GROUND-BASED SIGNATURES FOR THE NIGHTSIDE POLAR-CAP BOUNDARY

This work describes an application of incoherent scatter (IS) radar an d all-sky imaging techniques to the identification of signatures for t he nightside amoral zone/polar cap boundary. This identification is ma de by estimating the characteristic energy of precipitating electrons across all latitudes measured by the IS radar. The primary method used to discern the boundary is based on an algorithm wherein the detailed shape of E region ionization profiles is mapped to the required numbe r flux from a Library of monoenergetic ionization profiles. This proce dure results in a reconstructed energy distribution for the precipitat ing electrons from which a characteristic energy is derived. The latit udinal gradient of the characteristic energy is subsequently used to i dentify the boundary. Monochromatic all-sky imagers are used to establ ish the time history of the boundary and estimate characteristic energ y (at magnetic zenith). Additional plasma signatures such as amoral io nospheric cavities and localized F region T-e enhancements are shown t o provide contextual clues for the location of the boundary. Three in- depth case study periods are used to qualify the various boundary iden tification methods. A detailed study of 30 IS radar scans in the geoma gnetic meridian is used to conclude that a latitudinal gradient in cha racteristic energy of -5 to -11 keV per degree, when measured adjacent to an extended region of low-energy precipitation, can locate the bou ndary with a latitudinal precision of 0.2 degrees.