Abstract
Very Low Frequency (VLF) probing has recently emerged as a powerful tool for remote sensing of transient D region ionospheric disturbances. During nighttime, when relatively low electron densities make the D region inaccessible to probing with radar and ionosounds, VLF transmitter signals propagating in the earth/ionosphere waveguide are sensitive even to slight (10%) conductivity changes at ~85 km altitudes. Past studies have revealed VLF signal perturbations associated with some lightning discharges, indicating both direct and indirect energetic coupling mechanisms between the troposphere and ionosphere. However, the assessment of the geophysical and global significance of these processes requires the determination of their spatial distribution and frequency of occurrence. A new VLF receiver system has been designed for this goal, incorporating PC based real-time DSP, GPS timing, Internet remote control, and automated data transfer. The Holographic Array for Ionospheric Lightning research (HAIL) consists of nine of these receivers deployed in the Midwest and operating year-around. Resulting multi-station data from the HAIL array has shed light on both types of coupling interactions.

"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 30=BA, and are shown to be consistent with horizontal extents of 90 =B1 30 km when compared with a three dimensional model of VLF propagation.=20 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.=20 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.