EE 350 Radioscience Seminar
Professor Umran S. Inan
Date: Wednesday, March 12, 2003
Time: 4:15 PM – Refreshments at 4:00
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PhD Oral Defense:
Precipitation of energetic electrons from the Earth’s radiation
belts due to lightning-generated very low frequency waves
VLF, STAR Lab, Stanford University
Terrestrial cloud-to-ground lightning strokes generate a broad spectrum of waves.
In this talk, we focus on electromagnetic radiation in the very low frequency (VLF) band
[3-30 kHz], and in particular, on the impact of such waves in precipitating energetic
electrons from the Earth's radiation belts (also known as the Van Allen belts). This
precipitation occurs when VLF waves deflect the trajectories of trapped electrons in such a
way that they will ultimately be lost into the dense upper atmosphere.
Using an extensive raytracing and interpolation methodology, we calculate the detailed
frequency-time signature of a magnetospherically reflecting whistler wave (caused by a
single lightning stroke) at any point in the magnetosphere. By repeating this process at
a number of latitude points along a single magnetic field line (L-shell), we can estimate
the wave structure as a function of frequency and time. Finally, the flux of precipitating
particles is estimated by following the trajectories of a large number of geomagnetically
trapped electrons, and integrating the Lorentz force equation for each one to determine the
pitch angle deflections due to the wave.
Our results show that precipitating flux signatures have broad and well-defined features,
strongly correlated with the underlying wave structure. The location of the precipitating
electrons is generally displaced by thousands of kilometers from the causative lightning
strike, moves slowly to higher latitudes as a function of time, and can last for tens of
seconds. By examining the time and longitude-integrated curves of precipitating electrons,
a tantalizing relationship is suggested between lightning activity and a region in space with
highly depleted electron fluxes, known as the slot-region.