The Day the Solar Wind Disappeared at Mars

Abstract

In December 2022, an extremely low‐density solar wind stream encountered first the Earth and then Mars, shortly after the radial alignment of the two planets (i.e., Mars opposition). As the low‐density stream passed Mars, the properties of the Mars‐solar wind interaction changed dramatically in response to the low solar wind Alfvénic Mach number and dynamic pressure. The Martian magnetosphere and its boundaries expanded by thousands of kilometers, extending outside of the nominal average bow shock location. The low upstream Mach number resulted in a low ratio of plasma to magnetic field pressure in the magnetosheath, allowing the formation of a very high but stable ion temperature anisotropy, together with a very low level of electromagnetic instabilities. Meanwhile, the decrease in solar wind dynamic pressure caused the Martian upper ionosphere at the terminator to transition from a magnetized state to an unmagnetized state. This event provides an opportunity to study a unique end‐member state of the Mars‐solar wind interaction.

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

“…Similar conditions were observed during the inbound segment of the following orbit (but not shown here). An interesting point to note here is that as MAVEN transitioned from the magnetosheath into the ionosphere it observed signatures that are both consistent and inconsistent with the photoelectron boundary (PEB), described by for example, Xu et al (2023). Specifically, SWEA observed both the "CO 2 photoelectron peaks" at 22-27 eV, and high energy (greater than 500 eV) electrons, both traveling isotropically relative to the local magnetic field (Figure 1g summarizes this; the detailed energy spectra and pitch angle distributions are not shown here).…”

“…On 26 December 2022 the solar wind density dropped by over an order of magnitude, observed at both Earth and Mars, and an overview of this "disappearing solar wind" event is given by Halekas et al (2023). We have analyzed the time period surrounding this density minimum using MAVEN observations at Mars, with a particular focus on the orbit immediately following this event and the goal of understanding how such sudden and extreme changes in the solar wind affect the Mars-solar wind interaction.…”

“…On December 26th, over the course of a few hours, the solar wind density dropped by over an order of magnitude (from 2 to 3 cm −3 to 0.1-0.2 cm −3 at Mars). This density minimum lasted for approximately one day and gradually recovered to its pre-drop value over the course of roughly a second day (Figure 1 in Halekas et al (2023)). In this study we focus on MAVEN observations made during the orbit immediately following the minimum in solar wind density, to understand how the Mars-solar wind interaction changed during such an extreme and sudden drop in solar wind density.…”

“…To place the observations and results from Section 3 into context with the bigger picture we also determine how the ionosphere-solar wind interaction system responds to the changes in solar wind density over the disappearing solar wind time period. Our goal here is to provide context for the observed changes in interaction states (magnetized vs. unmagnetized upper ionosphere) and we do not focus on the specific locations of the various Martian plasma boundaries and regions, which are covered in detail by other studies (see Halekas et al, 2023).…”

“…Contrary to typical sheath characteristics, the plasma environment was relatively stable: there was very little fluctuation in the magnetic field amplitude and vector (panels a and b), nor the proton energy spectra (panel d). This "calm" plasma environment in the sheath was a result of the low mach number solar wind during the solar wind density minimum: the low mach number significantly raised the proton temperature anisotropy threshold required for sheath proton distributions to become unstable and to produce electromagnetic waves (Halekas et al, 2023).…”