On the fine structure of dipolarization fronts

Abstract

Measurements from the closely spaced Cluster spacecraft are used to study the structure of the magnetic and electric fields within the magnetic ramp of dipolarization fronts (DF) observed close to the neutral sheet and the midnight meridian (YGSM<3RE). The spacecraft separation was small enough (<300 km) to treat the magnetic ramp of the DF front as a planar structure as indicated from variance analysis. The finite value of the magnetic field along the minimum variance direction for the events studied indicates that the dipolarization front structure was distinct from a tangential discontinuity. In addition to the main increase of the magnetic field in the maximum variance component, strong oscillations were observed in the intermediate component. The presence of this oscillatory structure results in an expansion of the region in which a change of magnetic pressure occurs, the size of which is typically an ion Larmor radius or greater. This widening is important in maintaining the pressure balance at the edge of the DF. This phenomenon resembles observations of intense current sheets in the magnetotail and also laboratory experiments of current sheet formation, in which a similar widening of the ramp region has been observed. In this paper we argue against the idea that an electron temperature anisotropy, resulting in electron curvature currents, can explain the formation of the oscillatory structures observed at DFs. These oscillations can be explained as eigenmode waves of the plasma that propagate away from the disturbance (DF) that is moving at subsonic speeds. Oscillations observed within the magnetic ramp indicate field‐aligned currents that are expected to be associated with DF.

Contrasting dynamics of electrons and protons in the near-Earth plasma sheet during dipolarization

“…The spatial scale of B Z pulses, L pulse , is typically of the order of a few ρ T , while the L F is less than ρ T for all analyzed pulses. This is in agreement with previous results reported for the DFs (e.g., Angelopoulos et al, 2013;Balikhin et al, 2014).…”

“…They have shown the presence of intense localized and approximately fieldaligned electric currents near the leading and trailing edges of the B Z pulse. These currents can be a part of a multiscale electric current system usually associated with DFs (e.g., Fu et al, 2012;Liu et al, 2013;Balikhin et al, 2014).…”

Electron‐Scale Measurements of Dipolarization Front

“…We find that J m is positive inside the dip region but mainly negative at the front, suggesting that dawnward current exists inside the dip region, while duskward current exists at the front (Figure 3b), consistent with previous observations (e.g., Fu et al, 2012a;Yao et al, 2013). Such difference may stem from the fact that at electron scale, DF may be more like a nonlinear structure (e.g., Balikhin et al, 2014), as seen from the conspicuous fluctuation of currents. Note that J n has a finite value with a maximum approaching 20 nA/m 2 at the front (Figure 3c), indicating that plasma can flow across the DF at electron scale.…”

“…Interestingly, we see that J m also shows a localized positive value (in the dusk-dawn direction) in the low-density side of the DF. J l also shows a finite value at the front, with a peak approaching 13 nA/m 2 and a valley near À11 nA/m 2 (Figure 2d), indicating field-aligned/parallel currents that are expected to be associated with the DF (e.g., Balikhin et al, 2014;Liu et al, 2018;Sun et al, 2013;Yao et al, 2016). This is different from previous studies that suggested that DF may be a tangential discontinuity at subproton scale (e.g., Fu et al, 2012a;Khotyaintsev et al, 2011).…”

“…To date, DF structure at ion scale has been well investigated by spacecraft observations and numerical simulations (e.g., Balikhin et al, 2014;Drake et al, 2014;Divin, Khotyaintsev, Vaivads, André, Markidis, et al, 2015;Fu et al, 2012a;Pritchett et al, 2014;Vapirev et al, 2013;Yao et al, 2016), while DF structure at electron scale still remains not fully understood due to instrumental limitations and spacecraft separation distances. Studying the DF dynamics is of particular interest for the magnetotail physics.…”