Same as above, with a box showing the field of view of the corresponding IRIS image (top).
Cambridge, MA - Why
is the Sun's million-degree corona, or outermost atmosphere, so much
hotter than the Sun's surface? This question has baffled astronomers for
decades. Today, a team led by Paola Testa of the Harvard-Smithsonian
Center for Astrophysics (CfA) is presenting new clues to the mystery of
coronal heating using observations from the recently launched Interface
Region Imaging Spectrograph (IRIS). The team finds that miniature solar
flares called "nanoflares" - and the speedy electrons they produce -
might partly be the source of that heat, at least in some of the hottest
parts of the Sun's corona.
A solar flare occurs when a patch of the Sun brightens dramatically
at all wavelengths of light. During flares, solar plasma is heated to
tens of millions of degrees in a matter of seconds or minutes. Flares
also can accelerate electrons (and protons) from the solar plasma to a
large fraction of the speed of light. These high-energy electrons can
have a significant impact when they reach Earth, causing spectacular
aurorae but also disrupting communications, affecting GPS signals, and
damaging power grids.
Those speedy electrons also can be generated by scaled-down versions
of flares called nanoflares, which are about a billion times less
energetic than regular solar flares. "These nanoflares, as well as the
energetic particles possibly associated with them, are difficult to
study because we can't observe them directly," says Testa.
Testa and her colleagues have found that IRIS provides a new way to
observe the telltale signs of nanoflares by looking at the footpoints of
coronal loops. As the name suggests, coronal loops are loops of hot
plasma that extend from the Sun's surface out into the corona and glow
brightly in ultraviolet and X-rays.
IRIS does not observe the hottest coronal plasma in these loops,
which can reach temperatures of several million degrees. Instead, it
detects the ultraviolet emission from the cooler plasma (~18,000 to
180,000 degrees Fahrenheit) at their footpoints. Even if IRIS can't
observe the coronal heating events directly, it reveals the traces of
those events when they show up as short-lived, small-scale brightenings
at the footpoints of the loops.
The team inferred the presence of high-energy electrons using IRIS
high-resolution ultraviolet imaging and spectroscopic observations of
those footpoint brightenings. Using computer simulations, they modeled
the response of the plasma confined in loops to the energy transported
by energetic electrons. The simulations revealed that energy likely was
deposited by electrons traveling at about 20 percent of the speed of
light.
The high spatial, temporal, and spectral resolution of IRIS was
crucial to the discovery. IRIS can resolve solar features only 150 miles
in size, has a temporal resolution of a few seconds, and has a spectral
resolution capable of measuring plasma flows of a few miles per second.
Finding high-energy electrons that aren't associated with large
flares suggests that the solar corona is, at least partly, heated by
nanoflares. The new observations, combined with computer modeling, also
help astronomers to understand how electrons are accelerated to such
high speeds and energies - a process that plays a major role in a wide
range of astrophysical phenomena from cosmic rays to supernova remnants.
These findings also indicate that nanoflares are powerful, natural
particle accelerators despite having energies about a billion times
lower than large solar flares.
"As usual for science, this work opens up an entirely new set of
questions. For example, how frequent are nanoflares? How common are
energetic particles in the non-flaring corona? How different are the
physical processes at work in these nanoflares compared to larger
flares?" says Testa.
The paper reporting this research is part of a special issue of the journal Science focusing on IRIS discoveries.
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.
For more information, contact:
David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462
daguilar@cfa.harvard.edu
Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
617-495-7463
cpulliam@cfa.harvard.edu
David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462
daguilar@cfa.harvard.edu
Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
617-495-7463
cpulliam@cfa.harvard.edu
