Magnetic Holes Upstream of the Martian Bow Shock: MAVEN Observations

Abstract

Magnetic holes (MHs) are pressure-balanced structures characterized by distinct decreases in the interplanetary magnetic field strength in otherwise unperturbed solar wind. In this paper we present an analysis of MHs upstream of the Martian bow shock based on 3 months of observations by the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. Plasma properties within and around these structures as well as their shape characteristics are examined. We find an occurrence rate of around 2.1 events per day. About 51% of all events are of linear type with magnetic field rotation across the hole less than 10 • . We observe linear MHs both as isolated events and as part of a train of MHs. The proton temperature anisotropy inside MHs increases, while alpha particles remain mostly isotropic. The average electron temperature inside MHs modestly increases with increasing hole depth. The duration of linear holes at 1.5 AU shows an increase compared to durations at smaller heliocentric distances, but the structures remain asymmetrical and ellipsoid. A case study of MHs accompanied by a population of heavy pickup ions is also discussed. Plain Language SummaryThe solar wind, a supersonic flow of electrons and ions flowing radially outward from the Sun, carries a magnetic field. Magnetic holes are structures with reduced magnetic field strength that last between a few seconds to a few minutes. These structures have been observed at many places throughout the solar system. A common formation mechanism for magnetic holes is a wave mode instability that converts magnetic energy into ion thermal energy, known as the mirror mode. Mirror mode instabilities grow when the temperature of ions moving perpendicular to the magnetic field is higher than that of ions moving in the parallel direction. Mirror waves alter the magnetic field and the spatial structure of the plasma. In this paper, we use observations from the Mars Atmosphere and Volatile EvolutioN spacecraft to study magnetic holes in the space environment around Mars.

Statistical study of linear magnetic hole structures near Earth

“…The characteristics of the ubiquitous magnetic hole structures, that are indicative of temperature asymmetries in space plasmas, were studied just outside the Earth's bow shock (R ≥ 15R E ) with the MMS1 spacecraft. Naturally, due to the dynamics of the bow shock (see, for example, Meziane et al, 2014) 15R E may not suffice in some cases, which will then be magnetosheath structures. A test has been performed on the closest bin, 15 − 16R E , to see how many of the 22 events were in the magnetosheath, which resulted in only three events, all on the same day.…”

“…November 2017 through March 2018 and December 2018 through March 2019. An additional limit was set to the location of the spacecraft, namely that it be farther out than 15 R E , to avoid influence from the Earth's bow shock, which can move further outward in times of low solar wind pressure (Meziane et al, 2014). However this will not exclude the foreshock region from the data set, which extends much further upstream (Heppner et al, 1968;Greenstadt et al, 1968;Scarf et al, 1970).…”

Nonstationary Quasiperpendicular Shock and Ion Reflection at Mars

“…The magnetic field data are from the Magnetometer sensor, which measures the magnetic field with up to 32 Hz sampling rate (Connerney et al, 2015). The ion data are from the Solar Wind Ion Analyzer (SWIA) instrument (Halekas et al, 2015), which measures ions in the 25 eV to 25 keV energy range with a 22.5° angular resolution over a total field of view of 2.8 π solid angle every 8 s. The ion moments are calculated in a similar way as in Halekas et al (2017) and Madanian et al (2019).…”

Magnetic Holes in the Solar Wind and Magnetosheath Near Mercury

“…The temporal scale sizes of magnetic holes have been reported to vary between a few seconds and several minutes (Madanian et al., 2019; Sperveslage et al., 2000; Turner et al., 1977; Volwerk et al., 2020; Winterhalter et al., 1994; Xiao et al., 2010; Zhang, Russell, Baumjohann, et al., 2008). The variation of scale size with heliocentric distance is unclear.…”