For each source observed near and after the Hunters Trophy event, maps of TEC gradient fluctuation power as a function of east-west and north-south spatial frequency. Here, the peak spectral power over all temporal frequencies is shown as an image in each panel with contours representing the mean power over all temporal frequencies. The range in azimuths expected for waves propagating from Hunters Trophy for altitudes between 100 and 900 km is shown in each panel with black lines. (Credit: U.S. Naval Research Laboratory) Enlarge

Feb. 19, 2013 — U.S. Naval Research Laboratory radio astronomer, Joseph Helmboldt, Ph.D., and researchers at Ohio State University Department of Civil, Environmental, and Geodetic Engineering analyzed radio telescope interferometry and Global Positioning Satellite (GPS) data recorded of the ionosphere during one of the last underground nuclear explosions (UNEs) in the U.S., codenamed Hunters Trophy.

Situated in the Plains of San Agustin, 50 miles west of Socorro, New Mexico, twenty-seven 25-meter parabolic dish antennas collectively make up the National Radio Astronomy Observatory's Very Large Array (VLA) radio telescope.

The VLA is an interferometer, meaning it operates by multiplying the data from each pair of telescopes together to form interference patterns. The structure of those interference patterns, and how they change with time as Earth rotates, reflect the structure of radio sources in the sky.

Designed as a radio synthesis telescope, observing bands between 1 and 50 gigahertz (GHz), the VLA is chiefly used to observe cosmic sources. While such observations require detailed calibration schemes to remove the effects of the ionosphere, this calibration data is seldom used to actually study the ionosphere.

During the Hunters Trophy event at the Nevada Test Site, Sept. 18, 1992, the VLA was observing a series of relatively bright cosmic sources at 1.4 GHz and positioned in a compact D-configuration, allowing an increased sensitivity to smaller-scale fluctuations. The data for these observations were retrieved from the VLA archives and self-calibration was performed using each source to obtain differences in total electron content (ΔTEC) time series for each antenna.

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