Disappearing Solar Wind at Mars: Changes in the Mars‐Solar Wind Interaction

Abstract

On 26 December 2022 the solar wind density dropped by over an order of magnitude and remained low for about a day. We have utilized in‐situ plasma measurements made by the Mars Atmosphere and Volatile EvolutioN mission to determine how this change affected the Mars‐solar wind interaction. During this time period, on inbound orbit segments, MAVEN sampled the terminator ionosphere, which switched from a magnetized to unmagnetized state immediately following the minimum in solar wind density. The magnetic field amplitude was typically 5–10 nT within the upper ionosphere prior to the event and consistently <1 nT after. During the event the magnetic pressure dominated immediately above the ionosphere while within the ionosphere the ionospheric plasma pressure dominated. The high altitude terminator ionosphere remained in this unmagnetized state throughout the event, suggesting that it was the new equilibrium state of the system. The terminator upper ionosphere returned to its original magnetized state once the solar wind density had recovered. The outbound orbit segments sampled the dayside subsolar region which remained magnetized throughout the event: the magnetization state of the ionosphere varied locally, dependent upon the solar zenith angle and corresponding incident solar wind dynamic pressure. Such conditions are different to the commonly reported unmagnetized ionospheric state at Venus during solar maximum conditions, where the interplanetary magnetic field is repelled from the entire dayside ionosphere. Drastic changes in the upstream solar wind are able to change the Mars‐solar wind interaction state on timescales less than one MAVEN orbit (∼3.5 hr).

Ion-acoustic solitary waves in Mars’ lower ionosphere

“…Several investigations have confirmed the existence of superthermal electrons in the Martian ionosphere. These findings include [13][14][15][16]32]…”

“…According to the measurements conducted by MAVEN in the vicinity of the preapsis region at altitudes ranging from 180 to 500 km, it has been shown that the dayside ionosphere of Mars is primarily influenced by the presence of O 2 + , O + , and CO 2 + ions. These ions are believed to constitute a substantial constituent of the Martian ionosphere [8][9][10][11][12], the electrons are presented as a superthermal charges [13][14][15][16]. The interaction between the solar wind (SW) and the ionosphere can generate various electromagnetic and electrostatic waves.…”