The nature of electrodynamic coupling between lightning discharges and the overlying ionosphere and radiation belts is explored by using (and expanding) an array of very-low-frequency (VLF) receivers known as the Holographic Array for Ionospheric/Lightning research (HAIL). That array provides high-resolution measurements of local VLF disturbances generated in the ionosphere from the receivers distributed across the continental United States at rural high schools and community colleges. The VLF disturbances are correlated with lightning data from the national Lightning Detection Network and with extremely low frequency (ELF) and VLF data from a broadband receiver in Indiana. Specific objectives include development of a physical description of thunderstorm coupling to the radiation belts, a description of electrodynamic coupling between the troposphere, mesosphere, lower ionosphere and magnetosphere, and the effect of geomagnetic storms on ionospheric variability. In addition, correlation of the HAIL VLF data with satellite measurements is used to quantify the influence of geomagnetic conditions on the loss of radiation belt electrons due to lightning generated whistler waves.
The Taylor University VLF Ground Station is one of the Very-low-frequency (VLF) receivers known as the Holographic Array for Ionosphere and Lightning Research (HAIL) to measure ionospheric effects of lightning and under the collaborative research under the direction of the Stanford University VLF Group. The data presented by the Stanford group using the Taylor University VLF ground station data and others have found many interesting and unexplained phenomena. These include early-fast coupling, Sprite events with earth to ionosphere discharges, natural hiss, chorus, plasmapause VLF electron precipitation, VLF trapping boundary, auroral acceleration, and many other relevant breakthroughs. Early satellite measurements (Voss et. al. 1983, and Voss et. sl. 1998) of Lightning Induced Electron Precipitation (LEP) showed detailed LEP electron precipitation data related to pitch angle, fine energy resolution, dynamic tine structure (3ms), echo effects, geographic distribution, association with radiation belt filling after magnetic storms, efficient VLF coupling with ionospheric irregularities (ducts), and frequency of occurrence to estimate radiation belt loss rates. Radiation from lightning can directly affect the ionosphere through direct impact ionization and heating, and indirectly through lightning-induced electron precipitation (LEP). Both direct and indirect effects appear as rapid perturbations to sub-ionospheric VLF transmitter signals By measuring direct effects of lightning, we hope to ascertain the physical mechanism that produces these events, and measure their effect in terms of energy and ionization. By measuring the indirect LEP events, we can study the effects of lightning on the Earth's magnetosphere and radiation belts, by quantifying the depletion of energetic electrons from the radiation belts due to lightning. In the current five-year effort, Stanford University has made great discoveries in collaboration with Taylor University with respect to LEP events. Many undergraduate and graduate students have been impacted. In his Stanford Ph,D thesis work, Dr. William Peter was able to quantify the loss of radiation belt electrons due to LEP events with only the VLF measurements. He achieved this result through a combination of data analysis, studying many years' worth of LEP events, and through detailed modeling of the physical processes that lead to electron precipitation. He found that a single lightning strike depletes a "flux tube" from the radiation belts of 0.001% of its electrons. This may seem like a small number, of often hundreds or thousands of such events occur due to a single thunderstorm.
