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Structure and variability of the auroral acceleration region

5 February 2013

The Aurora Australis seen from the International Space Station (ISS-029). Courtesy: NASA

Bright auroral arc appear when charged particles from the magnetosphere are accelerated into the upper atmosphere. Collisions between charged particles and neutrals excite the electrons in the neutral particles which then de-excite by emitting auroral light. Particles, in particular electrons, are accelerated out of the magnetosphere and into the atmosphere by magnetic-field-aligned electric potential drops in a region known as the auroral acceleration region (AAR). In a recent paper, Forsyth et al. [2012] investigated the temporal variability and spatial structure in one such region.

In their study, Forsyth et al. used data from the Cluster spacecraft as they passed through an auroral acceleration region at altitudes between 4000 and 7000 km altitude. Using two spacecraft that were on the same magnetic field lines but separated by 1500 km, Forsyth et al. determined that up to 500 V of the potential drop along the magnetic field was located between the two spacecraft and that the majority of the remainder of the potential drop was concentrated below the lower spacecraft. Forsyth et al. were also able to use observations from a third Cluster spacecraft that passed through the AAR some three minutes prior to the two conjugate spacecraft to infer that the variation in the potential drop along the magnetic field was also concentrated at low altitudes.

Figure showing the change in the electric potential structure of the AAR as three of the Cluster spacecraft passed through it. During the second crossing, two of the spacecraft were magnetically conjugate, allowing for an accurate determination of the vertical structure of the AAR. Taken from Forsyth et al. [2012]

These observations are the first such observations, allowing the determination of the instantaneous vertical structure of the AAR to be determined and showing that the potential drop tends to be variable at low altitude. As other such observations become available (see Forsyth & Fazakerley, 2012), we will be able to gain a greater understanding of the processes that control the aurora and their link to the wider protective magnetospheric system.

References:

Forsyth et al. [2012], Temporal evolution and electric potential structure of the auroral acceleration region from multispacecraft measurements, J. Geophys. Res, 117 (A12), doi:10.1029/2012JA017655

Forsyth & Fazakerley [2012], Multispacecraft observations of auroral acceleration by Cluster, in Auroral Phenomenology and Magnetospheric Processes: Earth and Other Planets, Geophys. Monogr. Ser., vol. 197, 261–270,doi:10.1029/2011GM001166

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