Ionospheric height changes at two closely separated equatorial stations and implications in spread F onsets

Abstract

Changes in the ionospheric height during post-sunset hours are compared for two stations near the magnetic equator: Cebu Island ( 123.9° E, 10.3° N; 2.0° N MagLat at h=250 km ) and Manila ( 121.1° E, 14.7° N; 7.1° N MagLat at h=250 km ), both in the Philippines. Ionograms were obtained every 5 min during the WestPac98 (Western Pacific) ionospheric campaign conducted in March 1998. The ionospheric height changes during evening hours at the two stations showed various features from day to day. We summarized them into three cases: (a) the heights at the two stations varied almost identically; (b) they varied similarly but the height at Cebu was always 20– 30 km higher than at Manila; and (c) they varied in a fairly complicated way. Equatorial spread F during evening hours was observed on two case-c days. The height changes on those days are analyzed in detail. The pre-reversal enhancement in upward E × B drift started simultaneously at both stations, but drift direction reversed 1 h earlier at Manila than at Cebu. The difference in drift direction between the two stations was presumed to be the difference at the two corresponding heights over the magnetic equator. One possible explanation for this complexity is the manifestation of a vortex-like convection of E × B drift. Although we only have observations for 2 days during the campaign, the spread F occurrence and the peculiar height changes seem to be related to each other.