Measurements from instruments on board Dynamics Explorer 1 are used to study regions of diminished electron density in the nightside auroral zone. Local electron densities are obtained from the electron plasma frequency cutoff of the whistler mode auroral hiss. Electron density profiles are highly variable through the nightside auroral zone. Sharply defined regions of low electron densities are a common feature of auroral zone crossings from the predusk hours until the early morning hours at all radial distances up to at least 4.6 RE. Electron densities in these regions are strongly depleted in relation to the adjacent polar cap and plasmaspheric densities, forming a low-density cavity at 70¿¿5¿ invariant latitude. Minimum densities in the auroral cavity frequently fall to values below 0.3 cm-3 and rarely exceed 3 cm-3 at radial distances of 2--4.6 RE. Within the cavity the electron density profile exhibits extreme variability, with variations of a factor of 2 or more on spatial scales of tens of kilometers. Electron plasma frequency to electron cyclotron frequency ratios of 0.02--0.4 are found inside the auroral cavity, consistent with the values required by theory for the generation of auroral kilometric radiation and Z mode radiation. A comparison of the electron density depletions with simultaneously measured low- and high-energy particle measurements indicates a correspondence between low auroral plasma densities and upward directed ion beams and conics. Low-energy (1 keV) precipitating auroral electrons do not correlate with the poleward edge of the auroral cavity, the peak energies of the precipitating electron population do occur inside the cavity interval. These correlations show that density depletions in the nightside auroral zone are directly associated with auroral acceleration processes. ¿ American Geophysical Union 1988