Dipolarization Fronts: Tangential Discontinuities? On the Spatial Range of Validity of the MHD Jump Conditions

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.sides of this type of discontinuity. For RDs, on the other hand, all field and plasma quantities are continuous across it and only a rotation of the tangential velocity and magnetic field vectors within the discontinuity take place. This implies that for RDs, a change of the plasma velocity and the Alfvén velocity across the discontinuity must change accordingly. This relation is called the Walén relation (e.g. Hudson, 1970) and is valid for linear and nonlinear Alfvén waves (RDs are nonlinear Alfvénic waves). Note that the alignment between the plasma and Alfvén velocity alone is not sufficient to discriminate TDs from RDs, since such an alignment would be also present, for example, for Alfvénic surface waves traveling along a TDs Denskat and Burlaga (1977). The ratio of the magnitude changes of plasma and Alfvén velocity, however, may be a distinctive feature. Previous studies often used this Walén relation as a criteria to distinguish RDs and TDs in the solar wind (e.g. Neugebauer et al., 1984) or at the magnetopause (e.g. Paschmann et al., 1986).

Electron‐Scale Front of Magnetic Pile‐Up Region in Reconnection Exhaust

“…In addition, the average value of B N is only ∼0.13 of the average magnetic field strength ($$\overline{\vert B\vert }$$) (Figure 2e). Therefore, this front is a tangential discontinuity (Schmid et al., 2019). Due to the underestimate of ion moments (Figure 1h), the current density was calculated by curlometer method ($$\mu \mathbf{J}=\nabla \times \mathbf{B}$$).…”

Statistical investigation of electric field fluctuations around the lower-hybrid frequency range at dipolarization fronts in the near-earth magnetotail

“…For the investigation, we use data from the Magnetospheric Multiscale (MMS) Mission (Burch et al, 2016). We use the initial database from Schmid et al (2019b), comprising 203 events but select only events that were obtained during June, July, and August of 2017 and 2018 when the apogee of MMS was clearly in the magnetotail.…”

“…Embedded into BBFs are so-called dipolarization fronts (DFs) (Nakamura et al, 2002), which are understood as propagating, transient magnetic boundary structures with velocities in the range of several hundred km/s (Runov et al, 2009). In spacecraft measurements, they are identified by a sharp increase in the B z magnetic field component in Geocentric Solar Magnetic (GSM) coordinates (Runov et al, 2009), often preceded by a small dip in the B z component (Shiokawa et al, 2005;Schmid et al, 2019b) and followed by a longer structure of increased B z , termed the flux pileup region (Khotyaintsev et al, 2011) or dipolarizing flux bundle (Liu et al, 2013). Dipolarization fronts are structures at scales comparable to the ion inertial length, separating cold, dense plasma ahead from the hotter but diluted plasma within the trailing flux bundle (Sergeev et al, 2012).…”

Are Dipolarization Fronts a Typical Feature of Magnetotail Plasma Jets Fronts?

“…DFs are thought to be a result of transient unsteady magnetic reconnection (Sitnov et al., 2009) or detachment of interchange heads in the nonlinear stage of the kinetic ballooning interchange instability (Pritchett & Coroniti, 2010). Because of their distinct signature, DFs and their contribution to the magnetotail energy budget have been extensively investigated using case studies (Angelopoulos et al., 2013; Artemyev et al., 2012; Fu et al., 2011; Khotyaintsev et al., 2017) and statistical studies (Fu et al., 2012; Runov et al., 2011; Schmid et al., 2011, 2019). These studies focused on DFs, and thus omitted other JFs with more complicated magnetic field structures.…”