Auroral particle acceleration by strong double layers: The upward current region

Abstract

[1] Satellite observations have established that parallel electric fields of both upward and downward current regions of the aurora are supported, at least in part, by strong double layers. The purpose of this article is to examine the role of double layers in auroral electron acceleration using direct measurements of parallel electric fields and the accompanying particle distributions, electrostatic waves, and nonlinear structures; the concentration is on the upward current region. Direct observations of the ionospheric boundary of the auroral cavity suggest that a stationary, oblique double layer carries a substantial, albeit a minority fraction ($10% to $50%) of the auroral potential. An order of magnitude density gradient results in an asymmetric electric field signature. Oblique double layers with amplitudes greater than 100 mV/m have been verified in $3% and may occur in up to 11% of auroral cavity crossings, so it is feasible that strong double layers are a principal acceleration mechanism. In this article we also present a second type of double layer that has a symmetric electric field signature and is seen inside of the auroral cavity. These structures are a possible signature of a midcavity or high-altitude acceleration mechanism. Numerical solutions of the Vlasov-Poisson equations support the possibility of midcavity double layers and indicate that trapped electrons can play an important role in the double-layer structure.

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

“…Likewise, Figures 15b and 15c show the ion distributions above and below the DL, reproduced from Figure 6 of Ergun et al [2004]. Above the DL both FAST and simulated data show upgoing ion beam.…”

“…Figure 14a shows the electron velocity distribution functions above the DL in the height range 4100 < y/l d < 4800 (solid line) and below the DL for 1900 < y/l d < 2600 (dashed line). Figures 14b and 14c, reproduced from Figure 6 of Ergun et al [2004], are measured distributions above and below a DL, respectively. Note that in the FAST data positive (negative) velocities are downward (upward), exactly opposite to our simulations.…”

“…[56] Ergun et al [2004] have reported electron and ion velocity distribution functions above and below a measured double layer at an altitude of about 4000 km as observed from FAST. We compare the overall features of the measured distributions with those found above and below the double layer at t = 51,000 w po −1 shown in Figures 4 and 6a.…”

Dynamical behavior of U-shaped double layers: cavity formation and filamentary structures

“…; Mozer and Hull (2001) and more recently Ergun et al (2004) reported that auroral density cavities are the locations for large parallel electric fields (E || ), which are invariably accompanied with perpendicular electric fields (E ⊥ ) in the upward current region (UCR) of the auroral plasma. The ratio of E || to (E ⊥ ) ranges from ∼0.25 to O(10).…”

Vlasov simulations of trapping and loss of auroral electrons

“…However, this double layer only carries a small part of the total voltage. Ergun et al (2004) observed mid cavity double layers that held a small fraction of the potential drop in the presence of wave turbulence. It is thus likely that midcavity double layers are dynamic in nature.…”