Ion and electron characteristics in auroral density cavities associated with ion beams: No evidence for cold ionospheric plasma

Low-density cavities associated with upgoing ion beams were identified in t he auroral regions over two decades ago. In order to understand the waves, double layers, and solitary structures observed within these cavities, accu rate measurements of the plasma distribution function are required. Althoug h measurements by DE 1 indicated that these cavities were composed primaril y of hot plasma in the form of ion beams, plasma sheet ions, and inverted V electrons, later reports from Viking showed these cavities contained a col d plasma component whose density was an order of magnitude larger than the hot component. Recent measurements by the FAST satellite contrast sharply w ith the Viking results and support the earlier DE 1 observations, Regions o f upgoing ion beams observed by FAST are shown to contain little or no cold plasma. The hot electron densities (>100 eV) and the combined plasma sheet ion and upgoing ion beam densities (>30 eV) agree remarkably well. Further more, no cold ions (0-30 eV) are measured at low energies by the mass spect rometer, which precludes the presence of significant (>20%) cold electrons to preserve charge neutrality. Characteristics of the plasma for 1 1 ion be am events are tabulated.