Recovery and validation of Mars ionospheric electron density profiles from Mariner 9

Abstract

Electron density profiles from the ionosphere of Mars that were obtained by the Mariner 9 radio occultation experiment in 1971-1972 have unique scientific value because they extend to higher altitudes than comparable datasets and were acquired during a tremendous dust storm that had substantial and poorly understood effects on the ionosphere. Yet these profiles are not publicly available in an accessible format. Here, we describe the recovery of these profiles, which are made available as part of this article. The validity of the profiles was tested by using them to explore the effects of a dust storm on the topside ionosphere, the morphology of the topside ionosphere, the behavior of the M1 layer, and possible meteoric layers. The dust storm that waned over the course of the primary mission (November-December 1971) had major effects on the ionosphere of Mars. It elevated the M1 and M2 layers of the ionosphere by 20-30 km, but the separation of the two layers stayed fixed throughout the primary mission, which suggests that the neutral atmosphere at these altitudes was not heated during the dust storm. However, the altitude of the 1500 cm −3 density level, a proxy for the top of the ionosphere, decreased steadily by 74 ± 12 km over the course of the primary mission. Mariner 9 observations of the topside ionosphere differ from comparable Mars Express observations. Compared to Mars Express, the Mariner 9 data, which were acquired during a period of relatively high solar wind dynamic pressure, have lower densities at high altitudes. They are also more likely to have a "one scale height" morphology than a "two scale height" morphology. The peak density of the M1 layer depends on solar zenith angle and solar irradiance similarly to previous studies with Mars Global Surveyor observations, which indicates that dust storms do not affect the behavior of the peak density. No clear meteoric layers were identified.

Annual variations in the Martian bow shock location as observed by the Mars Express mission

“…In this season, as Mars approaches the Sun, increased solar irradiance and temperatures lead to a warmer and more circulated Martian atmosphere, which then lifts dust off the surface of Mars and causes the so-called dust storms. These dust storms have been previously shown to interact with the upper atmosphere and ionosphere of Mars in various ways, including the possibility of modulating the neutral densities and TEC [e.g., Wang and Nielsen, 2004;Mouginot et al, 2008;Liemohn et al, 2012;Withers, 2009;Withers and Pratt, 2013;Withers et al, 2015;Xu et al, 2014;Haider et al, 2015;Němec et al, 2015]. However, we do not draw any further conclusions on how the dust storms could directly impact the location of the Martian bow shock and leave such an investigation to a future study.…”

Spatial, Seasonal, and Solar Cycle Variations of the Martian Total Electron Content (TEC): Is the TEC a Good Tracer for Atmospheric Cycles?

“…This produces a thermal expansion of the lower atmosphere that is also observed in the upper atmosphere and as a consequence produces an increase in the altitude of the ionospheric peak (e.g., Hantsch & Bauer, ). Furthermore, Withers et al () recently reanalyzed the Mariner 9 radio‐occultation profiles that were recorded during a severe global dust storm (Kliore et al, ) and found a similar result; the ionosphere was systematically lifted upward by 20–30 km, although the peak density of the ionosphere was not affected. These observations suggest that while the ionosphere/thermosphere system was only moved upward, the total electron/ion column density, the TEC, was not affected.…”

Characterizing Average Electron Densities in the Martian Dayside Upper Ionosphere

“…First, radio occultation experiments can be used to derive electron density profiles, that is, electron densities as a function of the altitude, spanning the altitudes both below and above the peak altitude. These were performed using the Mariner 9 spacecraft (Kliore et al, , ; Withers, Weiner, et al, ), Mars Global Surveyor spacecraft (Hinson et al, ), Mars Express spacecraft (Grandin et al, ; Pätzold et al, ), and recently using the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft (Withers et al, ). The main disadvantage of these profiles is that their spatial coverage is restricted by geometric constraints imposed by the orbits of Earth and Mars (Tyler et al, ; Withers et al, ).…”

“…First, radio occultation experiments can be used to derive electron density profiles, that is, electron densities as a function of the altitude, spanning the altitudes both below and above the peak altitude. These were performed using the Mariner 9 spacecraft (Kliore et al, 1972a(Kliore et al, , 1972bWithers, Weiner, et al, 2015), Mars Global Surveyor spacecraft (Hinson et al, The second possible type of measurements is direct in situ measurements of electron/ion densities. Historically, a couple of such profiles was obtained by Viking 1 and 2 landers (Hanson & Zuccaro, 1977).…”