VLF remote sensing of high-energy auroral particle precipitation

Ground-based measurements of VLF transmitter signals propagating in the Earth-ionosphere waveguide can be used to determine the location of nighttime high-energy (greater than or similar to 100 keV) auroral particle precipitation. When the region of auroral particle precipitation passes over a VLF propagation path, disturbances in the D region of the ionosphere created by the high-energy particles perturb the amplitude of VLF signals propagating below in a characteristic manner. Continuous nighttime observations of the amplitude of the signal from the NLK transmitter (24.8 kHz, Jim Creek, Washington) were made in Gander, New-foundland, during November 1993 and January 1994. Simultaneous images of atmospheric X rays created by auroral particle precipitation taken by the AXIS instrument on the UARS satellite were examined for times when large-scale auroral particle precipitation extended over the NLK-Gander propagation path. Quantitative characteristics of the precipitation-associated NLK signal perturbations are established from days which clearly exhibit good correlation between the AXIS images and VLF data, and a larger data set from the months of November 1993 and January 1994 is examined statistically to determine the effectiveness of the 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 amplitude decreases are consistent with propagation under conditions of enhanced lower ionosphere electron density caused by auroral electron precipitation and suggest that electrons with energies greater than 100 keV are responsible for the VLF amplitude perturbations.