Analysis of Two Selected Solar Events in 2011 and 2015 With Mars Express Radio Occultation Data

Abstract

Martian ionosphere has a stratified structure with two main layers in its electron density profile (Ne). The primary layer (M2 layer) is formed by solar EUV radiation (~20-90 nm) and has a peak electron density at around 120-140 km altitude with a peak density of ~1011 m-3. The second layer (M1 layer) occurs at a lower altitude with a peak electron density of ~109 m-3 and is formed by solar X-ray and electron impact ionization. The electron densities and the altitudes at which these peaks occur vary with space weather activities. Radio Occultation (RO) experiments provide vertical electron density profiles that span the entire ionosphere. Therefore, RO experiments are ideal for understanding the variabilities of Martian ionospheric parameters (peak density, peak altitude, and Total Electron Content (TEC)). Here, we study the effect of solar flares and interplanetary coronal mass ejections (ICMEs) on the Martian ionosphere for two selected solar events in 2011 and 2015, using the publicly available Mars EXpress (MEX) radio occultation (RO) data (MaRS). The 2011 event was associated with a single flare and ICME1 while the 2015 event includes a series of ICMEs and flares2. The MaRS residual Doppler data for the selected periods were processed to obtain the electron density profiles using RO data processing pipeline developed at the Royal Observatory of Belgium3. For both events, the temporal variations of total electron content (TEC) and electron density profiles are retrieved to analyze and quantify the ionospheric response due to solar flares and CMEs. The analysis showed that the effects of solar events were observable in Mars upper atmosphere for up to several weeks, with the influence gradually decaying following the peak intensity at the arrival of CME. The overall electron density structure showed no evident changes in both events, but a gradual decrease in M2 peak altitude was observed for the 2011 event. An abrupt change in scale height was also observed for some of the profiles in 2011 and 2015, following a high-impact flare or CME. The overall trend of the measured TEC showed a good agreement with the predictions, however, no clear signs of variation due to solar events were observed. All the RO measurements available for this study were 1-4 days earlier or later than the peak events. Thus, this study also points to necessity of having more frequent RO measurements and multi-instrument monitoring of the ionosphere. Figure 1: The electron density profiles shifted 0.5 units along the x-axis, showing the gradual decrease in M2 peak altitude following the 2011 solar event. Figure 2:  The SZA, M2 peak density, and M2 peak altitude obtained from MaRS data (black) with 2015 solar events (vertical-coloured lines). The M2 peak density and M2 peak altitude are compared with the NeMars model predictions (green). The NeMars gives these parameters without considering the solar event.