William Clarence "Bill" Erickson

William Clarence "Bill" Erickson, a pioneering radio astronomer, died on September 5, 2015 in Hobart, Australia. Born in Chicago, Illinois on November 21, 1930. A descendent of Scandinavian immigrants, Bill spent his youth in Duluth, Minnesota where his father worked in the steel industry. He received undergraduate degrees in math and physics as well as a Ph. D in physics (1956) from the University of Minnesota. His thesis considered if spinning dust could be responsible for the recently discovered cosmic "radio noise."

Bill was a consummate intellectual, and an outstanding instrumentalist and astronomer. He built his first telescope at age 14 and early on became an amateur radio operator. He happened to visit the Department of Terrestrial Magnetism of the Carnegie Institution of Washington in 1955, just after B. F. Burke and K. L. Franklin discovered Jovian decametric emission and his encounter with this exciting discovery became his motivation to pursue low frequency radio astronomy. He later remarked: "It gave me the mistaken impression that low frequency work was easy. All you needed to do was to erect some poles, string some dipoles between them, and great discoveries would result."

After a Carnegie fellowship (1956-1957), pursuing HI observations under Howard Tatel, Bill was hired by the Convair Corporation's Scientific Research Laboratory, near San Diego, where, he said, "We were given complete freedom to choose our research projects within the confines of a very limited budget." His task was to search for a flat site in the San Diego area to conduct low-frequency radio astronomy. He located Clark's Dry Lake in the Anza-Borrego desert, where from 1958 through the late 1980s, he developed a succession of innovative low frequency radio telescopes with a hardy band of colleagues and students. While at Convair, he also developed a passion for flying, second only to his love for sailing.

Bill was offered and accepted a faculty position at the University of Maryland, College Park in 1961. Before starting work there and after convincing Maryland to take over the Clark Lake facility, he was hired for a couple of years by Prof. Jan Oort to help develop the Benelux Cross Antenna. Working with J. Hügbom and Ch. Christiansen, they soon recognized that aperture synthesis, recently pioneered, made possible a much simpler design than the one originally proposed. This realization gave origin to the Westerbork Synthesis Radio Telescope, that remains today a premier research instrument.

Bill assumed his position at Maryland in 1963, and spent much of the next 25 years developing instruments at Clark Lake and training students. His finest telescope, the TPT (a T-shaped array of conical, log-spiral antennas), was far ahead of its time. It was a broad-band array, electronically steerable that could be tuned in fractions of a second and form images in near real time, a level of innovation only now being matched, that was required e.g. for the study of solar activity in the corona, another of Bill's interests. Clark Lake was dismantled in the early 1990s due to a reduction in funding from the National Science Foundation, but perhaps more importantly due to a fundamental limitation. Ionospheric waves introduce rapid phase variations in the visibility of interferometer elements separated by more than a few kilometers, and this limits low frequency (> 100 MHz) observations to poor angular resolution and sensitivity. When self-calibration was developed, Bill recognized that the technique could lift the short-baseline limits imposed on the resolution and sensitivity of low frequency interferometry. Unfortunately, this came too late to save the TPT.

The closure of Clark Lake marked the start of a new chapter in Bill's career. With Rick Perley, a former student at the National Radio Astronomy Observatory (NRAO), he proposed a low-frequency array based on the infrastructure of the VLA. Funding for this project was denied, and Bill with another of his former students, Namir Kassim at the Naval Research Laboratory (NRL), embarked on a much less expensive system, intended to demonstrate that sub-arcminute angular resolution, low-frequency radio astronomy was possible. This system, based on the VLA dishes fed by 74 MHz dipole feeds, successfully overcame the "ionospheric barrier" and performed pioneering observations that are still unsurpassed.

Bill was one of the early generation of radio astronomers with balanced scientific and technical background. For many years, he served as informal advisor to the technical group at the NRAO that designed the VLA, and later he played an important role in the design of the Berkeley-Illinois-Maryland Array antennas at Hat Creek, California. His technical savvy was often sought for projects outside low frequency radio astronomy, not only in the USA but worldwide, e.g. in Australia, France and India. He authored papers on instrumentation, as well as solar and planetary radio science, Galactic and extragalactic radio astronomy, and interplanetary and interstellar scintillation. Many of the sky surveys he motivated are still in use, e.g. the VLA Low Frequency Sky Survey (VLSS) is still serving as a fundamental calibration grid. He helped lead the first successful experiments in Very Long Baseline Interferometry, and conducted one of the earliest searches for emission from magnetized extra-solar planets. His observation, that the radio source 4C21.53 had a steep spectrum akin to that of the Crab pulsar foreshadowed the discovery of the millisecond pulsar. He also discovered a class of travelling ionospheric disturbances using radio interferometry, presaging a new field of ionospheric remote sensing.

The closure of Clark Lake led to Bill's early retirement from the U of MD and his move to Bruny Island, off the coast of Tasmania, Australia, together with Hilary Cane, his wife who begun her career in low frequency radio astronomy. For many years, Bill continued to be scientifically and technically active. He and Hilary spent twenty summers in the USA, allowing Bill to collaborate with NRL and Hilary to work at the NASA Goddard Space Flight Center. They shared a love for the outdoors, and Bill continued to be an avid sailor, circumnavigating Tasmania twice, complementing a transatlantic voyage in the 1970s. His last instrument was a broad-band, solar radio spectrometer that provided valuable data until about a year before his passing and that was copied at several sites around the world.

Bill was the first recipient of the Grote Reber Medal (2005) "for lifetime innovative contributions to radio astronomy." A workshop: "From Clark Lake to the Long Wavelength Array: Bill Erickson's Radio Science" was organized by his students in his honor on occasion of his 74th birthday (2004).

Bill disliked teaching in a formal classroom setting, much preferring to dialogue and interact with his students on a one-on-one basis. One of his Ph. D. students remarked that some of the most memorable science discussions with his adviser took place over steak and a beer at Clark Lake. He kept in touch lifelong with most of his students, and continued to collaborate with some of them decades after they got their degree. In one of his last papers, he wrote that "The most enduring legacy of Clark Lake is the students that were trained there," many of whom went on to play active roles in modern astronomy. Always a modest person, Bill attributed the legacy to Clark Lake, but the merit was entirely his!

Prof. Erickson is survived by his wife Hilary, three sons and eight grandchildren. He will be greatly missed.