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Statistical altitude distribution of the auroral density cavity

The statistical altitude distribution of auroral density cavities located between 3.0 and 6.5 RE is investigated using in situ observations from flux tubes exhibiting auroral acceleration. The locations of the observations are described using a pseudo altitude derived from the distribution of the parallel potential drop above and below the satellite. The upper edge of the auroral acceleration region is observed between 4.375 and 5.625 RE. Above 6.125 RE, none of the events exhibit precipitating inverted V electrons, though the upward ion beam can be observed. This indicates that the satellites are located inside the same flux tube as, but above, the auroral acceleration region. The electron density decreases as we move higher into the acceleration region. The spacecraft potential continues to decrease once above the acceleration region, indicating that the density cavity extends above the acceleration region. From 3.0 to 4.375 RE the pseudo altitude increases by 0.20 per RE, consistent with a distributed parallel electric field. Between 4.375 and 5.625 RE the pseudo altitude increases weakly, by 0.01 per RE, due to an increasing number of events per altitude bin, which are occurring above the acceleration region. Above 5.625 RE the pseudo altitude increases by 0.28 per RE, due to a rapid increase in the number of events per altitude bin occurring above the acceleration region, indicating that the remaining parallel potential drop is concentrated in a narrow region at the upper edge of the acceleration region, rather than in a distributed parallel electric field.