X-ray scanning of overhead aurorae from rockets

Abstract

As part of Project Aurorozone I, conducted at Poker Flat Research Range, Alaska (65.1°N, 147.5° W) in September 1976, two Nike Tomahawk rocket payloads were launched into energetic auroral events to investigate their structure and effects on the atmosphere. The instrument complement included X-ray scintillation detectors with energy discrimination in four ranges (5–10, 10–20, 20–40, >40 keV) to measure the deposition of bremsstrahlung produced X-rays within the stratosphere and mesosphere. For this purpose, each instrument was designed for wide angle viewing; however, properties of the rocket motion have permitted coarse observation of distinct spatial X-ray structure. The detectors were mounted at a 45 angle with respect to the payload axis to permit scanning of the upper hemisphere, with rocket spin rates near 5 c/s during the upleg portion of each flight. Here, atmospheric shielding reduced energetic particle contamination effects to insignificant values below 65–75 km. Iterative computer techniques were used to reconstruct X-ray source maps at 100 km, taking atmospheric absorption effects into account. Payload 18.178 was launched on 21 September (0302 LMT) into an aurora observed to have two distinct azimuthal regions of optical brightness. The same features were detected with the X-ray scanner; overlays of the X-ray source maps on all-sky photographs show spatial coincidence of the X-ray with optical features at the lower energies but not above 40 keV. Payload 18.179 (23 September, 0147 LMT) was launched into an aurora of more diffuse character. In this case, some optical structure was observed to occur but did not coincide as well with the measured X-ray structure. The presence of a two component spectrum is indicated for each event with the hard component originating in the more diffuse, optically faint regions.