MMS Observations of a Compressed Current Sheet: Importance of the Ambipolar Electric Field

Spacecraft data reveal a nonuniform ambipolar electric field transverse to the magnetic field in a thin current sheet in Earth’s magnetotail that leads to intense $\mathbf{E}\times \mathbf{B}$ velocity shear and nongyrotropic particle distributions. The $\mathbf{E}\times \mathbf{B}$ drift far exceeds the diamagnetic drift and thus drives observed lower hybrid waves. The shear-driven waves are localized to the magnetic field reversal region and are therefore ideally suited for the anomalous dissipation necessary for reconnection. It also reveals substructures embedded in the current density, indicating a compressed current sheet.

Figure 1

Boxcar averaged MMS1 plasma parameters as a function of time (bottom axis) and distance normalized to ${\rho }_i=841\mathrm{km}$ (top axis). (a) Magnetic field burst data ($128\mathrm{S}/\mathrm{s}$) measured by the fluxgate magnetometer [54], (b) ion velocity, (c) electron velocity, and (d) electric field burst data ($8192\mathrm{S}/\mathrm{s}$) measured by FIELDS [53] rotated into LMN coordinates ($\mathrm{blue}=\widehat{L}$, $\mathrm{green}=\widehat{M}$, $\mathrm{red}=\widehat{N}$), (e) parallel (teal) and perpendicular (orange) electron temperature, and (f) electron density. Velocities, densities, and temperatures are obtained from the fast plasma investigation (FPI) [55] burst measurements ($\mathrm{ions}=6\mathrm{S}/\mathrm{s}$, $\text{electrons}=33\mathrm{S}/\mathrm{s}$). The unshaded region highlights $\pm 1$ second around the $B_L$ reversal (vertical dashed line), around which the electron diffusion region is typically located. The magenta box indicates the region during which electrostatic LH fluctuations are present.

Figure 2

Spectrogram showing power spectral density of electrostatic LH fluctuations as a function of time with (a) $E_N$ (solid, blue) and $n_e$ (dashed, orange), (b) total electron (gray, dot-dashed) and $\stackrel{\rightharpoonup }{E}\times \stackrel{\rightharpoonup }{B}$ shear (blue, solid) velocities in $\widehat{L}$, (c) shear (blue, solid) and diamagnetic drift (dashed, orange) frequencies overlaid. The top axis shows distance normalized to ${\rho }_i$. The vertical dotted line indicates the $B_L$ reversal time.

Figure 3

Kinetic equilibrium model (solid, red) compared to MMS data (dashed, blue) as a function of distance normalized to ${\rho }_i$ for (a) magnetic field in $\widehat{L}$, (b) ion velocity in $\widehat{L}$, (c) electron velocity in $\widehat{L}$, (d) ambipolar electric field in $\widehat{N}$, (e) electron current density in $\widehat{L}$. (f) The modeled nongyrotropic electron distribution function that arises due to the ambipolar electric field, where the axes represent the electron flow in the direction of $E_N$ (vertical) and parallel to $V_{E\times B}$ (horizontal) normalized to the thermal velocity. The color bar represents the log of the phase space density in arbitrary units.

Figure 4

(a) Spectrogram showing power spectral density of electrostatic LH fluctuations as a function of distance normalized to ${\rho }_i$ with the real (solid, red) and imaginary (dashed, blue) eigenmodes, $\varphi$, overlaid. The vertical dotted line indicates the $B_L$ reversal. (b) The frequency (solid) and growth rate (dashed) normalized to ${\omega }_{LH}$ plotted as a function of $k_y{L}_E$.