Equatorial spread-F campaign over SHAR

Abstract

A RH-560 rocket instrumented with a Langmuir Probe and two pairs of double probes was launched from SHAR (dip 14 ° N) at 2130 hr LT (1ST) on 4 October 1988 during spread-F conditions. As a part of this campaign VHF scintillations at 136.1 MHz were also recorded, using the beacon onboard the geostationary satellite ETS-II (130 ° E). A new digital ionosonde (KEL) installed at SHAR during the first week of September 1988 provided the basic ionospheric information. Ionosonde and scintillation data during this campaign (15 September–4October), along with the Langmuir probe data, are described. The occurrence of scintillations and spread-F during the campaign period was very high, with maximum hourly percentage occurrences of about 80 and 70%, respectively. Generally the onset of spread-F occurred slightly before 1900 hr LT when scintillations in the satellite signal also start appearing. This is followed soon by the range spreading on the ionosonde trace. On days with spread-F the minimum virtual height of the F-layer, h′F, rose to about 400 km in the post-sunset period while on days without spread-F it rose to only about 300–320 km. The electric field reversal, as inferred from the h′F variations, occurred around 1930 hr LT on spread-F days and around 1900 hr LT on days without spread-F. The ionograms, taken at 5 min interval on several nights, were used to determine the vertical drift velocities and these exceeded 50 m/s on spread-F days in the post-sunset period. At the time of the rocket launch, a strong blanketing type of sporadic-E with several multiple reflections appeared around 105 km. The electron density profile obtained from the Langmuir probe data also showed an extremely sharp layer at 105 km and another prominent layer at 130 km, both during the ascent and the descent. The electron density irregularities associated with spread-F were seen in different altitude regions, the most prominent being from 210 to 250 km and from 300 to 320 km. The power spectra of the electron density fluctuations have been obtained both from the in situ probe data and the scintillation data.