Climatology of midlatitude ionospheric disturbances from the Very Large Array Low-frequency Sky Survey

The results of a climatological study of ionospheric disturbances derived from observations of cosmic sources from the Very Large Array (VLA) Low-frequency Sky Survey (VLSS) are presented. We have used the ionospheric corrections applied to the 74 MHz interferometric data within the VLSS imaging process to obtain fluctuation spectra for the total electron content (TEC) gradient on spatial scales from a few to hundreds of kilometers and temporal scales from less than one minute to nearly an hour. The observations sample nearly all times of day and all seasons. They also span latitudes and longitudes from 28 degrees N to 40 degrees N and 95 degrees W to 114 degrees W, respectively. We have binned and averaged the fluctuation spectra according to time of day, season, and geomagnetic (K-p index) and solar (F10.7) activity. These spectra provide a detailed, multiscale account of seasonal and intraday variations in ionospheric activity with wavelike structures detected at wavelengths between about 35 and 250 km. In some cases, trends between spectral power and K-p index and/or F10.7 are also apparent. In addition, the VLSS observations allow for measurements of the turbulent power spectrum down to periods of 40 s (scales of similar to 0.4 km at the height of the E-region). While the level of turbulent activity does not appear to have a strong dependence on either K-p index or F10.7, it does appear to be more pronounced during the winter daytime, summer nighttime, and near dusk during the spring.