Altitude Distribution and Position of Auroral Density Cavities in the Auroral Acceleration Region

Abstract:

We have investigated the statistical distribution of auroral density cavities (ADC) in the auroral acceleration region (AAR) using data from the Cluster satellites. The data was collected from 2002 to 2007, at geocentric altitudes of 3.0-6.5 R_E and 60-80 degrees invariant latitude. All time intervals containing upward ion beams events were manually inspected for the presence of inverted-V electrons and the spacecraft potential determined as a proxy for the electron density. The parallel potential drops above and below the satellite was estimated in order to determine the satellite's position relative the AAR.

Between 4.0 and 5.5 R_E several crossings of the upper edge of the AAR were observed. The maximum rate of occurrence was found between 4.75 and 5.0 R_E. Between 3.75 and 6.5 R_E many of the events exhibits an ion beam but no inverted-V electrons. This is consistent with the satellite being located inside the flux tube of the AAR but above the AAR. A sharp increase in the occurrence rate of ion beams without inverted-V electrons is found above 5.25 R_E. The maximum occurrence rate is found between 6 and 6.5 R_Ewhere none of the events exhibits any inverted-V.

The spacecraft potential exhibits a monotonic decrease with the geocentric altitude, though the rate of decrease is very small between 4.5 and 5.75 R_E. Above 5.75 R_E, where a large number of events do not exhibit any inverted-V electrons, the spacecraft potential exhibits a rapid decrease. This is consistent with entering a distinct region of low electron densities.

The observations indicate that the AAR extends considerably higher that the 2.0-3.0 R_E which is often cited. The region between 4.5 and 5.75 R_E appears to be a transition region based on the behavior of the spacecraft potential and loss of inverted-V electrons. We interpret this as being the upper edge of the AAR. However, the ADC does not appear to be confined by the AAR and will in many cases both extend above the AAR and in many cases become more pronounced above the AAR.