On the Origin of Perpendicular Ion Anisotropy Inside Dipolarizing Flux Bundles

Abstract

Perpendicular anisotropy of suprathermal ions, observed inside some of the dipolarizing flux bundles (DFBs) in the magnetotail plasma sheet, have been attributed to successive, betatron-type accelerations during the DFB entry of ambient ions. It has been unclear, however, where and how these ions enter the DFBs. The proposed locations include the DFB flanks where cross-tail drifting ions are picked up, and the DFB leading edge with sharp magnetic field gradient (the dipolarization front, DF). Here we examine the latter scenario, based on a simplistic, test particle approach, to predict the preferred conditions for the appearance of the DFB ion anisotropy. Our model predicts that the ion anisotropy would be stronger at locations closer to the neutral sheet and would appear preferentially in the DFB dawnside and central sectors rather than the duskside sector. We also predict that the ion anisotropy would more likely be observed in DFBs with higher propagation speeds. These properties can be understood in our model by the dawnward drift of ions during their DF penetration (attributed to the large magnetic gradient). To examine these predictions, we carry out a statistical survey based on observations from the THEMIS (Time History of Events and Macroscale Interactions during Substorms) mission, to show a clear dependence of the ion anisotropy on spacecraft location and the DFB propagation speed. These findings, therefore, are consistent with the scenario that the perpendicular ion anisotropy originates from the ion acceleration and penetration across sharp DFs.

Ionospheric Cold Ions Detected by MMS Behind Dipolarization Fronts

“…They are structures with a typical thickness comparable to the ion inertial length (e.g., Fu et al, 2012b;, and usually embedded inside bursty bulk flows (e.g., Angelopoulos et al, 1992;Cao et al, 2006Cao et al, , 2013Chen et al, 2019). DFs play an important role in the Earth's magnetotail plasma sheet dynamics, contributing to the particles acceleration (e.g., Birn et al, 2013;Duan et al, 2014;Zhou et al, 2010Zhou et al, , 2019, energy conversion (e.g., Angelopoulos et al, 2013;Huang et al, 2015;Khotyaintsev et al, 2017;Yang et al, 2017;Yao et al, 2017), and magnetic flux transport (e.g., Liu et al, 2014;Nakamura et al, 2011) during substorms. DF, as a discontinuity separating the tenuous plasma from ambient dense plasma, is often accompanied with energetic particles (tens to hundreds of keV).…”

“…They are structures with a typical thickness comparable to the ion inertial length (e.g., Fu et al, ; Lu, Lu, et al, ), and usually embedded inside bursty bulk flows (e.g., Angelopoulos et al, ; Cao et al, , ; Chen et al, ). DFs play an important role in the Earth's magnetotail plasma sheet dynamics, contributing to the particles acceleration (e.g., Birn et al, ; Duan et al, ; Fu, Khotyaintsev, et al, ; Liu, Fu, Cao, Xu, et al, ; Liu & Fu, ; Zhou et al, , ), energy conversion (e.g., Angelopoulos et al, ; Huang et al, ; Khotyaintsev et al, ; Yang et al, ; Yao et al, ), and magnetic flux transport (e.g., Liu et al, ; Nakamura et al, ) during substorms.…”

Testing adiabatic models of energetic electron acceleration at dipolarization fronts

“…The presence of E y suggests that the DF is not a true tangential discontinuity, which would allow particles to penetrate across the boundary. Although this is not measured in the frame of the DF, other studies have found E y in the DF rest frame that supports the idea that there is plasma exchange across the boundary (e.g., Zhou et al, 2019).…”

Fast Ion Isotropization by Current Sheet Scattering in Magnetic Reconnection Jets

“…However, observations show a dominance of isotropic ion VDFs already at distances ∼ 50 d i from the reconnection region [15]. This disagreement between the simulations and observations suggests that other mechanisms relax the temperature anisotropy in the reconnection jets (e.g., betatron acceleration [17], Fermi acceleration [18], or current sheet pitch-angle scattering [19,20]). Since, these mechanisms can efficiently heat the reconnection jet content, studying the processes limiting the ion temperature anisotropy is crucial to understanding the energy budget and partition in the reconnection jets.…”