The Effects of Crustal Magnetic Fields and Solar EUV Flux on Ionopause Formation at Mars

Abstract

We study the ionopause of Mars using a database of 6,893 ionopause detections obtained over 11 years by the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) experiment. The ionopause, in this work, is defined as a steep density gradient that appears in MARSIS remote sounding ionograms as a horizontal line at frequencies below 0.4 MHz. We find that the ionopause is located on average at an altitude of 363±65 km. We also find that the ionopause altitude has a weak dependence on solar zenith angle and varies with the solar extreme ultraviolet (EUV) flux on annual and solar cycle time scales. Furthermore, our results show that very few ionopauses are observed when the crustal field strength at 400 km is greater than 40 nT. The strong crustal fields act as minimagnetospheres that alter the solar wind interaction and prevent the ionopause from forming.

Martian Ionopause Boundary: Coincidence With Photoelectron Boundary and Response to Internal and External Drivers

“…The ionopause altitude above the strong crustal fields is higher on the dayside, by more than 200 km for some SZA ranges. This result is also consistent with Garnier et al (2017), who showed that the altitude of the PEB depends strongly on the crustal magnetic fields, which increase the altitude on average by about 150 km, and with Chu et al (2019) who found that the crustal magnetic fields increase the altitude of the ionopause by ~1.58 km per nT.…”

“…The occurrence rate is slightly less near the poles; however, the difference is not very significant. Considering the data coverage is homogeneous with respect to geographic longitude (Chu et al, 2019), we find no evidence for an inhomogenous distribution of ionopause locations. We find that the ionopause occurs with approximately the same frequency at all latitudes and longitudes.…”

“…The ionopause can also be observed by remote sounding with MARSIS. The steep density gradients show up as a short horizontal line at low frequencies, below about 0.5 MHz, with the ionospheric echo being observed at higher frequencies and longer time delays (Duru et al, 2008; Chu et al, 2019). To detect the ionopause remotely, it has to be below the spacecraft.…”

“…However, as the crustal field magnitude becomes high, it may prevent the formation of the steep density gradients. According to Chu et al (2019), the crustal fields have a clear negative effect on the formation of the ionopause. Although we do not find a strong correlation in our data, we can say that there may be a slight effect for crustal field values greater than 100 nT at 400 km.…”

“…Due to their more generous definition of steepness, they observed the ionopause in about 50% of the cases. Recently, Chu et al (2019) studied the steep density gradients using MARSIS remote sounding data and showed that the occurrence rate is affected by the extreme ultraviolet (EUV) radiation and crustal magnetic fields.…”