Photoelectrons are an important population of the dayside Martian ionosphere, which are produced by solar Extreme Ultraviolet and X-ray ionization of atmospheric neutrals. A typical photoelectron energy spectrum is characterized by a distinctive peak near 27 eV, related to the strong solar HeII emission line at 30.4 nm, and an additional peak near 500 eV, related to O Auger ionization. In this study, the solar driven scenario for photoelectrons is verified with the aid of the extensive measurements made by the Solar Wind Electron Analyzer on board the recent Mars Atmosphere and Volatile Evolution spacecraft. Our analysis reveals that the photoelectron intensities at the centers of both peaks increase steadily with increasing solar ionizing flux below 90 nm and that the observed solar cycle variation is substantially more prominent near the O Auger peak than near the HeII peak. The latter observation is clearly driven by a larger variability in solar irradiance at shorter wavelengths. When the solar ionizing flux increases from 1 mW m-2 to 2.5 mW m-2, the photoelectron intensity increases by a factor of 3.2 at the HeII peak and by a much larger factor of 10.5 at the O Auger peak, both within the optically thin regions of the Martian atmosphere.
The photoelectron peak at 22–27 eV, a distinctive feature of the energetic electron distribution in the dayside Martian ionosphere, is a useful diagnostic of solar extreme ultraviolet (EUV) and X-ray ionization as well as of large-scale transport along magnetic field lines. In this work, we analyze the pitch angle distribution (PAD) of energetic electrons at 22–27 eV measured during several representative Mars Atmosphere and Volatile Evolution (MAVEN) orbits, based on the electron spectra gathered by MAVEN’s Solar Wind Electron Analyzer (SWEA) instrument. On the dayside, most photoelectron spectra show an isotropic PAD as is expected from production via solar EUV/X-ray ionization. The photoelectron spectra occasionally observed on the nightside show instead a strongly anisotropic PAD, indicative of cross-terminator transport along ambient magnetic field lines. This would in turn predict the presence of dayside photoelectrons, also with a strongly anisotropic PAD, which was indeed revealed in SWEA data. Comparison with magnetic field measurements made by the MAVEN Magnetometer suggests that on average the photoelectrons with anisotropic PAD stream away from Mars on the dayside and towards Mars on the nightside, further supporting the scenario of day-to-night transport. On both sides, anisotropic photoelectrons tend to be observed above the photoelectron exobase at ~160 km where photoelectron transport dominates over local production and energy degradation.
O++ is an interesting species in the ionospheres of both the Earth and Venus. Recent measurements made by the Neutral Gas and Ion Mass Spectrometer (NGIMS) on board the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft provide the first firm detection of O++ in the Martian ionosphere. This study is devoted to an evaluation of the dominant O++ production and destruction channels in the dayside Martian ionosphere, by virtue of NGIMS data accumulated over a large number of MAVEN orbits. Our analysis reveals the dominant production channels to be double photoionization of O at low altitudes and photoionization of O+ at high altitudes, respectively, in response to the varying degree of O ionization. O++ destruction is shown to occur mainly via charge exchange with CO2 at low altitudes and with O at high altitudes. In the dayside median sense, an exact balance between O++ production and destruction is suggested by the data below 200 km. The apparent discrepancy from local photochemical equilibrium at higher altitudes is interpreted as a signature of strong O++ escape on Mars, characterized by an escape rate of 6×1022 s–1.
Photoelectrons are produced by solar Extreme Ultraviolet radiation and contribute significantly to the local ionization and heat balances in planetary upper atmospheres. When the effect of transport is negligible, the photoelectron energy distribution is controlled by a balance between local production and loss, a condition usually referred to as local energy degradation. In this study, we examine such a condition for photoelectrons near Mars, with the aid of a multi-instrument Mars Atmosphere and Volatile Evolution data set gathered over the inbound portions of a representative dayside MAVEN orbit. Various photoelectron production and loss processes considered here include primary and secondary ionization, inelastic collisions with atmospheric neutrals associated with both excitation and ionization, as well as Coulomb collisions with ionospheric thermal electrons. Our calculations indicate that photoelectron production occurs mainly via primary ionization and degradation from higher energy states during inelastic collisions; photoelectron loss appears to occur almost exclusively via degradation towards lower energy states via inelastic collisions above 10 eV, but the effect of Coulomb collisions becomes important at lower energies. Over the energy range of 30–55 eV (chosen to reduce the influence of the uncertainty in spacecraft charging), we find that the condition of local energy degradation is very well satisfied for dayside photoelectrons from 160 to 250 km. No evidence of photoelectron transport is present over this energy range.
The exobase is defined as the interface between the strongly collisional and the collisionless parts of an atmosphere. Although in reality the exobase is a transition region of finite depth, it is conventionally defined as the boundary above which an upwardly ejected neutral particle makes one collision at higher altitudes. Such an idealized definition is of practical use and serves as a good tracer of the overall size of an atmosphere as it expands and contracts under the influences of both external and internal sources. Knowledge of the atmospheric properties near the exobase is crucial to first-order estimates of atmospheric escape rates on terrestrial planets. Since its arrival at Mars on 21 September 2014, the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft has provided comprehensive maps of the Martian upper atmosphere under a variety of conditions. This allows, for the first time, a thorough investigation of the variations of the exobase altitude on this red planet. In this study, we use the N2 density measurements accumulated by MAVEN’s Neutral Gas and Ion Mass Spectrometer from October 2014 to November 2018 to determine the exobase altitudes for a large number of MAVEN orbits. Our analysis reveals clearly the variations of exobase altitude with local time and solar extreme ultraviolet (EUV) flux, as well as tentative evidence for the impact of global dust storms. These observations are indicative of thermal expansion of the Martian upper atmosphere, driven either externally by solar EUV energy deposition or internally by global dust storms.