PHOTO: UNIVERSITY OF CHICAGO
SCIENCE science.orgBy Stuart D. BaleA
strophysicist Eugene N. (“Gene”)
Parker, who led the field of solar
physics into the space age, died on 15
March. He was 94. Gene was first to
identify many of the fundamental con-
cepts of Solar System plasma physics,
including the physics of solar wind accelera-
tion, coronal heating and magnetic reconnec-
tion, cosmic ray diffusion, and a conceptual
structure of the heliosphere and its interac-
tion with the interstellar medium. His foun-
dational work on the theory of the solar wind
led to the flagship NASA mission that bears
his name: the Parker Solar Probe.
Gene was born in Houghton, Michigan, in
1927 and attended Michigan State University,
where he received his BS in physics
in 1948. He earned his PhD at the
California Institute of Technology
(Caltech) in 1951 working with physi-
cist Leverett Davis Jr. on problems of
the interaction of dust with the inter-
stellar medium. After a brief appoint-
ment at the University of Utah, Gene
joined the faculty at the University of
Chicago, where he remained until his
retirement in 1995.
Gene’s early career coincided with
the beginning of the space age. At
that time, the prevailing wisdom was
that interplanetary space was either
a vacuum with sporadic solar per-
turbations or a solar atmosphere in
hydrostatic equilibrium. However,
subtle clues suggested something
quite different. German astronomer Ludwig
Biermann had made a convincing case that
one component of a bifurcated comet tail al-
ways pointed away from the Sun, guided by
a hypothetical “solar corpuscular radiation,”
whereas the other part was dominated by
photon pressure and Kepler’s laws.
Gene recognized that Biermann’s cor-
puscular radiation was consistent with a
hydrodynamic, transonic, escaping atmo-
sphere that he called the “solar wind.” His
controversial 1958 solar wind theory, a kind
of velocity-space analog of a rocket engine
nozzle, was published only after the editor
of the Astrophysical Journal overruled thereviewers. However, the theory provided pre-
dictions for measurements near Earth and
suggested the existence of a “heliosphere,”
a plasma bubble surrounding the Sun with
a discontinuous interface to the interstel-
lar medium. The general framework of the
model was verified by Soviet and American
spacecraft measurements in one of the early
scientific triumphs of the space age.
A 1960 Space Science Board report (the so-
called Simpson Report) recommended both
a solar probe to investigate the origins of
Gene’s solar wind and an outer Solar System
probe to study the interaction of the helio-
sphere with the interstellar medium. Just
2 years after the founding of NASA, Gene’s
work was already setting the goalposts for
major space mission objectives.Gene went on to define, and often resolve,
many of the major problems of solar astro-
physics. He proposed the concept of mag-
netic buoyancy to describe the origin and
dynamics of sunspots, and he pioneered the
models of cosmic ray diffusion and transport.
He made major contributions to the theory
of magnetic reconnection, the process that
triggers solar flares and magnetospheric
substorms. He elucidated the nature of the
Maxwell stress tensor, showing that the low-
est energy states are sheets of plasma cur-
rents. The nanoflare model that Gene pro-
posed to explain solar coronal heating is still
used today. In a major contribution to tur-
bulent dynamo theory, he showed that a po-
loidal magnetic field arises directly from the
dynamics of a toroidal field, an insight that
completed the alpha-omega dynamo theorythat explains the origin of planetary and as-
trophysical magnetic fields. Gene wrote four
books on topics of astrophysical plasmas,
each in his trademark dialectical style.
In 2010, NASA selected my team to design
and build scientific instrumentation for the
mission to investigate the origin and accel-
eration of the solar wind. The solar probe of
the 1960 Simpson Report was finally coming
to fruition. I asked Gene to join our team as
a senior adviser, and he happily accepted
the role. “My advice is free and worth every
penny of that,” he quipped. As we waded
through theories to define our measurement
requirements, he urged us to keep it simple.
He stood up at one of our early meetings to
remind us thoughtfully that “hydrodynamics
and magnetohydrodynamics are fundamen-
tal, regardless of popular opinion.”
Our Solar Probe Plus spacecraft—renamed
the Parker Solar Probe in Gene’s honor—
launched in August 2018. It was the first
time that a NASA spacecraft had carried the
name of a living scientist. The Parker Solar
Probe spacecraft has so far made 11 orbits
of the Sun, diving to within 13 solar radii of
the solar photosphere. Gene stayed
abreast of the Parker Probe results as
we sent him preprints and publica-
tions. As his health declined, he was
content to cheer us on from the side-
lines with the occasional handwritten
letter. Many of Gene’s predictions and
theories are being borne out by Parker
Solar Probe measurements, notably
the existence of an underlying hydro-
dynamic wind speed profile—popular
opinion be damned.
Gene was known as a supportive
and inspiring mentor to his students.
He was deeply committed to striking
a good work-life balance; his former
student Bill Collins recalls him saying,
“The University of Chicago pays me
for 40 hours per week and that’s what
they’re going to get.” Clearly, he put that time
to good use: His many honors include the US
National Medal of Science, the Crafoord Prize
awarded by the Royal Swedish Academy of
Sciences, the Kyoto Prize, the Arctowski Medal
of the US National Academy of Sciences, and
the American Physical Society Medal for
Exceptional Achievement in Research.
The history of modern solar physics fol-
lowed the arc of Gene’s career, and his death
is an opportunity to reflect on both his
achievements and the state of the field. Solar
and heliospheric physics are now entering
a golden age, with extraordinary commit-
ments from NASA, the US National Science
Foundation, and the European Space Agency.
Gene Parker’s many insights played a funda-
mental role in making that possible. j
10.1126/science.abq3164RETROSPECTIVEEugene N. Parker (1927–2022)
Pioneer of theoretical solar and plasma astrophysics
Department of Physics and Space Sciences
Laboratory, University of California, Berkeley, CA, USA.
Email: [email protected]29 APRIL 2022 • VOL 376 ISSUE 6592 461