Coincident Airglow, VHF Radar, and Ionosonde Observations of Electrified Medium‐Scale Traveling Ionospheric Disturbances in the Equatorial Latitudes

The occurrence of late night spread F in the low and equatorial latitude regions during solar minimum is not properly understood. Radar observations have revealed occurrence of postmidnight echoes at low and equatorial latitudes during solar minimum periods. This work discusses the coordinated airglow, VHF radar, and ionosonde observations of nighttime electrified medium-scale traveling ionospheric disturbances (EMSTIDs) made from the low and equatorial latitude Indian region. Two nights of observations are discussed in detail in which EMSTIDs are seen to coexist with spread F in ionograms and VHF backscatter. These observations reveal that the EMSTIDs propagate to latitudes very close to dip equator at times and they can be responsible for a reasonable portion of the postmidnight radar echoes and spread F events observed at low and equatorial latitudes during low solar activity period. Plain Language Summary The low-latitude and equatorial F region ionosphere is susceptible to the formation of plasma instabilities affecting radio wave propagation. They are commonly referred to as equatorial spread F. Generally, they are known to form soon after the sunset. However, during solar minimum periods, spread F is often observed in ionosonde and coherent backscatter VHF radar observations in the later hours of the night including postmidnight period. The cause of such instabilities is not properly understood. In this work, using airglow imaging of OI 630.0-nm redline, VHF radar, and ionosonde observations, we show evidence for the existence of midlatitude-type instabilities causing the spread F over low and equatorial latitudes in later hours of the night. These observations ascertain that a portion of the late night spread F events observed during solar minimum period are caused directly by midlatitude-type electrified medium-scale traveling ionospheric disturbances propagating into the low latitudes.