Abstract
We present an in‐depth analysis of a time interval when quasi‐linear mirror mode structures were detected by magnetic field and plasma measurements as observed by the NASA/Mars Atmosphere and Volatile EvolutioN spacecraft. We employ ion and electron spectrometers in tandem to support the magnetic field measurements and confirm that the signatures are indeed mirror modes. Wedged against the magnetic pile‐up boundary, the low‐frequency signatures last on average ∼10 s with corresponding sizes of the order of 15–30 upstream solar wind proton thermal gyroradii, or 10–20 proton gyroradii in the immediate wake of the quasi‐perpendicular bow shock. Their peak‐to‐peak amplitudes are of the order of 30–35 nT with respect to the background field, and appear as a mixture of dips and peaks, suggesting that they may have been at different stages in their evolution. Situated in a marginally stable plasma with β ‖ ∼ 1, we hypothesize that these so‐called magnetic bottles, containing a relatively higher energy and denser ion population with respect to the background plasma, are formed upstream of the spacecraft behind the quasi‐perpendicular shock. These signatures are very reminiscent of magnetic bottles found at other unmagnetized objects such as Venus and comets, also interpreted as mirror modes. Our case study constitutes the first unmistakable identification and characterization of mirror modes at Mars from the joint points of view of magnetic field, electron and ion measurements. Up until now, the lack of high‐temporal resolution plasma measurements has prevented such an in‐depth study.
Highlights
We present here an in-depth analysis of one time interval when quasi-linear mirror mode structures were detected by magnetic field and plasma measurements as observed by the NASA/Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft
We showed for the first time the unambiguous presence of a train of linear Mirror mode-like (MM) in the magnetosheath of Mars, as confirmed by magnetic field and plasma measurements taken from MAVEN, in the early part of the mission
These MMs are situated on the dayside in the wake of a highly quasi-perpendicular shock, deep in the magnetosheath (Rsc ∼ 1.35 Rp, solar zenith angle S ZA ∼ 60 ◦) and close to the transition into the magnetic pile-up boundary (MPB): this is precisely this region where Ruhunusiri et al (2015) statistically found waves matching MM characteristics
Summary
Mirror mode-like (MM) structures have been found everywhere in solar system plasmas (Tsurutani, Lakhina, et al, 2011), from the solar wind (Kaufmann et al, 1970; Winterhalter et al, 1995; Bale et al, 2009) to Earth (Tsurutani et al, 1984; Lucek et al, 1999b), Mars (Bertucci et al, 2004; Espley et al, 2004) and Venus (Volwerk et al, 2008a, 2008b), Jupiter Whereas slow mode waves (Alfvén waves and quasi-parallel slow waves) were statistically found to dominate both in the solar wind and in the magnetosheath region, waves consistent with mirror modes were on average confined on the dayside to the region closest to the magnetic pile-up boundary (MPB) and extending on the nightside in the magnetotail This would seem to confirm the theoretical picture that MM structures, if originating upstream in the solar wind as so-called magnetic holes (MH) where they are routinely detected (Madanian et al, 2020), need time to grow when crossing the bow shock (BS) region. One of the purposes of this paper is to characterise these structures fully and validate the B-field-only detection criteria in order to prepare for a statistical analysis of the whole 2014-2021 MAVEN dataset
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