Location, spatial scale and motion of radio wave absorption in the cusp-latitude ionosphere observed by imaging riometers

Characteristic examples of the location, spatial scale and motion of radio wave absorption events in the cusp-latitude ionosphere are obtained from daytime observations on 18 September and 17 October, 1992, by imaging riometers in the Arctic region. One case observed near local magnetic noon at Ny-Ålesund, Svalbard (invariant lat. = 76.1 °) displays small-scale absorption events of 100–200 km in extent superposed on large-scale absorption features extending at least 700 km in longitude toward the prenoon sector. Many of the small-scale absorption events show quasi-periodic variations with repetition periods of 3–5 min which correlate well with local magnetic variations. Short-lived, impulsive absorption events (1–3 min duration) found among the quasi-periodic variations corresponded to impulsive magnetic variations observed over a wide range of magnetometer stations. Some of these impulsive events showed northward or northeastward motions. This case is interpreted in terms of the variable precipitation of high-energy substorm electrons. Another characteristic case observed in the noon sector at cusp-latitudes is an event of slowly varying absorption intensities associated with magnetic bays in the cusp and polar cap regions during conditions of strongly negative IMF-By component (Bz≈0). An interesting feature of this event is the observed antisunward motion of the front-like absorption features extending over 700 km in longitude. From these characteristics the slowly varying absorption intensities are interpreted in terms of E-region ionospheric disturbances related to the east-west oriented DPY currents in the cusp and polar cap.