Observations of Double Layers in Earth’s Plasma Sheet

Abstract

We report the first direct observations of parallel electric fields (E_{ parallel}) carried by double layers (DLs) in the plasma sheet of Earth's magnetosphere. The DL observations, made by the THEMIS spacecraft, have E_{ parallel} signals that are analogous to those reported in the auroral region. DLs are observed during bursty bulk flow events, in the current sheet, and in plasma sheet boundary layer, all during periods of strong magnetic fluctuations. These observations imply that DLs are a universal process and that strongly nonlinear and kinetic behavior is intrinsic to Earth's plasma sheet.

Mesoscale PIC simulation of double layers and electron holes affecting parallel and transverse accelerations of electrons and ions

“…In order to have V eb ∼ 10 8 m/s, the required potential drop across the double layers is DL ∼ 28 kV. Although Andersson et al [2009] do not report observation of double layers in association with the electron holes, but Ergun et al [2009] report strong sequential relation between double layers and electron holes, like that shown in Figure 8b. Figure 1a of Andersson et al [2009] shows intense electron fluxes extending to about 30 keV energy range.…”

“…When the potential profile is averaged over one ion plasma period, only the ramp containing the multiple double layers is clearly seen at the scale used in Figure 2. However, the instantaneous parallel electric fields at an enlarged horizontal time scale in Figure 8b [60] Andersson et al [2009] have reported data on strong electron hole turbulence observed from THEMIS in the plasma sheet as in the work by Ergun et al [2009]. They find that speed of the electron holes on the average is about 10 8 m/s, faster than the electron thermal velocity V te ∼ 4 × 10 7 km/s.…”

“…The speed for the downward motion is V DL ≈ 0.3V teo ∼ V tih , the thermal speed of the magnetospheric hot ions. For an example, for a hot ion temperature T ih = 5 keV, V DL ∼ 700 km/s and it could cross the auroral acceleration region, spanning over 2R e in altitude , in about 20 s. Later in section 7 we compare the speeds of double layers with those from observations of moving double layers in the hot ion environment of plasma sheet [Ergun et al, 2009]. The EHs move much faster and we will discuss their motion later in this section.…”

“…The THEMIS measurements are from the plasma sheet environment with hot ions having temperatures in the range from 2.5 to 20 keV. Ergun et al [2009] estimated the velocity of the moving double layers ranging from 250 km/s to 1400 km/s. We found here that the DL moves with a hot ion front with a velocity nearly the thermal velocity of the hot ions.…”

“…[58] Recently Ergun et al [2009] reported time series of parallel electric fields (E k ) of double layers and electron holes in the Earth's plasma sheet from THEMIS observations. The time series consists of electron hole turbulence suddenly ending in a double layer, very much like the time series plot in Figures 8a and 8b.…”