Abstract
AbstractEnergy conversion on the dipolarization fronts (DFs) has attracted much research attention through the suggestion that intense current densities associated with DFs can modify the more global magnetotail current system. The current structures associated with a DF are at the scale of one to a few ion gyroradii, and their duration is comparable to a spacecraft's spin period. Hence, it is crucial to understand the physical mechanisms of DFs with measurements at a timescale shorter than a spin period. We present a case study whereby we use measurements from the Magnetospheric Multiscale (MMS) Mission, which provides full 3‐D particle distributions with a cadence much shorter than a spin period. We provide a cross validation amongst the current density calculations and examine the assumptions that have been adopted in previous literature using the advantages of MMS mission (i.e., small‐scale tetrahedron and high temporal resolution). We also provide a cross validation on the terms in the generalized Ohm's law using these advantageous measurements. Our results clearly show that the majority of the currents on the DF are contributed by both ion and electron diamagnetic drifts. Our analysis also implies that the ion frozen‐in condition does not hold on the DF, while electron frozen‐in condition likely holds. The new experimental capabilities allow us to accurately calculate Joule heating within the DF, which shows that plasma energy is being converted to magnetic energy in our event.
Highlights
A dipolarization front (DF), characterized by a sharp increase in the northward magnetic field component Bz, is often observed during bursty bulk flows (BBFs) in the magnetotail [Nakamura et al, 2002]
The currents associated with DFs usually have a strong field-aligned component [Hwang et al, 2011; Liu et al, 2013a, 2013b; Sun et al, 2013; Zhou et al, 2013], which will act to couple the magnetosphere and ionosphere or even contribute to the substorm current wedge (SCW) [McPherron et al, 1973]
We study a dipolarization front (DF) using the high temporal plasma and field measurements from Magnetospheric Multiscale (MMS) on 12 August 2015
Summary
A dipolarization front (DF), characterized by a sharp increase in the northward magnetic field component Bz, is often observed during bursty bulk flows (BBFs) in the magnetotail [Nakamura et al, 2002]. A reduction of the north-south component of the magnetic field (Bz) is usually observed immediately ahead of the sharp Bz increase representing the DF [Shiokawa et al, 2005; Runov et al, 2011b]. Using these criteria, Yao et al [2013] classified both current regions according to their Bz features: the “magnetic dip current” preceding the DF and the “front layer current” flowing along the DF itself. The high-resolution ion measurements combined with the magnetic fields from MMS tetrahedron provide a cross validation to the electron dynamics. Using high-cadence plasma measurements, we determine the dominant current carriers, present an analysis of the charged particle terms in the generalized Ohm’s law, and reveal how energy is converted between the field and the plasma at this dipolarization front
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