Space Instrument Adds
Big Piece to the Solar Corona Puzzle
The Sun is a fascinating object and is not fully understood. It's a complex entity and a recent scientific instrument has helped to unlock some of its secrets.
The Sun's visible surface, or photosphere,
is 10,000 degrees Fahrenheit. As you move outward from it, you pass through a
tenuous layer of hot, ionized gas or plasma called the corona. The corona is
familiar to anyone who has seen a total solar eclipse, since it glimmers
ghostly white around the hidden Sun.
But how can the solar atmosphere get hotter, rather than
colder, the farther you go from the Sun's surface? This mystery has puzzled
solar astronomers for decades. A suborbital rocket mission that launched in
July 2012 has just provided a major piece of the puzzle.
The High-resolution Coronal Imager, or Hi-C, revealed one of
the mechanisms that pumps energy into the corona, heating it to temperatures up
to 7 million degrees F. The secret is a complex process known as magnetic
reconnection.
"This is the first time we've had images at high enough
resolution to directly observe magnetic reconnection," explained
Smithsonian astronomer Leon Golub (Harvard-Smithsonian Center for Astrophysics).
"We can see details in the corona five times finer than any other
instrument."
"Our team developed an exceptional instrument capable
of revolutionary image resolution of the solar atmosphere. Due to the level of
activity, we were able to clearly focus on an active sunspot, thereby obtaining
some remarkable images," said heliophysicist Jonathan Cirtain (Marshall
Space Flight Center).
Magnetic braids and
loops
The Sun's activity, including solar flares and plasma
eruptions, is powered by magnetic fields. Most people are familiar with the
simple bar magnet, and how you can sprinkle iron filings around one to see its
field looping from one end to the other. The Sun is much more complicated.
The Sun's surface is like a collection of thousand-mile-long
magnets scattered around after bubbling up from inside the Sun. Magnetic fields
poke out of one spot and loop around to another spot. Plasma flows along those
fields, outlining them with glowing threads.
The images from Hi-C showed interweaved magnetic fields that
were braided just like hair. When those braids relax and straighten, they
release energy. Hi-C witnessed one such event during its flight.
It also detected an area where magnetic field lines crossed
in an X, then straightened out as the fields reconnected. Minutes later, that
spot erupted with a mini solar flare.
Hi-C showed that the Sun is dynamic, with magnetic fields
constantly warping, twisting, and colliding in bursts of energy. Added
together, those energy bursts can boost the temperature of the corona to 7
million degrees F when the Sun is particularly active.
Selecting the target
The telescope aboard Hi-C provided a resolution of 0.2
arcseconds - about the size of a dime seen from 10 miles away. That allowed
astronomers to tease out details just 100 miles in size. (For comparison, the
Sun is 865,000 miles in diameter.)
Hi-C photographed the Sun in ultraviolet light at a
wavelength of 19.3 nanometers - 25 times shorter than wavelengths of visible
light. That wavelength is blocked by Earth's atmosphere, so to observe it
astronomers had to get above the atmosphere. The rocket's suborbital flight
allowed Hi-C to collect data for just over 5 minutes before returning to Earth.
Hi-C could only view a portion of the Sun, so the team had to
point it carefully. And since the Sun changes hourly, they had to select their
target at the last minute - the day of the launch. They chose a region that
promised to be particularly active.
"We looked at one of the largest and most complicated
active regions I've ever seen on the Sun," said Golub. "We hoped that
we would see something really new, and we weren't disappointed."
Next steps
Golub said that data from Hi-C continues to be analyzed for
more insights. Researchers are hunting areas where other energy release
processes were occurring.
In the future, the scientists hope to launch a satellite
that could observe the Sun continuously at the same level of sharp detail.
"We learned so much in just five minutes. Imagine what
we could learn by watching the Sun 24/7 with this telescope," said Golub.
This research is
being published in the journal Nature in a paper co-authored by Cirtain, Golub,
A. Winebarger (Marshall), B. De Pontieu (Lockheed Martin), K. Kobayashi
(University of Alabama - Huntsville), R. Moore (Marshall), R. Walsh (University
of Central Lancashire), K. Korreck, M. Weber and P. McCauley (CfA), A. Title
(Lockheed Martin), S. Kuzin (Lebedev Physical Institute), and C. DeForest
(Southwest Research Institute).
Headquartered in
Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a
joint collaboration between the Smithsonian Astrophysical Observatory and the
Harvard College Observatory. CfA scientists, organized into six research
divisions, study the origin, evolution and ultimate fate of the universe.
