Mapping a bubble at dip equator and anomaly with oblique ionospheric soundings of range spread F

Abstract

Multiple ionospheric sounders are employed in the first attempt to detect and map remotely an equatorial bubble near the dip equator and simultaneously near the Appleton anomaly crest. The sounders also provide latitudinal profiles of electron density through ±30° dip latitude (DIPLAT). Conditions are solar maximum and low Kp. Using oblique range spread F (RSF) echoes from the bubble boundaries, four sounders located near the dip equator detect a single isolated bubble at ranges as great as 1020 km and track it for 2.25 hours at 5‐min intervals as it drifts eastward through 15° geomagnetic longitude (MLONG). Maximum velocity is 185 m/s, and width is 375 km. Near the northern anomaly crest, a sounder at 20.3° DIPLAT observes the western boundary for a duration of 1 hour concurrently with and magnetically conjugate to the equatorial observations; The boundary here has nearly the same drift velocity and MLONG as at the equator, implying that the boundary location and velocity are nearly independent of altitude above the dip equator. Subsequently, in the south a sounder at 19.9° DIPLAT observes the bubble to have a velocity approximately half that seen by the other stations and a MLONG that is at least 3° west of the others, This difference may be related to the north‐south asymmetry of the anomaly crest, which is at the location of the northern sounder but equatorward of the southern sounder, although the two are at the same DIPLAT. The onset of the bubble is recorded in the first appearance of RSF at 1915 local mean time; 20 min later the bubble boundary appears and begins accelerating eastward, the movement occurring within about 5 min of electric field reversal as inferred from F layer vertical motion.