No abstract

The Mars Atmosphere and Volatile EvolutioN spacecraft provides an opportunity to observe the response of the Martian upper atmosphere to the passage of interplanetary coronal mass ejections. We study the response of Martian ionosphere to two successive solar disturbances during 3 and 4 March and 8 and 9 March 2015 using the observations from Neutral Gas and Ion Mass Spectrometer and Langmuir Probe and Waves aboard Mars Atmosphere and Volatile EvolutioN. During these events, the ionospheric boundary was significantly lowered. During 3 March the ionopause is seen to be at a lower altitude, compared to that during 8 March. The higher ionopause seen on 8 March could be due to the sustained effect of heating and inflation by the 3 March interplanetary coronal mass ejection event or due to the heating by the enhanced particle precipitation. The comparison of the O+/O 2+ ratio indicates that 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.

Solar cycle 24 is one of the weakest solar cycles recorded, but surprisingly the declining phase of it had a slow CME which evolved without any low coronal signature and is classified as a stealth CME which was responsible for an intense geomagnetic storm at Earth (Dst = -176 nT). The impact of this space weather event on the terrestrial ionosphere has been reported. However, the propagation of this CME beyond 1 au and the impact of this CME on other planetary environments have not been studied so far. In this paper, we analyse the data from Sun-Earth L1 point as well as from the Martian orbit (near 1.5 au) to understand the characteristics of the stealth CME as observed beyond 1 au. The observations near Earth are using data from the Solar Dynamics Observatory (SDO) and the Advanced Composition Explorer (ACE) satellite located at L1 point whereas those near Mars are from the instruments for plasma and magnetic field measurements on board Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. The observations show that the stealth CME has reached 1.5 au after 7 days of its initial observations at the Sun and caused depletion in the nightside topside ionosphere of Mars, as observed during the inbound phase measurements of the Langmuir Probe and Waves (LPW) instrument on board MAVEN. These observations have implications on the ion escape rates from the Martian upper atmosphere.

The response of Martian ionosphere to the passage of corotating interaction region (CIR) of June 2015 is studied using observations from several instruments aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. An intense CIR arrived at Mars on 22 June 2015, during which the upstream solar wind and interplanetary magnetic field conditions were monitored by the Solar Wind Ion Analyzer (SWIA), Solar Wind Electron Analyzer (SWEA), Solar Energetic Particle (SEP), and Magnetometer (MAG) instruments aboard MAVEN. The CIR event was characterized by enhancements in solar wind density, velocity, and dynamic pressure, and increased and fluctuating interplanetary magnetic field and was associated with enhanced fluxes of Solar Energetic Particles. The Langmuir Probe and Waves (LPW) instrument onboard MAVEN provided the ionospheric observations such as electron density and electron temperature during this period. The dayside ionosphere is significantly compressed only near the peak of solar wind dynamic pressure enhancement (∼14 nPa). In contrast, on the nightside, the electron density remains depleted for a longer period of time. The electron temperatures are also enhanced during the period of electron depletion on the nightside. The Suprathermal and Thermal Ion Composition (STATIC) measurements show enhanced fluxes of suprathermal heavy ions in the Martian exosphere during CIR period, and evidences for enhanced tailward flow of these pickup ions. The analysis suggests that the nightside ionosphere is primarily controlled by the precipitating Solar Energetic Particles and pickup ions transported across the Martian terminator and depletes significantly when the heavy ion flux in the exosphere enhances.

The dearth of observations between 1 AU and 3 AU limits our understanding of energetic particle acceleration processes in interplanetary space. We present the first-of-their-kind observations of the energetic particle acceleration in a Corotating Interaction Region (CIR) using data from two vantage points, 1 AU (near Earth) and 1.5 AU (near Mars). The CIR event of June 2015 was observed by the particle detectors aboard the Advanced Composition Explorer (ACE) satellite as well as the SEP (Solar Energetic Particle) instrument aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft situated near 1.5 AU. We find that a CIR shock can accelerate a significant number of particles even at 1.5 AU. During this event the acceleration by the shocks associated with the CIR could cause an enhancement of around two orders of magnitude in the SEP energetic ion fluxes in the ∼500 keV to 2 MeV range when the observations near 1 and 1.5 AU are compared. To demonstrate the differences between SEP acceleration in CIR and other impulsive events, we show the energetic ion flux observations during an intense CME period in March 2015, in which case the enhanced SEP fluxes are seen even at 1 AU. These observations provide evidence that CIR shock can accelerate particles in the region between Earth and Mars, that is, only within the short heliocentric distance of 0.5 AU, in interplanetary space.