Donald Leland Carpenter, a pioneering space scientist and electrical engineering professor, passed away in Aptos, California, on February 5, 2019, at the age of 91. Although Don initially contemplated making a career in international relations, he began studying space topics as a graduate student at Stanford and ultimately became best known for his use of the lightning generated radio signals called whistlers for precise measurements of the ionized gases in the Earth’s outer atmosphere, and, more specifically, for the discovery of the plasmapause, an important boundary defining two regions in the outer atmosphere. This discovery played a crucial role in our understanding of the space environment near the Earth right at the beginning of the space age, as satellites began getting placed in orbit around the Earth and manned spaceflights commenced. Don was a member of the Electrical Engineering faculty for over 40 years.
Don was born on January 3, 1928, in Spokane, Washington. He graduated from Grant High School in Portland, Oregon, and then served as an electronics technician in the U.S. Navy from 1946 to 1948. He then studied political science and philosophy at Willamette University in Salem, Oregon, gaining a B.A. in 1951. Following his graduation, with his eye on the developing cold war, he moved to New York City to pursue a master’s degree in political science at Columbia University, enrolling in its Russian Institute, a newly established center for training specialists on Eastern Europe and the U.S.S.R. He spent two years at Colombia followed by a third as a visiting student in Belgrade, Yugoslavia, before graduating with an M.A. in Public Law and Government in 1954.
Don then made one of the major decisions in his life by moving to the San Francisco Bay area while applying for U.S. government jobs in international relations. This was a major decision because, first, it led to him meeting his future wife, Betty Jane Donovan, and secondly, as no jobs came through, he applied for and obtained a graduate student position in the Electrical Engineering Department at Stanford University (possibly due his electronic training in the U.S. Navy). He joined a group headed by Professor Robert Helliwell that was conducting pioneering new research on the upper atmosphere by using natural and manmade very low frequency (VLF) radio waves, working initially as a data aid. Ultimately he became a key member of this group as he completed his M.S. in 1959 and his Ph.D. in 1962, both degrees in Electrical Engineering. He married Betty Donovan in 1956.
Don’s research over the years covered many different if related topics involving the ionized upper atmosphere of the Earth, but a key element and the secret of his success was his use of VLF whistlers as a probe of the state of the upper atmosphere. These whistlers had been noted in earlier measurements prior to the 1950’s and indeed they were rather hard to miss since they consisted of an infrequent musical-sounding downward gliding tone in measurements of the VLF radio background. In 1953 Owen Storey, a recent graduate of Cambridge University, provided great impetus to further research by showing that the whistlers were generated by lightning and that their waves could echo back and forth along magnetic field lines in the Earth’s outer atmosphere, or, as it came to be called, the magnetosphere. Helliwell’s group, and numerous other scientists in the US and overseas, were greatly stimulated by these developments and it was not long before Helliwell and his group had identified another form of whistler that they referred to as a “nose” whistler. Don increasingly concentrated his efforts on mapping the structure of the thermal plasma
in the magnetosphere and around 1960 he began presenting evidence for a change in this structure which showed up as a kink or bend when the plasma density was plotted against distance from the surface. Don began referring to this bend as a “knee” in his presentations and soon, as it became recognized as an important permanent feature of the magnetosphere, everyone took to calling it “Carpenter’s knee” in their presentations. Don himself only referred to it as a knee and as it became increasingly clear that a more scientific name was required he adopted the term plasmapause (in 1966), which is now the official term.
Although Don started as a data aid in Helliwell’s group his work gradually gave him more of a leadership role and his discoveries and completion of his M.S. and Ph.D. degrees led to him being appointed a Research Associate in Electrical Engineering in 1962 followed by Senior Research Associate in 1968. Finally, in 1975 he joined the faculty as a Professor (Research) in Electrical Engineering. He converted to Emeritus status in 1991 and at the time of his death he had been a member of the faculty for over 40 years.
Don was an accomplished runner, usually placing high in his age bracket in local races. He helped form a Stanford running group called the Angell Field Ancients that would startle the students and visitors by pounding through the campus at lunchtimes.
Don was an author/coauthor of over 115 scientific papers in refereed journals and he was the author of chapters in several books, including in particular Discovery of the Magnetosphere by C. Stewart Gillmor and J. R. Spreiter, American Geophysical Union, 1997, and The Earth’s Plasmasphere by J. Lemaire and K. Gingauz, Cambridge Press, 1998. In a book published in 2015, Very Low Frequency Space Radio Research at Stanford 1950 – 1990,
Don summarizes decades of the space research conducted initially by Robert Helliwell’s group, leading to 35 Ph.D. graduates, and then by its continuation, Umran Inan’s group, leading to the graduation of another 60 Ph.D. students, in Stanford’s Electrical Engineering Department. It is little known that these groups, largely of students, operated a station (Siple Station) in the Antarctic for over a decade. Don served as Associate Editor of the Journal of Geophysical Research, 1971 – 1972, and Radio Science, 1967 – 1968, and he also served on numerous scientific committees. He was a long-time member of Commissions G and H of the International Scientific Radio Union (URSI) and of the American Geophysical Union, where he was elected a Fellow in 2002. The same year he was nominated by Belgium and awarded the John Howard Dellinger Medal by URSI “For his discovery of the plasmapause, for pioneering studies of the plasmasphere structure and dynamics and for development and use of whistler-mode waves as diagnostic probes of the magnetosphere.” Don was truly loved by his colleagues, students, friends and family alike.
Don’s wife Betty died at their home on June 3, 2009. He is survived by his two sons, Frederic Carpenter (with wife Rose), and Jesse Carpenter (with wife Erika and daughter Megan), and by his brother Richard.
This Resolution in memory of the late Donald Carpenter, Research Professor of Electrical Engineering, was presented to the Senate of Stanford's Academic Council on February 20, 2020, by Antony Fraser-Smith. The Resolution was prepared by a faculty committee consisting of Antony Fraser-Smith and Umran S. Inan.