OEDIPUS‐C topside sounding of a structured auroral E region

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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.