MGS Electron Density Profiles: Analysis and Modeling of Peak Altitudes

We have analyzed most of the electron density profiles returned from the Mars Global Surveyor Radio Science Experiment, with a view toward investigating the shapes of the profiles and the altitudes of the upper (F1) and lower (E) peaks as a function of solar zenith angle and solar activity. We first categorize the shapes of the profiles according to the morphology of the F1 and E peaks, and find that there is an expected variation with solar activity of the two major types of shapes, those in which the E-peak appears as a shoulder on the bottomside of the F1 peak, and those in which the E peak is separated by a small minimum from the F1 peak. Since the peak altitudes have been shown to vary with planetocentric longitude, we divide the data into 36 10°E. longitude bins. We have plotted the peak altitudes of both the upper and lower peaks as a function of ln(effective secant χ), where χ is the solar zenith angle. The “effective secant” is derived by integrating the densities along the line of sight to the Sun and dividing the result by the vertically integrated densities. We then fit the peak altitudes in each longitude bin with linear least squared regressions, and report the slopes and intercepts of the lines, which, in Chapman theory, correspond to the scale heights and the subsolar peak altitudes, respectively. We find that both parameters are highly variable, and the median slopes for the F1 and E peaks are about 7 and 4 km, respectively. If interpreted as a scale height, the latter value implies a temperature in the unrealistic range of 70–75 K. In Chapman theory, there is no solar activity variation of the peak heights. When we plot the peak altitudes versus F10.7 for each longitude bin, however, and fit them to a trendline, we find that the mean slopes are negative for both the F1 and E peaks, although the slopes are in general smaller for the F1 peaks. We conclude that the E peak heights are inversely correlated with solar activity, but the evidence is not as strong for the F1 peaks. We compare electron density profiles from a numerical model to Chapman profiles, and show that the fit is poor for one Chapman profile, but is improved with a superposition of two Chapman profiles, although there are still large deviations, especially on the topside. Finally, we plot the peaks of the near-terminator numerical models for low and high solar activities as a function of ln(effective secant χ), and find that the linear fits appear to be good, but the slopes are not indicative of the scale heights in our models, nor are the intercepts the same as the peak heights of our 0° models. We conclude from these various studies that there is considerable evidence for non-Chapman behavior in the ionosphere of Mars