VLF/LF pulse reflection measurements of the polar D-region during quiet and disturbed ionospheric conditions

A short pulse VLF/LF ionosounder to determine the characteristics of the polar D-region during quiet and disturbed conditions is described. Data obtained at Thule AB, Greenland, near the center of the polar cap, are presented giving information on the D-region's height and reflectivity as a function of time. Under quiet conditions there are marked variations in the D-region due to the effects of diurnal and seasonal changes in solar illumination. Energetic particle events produce dramatic changes in both the height and reflectivity of the D-region which can exhibit a complex behavior throughout the duration of the event. The severity of the D-region effects is highly dependent on seasonal illumination conditions. Data on ionospheric reflectivity illustrate normal seasonal and diurnal variations, data from disturbed periods show the interaction between propagation parameters and energetic particle ionization, solar ionizing and photodetaching radiations.