HR: 1330h
AN: AE42A-0798    [PDF]
TI: The Use of the Waveguide Cutoff Region to Monitor Ionospheric Height Through the Day-Night Boundary
AU: * Surana, K K
EM: kunal@mit.edu
AF: Massachusetts Institute of Technology, Electrical Engineering and Computer Science, Cambridge, MA 02139 United States
AU: Williams, E
EM: earlew@ll.mit.edu
AF: Massachusetts Institute of Technology, Parsons Laboratory, 48-211, Cambridge, MA 02139 United States
AB: The use of electromagnetic waves to measure the height of the ionosphere is a well-known concept, as in any ionosonde. However, the use of lightning as a tool to measure the ionospheric height is less well explored. Using a wideband electric antenna in West Greenwich, Rhode Island with a flat frequency response across the ELF-to-VLF transition region, this study purports to do just that. A preliminary study for a single day has revealed that the effective ionospheric height rises from 60 km during the day to almost 90 km at night, and that this change in height occurs over a short period of time - less than 30 minutes. Given that the effects of dispersion and waveguide attenuation at VLF are different for day and night, different approaches are used at different times. The two main approaches are outlined below:\\ 1) During nighttime, tweek sferics are prevalent and the tranverse resonances of the Earth-ionosphere waveguide are weakly damped. In this case, frequency-time spectrograms are used to determine the cutoff frequency and asymptotically, the tranverse resonance frequencies. The effective ionospheric height is easily calculated from these results.\\ 2) During daytime, VLF attenuation is enhanced and the quality factor of the transverse resonances much reduced. Consequently, we appeal to the extraction of the waveguide cutoff frequency from simple FFT's of individual sferics waveforms. This frequency is picked in the steep rise to VLF energy out of the broader frequency region with reduced waveguide energy. Attention will also be given to the locations and characteristics of lightning flashes that provide the best estimates for effective ionospheric height, by any method.
DE: 0674 Signal processing and adaptive antennas
DE: 3304 Atmospheric electricity
DE: 3324 Lightning
DE: 5435 Ionospheres (2459)
DE: 6964 Radio wave propagation
SC: Atmospheric and Space Electricity [AE]
MN: 2003 Fall Meeting