Large-scale destruction of magnetic fields in the sun’s
atmosphere likely powers enormous solar explosions, according
to a new observation from NASA’s Ramaty High Energy Solar
Spectroscopic Imager (RHESSI) spacecraft.
The explosions, called solar flares, are capable of releasing
as much energy as a billion one-megaton nuclear bombs. The
destruction of magnetic fields, called magnetic reconnection,
was a leading theory to explain how solar flares could
suddenly release so much energy, but there were other
possibilities as well. The new picture from RHESSI confirms
large-scale magnetic reconnection as the most likely scenario.
“Many observations gave hints that magnetic reconnection over
large areas was responsible for solar flares, but the new
pictures from RHESSI are the first that are really
convincing,” said Linhui Sui of the Catholic University of
America, Washington. “The hunt for the energy source of flares
has been like a story where villagers suspect a dragon is on
the loose because something roars overhead in the middle of
the night, but only something resembling the tail of a dragon
is ever seen. With RHESSI, we’ve now seen both ends of the
dragon.” Linhui is lead author of a paper on this research
published October 20 in Astrophysical Journal Letters.
Magnetic reconnection can happen in the solar atmosphere
because it is hot enough to separate electrons from atoms,
producing a gas of electrically charged particles called
plasma. Because plasma is electrically charged, magnetic
fields and plasma tend to flow together. When magnetic fields
and plasma are ejected from the sun, the ends of the magnetic
fields remain attached to the surface. As a result, the
magnetic fields are stretched and forced together until they
break under the stress, like a rubber band pulled too far, and
then reconnect — snap — to a new shape with less energy.
The thin region where they reconnect is called the
reconnection layer, and it is where oppositely directed
magnetic fields come close enough to merge. Magnetic
reconnection could power a solar flare by heating the sun’s
atmosphere to tens of millions of degrees, and accelerating
electrically charged particles that comprise the plasma
(electrons and ions) to almost the speed of light.
At such high temperatures, solar plasma will shine in X-rays,
and RHESSI observed high-energy X-rays, emitted by plasma,
heated to tens of millions of degrees in a flare on April 15,
2002. The hot, X-ray emitting plasma initially appeared in the
RHESSI images as a blob atop an arch of relatively cooler
plasma protruding from the sun’s surface. The blob-and-arch
structure is consistent with reconnection, because the X-ray
blob could be heated by reconnection, and the part of the
magnetic field that breaks and snaps back to the solar surface
will assume an arch shape.
These structures have been seen before and hinted at
reconnection, but the observations were not conclusive.
However, as RHESSI made images of the 20-minute long flare,
over the course of about four minutes during the most intense
part of the flare, the X-ray emitting blob exhibited two
characteristics consistent with large-scale magnetic
reconnection.
First, the blob split in two, with the top part ultimately
rising away from the solar surface at a speed of about 700,000
miles per hour, or around 1.1 million km/hr. This is expected
if extensive reconnection is occurring, because as the
magnetic fields stretch, the reconnection layer also
stretches, like taffy being pulled. Plasma heated by
reconnection squirts out of the top and bottom of the
reconnection layer, forming the two X-ray blobs in the RHESSI
pictures, when the top and bottom are sufficiently far apart
to be resolved as distinct areas.
Second, in both blobs, the area closest to the apparent
reconnection layer was hottest, and the area farthest away was
coolest, according to temperature measurements by RHESSI. This
is also expected if reconnection is occurring, because, as the
magnetic fields break and reconnect, other magnetic fields
nearby move in to the reconnection region and reconnect as
well, since the overall, large-scale field continues to
stretch. Thus, plasma is continuously heated and blasted out
from the reconnection layer. The plasma closest to the
reconnection area is the most recently expelled and therefore
the hottest. Plasma farther away was ejected earlier and had
time to cool.
“This temperature gradient in the hot plasma was the clincher
for me,” said Dr. Gordon Holman, a Co-Investigator on RHESSI
and co-author of the paper, at NASA’s Goddard Space Flight
Center, Greenbelt, Md. “If some other process was powering the
flare, the hot plasma would not appear like this.” For images,
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