EQUATORIAL SPREAD-F CAMPAIGN OVER SHAR

A RH-560 rocket instrumented with a Langmuir Probe and two pairs of do uble probes was launched from SHAR (dip 14 degrees N) at 2130 hr LT (I ST) on 4 October 1988 during spread-F conditions. As a part of this ca mpaign VHF scintillations at 136.1 MHz were also recorded, using the b eacon onboard the geostationary satellite ETS-II (130 degrees E). A ne w digital ionosonde (KEL) installed at SHAR during the first week of S eptember 1988 provided the basic ionospheric information. Ionosonde an d scintillation data during this campaign (15 September-4 October), al ong with the Langmuir probe data, are described. The occurrence of sci ntillations and spread-F during the campaign period was Very high, wit h maximum hourly percentage occurrences of about 80 and 70%, respectiv ely. Generally the onset of spread-F occurred slightly before 1900 hr LT when scintillations in the satellite signal also start appearing. T his 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, ro se to about 400 km in the post-sunset period while on days without spr ead-F it rose to only about 300-320 km. The electric field reversal, a s inferred from the h'F variations, occurred around 1930 hr LT on spre ad-F days and around 1900 hr LT on days without spread-F. The ionogram s, taken at 5 min interval on several nights, were used to determine t he vertical drift velocities and these exceeded 50 m/s on spread-F day s in the post-sunset period. At the time of the rocket launch, a stron g blanketing type of sporadic-E with several multiple reflections appe ared around 105 km. The electron density profile obtained from the Lan gmuir probe data also showed an extremely sharp layer at 105 km and an other prominent layer at 130 km, both during the ascent and the descen t. The electron density irregularities associated with spread-F were s een in different altitude regions, the most prominent being from 210 t o 250 km and from 300 to 320 km. The power spectra of the electron den sity fluctuations have been obtained both from the in situ probe data and the scintillation data. Copyright (C) 1996 Elsevier Science Ltd.