Electrical Engineering, Stanford University
"Early/Fast" VLF signal perturbations, so called because they occur immediately after a lightning discharge (<20 ms, i.e., "early") and exhibit a rapid onset (<20 ms, i.e., "fast"), are associated with ionospheric disturbances located above the associated lightning discharge. These direct coupling disturbances are observed with forward scattering patterns which exhibit 15 dB beamwidths of less than 30º, and are shown to be consistent with horizontal extents of 90 ± 30 km when compared with a three dimensional model of VLF propagation. However, they are only observed in association with a small fraction of the frequent lightning activity in North America. In order to quantify their global significance, observations now show that only that lightning with an unusually long duration intracloud component is associated with the direct coupling events - consistent with unusually large charge dipole moments.
Lightning-Induced Electron Precipitation (LEP) VLF events are known to be caused by an indirect lightning-ionosphere interaction. A fraction of the energy radiated by a lightning discharge escapes into the magnetosphere and propagates as a whistler-mode wave where it exchanges energy with trapped radiation belt electrons, causing those close to the loss cone to precipitate into the D region. These high energy electrons cause secondary ionization, modifying the ionospheric conductivity which in turn is manifested as a VLF perturbation. Previous and experimental work on the LEP phenomena has mostly emphasized interactions with ducted whistler waves, which are constrained to propagate in field-aligned ducts of enhanced ionization in the magnetosphere. The first two-dimensional images of these indirect conductivity disturbances, however, now show that the LEP phenomenon does occur under more general conditions, without the existence of magnetospheric ducts. A recent theoretical study of electron precipitation from these oblique whistlers made several predictions about their expected energy spectra, size, location, and intensity - all of which are here shown to be quantitatively consistent with empirical observations with the HAIL array in conjunction with two dimensional earth ionosphere propagation modeling. Results suggest that the lightning-induced electron precipitation process is likely to be an important contributor to the loss of radiation belt electrons on a global scale, since oblique whistler waves are naturally excited under a broad range of magnetospheric conditions, and continuously populate large regions of the magnetosphere