Dipolarization Fronts: Tangential Discontinuities? On the Spatial Range of Validity of the MHD Jump Conditions. Issue 12 (5th December 2019)

• Record Type: Journal Article
• Authors: Schmid, D.; Volwerk, M.; Plaschke, F.; Nakamura, R.; Baumjohann, W.; Wang, G.Q.; Wu, M.Y.; Zhang, T.L.
• Is Part Of: Journal of geophysical research. Volume 124:Issue 12(2019)
• Page Start: 9963
• Page End: 9975
• Publication Date: 2019-12-05
• Subjects: Plasma Physics -- Magnetotail -- Dipolarization Front -- Plasma Sheet; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1029/2019JA027189
• Languages: English
• ISSNs: 2169-9380

Abstract

Abstract: Magnetotail dipolarization fronts (DFs) are referred as the sharp increase of the northward magnetic field component, embedded in bursts of fast earthward moving plasma flows, so called bursty bulk flows. Earlier studies often considered DFs as tangential discontinuities (TDs), which can be understood as thin vertical current layers of earthward moving flux tubes, so called dipolarzing flux bundles (DFBs), which separate the ambient plasma sheet plasma from the low entropy plasma within the DFB. Here we present a statistical study of 23 DFs observed by the Magnetospheric Multiscale mission during 2017 and 2018 when the apogee was at 25 R E in the magnetotail. We perform a test of the Walén relation to distinguish whether the observed DFs have rather a TD or rotational discontinuity character and evaluate the plasma flow across the DFs in detail. The results show that on MHD large scales, all 23 DFs can be considered as TD like, but sometimes may have a significant normal plasma flow across it: for 16 events (∼70 % ), the plasma flows mainly tangential to the DFs, while for seven events (∼30 % ), the plasma flows mainly across the DFs. Based on the findings present in this study, we further hypothesize that the DF structure becomes more distorted and unstable in a (locally) more dipolarized background magnetic field region, which may additionally facilitate the plasma flow across the front. Key Points: Walén test shows that dipolarization fronts (DFs) can generally be classified as tangential discontinuity (TD)‐like structures Plasma flow across DFs are observed, indicating that they can not be characterized as strict TDs Plasma flow across DFs is larger when background northward magnetic field is stronger