VLF REMOTE-SENSING OF HIGH-ENERGY AURORAL PARTICLE-PRECIPITATION

Ground-based measurements of VLF transmitter signals propagating in th e Earth-ionosphere waveguide can be used to determine the location of nighttime high-energy (greater than or similar to 100 keV) auroral par ticle precipitation. When the region of auroral particle precipitation passes over a VLF propagation path, disturbances in the D region of t he ionosphere created by the high-energy particles perturb the amplitu de of VLF signals propagating below in a characteristic manner. Contin uous nighttime observations of the amplitude of the signal from the NL K transmitter (24.8 kHz, Jim Creek, Washington) were made in Gander, N ew-foundland, during November 1993 and January 1994. Simultaneous imag es of atmospheric X rays created by auroral particle precipitation tak en by the AXIS instrument on the UARS satellite were examined for time s when large-scale auroral particle precipitation extended over the NL K-Gander propagation path. Quantitative characteristics of the precipi tation-associated NLK signal perturbations are established from days w hich clearly exhibit good correlation between the AXIS images and VLF data, and a larger data set from the months of November 1993 and Janua ry 1994 is examined statistically to determine the effectiveness of th e VLF technique in capturing particle precipitation events. The number of particle precipitation onsets seen in the AXIS images that can be readily identified in the VLF amplitude data is found to be almost 94% . VLF propagation model calculations show that the observed VLF amplit ude decreases are consistent with propagation under conditions of enha nced lower ionosphere electron density caused by auroral electron prec ipitation and suggest that electrons with energies greater than 100 ke V are responsible for the VLF amplitude perturbations.