Coupling between the lower and upper atmosphere, combined with loss of gas from the upper atmosphere to space, likely contributed to the thin, cold, dry atmosphere of modern Mars. To help understand ongoing ion loss to space, the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft made comprehensive measurements of the Mars upper atmosphere, ionosphere, and interactions with the Sun and solar wind during an interplanetary coronal mass ejection impact in March 2015. Responses include changes in the bow shock and magnetosheath, formation of widespread diffuse aurora, and enhancement of pick-up ions. Observations and models both show an enhancement in escape rate of ions to space during the event. Ion loss during solar events early in Mars history may have been a major contributor to the long-term evolution of the Mars atmosphere.

[1] This paper reports a new global multispecies single-fluid MHD model that was recently developed for Venus. This model is similar to the numerical model that has been successfully applied to Mars. Mass densities of proton and three important ionospheric ion species (O + , O 2 + , and CO 2 + ) are self-consistently calculated in the model by including related chemical reactions and ion-neutral collision processes. The simulation domain covers the region from 100 km altitude above the surface up to 24 R V in the tail. An adaptive spherical grid structure is constructed with radial resolution of about 5 km in the lower ionosphere. Bow shock locations are well reproduced for both solar-maximum and solar-minimum conditions using appropriate solar wind parameters for each case. It is shown that the shock locations are farther from the planet during the solar maximum condition, because of both the enhanced solar radiation strength and the relatively small Mach number. The simulation results also agree well with Venus Express observations, as shown by comparisons between model results with magnetic fields observed by the spacecraft.

As a weakly magnetized planet, Mars ionosphere/atmosphere interacts directly with the shocked solar wind plasma flow. Even though many numerical studies have been successful in reproducing numerous features of the interaction process, these earlier studies focused mainly on interaction under steady solar wind conditions. Recent observations suggest that plasma escape fluxes are significantly enhanced in response to solar wind dynamic pressure pulses. In this study, we focus on the response of the ionosphere to pressure enhancements in the solar wind. Through modeling of two idealized events using a magnetohydrodynamics model, we find that the upper ionosphere of Mars responds almost instantaneously to solar wind pressure enhancements, while the collision dominated lower ionosphere (below~150 km) does not have noticeable changes in density. We also find that ionospheric perturbations in density, magnetic field, and velocity can last more than an hour after the solar wind returns to the quiet conditions. The topside ionosphere forms complicated transient shapes in response, which may explain unexpected ionospheric behaviors in recent observations. We also find that ionospheric escape fluxes do not correlate directly with simultaneous solar wind dynamic pressure. Rather, their intensities also depend on the earlier solar wind conditions. It takes a few hours for the ionospheric/ atmospheric system to reach a new quasi-equilibrium state.

We conduct an extended survey of reconnection signatures observed in the Martian magnetotail by the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. Although it is well established that magnetic reconnection plays a fundamental role in the dynamics of intrinsic magnetospheres, the role of reconnection in the dynamics of induced magnetospheres remains poorly understood. Based on comprehensive plasma and field measurements by MAVEN in the Martian magnetotail, we first identified 776 current sheet crossings and then selected 34 crossings with Hall magnetic field signatures, which are indicative of the ion diffusion region of tail reconnection. For the majority of the identified Hall field events, we observe ion flow enhancements in the directions consistent with the reconnection outflow directions expected from the Hall magnetic field polarity. The reconnection signatures are preferentially observed in the −E hemisphere of the near‐Mars magnetotail at ∼1–2 Mars radii downstream from Mars. We have found no strong correlation of the likelihood of observing reconnection signatures with local crustal field strengths or with upstream drivers. The duty cycle of tail reconnection is estimated to be ∼1–10% or even higher. The MAVEN observations suggest that magnetic reconnection can play an important role in the dynamics of the Martian magnetotail.

The identification of magnetic reconnection on the dayside of Mars has been elusive owing to the lack of comprehensive plasma and field measurements. Here we present direct measurements of dayside in situ reconnection signatures by the comprehensive particles and fields package on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft over strong crustal magnetic fields in the southern hemisphere of Mars. During a crossing of a bifurcated current sheet consisting of northward and southward magnetic fields, MAVEN recorded (i) ionospheric photoelectrons trapped on closed magnetic field lines, (ii) Hall magnetic fields and a nonzero normal field with polarity consistent with a crossing northward of the X line, and (iii) northward Alfvénic ion jets. Dayside magnetic reconnection on crustal magnetic fields could control the global configuration and topology of the Martian magnetosphere and alter the ion escape pattern from the dayside ionosphere.