Connection between high-latitude arcs and the low-latitude boundary layer during periods of northward IMF

Abstract:

High-latitude auroral arcs are a typical feature of periods of northward IMF. They consist in thin and elongated optical emission similar to discrete auroral arcs but located in the polar ionosphere. Their formation mechanism and the magnetospheric regions to which they are connected are still not well understood.

On November 10, 2005, high-latitude arcs were detected by an all-sky camera at Resolute Bay in Canada and by the TIMED/GUVI and DMSP/SUSIE space-based imagers. These observations indicate that they were detaching from the duskside auroral oval and then drifting poleward while pointing in the cusp direction. The same day, the Cluster spacecraft were flying in the dawn-dusk direction from the lobe region at altitudes ~5 R_E to the magnetospheric equatorial plane at geocentric distances ~19 R_E. Cluster observations reveal the presence of field-aligned acceleration regions above the polar ionosphere associated with the high-latitude arcs detected by the imagers.

We analyze Cluster particle observations from the lobe region to the duskside magnetopause. In the high-latitude arcs region, Cluster detects upgoing ions and precipitating electrons accelerated by a quasi-static electric field. These accelerated particles coexist with plasmasheet-like plasma embedded in the lobe region. A comparison between the 4 Cluster spacecraft electron measurements for the most poleward arc reveals that the plasmasheet-like electron population is vanishing on a time scale of a few minutes while the plasmasheet-like ion population doesn’t display any temporal evolution. The most equatorward arc is separated from the auroral oval by a “transition” region where weak fluxes of ions with plasmasheet like temperatures are detected. Then the Cluster spacecraft cross the plasmasheet until they reach the low-latitude boundary layer (LLBL) characterized by a mixture of plasmasheet and magnetosheath plasma. The “transition” region and the LLBL are magnetically connected. Using Cluster observations we show that these two regions display many similar features which suggest that the origin of high-latitude auroral arcs may be related to processes occurring in the LLBL during periods of northward IMF. We’ll discuss the implication of these observations on the formation mechanisms of high-latitude auroral arcs.