EE 350 RADIOSCIENCE SEMINAR
Professor Len Tyler
Date: Wednesday, October 18, 2000
Time: 4:15-5:30 PM; Refreshments at 4:00 PM
LIGHTNING-DRIVEN ELECTRODYNAMICS OF THE NEAR-EARTH SPACE ENVIRONMENT
Prof. Umran Inan
Electrical Engineering, Stanford University
A menagerie of complex phenomena have been uncovered during the past decade that collectively indicate that tropospheric thunderstorms and lightning discharges are strongly coupled (electrodynamically) to the overlaying upper atmospheric regions, ranging from the mesosphere to the lower ionosphere and extending to the radiation belts. Lightning phenomena at cloud altitudes (<20 km) affect the upper atmosphere at altitudes >40 km either via the release of intense electromagnetic pulses (EMPs) and/or the production of intense quasi-static electric (QE) fields at mesospheric altitudes. Lightning involves rapid removal of charge built-up within the thundercloud, either by intracloud discharges involving charge motion and neutralization within the cloud or by cloud-to-ground (CG) discharges which move charge to the ground. An average lightning discharge radiates an EMP of ~20 GW peak power, which propagates though the ionosphere and couples into the radiation belts, heating and ionizing the former and precipitating trapped energetic electrons from the latter. In addition, lightning discharges often produce intense transient QE fields of up to ~1 kV/m at 40--80 km altitudes, which for positive CG discharges is directed downwards, and which can thus avalanche accelerate upward-driven runaway MeV electron beams, producing gamma radiation, both in the hemisphere of the parent lightning and in the conjugate hemisphere, and contributing to the population of trapped radiation belt particles. A spectacular manifestation of these intense fields is the so-called ``Sprites'', large luminous discharges which appear in the altitude range of ~40 km to 90 km. The so-called ``Elves'' are optical flashes which last much shorter (<1 ms) than sprites, and are typically limited to 80-95 km altitude, being produced by the heating, ionization, and optical emissions due to the EMPs radiated by both positive and negative lightning discharges. The potential global importance of these lightning-driven electrodynamic stresses of the upper atmosphere are underscored by the fact that up to ~2000 thunderstorms are active on the surface of the Earth at any time, with ~40--100 lightning discharges occurring every second. A brief background and a discussion of recent results will be provided, much of it realized in the context of the recent PhD dissertations completed by N. Lehtinen (Mar 2000), M. Johnson (Sept 2000), and C.Barrington-Leigh (Sept 2000). We will also present some very recent SAMPEX satellite observations of lightning-induced precipitation of energetic (>150 keV) radiation belt electrons.