OEDIPUS-C topside sounding of a structured auroral E region

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 paylo ad included a tethered HF transmitter-receiver pair which acted as a topsid e sounder. The bistatic (two-point) configuration allowed an in situ calibr ation of the radiated power. The conditions in the magnetosphere and ionosp here during the experiment were monitored by a groundbased network of instr uments and by instruments on the GOES 7 satellite in a geosynchronous orbit . In this paper we present results of the data analysis of topside ionogram s that were obtained during the down-leg flight of OEDIPUS-C (OC). The rela tively 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 th e down leg. Near apogee at 824 km, extremely low electron densities (simila r to 100 cm(-3)) were observed. The monotonic rise in electron density at t he payload from apogee to reentry clearly showed that there was no ionosphe ric 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 ion ospheric reflections plus strong Q-mode ground reflections were observed. O C thus has provided a close-hand view of a thick, highly structured, aurora l E layer sounded at small ranges. The RF signal was efficiently guided alo ng the magnetic field aligned density depletions that were located at the e quatorward edges of auroral arcs. Large pulse-to-pulse variations in the am plitude of the ionospheric reflection are not explained by ducting in the g eometric-optics sense.