Temporal evolution of the electric field accelerating electrons away from the auroral ionosphere

Abstract

The bright night-time aurorae that are visible to the unaided eye are caused by electrons accelerated towards Earth by an upward-pointing electric field. On adjacent geomagnetic field lines the reverse process occurs: a downward-pointing electric field accelerates electrons away from Earth. Such magnetic-field-aligned electric fields in the collisionless plasma above the auroral ionosphere have been predicted, but how they could be maintained is still a matter for debate. The spatial and temporal behaviour of the electric fields-a knowledge of which is crucial to an understanding of their nature-cannot be resolved uniquely by single satellite measurements. Here we report on the first observations by a formation of identically instrumented satellites crossing a beam of upward-accelerated electrons. The structure of the electric potential accelerating the beam grew in magnitude and width for about 200 s, accompanied by a widening of the downward-current sheet, with the total current remaining constant. The 200-s timescale suggests that the evacuation of the electrons from the ionosphere contributes to the formation of the downward-pointing magnetic-field-aligned electric fields. This evolution implies a growing load in the downward leg of the current circuit, which may affect the visible discrete aurorae.

Outflow of low-energy O<sup>+</sup> ion beams observed during periods without substorms

“…A qualitative picture that is emerging from these preliminary observations is that the field-aligned O + ions are accelerated along the magnetic field direction by a potential drop very similar to ions accelerated during substorms (Marklund et al, 2010). The observed values of the streaming velocity ratios of H + / O + and H + / He + support the field-aligned acceleration interpretation.…”

“…While the O + beams can form from velocity filter effects (Liao et al, 2010(Liao et al, , 2012Nilsson et al, 2004), we will interpret our observations in terms of particle acceleration by parallel potential drops because the distributions we measured were very much confined to the parallel direction (Marklund et al, 2010). In this case, the energy per charge of an ion that has gone through a potential drop φ measured by CODIF is mv 2 /2q = (W th /q)+ φ, where W th is the thermal energy.…”

“…The measured energies of the escaping O + ions are typically ∼ 20-40 eV, but after correcting for the spacecraft potential, the ion beams have energies of ∼ 40 to 80 eV. These potentials are lower than the typical potentials associated with substorms which are > 100 eV to several kiloelectron volts (Wilson et al, 2004;Marklund et al, 2010;Cui et al, 2010).…”

Pitch angle distributions of energetic electrons at Saturn

“…According to one model of terrestrial aurora [Marklund et al, 2001], quasi-static electric fields appear above the aurora and accelerate electrons down and ions upward, giving rise to ion conics. The current closes in the ionosphere through Pedersen currents perpendicular to the magnetic field, and then turns upward at a different latitude where a second parallel field accelerates electrons upward and ions downward.…”

“…While some investigations suggested an auroral source for the fieldaligned distributions [e.g., Klumpar et al, 1988;Klumpar, 1993], the consensus had developed that such distributions arise either locally or remotely from Fermi-type acceleration [e.g., Hada et al, 1981;Smets et al, 1999;Shiokawa et al, 2003;Vogiatzis et al, 2006;Wu et al, 2006]. Field-aligned electron fluxes have certainly been observed in the Earth's magnetosphere at low altitudes above the aurora and associated with particle acceleration in field-aligned potentials [e.g., Carlson et al, 1998;Marklund et al, 2001].…”