Thesis and Dissertation Defenses
Seasonality of VLF Attenuation Through the Ionosphere
Presented by Paul T. Greninger
We address the question of seasonality in the attenuation of Very Low Frequency (VLF) radiation transiting the ionosphere. To calculate this transionospheric attenuation we constructed a program we call "Iriatten." Our program implements the quasi-longitudinal approximation for the propagation of radiation put forth by Helliwell [1965], where the k vector of the wave is nearly aligned with the Earth's magnetic field. Inputs are electron densities taken from the International Reference Ionosphere (IRI); Earth's geomagnetic field comes from the International Geomagnetic Reference Field (IGRF); and neutral particle densities come from the Mass Spectrometer and Incoherent Scatter Radar (MSISE) code. Calculations are a function of latitude and longitude, year, month, day, and hour. Expressions for e- and ion–neutral particle collisions, including electron temperature dependence, come from Banks [1966]. In the literature Helliwell famously presented a series of atmospheric attenuation curves for day and night, 2 and 20 kHz, as a function of latitude. Iriatten is used to re-calculate Helliwell's curves as a check against his published answer and then to assess seasonality. High power VLF transmitters are utilized as a reference signal and two physical models are used to simulate them. In the Straight Up model waves are launched 'straight up' from the location of the VLF transmitter. In the Crary model waves are launched from a dipole source and attenuation is applied for large distances over the Earth's surface prior to calculating ionospheric attenuation with Iriatten. Results show a seasonality pattern that is unmistakable. Using the two models the ionospheric attenuation is calculated for four VLF transmitters and compared to DEMETER satellite observations. Plots are presented for electron, ion, and neutral particle densities, electron temperatures, and TEC. Results from the Crary vs Straight Up transmitter model more realistically portray seasonal variations exemplified by DEMETER satellite data. Crary errors are +/-4.5 dB, with a systematic bias of 8.8 dB. The motivation for the dissertation topic: understanding the anthropogenic contribution to VLF radiation observed in orbit. A code is presented that calculates ionospheric attenuation as a function of day/night, latitude, longitude and frequency. Parameters are taken from the IRI suite of codes.
1:00 pm, Friday, June 24, 2016
PAIS-2540, PAIS
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