Effect of the solar radiation in the topside atmosphere/ionosphere of Mars: Mars Global Surveyor observations

Abstract

The peak electron density in the ionosphere of Mars and the neutral atmosphere scale height are proportional to the solar radiation flux. The data of the radio occultation experiments on board Mariner 9 and Viking 1 and 2 have already been analyzed to derive the relationship between the peak electron density and neutral atmosphere scale height and F10.7 flux which was used as a proxy of the solar UV radiation. The data points from Mariner 9 and Viking 1 and 2 missions are distributed from −40° to +38° in latitude and rather nonuniformly distributed in longitude. The experiment with the Mars Global Surveyor (MGS) accelerometer has revealed significant diurnal variations and latitudinal and longitudinal variations in the neutral atmosphere density and scale height. The effect of the solar radiation can be more confidently established if the effects of diurnal, latitudinal, and longitudinal variations are minimized. In the northern hemisphere the 732 electron density profiles derived from the data of the MGS radio science experiment, which were collected during the mapping phase of the mission, are located in the narrow latitude interval from +63° to +77°. In the southern hemisphere, 219 profiles are located in the latitude interval from −69° to −64.5°. These profiles were also obtained within narrow intervals of local time and are practically uniformly distributed in longitude. The peak electron density and the effective scale height of the neutral atmosphere density in the vicinity of the ionization peak have been derived for each of the profiles studied. The daily averages of E10.7 index are derived from the solar radiation fluxes measured near Earth. Their values are then recalculated, accounting for the relative positions of Mars and the Earth, and are used as a proxy of the EUV radiation flux at Mars. For the time period November 2000 to January 2001 the calculated daily averages of the adjusted peak electron density and effective scale height have been compared with the daily averages of E10.7 index. Analysis has shown that Te was, on average, slightly larger in the southern hemisphere than it was in the northern hemisphere. The hot electrons could be actually trapped within the latitude interval from −69° to −64.5° and the strong crustal magnetic fields have detectable but modest effect in the southern hemisphere Te might be, on average, 50–60% larger than Te in the northern hemisphere. In the case of density profiles measured in the period 1–31 January 2001 in the northern hemisphere Te is likely primarily controlled by the solar wind interaction with the Martian atmosphere/ionosphere.

Effects of Low Energetic Neutral Atoms on Martian and Venusian Dayside Exospheric Temperature Estimations

“…However, their measurements can be used to derive vertical profiles of atmospheric density, pressure and temperature, but due to the higher entry mass compared to MPF the atmospheric structure reconstruction could only be achieved below altitudes of about 100 km (Withers and Smith, 2005), which does not allow so far any accurate prediction of exospheric temperatures. (1); the dotted lines correspond to a temperature of 350 K. The best fits for the medium solar activity in Figure 3a correspond to 190-200 K, which is in good agreement with exospheric temperatures inferred from models using MGS aerobraking data during medium solar activity (Bougher et al, 2000). Even at very high solar activity periods (F 10.7 = 220), the neutral gas temperature at about 150 km does not exceed ∼220 K (Figure 3b) and temperatures of 350 K or more appear very unlikely.…”

“…For their later Mars thermospheric simulations Bougher et al ( , 2000 assumed a 22% heating efficiency and obtained seasonal-solar cycle maximum exobase temperatures of up to 380 K which are not compatible with the equilibrium CO 2 exobase temperatures of less than 250 K for solar maximum estimated in the present study. Moreover, besides the EUV solar heating, also near-IR solar heating is present in the lower CO 2 -rich thermosphere.…”

“…An even lower thermospheric peak temperature of 153 K for low solar activity was obtained during the nighttime descent of Mars Pathfinder (MPF) in 1997 (Schofield et al, 1997;Magalhaes et al, 1999). Moreover, scale heights derived from aerobraking data of Mars Global Surveyor above 150 km altitude implied exospheric temperatures of about 220 K during moderate solar activity conditions (e.g., Keating et al, 1998b;Bougher et al, 2000).…”

“…In order to explain the apparently large variations of the Martian dayside exospheric temperature of 150-350 K from low to moderate and high solar activity periods, Bougher et al (2000) considered seasonal-solar cycle extremes for the exobase temperature calculations and estimated the temperature to vary from 200 K near aphelion during solar minimum to 370 K near perihelion during solar maximum. However, the results of various studies by Bauer and Hantsch (1989), Krasnopolsky andGladstone (1996), Breus et al (2004) and Lichtenegger et al (2004) indicate that such extreme variations of the average dayside exospheric temperature are in disagreement with the observed solar cycle dependence of the ionospheric peak plasma density as measured by the Viking Lander 1, the Viking Orbiters 1 and 2, Mariner 4, 6, 7, the Mars 2, 3, 4, 6 and Mariner 9 spacecraft.…”

“…The two aerobraking data sets yield modelled exospheric temperatures during medium solar activity of about 220 and 230 K, respectively (Bougher et al, 2000).…”