Kenneth John Frost

A total of 130 papers and presentations are listed in NASA ADS with Frost as co-author with 57 of them refereed and seven of these with him as lead author.[4] Two of his papers have had lasting significance in advancing our understanding of solar flares. The most famous paper, published in 1971, is based on observations of a solar flare with Frost's instrument on the fifth Orbiting Solar Observatory (OSO-5) and presents "Evidence from Hard X-Rays for Two-Stage Particle Acceleration in a Solar Flare."[5] It is still cited today with a total of 154 citations, with 5 in 2013 alone. A second paper, published in 1969 and entitled "Rapid Fine Structure in a Burst of Hard Solar X-Rays Observed by OSO-5," is less well known but still has 116 citations.[6]

At the start of the space program in the late fifties and early sixties, there was the recognition that X-rays and gamma-rays, not detectable from the ground, offered a new window on the universe. These high-energy photons had the potential for enabling new scientific discoveries that could rival or exceed all that had been achieved previously with ground-based optical astronomy. Hence, there was a great demand for flying ever-improving X-ray and gamma-ray instruments to determine the intensity and spectrum of the emission from the Sun and other more distant cosmic sources of this radiation.

When the first attempts to detect X-rays and gamma-rays from the Sun, it was found that the background flux in space was of the same order of magnitude as the maximum theoretical estimates for the primary flux. Thus, any detector had to be capable of suppressing this background. Lead shielding to reject this background and provide angular collimation had proved unsuccessful since it generated its own background through interactions with cosmic rays. Frost was the first to suggest the use of an active scintillation shield around the X-ray/gamma-ray detector with the two connected in electronic anticoincidence to reject unwanted charged particle events and to provide the required angular collimation.[7][8][9]

Drawing of an active anticoincidence collimated scintillation spectrometer designed for gamma-ray astronomy in the energy range from 0.1 to 3 MeV.[7]

The proposed instrument is shown in the figure to the right.

Frost developed this design in collaboration with Laurence E. Peterson, then at the University of Minnesota, who had been working independently on a similar idea. Together they built an instrument of this type and flew it on a high-altitude balloon to near the top of the atmosphere over Minneapolis on June 10, 1962. They were able to demonstrate the instrument's performance and establish upper limits on the flux of gamma-rays from the quiet (non-flaring) Sun between 160 and 800 keV that were "considerably below those previously available."[10]

Actively shielded X-ray and gamma-ray spectrometers of this type became the standard for making high sensitivity observations of solar and cosmic sources, and variations on this basic technique are still being used today.

Frost was the PI for active anticoincidence spectrometers on four of the eight Orbiting Solar Observatories, namely the OSO-1, 2, 5, and 8.[11][12][13][14] All the instruments were X-ray and gamma-ray spectrometers of the same basic design with an active anticoincidence collimator that Frost had originally suggested. Laurence E. Peterson at the University of California, San Diego (UCSD), Edward Chupp at the University of New Hampshire (UNH), and others, developed similar instruments that were flown on OSO-1, 3, and 7, so that there was an X ray and gamma-ray spectrometer on almost every one of the eight OSOs.

Frost was the NASA Project Scientist for the Solar Maximum Mission (SMM) that was launched on 14 February 1980. In 1984, the project included the first recorded in-orbit repair using the Space Shuttle STS-41C. SMM continued to operate until re-entry into the Earth's atmosphere on 2 December 1989. It included seven instruments specifically selected to provide coordinated observations of solar activity, particularly solar flares and coronal mass ejections (CMEs). Frost was PI of the Hard X-Ray Burst Spectrometer (HXRBS) that measured the hard X-ray spectrum of thousands of solar flares with high time resolution.[15] HXRBS observations became widely used as the primary indicator of energetic electrons in solar flares.[16] Many scientists, including those outside the immediate PI team, used HXRBS X-ray time histories and spectra of the flares they were interested in. The observations confirmed the energetic importance of accelerated electrons in solar flares, and for a decade they became the standard measure of solar hard X-ray emission. The free access to HXRBS data greatly increased the influence and importance of this relatively simple spectrometer that was, in fact, the back-up unit from Frost's OSO-5 spectrometer flown some ten years earlier.