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Edward J. Smith, a pioneer of space plasma research, passed away on 11 August 2019 at age 91. The magnetometers he built and developed were the first to measure magnetic fields near Venus, Mars, Jupiter, Saturn, and the Sun’s poles; in a comet’s tail; and in deep interplanetary space. His research contributed greatly to our understanding of the magnetic fields and processes that surround Earth, the Sun, and other planets in our solar system.

Launching a Career

Ed received a Ph.D. in physics from the University of California, Los Angeles, in 1959—his thesis adviser, Robert E. Holzer, was also a space plasma physicist and an AGU Fellow. Ed spent 1959 to 1961 at the Space Technologies Laboratory (now TRW) in El Segundo, Calif. He then went to work at the Jet Propulsion Laboratory (JPL), managed by the California Institute of Technology (Caltech) in Pasadena, where he spent the remainder of his scientific life. In his early years at JPL, Ed took a sabbatical at the Royal Institute of Technology in Stockholm, Sweden, with physicist (and, later, Nobel laureate) Hannes Alfvén. They became lifelong friends.

As was typical for scientists of that time, Ed built instruments in addition to making major scientific discoveries. Ed was unique among his peers in that he developed instruments—a fluxgate magnetometer, vector helium and scalar helium magnetometers, and dual vector/scalar mode magnetometers—and then sent these instruments into space. Ed was also the first to build and fly triaxial search coil magnetometers in space for electromagnetic plasma wave studies. He gave his design for search coil magnetometers to Europe and Japan, where other scientists copied and flew the basic designs in space.

Also during the 1960s, Ed was principal investigator for the search coil magnetometers on board Orbiting Geophysical Observatory (OGO) satellites OGO 1, 3, and 5. From the magnetometer data, Ed and his colleagues were the first to identify the major electromagnetic plasma waves in Earth’s magnetosphere: plasmaspheric hiss, outer zone chorus, and magnetosheath lion roars. Ed was the first to apply a technique called “minimum variance analysis” that was developed by his colleague Bengt Sonnerup of Dartmouth College. He found this analysis extremely useful for analyzing electromagnetic waves, and it is the main means of studying plasma wave details today.

Venturing Outward

Ed’s next endeavor was serving as principal investigator of the magnetometer experiments on the twin spacecraft Pioneer 10, launched in 1972, and Pioneer 11, launched in 1973. These were the first missions to go to the outer heliosphere, through the asteroid belts (then thought to be potentially dangerous) and to distant planets. Pioneer 10 encountered the giant planet Jupiter in 1973, approaching as near as 2.8 Jovian radii (132,000 kilometers). Jupiter’s intense radiation belt fluxes were even higher than predicted, but Pioneer 10 survived. Ed was the first to characterize Jupiter’s planetary magnetic fields and its dynamic ever-changing magnetosphere. Pioneer 10 crossed Jupiter’s bow shock 17 times during the encounter.

During the planning phase for Pioneer 11’s encounter with Jupiter, scientists realized that if Pioneer 11 could achieve an even closer approach, the gravitational boost could direct it to an encounter with Saturn! NASA decided to send Pioneer 11 inward to a distance one third that of Pioneer 10’s approach, on a trajectory that would miss the magnetic equator, where the particle radiation is most intense. The mission was successful, and Ed and his colleagues made even better measurements of the planetary magnetic fields.

During the transit to these distant planets, Ed and his colleagues made other discoveries. With John Wolfe, they were the first to identify an interaction between high-speed solar wind streams and slow streams. These interactions occur in regions of space that they named corotating interaction regions (CIRs), and they are important causes of geomagnetic activity on Earth. Ed and his colleagues discovered that fast forward and fast reverse collisionless shocks bounding the CIRs were accelerating energetic particles in deep interplanetary space between 3 and 10 astronomical units from the Sun.

Pioneer 11’s close encounter with Saturn gave it a gravitational boost, and it became the first spacecraft to escape the ecliptic plane. Although it cleared this plane by only 17°, that was enough for Ed and colleagues to determine the shape of the heliospheric current sheet. Ed remembered a somewhat obscure prediction by his old friend Hannes Alfvén that this current sheet would warp and fluctuate up and down as it spun, like a ballerina’s skirt. Thus, the heliospheric current sheet became known as the “Alfvén ballerina skirt.”

International Collaborations

Ed was a coinvestigator for the three spacecraft known as the International Sun-Earth Explorers (ISEE), the first joint NASA–European Space Agency (ESA) mission. He was also the principal investigator for the magnetometers aboard ISEE-3, a solar wind monitor placed in orbit around the L1 libration point—the first spacecraft to have achieved this orbit.

With combined ISEE-3 magnetometer and plasma data, Ed and colleagues identified and characterized what are now called coronal mass ejections, one of the causes of magnetic storms on Earth. Ed and other members of Sam Bame’s Los Alamos National Laboratory (New Mexico) plasma team were the first to identify and characterize interplanetary fast shocks using observational data.

The ISEE-1, -2, and -3 teams were the first to collectively study Earth’s foreshock regions, where back-streaming energetic electrons and ions propagate into the solar wind, generating plasma waves. Ed was also strongly involved in identifying a fundamental magnetosheath wave mode at Earth, Jupiter, and Saturn called the mirror mode. With several of his colleagues, Ed also studied quasiparallel shock particle acceleration for the first time.

The ISEE-3 team voted (again, not unanimously) to move the spacecraft from the L1 orbit to a series of deep magnetotail passes reaching ~240 Earth radii, using a series of lunar gravitational assists. From this new mission, Ed and colleagues determined that the magnetotail was not “fragmented” but maintained a two-lobe structure all the way out to 240 Earth radii. They also discovered slow shocks bounding the lobe–plasma sheet boundary and “plasmoids” flowing down the tail. Ed and his colleagues also identified electrostatic plasma wave modes in the tail and magnetosphere.

Ed discovered one of the biggest surprises of the cometary encounter, the detection of highly nonlinear plasma waves associated with neutral gas sublimated from the comet nucleus. As photoionization or charge exchange from the solar wind ionizes neutral water molecules, solar wind electric fields instantaneously affect these newly formed ions, accelerating them so that collectively, they are unstable and generate plasma waves.

After ICE’s encounter with comet Giacobini-Zinner, a spacecraft “armada” was sent to a 1986 encounter with comet Halley. ICE, a distant participant some 1 million kilometers away, managed to detect pickup ion waves. Hiroshi Oya (Tohoku University, Japan) invited Ed to become a coinvestigator on the Japanese Sakigake plasma wave investigation going to Halley.

For the 1990 joint NASA-ESA Ulysses mission to orbit the Sun, Ed served as NASA’s project scientist and also supplied a vector helium magnetometer as a coinvestigator. Using the magnetometer data, Ed was able to determine that the Sun’s magnetic field does not disappear and then reverse polarity or simply rotate its dipole every 11 years—a more complex field dominates during the field reversal interval.

Ed supplied the dual sensor scalar magnetometer for the Argentine science application satellite in 2000 and a vector/scalar helium magnetometer on the NASA-ESA Cassini mission, which orbited Saturn. Ed also was a coinvestigator of the Juno mission to understand the origin and evolution of Jupiter.

Professional Recognition

Ed was a member of AGU, the American Association for the Advancement of Science, and the American Astronomical Society. He was a recipient of two NASA medals for exceptional scientific achievement and a NASA Distinguished Service Medal. In 2005, he received the Arctowski Medal from the National Academy of Sciences in recognition of his contributions to solar-terrestrial research. Ed was the author or coauthor of more than 500 scientific articles, and his research interests included planetary magnetism and magnetospheric and solar-heliospheric physics. His achievements as a pioneer of space science were truly remarkable.

—Bruce Tsurutani ([email protected]), Pasadena, Calif., retired; and Marcia M. Neugebauer, University of Arizona, Tucson