Global assessment of precipitation of radiation belt electrons by electromagnetic waves from lightning [electronic resource]
Keywords: Electrical Engineering

Lightning discharges are well known sources of electromagnetic radiation in the frequency range of a few Hz to many MHz, with the most intense radiation typically being in the range of 5--10 kHz. Electromagnetic waves originating in lightning discharges often propagate through the most densely ionized regions of the Earth's atmosphere and populate the radiation belts. High-energy electrons in this region constitute a hazard to the increasing number of scientific and commercial spacecraft that orbit the Earth, and quantitative understanding of this radiation and its sources and losses are thus important. Electromagnetic whistler waves injected into the radiation belts by lightning discharges can pitch-angle scatter the energetic electrons and cause them to precipitate out of their stably trapped radiation belt orbits and onto the dense upper atmosphere of the Earth. This dissertation examines the detection of lighting-induced energetic electron precipitation via long-term analysis of in-situ observations of drift loss cone fluxes (i.e., fluxes destined to be precipitated over the South Atlantic Anomaly within ~2 hours). The primary measurement tool used is an energetic electron detector (IDP) on board the DEMETER satellite---a French micro-satellite in a sun-synchronous low Earth orbit. Energetic electron flux data are analyzed alongside ground-based lightning data recorded by the National Lightning Detection Network (NLDN) to determine the relationship between drift loss cone fluxes and lightning. While lighting-induced electron precipitation events occur globally, the best region for making in-situ observations of fluctuations in drift loss cone fluxes is over the continental United States. Measurements of VLF wave activity in the typical frequency range of lightning-generated whistler waves (5--10 kHz) on DEMETER show a substantial increase of electromagnetic wave power over the United States, particularly during the northern summer months when lightning activity is at its highest. Analysis of particle precipitation data on the DEMETER spacecraft over a three-year period shows that energetic electron fluxes in the drift loss cone exhibit a seasonal dependence consistent with lightning-induced electron precipitation (LEP) being an important source of loss of such energetic radiation. Over the United States, energetic electron fluxes in the slot region (2< L< 3) are significantly higher in the northern summer than in the winter, consistent with the seasonal variation of lightning activity in the Northern Hemisphere. The variation of electron precipitation in energy and L-shell is explored and found to be consistent with expected pitch-angle scattering by lightning-generated whistler waves, indicating that lightning is a significant contributor to the loss of slot region electrons. To quantitatively relate IDP fluxes with NLDN lightning activity, a physical model of lightning-induced energetic electron precipitation is utilized to determine the size and location of the expected precipitation hot spot for each causative lightning discharge. Incorporation of energy and L-dependent drift periods into the calculation of the precipitation region results in a forward estimation of expected energetic electron precipitation at the satellite location that is then used to quantify the association between lightning and electron precipitation. Assessment of the relationship between lightning and drift loss cone fluxes is performed by correlating the relative fluxes expected from the electron precipitation model with the measured fluxes on the IDP instrument. A peak correlation between measured and expected fluxes of 0.42 for 126 keV electrons at L~2.2 indicates that lightning is a significant contributor to the loss of 126 keV electrons. Determination of the energy ranges and L-shell regions for which a strong correlation between expected and measured fluxes exists implies that lightning plays a continuous role in affecting the lifetime of radiation belt electrons, particularly at low energies (100--300keV) within the slot region (2< L< 2.5).