Title:  D-region Ionospheric Imaging using Lightning Generated Sferics, Forward Modeling, and Tomography

Committee: 

Dr. Cohen, Advisor      

Dr. Romberg, Chair

Dr. Simon

Dr. Bibby

Abstract:

The objective of the proposed research is to produce and evaluate the capabilities of producing a 2D image of the D-region ionospheric parameterization. The D-region of the ionosphere (60-90 km altitude) is highly variable on timescales from fractions of a second to many hours, and on spatial scales up to many hundreds of km. VLF and LF (3-30 kHz, 30-300 kHz) radio waves are guided to global distances by reflections from the ground and the D-region. Therefore, information about the current state of the ionosphere is encoded in received VLF/LF signals. VLF transmitters, for example, have been used in the past for ionospheric remote sensing, with ionospheric disturbances manifesting as perturbations in amplitude and/or phase. The return stroke of lightning is also an impulsive VLF radiator, but unlike VLF transmitters, lightning flashes are spread broadly in space allowing for much greater spatial coverage of the D-region compared to VLF transmitter-based remote sensing in addition to the broadband spectral advantage over the narrowband transmitters. The challenge is that individual lightning-generated waveforms, or ‘sferics’, vary due to the lightning current parameters and uncertainty in the time/location information, in addition to D-region ionospheric variability. In part, proposal describes a technique to mitigate this variability to produce stable, high-SNR sferic measurements. Using a model, the received sferics can be used to infer an ionospheric profile that is interpreted as an average along the path from lighting stroke to receiver. Through a tomographic technique, these sparse and sporadic path average inferences can be combined to calculate a 2D image of the D-region electron density profile.