Lightning-induced Electron Precipitation Studies


Lightning-induced electron precipitation (LEP) is studied by the VLF Group mainly by monitoring man-made subionospheric VLF signals. The electron precipitation process has several steps. In the top left panel (which can also be seen as a larger image), ducted whistler waves launched by lightning strikes travel along geomagnetic field lines, and are able to interact with geomagnetically trapped electrons. The interaction can alter the mirroring height of the trapped electrons, allowing them to penetrate the ionosphere where they cause secondary ionization. This excess ionization perturbs the earth-ionosphere waveguide (top right panel), which can then disturb subionospheric VLF signals like the 23.4 kHz signal (call letters of the transmitter are NPM) which is shown in the lower left panel. The rapid onset of amplitude and phase changes caused by the sudden burst of precipitating electrons is followed by a slower recovery as the excess ionization is recombined. The event in the episode of events (bottom left panels) indicated by a red arrow is detailed in the lower right panels, along with the associated ducted whistler (recorded at Palmer Station, Antarctica). The broadband recording of the whistler has the causative radio atomospheric (or sferic) indicated by a red arrow. The sferic is also caused the lightning strike, but unlike the whistler wave it travels through the earth-ionosphere waveguide. A larger image of a whistler wave recorded at Palmer is available.



Currently underway is an international program on the Antarctic Peninsula for monitoring VLF signals. As can be seen from the map, large areas are now able to be covered by having more VLF paths. Interpretation of LEP events is easier than before with the same event being observed at more than one receiver and, in some cases, on more than one signal.



In addition to LEP, similar signatures are observed which are called "early/fast events." These events are caused by direct heating of the ionosphere by lightning, as shown in the figure above. Like LEP events, the ionosphere is perturbed suddenly and then relaxes at a slower rate. Below LEP (left) and Early/fast (right) events are compared. Note the difference in time delays between the causative sferic (time of lightning strike) and the event's onset in the two cases.


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Last Updated February 22, 1999
lev_tov@nova.stanford.edu