A global study of nighttime midlatitude topside spread echoes

Range spreading is frequently evident in midlatitude topside ionograms from the ISS-B satellite. These ionograms provide a geographically unbiased perspective for the first global investigation of nighttime spread echo events that focuses solely on the midlatitude domain. The topside echo occurrence probability data show significant longitudinal variations, along with very large winter/equinox asymmetries in several geographic regions. The spread echoes are divided into two types and found to have different geophysical dependences. Small-scale events that appear as spreading in one or two consecutive ionograms have a marked seasonal dependence on magnetic declination and no obvious correlation with auroral indices. Larger-scale events in which the spreading is observed over spatial domains of 1000 km or more have less dependence on magnetic declination but are anticorrelated with the AE index in periods 6-8 hours prior to their appearance. The declination dependence for the small-scale events suggests that the irregularities are most prevalent when the E region conductivity is low at both ends of the flux tube. This in turn suggests that electric fields mapping from the E region up to F region altitudes are likely involved in their generation. The correlation of the larger-scale events with depressed AE signatures suggests that traveling ionospheric disturbances caused by auroral heating are not the most likely cause of these large features at midlatitudes.