OEDIPUS-C topside sounding of a structured auroral E region

Abstract

The Observations of Electric-field Distributions in the Ionospheric Plasma - A Unique Strategy C (OEDIPUS-C) rocket payload was launched from Poker Flat, Alaska, into an evening aurora at 0638 UT, on November 7, 1995. The payload included a tethered HF transmitter-receiver pair which acted as a topside sounder. The bistatic (two-point) configuration allowed an in situ calibration of the radiated power. The conditions in the magnetosphere and ionosphere during the experiment were monitored by a ground-based network of instruments and by instruments on the GOES 7 satellite in a geosynchronous orbit. In this paper we present results of the data analysis of topside ionograms that were obtained during the down-leg flight of OEDIPUS-C (OC). The relatively low altitudes through which OC carried out topside sounding make the resulting ionograms a novel data set. Ionospheric reflections of the 10-W transmissions were detected at payload heights between 780 and 160 km on the down leg. Near apogee at 824 km, extremely low electron densities (∼100 cm-3) were observed. The monotonic rise in electron density at the payload from apogee to reentry clearly showed that there was no ionospheric F layer peak. The topside-sounding echoes came from all heights between the payload and the E layer peak around 100 km altitude. Strong X-mode ionospheric reflections plus strong O-mode ground reflections were observed. OC thus has provided a close-hand view of a thick, highly structured, auroral E layer sounded at small ranges. The RF signal was efficiently guided along the magnetic field aligned density depletions that were located at the equatorward edges of auroral arcs. Large pulse-to-pulse variations in the amplitude of the ionospheric reflection are not explained by ducting in the geometric-optics sense. Copyright 2000 by the American Geophysical Union.