Global quantification of lightning-induced electron precipitation using very low frequency remote sensing [electronic resource]
Keywords: Electrical Engineering

Observations of lightning-induced electron precipitation (LEP) events at three geographic regions show characteristics which systematically vary with both longitude and hemisphere. These observations are quantitatively interpreted with the use of a novel model of atmospheric backscatter designed to be used to predict the characteristics of LEP events at any longitude and mid-latitude L-shell by accounting for the effects of precipitating electrons which are backscattered from the atmosphere. The new model of atmospheric backscatter (ABS) is based upon the calculation of ~16,000 individual atmospheric backscatter responses for monoenergetic electron beams with a single incident pitch angle using a Monte Carlo model of atmospheric interactions which tracks the full gyration of each individual precipitating electron around the magnetic field line as it enters the atmosphere, accounting for the dynamic friction force and angular diffusion as well as the production of new electrons via ionization. The ABS model includes the effects of the asymmetric magnetic field in calculations of the pitch angle of backscattered electrons entering the conjugate hemisphere and accounts for the different strength of the magnetic field at conjugate points of the same field line. This magnetic field difference causes the equatorial loss cone angle to vary greatly between hemispheres and with longitude which results in significant and systematic differences in LEP signatures at various locations. A realistic distribution of precipitating electrons is inserted into the ABS model by calculating the energy and pitch angle distribution which results from the resonant interactions of a lightning-initiated magnetospherically reflecting whistler wave with trapped radiation belt particles. This calculation is accomplished by extensive magnetospheric ray-tracing, accounting for Landau damping as well as spatial and temporal dispersion of the whistler wave. This distribution of precipitating electrons is then inserted into the ABS model at three separate longitudes (260 degrees east (1N) and 290 degrees east (2N) in the Northern Hemisphere, and 295 degrees east (2S) in the Southern Hemisphere, corresponding to the Central United States, the East Coast of the United States, and Palmer, Antarctica, respectively) and the deposition results are compared with VLF remote sensing data collected on nearly north-south great circle paths (GCPs) allowing for isolation of longitudinal effects on LEP. Results predicted by the model and confirmed by data indicate that all four primary LEP characteristics exhibit longitudinal and hemispheric dependencies which can be explained in terms of backscatter of precipitating electrons from the atmosphere. The mean observed LEP onset delay exhibits a hemispheric dependence at these longitudes with events in the northern hemisphere delayed by one bounce period relative to direct precipitation and advanced by one-half bounce period in the southern hemisphere. The mean observed onset duration exhibits a longitudinal dependence with events observed at locations 1N and 2S persisting for three bounce periods, and at location 2N for two bounce periods. The amplitude change and recovery time also show a longitudinal and hemispheric dependence based upon the relative sizes of the loss cones at different longitudes with LEP events produced at location 1N consistently larger than at location 2N, and observed recovery times at location 2N longer than at 1N which are still longer than at 2S. All of these results are explained in terms of backscatter of precipitating electrons from the atmosphere and ABS model shows that by accounting for atmospheric backscatter it is possible to accurately predict all the observable characteristics of LEP events. Furthermore, by combining these effects with previously calculated radiation belt electron loss rates due to lightning at a single location, it is possible to estimate the global loss of radiation belt electrons due to lightning.