MAVEN Observations of the Response of Martian Ionosphere to the Space Weather Events of March 2015

The Mars Atmosphere and Volatile EvolutioN (MAVEN) mission spacecraft observed the interaction of interplanetary coronal mass ejections (ICMEs) with the Martian upper atmosphere during March 2015. We study the response of the ionosphere of Mars to two adjacent solar disturbances during 3 - 4 March 2015 and 8 - 9 March 2015 using the observations from Neutral Gas and Ion Mass Spectrometer (NGIMS) and Langmuir Probe and Waves (LPW) aboard MAVEN. During these events, the ionospheric boundary (ionopause-like density gradient) was observed at significantly lower altitudes. During 3 March, the ionopause was observed at a lower altitude, compared to that during 8 March 2015. The higher ionopause during second event could be because of two reasons: (a) The ionosphere was already heated and inflated by the 3 March ICME event, and its effects persisted. As is evident from the nightside data, this could bring about an observed increase of ∼40 km in the ionopause altitude during 8 March event. Hence, the diurnal effects and enhanced particle precipitations may also have changed the heating in the upper atmosphere. Detailed modeling of the response time of the atmosphere and the cooling efficiency of the Martian atmosphere is needed to understand these observations. (b) As evident from the dayside O⁺/O_2⁺ ratio, there is a preferential decrease of O_2⁺ and a relative enhancement in O⁺, probably due to increased dissociative recombination and charge exchange as a result of the higher solar wind densities during the 3 March event. The latitudes of periapsis passes during the orbits considered for study are in the northern hemisphere, where the crustal magnetic field influences are negligible as compared to the solar drivers. These observations of the Martian ionospheric response to two successive ICMEs are first of their kind and are important indicators of the complex interaction that happens in the Martian ionopause region.