Magnetosheath and heliosheath mirror mode structures, interplanetary magnetic decreases, and linear magnetic decreases: Differences and distinguishing features

Abstract

[1] There has been considerable confusion in the literature about what mirror mode (MM), magnetic decrease (MD), and linear magnetic decrease (LMD) structures are and are not. We will reexamine past spacecraft observations to demonstrate the observational similarities and differences between these magnetic and plasma structures. MM structures in planetary magnetosheaths, cometary sheaths, and the heliosheath have the following characteristics: (1) the structures have little or no changes in the magnetic field direction across the magnetic dips; (2) the structures have quasiperiodic spacings, varying from ∼20 proton gyroradii (r p ) in the Earth's magnetosheath to ∼57 r p in the heliosheath; and (3) the magnetic dips have smooth edges. Magnetosheath MM structures are generated by the mirror instability where b ? /b k > 1 + 1/b ? (b is the plasma thermal pressure divided by the magnetic pressure). In general, the sources of free energy for the mirror instability are reasonably well understood: shock compression, field line draping, and, in the cases of comets and the heliosheath, also ion pickup. The observational properties of interplanetary MDs are as follows: (1) there is a broad range of magnetic field angular changes across them; (2) their thicknesses can range from as little as 2-3 r p to thousands of r p , with no "characteristic" size; and (3) they typically are bounded by discontinuities. The mechanism(s) for interplanetary MD generation is (are) currently unresolved, although at least five different mechanisms have been proposed in the literature.

Statistical analysis of mirror mode waves in sheath regions driven by interplanetary coronal mass ejection

“…In addition, because MMs are relatively narrow structures, they cover only a small portion of the sheath, e.g., in our study only 3 % for the sheath that contained the largest number of MMs. Similar to planetary magnetosheaths (e.g., Tsurutani et al, 2011b), we found that MMs tend to occur in trains also in ICME sheaths. However, 60 % of all observed MMs occurred as singles or trains of two.…”

“…In planetary magnetosheaths the largest MM amplitudes are found in the inner magnetosheath (e.g., Erkaev et al, 2001) suggesting that both shock compression and field line draping may play an important role (Tsurutani et al, 2011b). As discussed previously, in ICME sheaths, the MMs are most abundant and have largest amplitudes close to the shock suggesting that the shock compression is the dominant source of free energy.…”

“…However, MMs have typically larger amplitudes behind quasi-parallel shocks (Génot et al, 2009b). In addition, while also isolated MMs exist (Winterhalter et al, 1994;Zhang et al, 2008), most MMs in planetary magnetosheaths occur as sequences of peaks or dips, so called MM trains (Tsurutani et al, 2011b).…”

“…In planetary magnetosheaths, two primary mechanism have been identified to produce temperature anisotropy: the quasi-perpendicular shock compression, which heats the ions perpendicular to the magnetic field, and magnetic field line draping, which happens near the magnetopause as the field lines drape around the magnetic obstacle (e.g., Erdős, 2002, 2005). The shock compression has been argued to be a source of free energy that generates peak-like MMs and likely both sources need to be available in order to generate the largest amplitude MMs (Tsurutani et al, 2011b). In the heliosheath and cometosheaths, the ion pickup process is also identified as a free energy source (e.g., Tsurutani et al, 1999Tsurutani et al, , 2011aSchmid et al, 2014).…”

“…We define a MM as a structure through which the magnetic field direction changes less than 10 • (Tsurutani et al, 2011b). Similarly to Zhang et al (2008Zhang et al ( , 2009, the start and end of each MM structure are defined as the nearest points of the magnetic field minimum or maximum that satisfy the requirements B >B − δ and B > 1.25 B MIN when a minimum is studied, and B <B + δ and B < 0.75 B MAX when a maximum is studied.…”