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

• Published in: Geophysical research letters 2019-09, Vol.46 (17-18), p.10257-10266
• Main Authors: Chu, F., Girazian, Z., Gurnett, D. A., Morgan, D. D., Halekas, J., Kopf, A. J., Thiemann, E. M. B., Duru, F.
• Format: Article
• Language: English
• Subjects: Altitude; crustal magnetic fields; Cycle time; Density gradients; Dependence; Field strength; Ionograms; Ionopause; Ionosphere; Magnetic field; Magnetic fields; Mars; Mars Express; MARSIS; Planetary magnetic fields; Radar; Solar cycle; Solar EUV; solar EUV flux; Solar magnetic field; Solar wind; Sounding
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2019GL083499

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. Key Points The Martian ionopause is found to have an average altitude of 363±65 km The ionopause altitude has seasonal variations in response to the solar EUV flux The ionopause rarely forms over strong crustal magnetic fields