Kinetic Equilibrium of Dipolarization Fronts

Abstract

The unprecedented high-resolution data from the Magnetospheric Multi-Scale (MMS) satellites is revealing the physics of dipolarization fronts created in the aftermath of magnetic reconnection in extraordinary detail. The data shows that the fronts contain structures on small spatial scales beyond the scope of fluid framework. A new kinetic analysis, applied to MMS data here, predicts that global plasma compression produces a unique particle distribution in a narrow boundary layer with separation of electron and ion scale physics. Layer widths on the order of an ion gyro-diameter lead to an ambipolar potential across the magnetic field resulting in strongly sheared flows. Gradients along the magnetic field lines create a potential difference, which can accelerate ions and electrons into beams. These small-scale kinetic effects determine the plasma dynamics in dipolarization fronts, including the origin of the distinctive broadband emissions.

Figure 1

MMS fields and particle data during a dipolarization front. (a,b) Low-frequency magnetic field data (128 Samples/s) in GSE coordinates. (c) Spacecraft floating potential (d) Electron and proton density. (e) Electron omni-directional energy flux. (f) Plasma flow velocity components, in GSE coordinates, estimated from E × B measurements. (g) Electric field wave power spectral density.

Figure 2

(a) Self-consistent electrostatic potential, Φ₀, across the dipolarization front. The abrupt changes in value are due to variation on the electron scale. The inset shows stronger variation on electron scale around $\overline{x}\sim -1.5$ in the electron layer. (b) Self-consistent model plasma density across the dipolarization front (blue) compared with the electron plasma density (orange) from the measurements. (c) Magnetic flux pile up (blue) due to the cross-field current generated by the model compared to the measured magnetic perturbation (orange).

Figure 3

Model-calculated equilibrium features across the magnetic field of the dipolarization front considered. The blue shading shows the location of the electron layer. (a) Ion (black) and electron (blue) flow velocities normalized by the ion thermal velocity. (b) Ion temperature component in the y-direction (blue) and anisotropy ratio (black). (c) In the dipolar magnetic field geometry an electric field component is generated along the magnetic field lines.

Figure 4

The presence of a non-uniform transverse electric field makes ion and electron distribution functions in the dipolarization front non-gyrotropic. The non-gyrotropy is more visible in the ion distribution function than in electrons. The distribution function is plotted at $x/{\rho }_i=0.0$.