New theory straightens out sun’s curved magnetic fields

A long-accepted
model of the sun’s magnetic fields holds that the fields radiate
outwards from the sun into space in great curving arcs in the sun’s
equatorial regions and growing ever more radial at higher solar latitudes.
That model has been proven only partly right by direct measurements of
magnetic fields by the Ulysses solar orbiter and other spacecraft.

Data
from Ulysses and other observations have yielded an interesting phenomenon.
While the sun’s magnetic fields are usually curved, as expected,
sometimes they’re not, actually changing from long arcs to straight
radial lines once or twice a month, on average, and at intervals lasting
two or three days. One possible explanation for this puzzle was presented
by Los Alamos National Laboratory researcher Jack Gosling to the annual
meeting of the American Geophysical Union meeting in Washington, D.C.,
today.

About five years ago Ulysses data confirmed the existence of straight
radially-oriented fields and their association with Coronal Mass Ejections
about 50 percent of the time. “These observations depart dramatically
from the standard model,” said Gosling, of Los Alamos’ Non-Proliferation
and International Security Division. “The fields are nearly radial,
or straight, for extended periods of time, almost always when the speed
of solar wind plasma observed in space is in decline, and about half the
time this occurs during a CME. There still is no definitive explanation
for why this is happening.”

Gosling’s
theory is that dramatic variations in the speed of the solar wind directly
associated with the speed at which plasma is ejected from the sun — specifically
a sudden drop in that speed — along a single magnetic field line produces
a straight-line interval along the normally curved field line. “Our
suggestion is that open and closed field lines get pinched together causing
a ‘charge reconnection’ that opens a closed field line and results
in a drop in speed,” said Gosling

Once generally confused with solar flares, images of CMEs are now commonplace.
Most of these images are supplied by the Solar and Heliospheric Observatory,
a project of NASA and the European Space Agency. Giant closed arcs of
ejecta reaching out into space are observed in these events. These arcs
illuminate the sun’s magnetic field lines that are carried into space
by the CME material, but remain rooted in the sun. Gosling and his colleague
Ruth Skoug suggest it is when the footpoints of field lines threading
the CMEs reconnect with closed field lines in the surrounding corona that
the sudden drops in outflow speed occur and are responsible for intervals
of radial magnetic field far out in space.

All of this is important to the understanding of the solar wind, and
space weather in general. The solar wind contains huge quantities of charged
particles, or ions, that travel at millions of miles per hour and are
responsible for geomagnetic events like the aurora phenomena of Earth’s
higher latitudes. These geomagnetic events have also caused major communications
outages and energetic charged particles found in the deep space solar
wind can pose a real threat to astronauts who wish to venture to other
planets.

Gosling and Skoug are working on computer simulations of these events
with colleagues in San Diego, Calif., to confirm the theory, but are confident
that their idea is consistent with data from basic observations. They
have also submitted a paper on this theory to the Journal of Geophysical
Research.

Gosling is a fellow of the AGU and is currently president of its Space
Physics and Astronomy Section. He will also be giving an invited talk
at the AGU meeting on what is now known about CME properties and behaviors.

Los Alamos National Laboratory is operated by the University of California
for the National Nuclear Security Administration (NNSA) of the U.S. Department
of Energy and works in partnership with NNSA’s Sandia and Lawrence Livermore
national laboratories to support NNSA in its mission.

Los Alamos enhances global security by ensuring safety and confidence
in the U.S. nuclear stockpile, developing technologies to reduce threats
from weapons of mass destruction and improving the environmental and nuclear
materials legacy of the cold war. Los Alamos’ capabilities assist the
nation in addressing energy, environment, infrastructure and biological
security problems.

The images used in this news release are available from the Los Alamos
National Laboratory Public Affairs Office.