MAVEN Observations of Solar Wind‐Driven Magnetosonic Waves Heating the Martian Dayside Ionosphere

Abstract

We present Mars Atmosphere and Volatile EvolutioN observations of large-amplitude magnetosonic waves propagating through the magnetosheath into the Martian ionosphere near the subsolar point on the dayside of the planet. The observed waves grow in amplitude as predicted for a wave propagating into a denser, charged medium, with wave amplitudes reaching 25 nT, equivalent to ∼40% of the background field strength. These waves drive significant density and temperature variations (∼20% to 100% in amplitude) in the suprathermal electrons and light ion species (H + ) that correlate with compressional fronts of the magnetosonic waves. Density and temperature variations are also observed for the ionospheric electrons, and heavy ion species (O + and O + 2 ); however, these variations are not in phase with the magnetic field variations. Whistler waves are observed at compressional wave fronts and are thought to be produced by unstable, anistropic suprathermal electrons. The magnetosonic waves drive significant ion and electron heating down to just above the exobase region. Ion heating rates are estimated to be between 0.03 and 0.2 eVs −1 per ion, and heavier ions could thus gain escape energy if located in this heating region for ∼10-70 s. The measured ionospheric density profile indicates severe ionospheric erosion above the exobase region, and this is likely caused by substantial ion outflow that is driven by the observed heating. The effectiveness of these magnetosonic waves to energize the plasma close to the exobase could have important implications for the long-term climate evolution for unmagnetized bodies that are exposed to the solar wind.

Subsolar Electron Temperatures in the Lower Martian Ionosphere

“…The electron temperature altitude profiles are consistent with previous results reported from the MAVEN/LPW instrument during normal, that is, nondeep dip operations (e.g., Fowler, Andersson, Ergun, et al, ; Fowler, Andersson, Peterson, et al, ; Peterson et al, ). The electron temperatures below ~200 km were also shown to be independent of magnetic field orientation, consistent with the results of Sakai et al ().…”

“…Other contributions to the variability above ~200 km come from temporal and spatial variations in the inputs to the heat equation not captured in the deep dip average values used in the calculations of cooling, advection, conduction, and expansion rates. See, for example, Fowler, Andersson, Ergun, et al, , Fowler, Andersson, Peterson, et al, ). Figure demonstrates that electron heating efficiency below ~200 km depends strongly on the electron temperature.…”

Solar Wind Induced Waves in the Skies of Mars: Ionospheric Compression, Energization, and Escape Resulting From the Impact of Ultralow Frequency Magnetosonic Waves Generated Upstream of the Martian Bow Shock

“…Particle tracing simulations by Halekas, McFadden, et al () showed that the driving mechanism for these time‐dispersed ion bursts is ion‐pickup from the variable electric field in the wave. A companion paper by Fowler et al () examines the impact on the Martian ionosphere of this phenomenon in more detail. As with the event presented here, significant ion heating was observed all the way down to just above the exobase, likely caused by the damping of the magnetosonic wave by the dense ionosphere, and the density of the upper ionosphere was significantly lower when compared to preceding and following orbits of MAVEN , as if the upper ionosphere had been drained of plasma.…”

Localized Heating of the Martian Topside Ionosphere Through the Combined Effects of Magnetic Pumping by Large‐Scale Magnetosonic Waves and Pitch Angle Diffusion by Whistler Waves

“…We have presented observations of large‐scale magnetosonic waves (likely produced by the Mars‐solar wind interaction) propagating into the Martian upper ionosphere. An earlier study of the event presented here demonstrated that such large‐scale waves were able to directly heat the heavy, cold ionospheric ions (Fowler et al, ). This study demonstrates that these magnetosonic waves are also able to heat ionospheric superthermal electrons via the magnetic pumping mechanism described by, for example, Lichko et al () and Borovsky et al ().…”

“…Data are from an inbound segment of MAVENs orbit, where the spacecraft is close to the subsolar point on the dayside of the planet. Data from this inbound orbit segment were analyzed by Fowler et al (), who demonstrated that compressive, large‐amplitude magnetosonic waves, likely generated by the Mars‐solar wind interaction, propagate planetward and heat the dayside ionospheric ions. Bursts of whistler waves were observed coincident with the large‐amplitude magnetosonic compressive wave fronts in the upper ionosphere (Figure , 05:55–05:58 in particular), but these whistler bursts were not studied in detail and are the focus of this present study.…”