Davis, Leverett Jr.
Born Elgin, Illinois, USA, 3 March 1914
Died Altadena, California, USA, 15 June 2003
American astrophysicist Leverett Davis, Jr., is probably best known for the proposal with Jesse Greenstein that starlight on its way to us is polarized by scattering by nonspherical dust grains aligned in the galactic magnetic field. He also coined the name “heliosphere” for the region of space around the solar system dominated by a wind of particles and magnetic fields coming from the Sun.
Davis was the eldest of four children of Louis Leverett Davis and Susan Gulick Davis. He married Victoria Stocker in June 1943. Davis earned his Bachelor of Science degree at Oregon State College in 1936. He started graduate studies at the California Institute of Technology, where his thesis advisor was William V. Houston. Davis received his PhD in 1941 for a thesis on electrical properties in nerves, and he briefly considered continuing research in biophysics. World War II intervened, and he became a member of the Caltech rocket program. As a result of his work during the war years, Davis wrote Exterior Ballistics, published by Van Nostrand in 1958.
Davis joined the faculty at the California Institute of Technology in 1946 and continued teaching there until his retirement in 1981, developing an enviable reputation for good teaching among his students. Some of Davis’s most important papers came out of a collaboration with Jesse Greenstein , which started soon after the latter moved from Yerkes Observatory to Caltech, stimulated by the discovery of the polarization of light by the interstellar medium by William Hiltner and John Hall in 1949. Their interpretation of the observed polarization of starlight involved the partial alignment of interstellar dust grains with the interstellar magnetic field. Their proposed mechanism involved the interaction between spinning nonspherical dust grains and interstellar magnetic fields. The interstellar magnetic field induces a field in the particle, such that, in a static situation, both fields would be parallel. In a spinning grain the adjustment of the induced field lags, and there is a constant misalignment. The result is a dissipative torque, favoring the particle with angular momentum parallel to the interplanetary field. Their mechanism as originally proposed required some 30 microgauss, larger than the 4–7 microgauss typically found in the plane of the galaxy. The basic idea of the Davis-Greenstein mechanism appears to apply, but the origin of coherent magnetic fields parallel to spiral arms of galaxies remains a puzzle. Thomas Gold proposed a competing mechanism at about the same time.
This collaboration with Greenstein converted Davis into an astrophysicist with a lifelong interest in interstellar and interplanetary magnetic fields. He wrote papers on the Fermi mechanism for acceleration of cosmic ray particles and their interaction with the galactic magnetic fields. Collaborations with Ludwig Biermann and Reimar Lüst in Germany led to a number of papers, including the discovery of an important new kind of nonlinear wave in collisionless plasmas.
In 1955, well before the continuous flowing solar wind was predicted or established, Davis proposed that the outward flow of particles from the Sun inflated a spheroidal cavity in the interstellar medium. With what little information was available at that time, Davis suggested the radius of the cavity would lie at or beyond 200 AU. He named the cavity the “heliosphere.” The radius of the heliosphere is not yet known for certain, but the Voyager I spacecraft may be close to passing through the heliopause (the region where the solar and interstellar plasmas and magnetic fields interact) somewhere beyond 120 AU. Davis explained modulation of the intensity of cosmic rays in synchronism with the solar cycle, observed as the Forbush decrease by scattering due to concentrated magnetic fields at the edge of the heliosphere. He described the outflow from the Sun as “solar corpuscular radiation,” which was named a few years later the “solar wind” in 1958 by Davis’s first graduate student, Eugene Parker.
In the 1960s, Davis’s interests in astrophysical and solar magnetic fields, energetic particles, and plasmas led him to major investigations in space physics, and he became a leader in studies of interplanetary plasmas and magnetic fields using direct measurements from spacecraft. He studied the basic physical processes governing the motions of trapped particles in the radiation belts of the Earth, Jupiter, and Saturn. He was a coinvestigator on early planetary missions to Venus in 1962 (Mariner 2), to Mars in 1964 (Mariner 4), to Jupiter in 1973–1974 (Pioneers 10 and 11), and to Saturn in 1989 (Pioneer 11).
In 1970, Davis received National Aeronautics and Space Administration’s Exceptional Scientific Achievement Award for his research into interplanetary magnetic fields. He served as president of the International astronomical Union, 1967–1970. He was a fellow of the American Physical Society and a member of the American Geophysical Union and the American Astronomical Society.