  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 deg. N to 40 deg. N and 95 deg. W to 114 deg. W,
respectively. We have binned and averaged the fluctuation spectra according to
time of day, season, and geomagnetic (Kp index) and solar (F10.7) activity.
These spectra provide a detailed, multi-scale 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 Kp 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 seconds (scales of ~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 Kp index or F10.7, it does appear to be more pronounced during the
winter daytime, summer nighttime, and near dusk during the spring.
