heating, a term not included in the energy equation used here, would increase the electron temperature.…”

Impact of Martian Crustal Magnetic Field on the Ion Escape

“…The mechanism for this process is similar to the one operating in the intrinsic magnetospheres—polar wind driven by the ambipolar electric field (Collinson et al, 2015; Dubinin et al, 2011; Ergun et al, 2016; Ma et al, 2019; Xu et al, 2018). Factors important for the escape in classical polar and auroral winds—the enhanced electron temperature and ion heating by wave power transmitted to the ionosphere—operate at Mars too (Ergun et al, 2016; Fowler et al, 2017). The existence of the tail lobes in the Martian tail filled by low‐energy ionospheric ions supports the analogy with the Earth's case (Dubinin et al, 2017).…”

MAVEN Observations of Solar Wind‐Driven Magnetosonic Waves Heating the Martian Dayside Ionosphere

“…Compressional wave modes similar to those studied here have also been observed at Comet P/Halley (e.g., Glassmeier et al, ; Mazelle et al, , ; Mazelle & Neubauer, ), suggesting that such waves may represent a fundamental element of the solar wind interaction with unmagnetized bodies. Transverse wave mode heating is also known to occur within the dayside Venusian ionosphere (Scarf et al, ; Taylor et al, ) and has been suggested as a source for ionospheric heating at Mars (e.g., (Ergun et al, ; Fowler, Andersson, Peterson, et al, ; Fowler, Ergun, et al, ). Thus, the solar wind interaction with unmagnetized bodies in general appears to produce a variety of wave modes that can propagate into the ionospheres of these bodies and deposit significant amounts of energy there.…”

“…MAVEN is the first dedicated aeronomy mission to Mars and as such is the most capable to date of characterizing the plasma environment within the ionosphere there. Enhancements in electron temperature within the upper dayside ionosphere have been observed to correlate with enhancements in electric field wave power (Fowler, Andersson, Peterson, et al, ), and significantly, heated ion populations have also been observed in the middle to upper dayside ionosphere (Fowler, Ergun, et al, ). Electric field wave power whose source was assumed to be within the magnetosheath has been observed to penetrate 100–200 km into the dayside upper ionosphere (Fowler, Andersson, Halekas, et al, ) and is a possible energy source for these ionospheric energization events.…”