Thesis (M. Eng. and S.B.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2004.Includes bibliographical references (leaves 93-96).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 is used to monitor the change in ionosphere height through the day-night boundary. 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. Nighttime observations are also compared against theoretical predictions (Porrat et al, 2001). Agreement is best for large peak current events ( > 100) kA). Some indication of a residual daytime effect in a nighttime ionosphere is also observed.by Kunal K. Surana.M.Eng.and S.B
