On the possible source of the ionization in the nighttime Martian ionosphere: 1. Phobos 2 Harp Electron Spectrometer measurements

Abstract

The measurements of electron spectra in the Martian magnetosphere by the HARP instrument on board the Phobos 2 orbiter are presented. The energy of the electrons (a few tens of electron volts) is sufficient for the impact ionization of the planetary neutral gas, and the characteristic flux of electrons (∼108 cm−2 s−1) could produce the nightside ionospheric layer with a peak density of a few thousands of electrons per cubic centimeter, which corresponds to densities observed earlier during radio occultations of the Mars 4 and 5 and Viking 1 and 2 spacecraft. The possibility of magnetospheric electron precipitation into the nightside atmosphere of Mars is in agreement with the mainly induced nature of the magnetic field in the planetary magnetotail (as at Venus), while the variability of the Martian nightside ionosphere may be explained by the partial screening of the atmosphere by a weak intrinsic magnetic field of the planet.

Nightside ionosphere of Mars studied with local electron densities: A general overview and electron density depressions

“…Therefore, sinks dominate sources, and the gradual recombination of ionized components leads to the depression in all three instruments. The partial screening of the nightside atmosphere of Mars by a weak intrinsic magnetic field of the planet was concluded by Verigin et al [1991] to be one of the reasons for the variability of the Martian nightside atmosphere.…”

“…They also stated that ion transport from the dayside and particle (mainly electron) impact are the two main sources for the nightside ionosphere at Mars. Verigin et al [1991] used Phobos 2 HARP spectrometer measurements and an analytic method to calculate the ionization rates. They stated that the magnetospheric electrons can be a possible source of ionization in the nightside ionosphere of Mars.…”

Solar EUV and Electron-Proton-Hydrogen Atom-Produced Ionosphere on Mars: Comparative Studies of Particle Fluxes and Ion Production Rates Due to Different Processes

“…The third objective of the paper is to reanalyze these radio occultation data by introducing solar wind electrons and protons as a new source of ionizations in the nighttime ionosphere of Mars. We have reported previously that maximum ionization occurred at about 158 km due to the precipitation of magnetotail electrons in the nighttime ionosphere (Verigin et al 1991, Haider et al 1992, Haider 1997. This calculation was carried out during a solar medium period, appropriate for Phobos 2 measurements.…”

Magnetic Field and Plasma Observations at Mars: Initial Results of the Mars Global Surveyor Mission

“…4). Impact ionization at Mars' night-side ionosphere was reported by Verigin et al (18). These cold electrons above the ionopause may indicate the presence of clouds or streamers of cold ionospheric plasma detaching from the ionopause, as was frequently observed at Venus (19).…”