HR: 14:28h
AN: AE53A-05    [Abstracts]
TI: The Use of the Waveguide Cutoff Region to Monitor Ionospheric Height Through the Day-Night Boundary
AU: * Surana, K
EM: kunal@mit.edu
AF: EECS Department, Massachusetts Institute of Technology, Cambridge, MA 02139
AU: Williams, E
EM: earlew@ll.mit.edu
AF: Parsons Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139
AB: The use of electromagnetic waves to measure the height of the ionosphere is a well-known concept, as in any ionosonde. The waveguide cutoff phenomenon provides information about the height of a waveguide. This cutoff for the earth-ionosphere waveguide is illuminated by lightning sferics and can be used to monitor the change in ionosphere height through the day-night boundary. 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. Study has revealed that the observed ionospheric heights are found to be dependent on the three kinds of source-receiver paths: daytime, nighttime and mixed. Accurate locations for individual events are obtained from the National Lightning Detection Network (NLDN) to enable the analysis. Tweek sferics, which asymptote towards the transverse resonance frequency of the waveguide, predominate in the night path giving the most precise information about ionospheric heights. Weak dispersion and steep transitions in one-dimensional spectra provide information about daytime heights. Propagation over mixed paths reveals both daytime and nighttime signatures. 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.
DE: 6929 Ionospheric physics (2409)
DE: 3324 Lightning
DE: 2439 Ionospheric irregularities
DE: 3304 Atmospheric electricity
SC: Atmospheric and Space Electricity [AE]
MN: 2004 AGU Fall Meeting