Parallel electric fields are understood to accelerate electrons and ions in the auroral regions. Auroral satellites have provided direct observation of double layers that are primarily associated with global field-aligned current systems and strong Alfvén wave-driven currents. The auroral observations prove that double layers naturally occur, but are limited to a region known to have highly-kinetic, non-ideal processes.

The THEMIS spacecraft have made the first direct observations of parallel electric fields carried by double layers well outside of the auroral acceleration region, in the plasma sheet. These double layers are observed during periods of strong magnetic fluctuations in several distinct regions of the plasma sheet including the boundary layer, the current sheet, and in association with rapid Earthward flows known as bursty bulk flows. This widespread observation of double layers not only suggests that parallel electric fields are universal in collisionless plasmas, but that many active plasma regions, including astrophysical plasmas, may be subject to strongly-nonlinear and non-ideal behavior.

THEMIS Observations

Figure 1. A ten-minute period of enhanced magnetic fluctuations observed by THEMIS D in the plasma sheet ~10 RE from Earth's center.

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Figure 1 displays a ten-minute period of enhanced magnetic fluctuations observed by THEMIS D in the plasma sheet ~10 RE from Earth's center. Figure 1a plots the electron differential energy flux as a function of energy (vertical axis) and time (horizontal axis). The overlying black trace is the electron temperature in eV derived from the electrostatic analyzer. Figure 1b plots the differential energy flux of ions in the same format. The plot shows high ion fluxes at several kilovolts which also are typical of the plasma sheet.

Figures 1c-d plot the magnetic field and the electric field in the GSM coordinate system: blue is towards the Sun, red is near Earth's magnetic north, and green completes the set. The black trace in Figure 1c is |B|. The black trace in Figure 1d represents E||. In Figure 1, E and B are at 128 samples/s after low-pass filtering to 50 Hz.

Figure 1e plots the quantity ExB/B2 low-pass filtered to 1 Hz. The flow perpendicular to B in the x-direction (towards Earth; blue trace) rises to over 1000 km/s from ~10:28 UT to ~10:30 UT, indicating a bursty bulk flow event . Such events are associated with magnetic reconnection occurring anti-Earthward of the spacecraft's position. During the bursty bulk flow event, the electron and ion energies increase and E and B display strong variations.

Figure 2. The auroral double layer is verified by an electron beam (Figure 2f) consistent with the double layer potential.

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At the time marked "Event 1" in Figure 1d, E was available at higher time resolution, 8192 samples/s (filtered to < 3.3 kHz). Figures 2a-c display E|| and two components of the perpendicular electric field. E|| displays strong turbulence in the form of electron phase-space holes (bipolar structures from -0.05 s to 0.10 s) and a unipolar electric field from 0.12 s to 0.14 s. This signature, a unipolar E|| structure adjacent to a turbulent region of electron phase-space holes, is identical to that of double layers observed in the auroral ionosphere and shown in Figures 2e-f. The auroral double layer is verified by an electron beam (Figure 2f) consistent with the double layer potential. It also is reproduced by open-boundary Vlasov simulations displayed in Figure 2g. The simulation shows a relatively smooth "ramp" region (corresponding to the unipolar E|| structure) adjacent to a series of electron phase-space holes. The THEMIS observations in the plasma sheet display these same characteristics.

Figure 3. Figure 3 displays the E|| signal of four more double layers in the PS observed by the THEMIS satellites at high time resolution (8192 samples/s).

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Figure 3 displays the E|| signal of four more double layers in the PS observed by the THEMIS satellites at high time resolution (8192 samples/s). All have the same characteristic signature: a nearly unipolar E|| structure adjacent to a turbulent region of electron phase-space holes. The polarity of the electron phase-space holes (negative then positive or positive then negative) is consistent with the high-potential side of the double layer (electron acceleration). All are observed during periods of strong fluctuations in B and E. Assuming that the double layers travel at the ion acoustic speed, all are on the order of 10 Debye lengths with potentials comparable to Te. The double layers are observed primarily in three plasma environments: (1) at the plasma sheet boundary layer, (2) near the current sheet where |B| is minimum, or (3) during bursty bulk flows events.

The THEMIS spacecraft have detected double layers in the Earth's plasma sheet during enhanced magnetic activity. The double layers are primarily at the plasma sheet boundary layer, near currents (|B| is minimum), and during bursty bulk flows. The occurrence of double layers is such that a significant fraction (1% or more) of the magnetic field lines are undergoing non-ideal behavior when active. These observations imply that strongly nonlinear, kinetic behavior is intrinsic to the Earth's plasma sheet and perhaps, to many space and astrophysical plasmas.