Cluster measurements of an auroral acceleration region
A comparison of the data returned by the PEACE (electrons) and CIS (ions) instruments on the Cluster C3 (left) and Cluster C1 spacecraft (right) during their passage through the auroral acceleration region (AAR) of Earth's dusk-side aurora. Time is shown on the horizontal axis and particle energy on the vertical axis, with colour indicating the particle flux level at that energy. The key acceleration signatures detected by C3 occurred between 17:00 and 17:03 UT, whereas the comparable signatures detected by C1 occurred between 17:04:45 and 17:08 UT. Both areas are indicated by blue vertical lines.
The top panels show the number of electrons moving down Earth's magnetic field lines toward the atmosphere. The lower panels show energy spectrograms of ions going up the field lines. At top left, there are two regions with enhanced flux of descending electrons with peak energies of 4 and 5 keV. At top right, there are similar peaks associated with downward moving electrons at energies of 0.4 keV and 1 keV (dark red). The observed widths and peak energies of the ascending ions observed by C1 match those of the descending electrons observed by Cluster 3, as illustrated by the dotted curves, demonstrating that the acceleration region was located between the spcaecraft.