Structure and Variability of the Martian Ion Composition Boundary Layer

Abstract

A complex boundary layer with a variety of charged particle and electromagnetic field signatures, including a transition between plasma predominantly of solar wind origin and plasma of planetary origin, lies between the Martian bow shock and the ionosphere. In this paper, we develop and utilize algorithms to autonomously identify and characterize this ion composition boundary (ICB), using data from the Mars Atmosphere and Volatile EvolutioN mission. We find an asymmetric ICB with a larger average thickness, lower altitude, and lower velocity shear in the hemisphere where the solar wind motional electric field points outward, as a result of the asymmetry of the mass loading process. The ICB thickness scales with the magnetosheath proton gyroradius at the top of the boundary layer but does not clearly vary with external drivers. The ICB location varies with solar wind ram pressure and crustal magnetic field strength, but does not clearly respond to solar wind Mach number or extreme ultraviolet irradiance. The ICB represents a distinct boundary for ion density and flow speed, but the magnetic field strength and direction typically do not vary significantly across the ICB. The plasma density and flow speed at the ICB vary seasonally, likely in response to variations in the neutral exosphere and/or atmosphere. However, the ICB on average remains at or below the altitude where pressure balance is achieved between the piled up magnetic field and the solar wind ram pressure, regardless of season or crustal magnetic field strength.

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

“…This behavior is expected when one considers pressure balance across the interface. At Mars, similar trends are found in plasma boundaries such as the bow shock, MPB, ion composition boundary, and pressure balance boundary (Crider et al, ; Edberg et al, ; Gruesbeck et al, ; Halekas et al, ; Matsunaga et al, ; Ramstad et al, ; Vignes et al, ). However, as shown in Figure b, the flaring of the ionopause is much weaker, suggesting that pressure balance may not be the only mechanism that controls the formation of the ionopause at Mars.…”

Ion Composition Boundary Layer Instabilities at Mars

“…The first case represents an apparently stable ICB, with a smooth transition in composition, a location and inferred thickness comparable to the average + E hemisphere ICB (Halekas et al, ), and a monotonic variation in plume density with altitude.…”

“…The first case represents an apparently stable ICB, with a smooth transition in composition, and a location and inferred thickness comparable to the average −E hemisphere ICB (Halekas et al, ). Close to the ICB, we find two populations of protons, likely consisting of magnetosheath particles that gyrate across the boundary (higher energy, limited in extent) and ionospheric H + ions (low energy, throughout the ionosphere).…”

“…Outside the ICB, we observe no significant heavy ion fluxes, with the few counts due to instrumental backgrounds associated with the high proton fluxes. We expect a clear separation between ion populations, given the forces due to motional electric fields in the −E hemisphere near the ICB, which should act to accelerate ionospheric ions inward and magnetosheath ions outward (Halekas et al, ; Simon et al, ).…”

“…Figure shows observations from two more ICB crossings during the same two‐day time period, but over the northern hemisphere of Mars, in the +E hemisphere. In the +E hemisphere, in contrast to the −E case, motional electric fields should act to accelerate magnetosheath ions inward and ionospheric ions outward near the ICB, mixing the species and leading to a broader ICB (Halekas et al, ). The acceleration of ionospheric ions results in the formation of a “plume” of ions in the +E hemisphere (Dong et al, ).…”