Scientists unlock the secrets of exploding plasma clouds on the sun

[Henk Elegeert]

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http://www.eurekalert.org/pub_releases/2010-11/aps-sut110310.php

 Scientists unlock the secrets of exploding plasma clouds on the sun News
from the 52nd annual meeting of the APS Division of Plasma Physics
<http://www.eurekalert.org/multimedia/pub/26969.php?from=172387>

*IMAGE:* <http://www.eurekalert.org/multimedia/pub/26969.php?from=172387> An
erupting "prominence " is observed using photons at wavelength 304 Å. A
prominence typically outlines the trailing part of a larger CME flux rope
structure. The striated plasma filaments are...
Click here for more
information.<http://www.eurekalert.org/multimedia/pub/26969.php?from=172387>

     The Sun sporadically expels trillions of tons of million-degree
hydrogen gas in explosions called coronal mass ejections (CMEs). Such
clouds—an example is shown in Figure 1a—are enormous in size (spanning
millions of miles) and are made up of magnetized plasma gases, so hot that
hydrogen atoms are ionized. CMEs are rapidly accelerated by magnetic forces
to speeds of hundreds of kilometers per second to upwards of 2,000
kilometers per second in several tens of minutes. CMEs are closely related
to solar flares and, when they impinge on the Earth, can trigger spectacular
auroral displays. They also induce strong electric currents in the Earth's
plasma atmosphere (i.e., the magnetosphere and ionosphere), leading to
outages in telecommunications and GPS systems and even the collapse of
electric power grids if the disturbances are very severe.

Since the first observation of a solar flare in 1859, solar eruptions
("explosions") have attracted much attention from scientists around the
world and have been studied with a succession of increasingly sophisticated
international satellite missions in the past three decades. A major
challenge has been that enormous and complicated plasma structures
accelerating away from the Sun can only be observed remotely. As a result,
it has been difficult to test theoretical models to establish a correct
understanding of the mechanisms that cause such eruptions. But in 2006, an
international twin-satellite mission called STEREO was launched to
continuously observe the erupting plasma structures from the Sun to the
Earth.

Now, using the data from STEREO, new research by scientists at the Naval
Research Laboratory (NRL) in Washington, D.C., demonstrates for the first
time that the observed motion of erupting plasma clouds driven by magnetic
forces can be correctly explained by a theoretical model. The work will be
presented at the 52nd Annual Meeting of the APS Plasma Physics Division.
        <http://www.eurekalert.org/multimedia/pub/26970.php?from=172387>

*IMAGE:* <http://www.eurekalert.org/multimedia/pub/26970.php?from=172387> This
is an artist's rendition of an expanding model CME flux rope, which is about
to impinge on the Earth. The dark blue represents a weak calculated magnetic
field (of...
Click here for more
information.<http://www.eurekalert.org/multimedia/pub/26970.php?from=172387>

     The theory, controversial when it was first proposed in 1989 by Dr.
James Chen of NRL, is based on the concept that an erupting plasma cloud is
a giant "magnetic flux rope," a rope of "twisted" magnetic field lines
shaped like a partial donut. Chen and Valbona Kunkel, a doctoral student at
George Mason University, have applied this model to the new STEREO data of
CMEs and shown that the theoretical solutions agree with the measured
trajectories of the ejected clouds within the entire field of view from the
Sun to the Earth.

The position of the leading edge (LE) of a CME that erupted on December 24,
2007 were tracked by the STEREO-A spacecraft from the earliest stages of
eruption to its arrival at 1 AU approximately five days later. The magnetic
field and plasma parameters were measured by the STEREO-B spacecraft. The
agreement between theory and data is within 1 percent of the measured
position of the LE. Chen and Kunkel's results show that the theoretically
predicted magnetic field and plasma properties are in excellent agreement
with the measurements aboard STEREO-B. This is the first model that can
replicate directly observed quantities near the Sun and the Earth as well as
the actual trajectories of CMEs. Prior to STEREO, the motion of CMEs in the
region corresponding to HI1 and HI2 data was not observed.

Interestingly, the basic forces acting on solar flux ropes are the same as
those in laboratory plasma structures such as tokamaks developed to produce
controlled fusion energy. The mechanism described by the theory is also
potentially applicable to eruptions on other stars.

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Henk Elegeert

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