Microstructure of the auroral acceleration region as observed by FAST

Abstract

Abstract. The Fast Auroral Snapshot (FAST) explorer satellite was designed to investigate the microscale structure of the auroral acceleration region that was unresolved by previous satellites. This paper will present highlights from the first 2 years of the FAST mission and compare them with previous observations and auroral models. In particular, we find good agreement with the overall field-aligned current systems previously discovered; however, we present evidence that the downward currents are often carried by energetic upgoing electrons. These upgoing electrons are correlated with diverging electrostatic shocks, indicating that quasi-static parallel electric fields are responsible for their energization. Some of these field-aligned fluxes contain large-amplitude fast solitary waves which produce strong modulations of the electrons. Observations in inverted-V arcs show that the parallel acceleration region contains narrow (-10 kin) fingers of potential that extend along the magnetic field. On these narrow kilometer scales, ion beam energies are found to agree with the inferred potential determined from the perpendicular electric field or from the widening of the electron loss cone, implying acceleration is typically quasi-static on ion transit times. We also find evidence that both the ion and electron upgoing beams produced by the parallel electric fields have plateaued parallel distribution functions. Fast solitary waves are a prime candidate to stabilize the electron beams and may provide the resistance that allows the downward directed parallel electric fields to form in the highly conducting ionosphere. Intense ion cyclotron waves and ion solitary waves are often observed during ion beams, but the stabilizing waves have not been identified. In addition, intense electromagnetic ion cyclotron waves are also observed in inverted-V arcs, along with strongly modulated electron fluxes, indicating that turbulent acceleration is occurring in addition to simple acceleration by a static potential drop.

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

“…The connection between double layers and electron holes is known since the very early simulations of double layers [Singh, 1982;Singh and Schunk, 1982a, 1982b, 1984Singh et al, 1987], including the plateauing of the DL-accelerated electron beams by the plasma turbulence mainly consisting of the electron holes Thiemann, 1980a, 1980b]. This connection was eventually confirmed by satellite observations both in the upward and downward current regions [Carlson et al, 1998;Ergun et al, 1998;McFadden et al, 1999]. However, rarer observations of electron holes in the auroral upward current region (AUCR) compared to the auroral downward current region (ADCR) remains a mystery.…”

“…Using magnetic conjunction between the Cluster satellites and DMSP, Vaivads et al [2003] reported that as the Cluster satellites 1, 2 and 3 crossed the auroral arc a density enhancement was found at 4.7R e geocentric altitude while at the low altitude of DMSP (∼800 km) an extended density cavity was found. While the formation of the lowaltitude density cavity is well known from satellites like FAST [McFadden et al, 1999], the density enhancement measurements at high altitude, perhaps in the source region of the auroral arcs, is an unexplained new finding. Our simulation discussed here reveals this feature of the density at high altitudes in an auroral potential structure associated with an arc.…”

“…The velocity of the electron hole depends on the velocity of the electron beam, in which they form and the beam velocity itself depends on the parallel potential drop in the potential structure. McFadden et al [1999] have reported the existence of electron holes in AUCR. Unlike in the ADCR [Ergun et al, 1998], electron holes are not often observed in the upward current region.…”

“…[2] Double layers (DLs) and electron holes (EHs) play significant roles in auroral acceleration processes [Ergun et al, 1998;Carlson et al, 1998;Mozer and Kletzing, 1998;McFadden et al, 1999;Mozer and Hull, 2001;Hull et al, 2010]. The formation of EHs in plasma permeated by electron beams has been known since the early work of Roberts and Berk [1967] and Morse and Nielson [1969].…”

Observations of the seasonal dependence of the thermal plasma density in the Southern Hemisphere auroral zone and polar cap at 1 RE

“…In the region of upward field-aligned currents, parallel electric fields (which energize plasma sheet electrons to produce the aurora) accelerate ion beams out of the ionosphere and also reflect low-energy electrons, trapping them between the potential structure and their low-altitude mirror points. FAST satellite measurements have shown that the dominant contribution to the electron density in the density cavities associated with ion beams is energetic plasma sheet electrons [Strangeway et al, 1998;McFadden et al, 1999]. Outside these regions, measurements from the S3-3 Langmuir probe [Kletzing et al, 1998] show that the electron density is primarily due to electrons with energies of a fraction of an eV (ionospheric electrons).…”