Kinetics of sub‐ion scale magnetic holes in the near‐Earth plasma sheet

Abstract

In collisionless space plasmas, the energy cascade from larger to smaller scales requires effective interactions between ions and electrons. These interactions are organized by sub‐ion scale plasma structures in which strong electric fields connect demagnetized ions to magnetized electrons. We consider one such structure, magnetic holes, observed by THEMIS spacecraft in the dipolarized hot plasma sheet. Magnetic holes are localized depressions of the magnetic field with strong currents at their boundaries. Taking advantage of slow plasma convection (∼10–20 km/s), we reconstruct the electron velocity distribution within magnetic holes and demonstrate that the current at their boundaries is predominantly carried by magnetized thermal electrons. The motion of these electrons is the combination of diamagnetic drift and E × B drift in a Hall electric field. Magnetic holes can effectively modulate the intensity of electron cyclotron harmonic waves, and thus the spatial distribution of thermal electron precipitation. They may also contain field‐aligned currents with magnitudes of ∼5 nA/m2 (1 order of magnitude smaller than the cross‐field current density). Therefore, sub‐ion scale magnetic holes can be important for ionosphere‐magnetosphere coupling.

Observations of a Kinetic‐Scale Magnetic Hole in a Reconnection Diffusion Region

“…It has been widely observed in various space plasmas, including the solar wind (Turner et al, 1977;Winterhalter et al, 1995;Zhang et al, 2008), the comet magnetosphere (Russell et al, 1987), the planetary magnetosheath (Balikhin et al, 2009;Cattaneo et al, 1998;Huang, Du, et al, 2017;Joy et al, 2006;Lucek et al, 1999;Tsurutani et al, 1982;Violante et al, 1995), and the magnetosphere (Ge et al, 2011;Gershman et al, 2016;Shi et al, 2009;Sun et al, 2012;Zhang et al, 2017). It has been widely observed in various space plasmas, including the solar wind (Turner et al, 1977;Winterhalter et al, 1995;Zhang et al, 2008), the comet magnetosphere (Russell et al, 1987), the planetary magnetosheath (Balikhin et al, 2009;Cattaneo et al, 1998;Huang, Du, et al, 2017;Joy et al, 2006;Lucek et al, 1999;Tsurutani et al, 1982;Violante et al, 1995), and the magnetosphere (Ge et al, 2011;Gershman et al, 2016;Shi et al, 2009;Sun et al, 2012;Zhang et al, 2017).…”

“…Subion scale MHs have been observed both in steady and turbulent plasmas (Gershman et al, 2016;Huang, Du, et al, 2017;Yao et al, 2017;Zhang et al, 2017). Subion scale MHs have been observed both in steady and turbulent plasmas (Gershman et al, 2016;Huang, Du, et al, 2017;Yao et al, 2017;Zhang et al, 2017).…”

Statistical Properties of Sub‐Ion Magnetic Holes in the Dipolarized Magnetotail: Formation, Structure, and Dynamics

“…In the absence of strong magnetic field gradients (absence of strong currents in the dipolarized magnetotail), hot rarefied plasma is embedded within significantly enhanced B z field (the so‐called magnetic flux pileup region or dipolarizing flux bundle, see Fu et al, ; Liu et al, ; i.e., the neutral sheet with B z > | B x | can occupy a large cross‐tail region) and contains very hot electrons (electron temperature in the dipolarized magnetotail can reach the ion temperature, see, e.g., Grigorenko et al, ). Such plasma is characterized by large electron β e (ratio of the electron thermal pressure to magnetic field pressure ), ∼1–10, and large electron anisotropy (electrons are often transversely anisotropic with the perpendicular temperature T e ,⊥ larger than the parallel temperature T e ,‖ , see Fu et al, ; Khotyaintsev et al, ; Zhang et al, ). This hot, anisotropic electron population supplies free energy for various plasma instabilities (e.g., Divin, Khotyaintsev, Vaivads, & André, ; Viberg et al, ; Zhang & Angelopoulos, ).…”

“…Although configuration of these holes resembles the configuration of mirror mode solitons (e.g., Kuznetsov et al, ), the magnetic field polarization of these holes is not linear (i.e., observed hole polarization differs from model expectations; see Balikhin et al, ). Moreover, there is no residual ion transverse anisotropy observed around or within sub‐ion holes (Ji et al, ; Li et al, ; Zhang et al, ), and thus, these holes are not likely related to the mirror mode instability (note recently investigated, the electron mode of the mirror instability could potentially explain formation of these holes; see Hellinger & Stverák, ). An alternative explanation for the formation of sub‐ion holes was proposed by Ji et al (), who considered sound solitary waves that develop on the electron scale. This model does not require ion anisotropy and is able to predict observed relation between the hole spatial scale and its magnetic field perturbation (Li et al, ).…”

“…Although this model allows holes of any spatial scale, in the absence of ion contribution, the typical scale is about electron inertial length (Ji et al, ). Thus, it remains unclear why scales of most observed sub‐ion holes are still much larger than the electron scales (typical scale of observed sub‐ion holes is tens of electron gyroradii or inertial length; see Ge et al, ; Li et al, ; Zhang et al, ). Low velocities of the sub‐ion hole motion and their hybrid scales (smaller than ion gyroradius but much larger than the electron gyroradius) suggest that these holes can result from drift instabilities (see, e.g., Petviashvili & Pokhotelov, ), often observed and modeled around the dipolarization front (e.g., Divin, Khotyaintsev, Vaivads, & André, ; Pritchett et al, ). However, this scenario cannot explain the increased transverse electron anisotropy within the holes (electrons resonating with drift waves are usually heated along magnetic field lines; see Zhou et al, ; Divin, Khotyaintsev, Vaivads, André, Markidis, et al, ).…”