Scientists have recently gathered some of the strongest evidence to date
to explain what makes the sun's outer atmosphere so much hotter than
its surface. The new observations of the small-scale extremely hot
temperatures are consistent with only one current theory: something
called nanoflares – a constant peppering of impulsive bursts of heating,
none of which can be individually detected -- provide the mysterious
extra heat.
NASA's EUNIS sounding rocket mission spotted evidence to explain why the sun's atmosphere is so much hotter than its surface.
Image Credit: NASA/Goddard/Duberstein. Download video
The EUNIS experiment undergoing tests before launch.
Image Credit: NASA
The sounding rocket carrying the EUNIS experiment launches from the White Sands Missile Range in New Mexico on April 23, 2013. Image Credit: NASA
What’s even more surprising is these new observations come from just
six minutes worth of data from one of NASA's least expensive type of
missions, a sounding rocket. The EUNIS mission, short for Extreme
Ultraviolet Normal Incidence Spectrograph, launched on April 23, 2013,
gathering a new snapshot of data every 1.3 seconds to track the
properties of material over a wide range of temperatures in the complex
solar atmosphere.
The sun's visible surface, called the photosphere, is some 6,000
Kelvins, while the corona regularly reaches temperatures which are 300
times as hot.
"That's a bit of a puzzle," said Jeff Brosius, a space scientist at
Catholic University in Washington, D.C., and NASA's Goddard Space Flight
Center in Greenbelt, Maryland. "Things usually get cooler farther away
from a hot source. When you're roasting a marshmallow you move it closer
to the fire to cook it, not farther away."
Brosius is the first author of a paper on these results appearing in the Aug. 1, 2014, edition of The Astrophysical Journal.
Several theories have been offered for how the magnetic energy
coursing through the corona is converted into the heat that raises the
temperature. Different theories make different predictions about what
kind of – and what temperature – material might be observable, but few
observations have high enough resolution over a large enough area to
distinguish between these predictions.
The EUNIS rocket, however, was equipped with a very sensitive version of
an instrument called a spectrograph. Spectrographs gather information
about how much material is present at a given temperature, by recording
different wavelengths of light. To observe the extreme ultraviolet
wavelengths necessary to distinguish between various coronal heating
theories, such an instrument can only work properly in space, above the
atmosphere surrounding Earth that blocks that ultraviolet light. So
EUNIS flew up nearly 200 miles above the ground aboard a sounding
rocket, a type of NASA mission that flies for only 15 minutes or so, and
gathered about six minutes worth of observations from above the
planet's air.
During its flight, EUNIS scanned a pre-determined region on the sun
known to be magnetically complex, a so-called active region, which can
often be the source of larger flares and coronal mass ejections. As
light from the region streamed into its spectrograph, the instrument
separated the light into its various wavelengths. Instead of producing a
typical image of the sun, the wavelengths with larger amounts of light
are each represented by a vertical line called an emission line. Each
emission line, in turn, represents material at a unique temperature on
the sun. Further analysis can identify the density and movement of the
material as well.
The EUNIS spectrograph was tuned into a range of wavelengths useful
for spotting material at temperatures of 10 million Kelvin –
temperatures that are a signature of nanoflares. Scientists have
hypothesized that a myriad of nanoflares could heat up solar material in
the atmosphere to temperatures of up to 10 million Kelvins. This
material would cool very rapidly, producing ample solar material at the 1
to 3 million degrees regularly seen in the corona.
However, the faint presence of that extremely hot material should
remain. Looking over their six minutes of data, the EUNIS team spotted a
wavelength of light corresponding to that 10 million degree material.
To spot this faint emission line was a triumph of the EUNIS instrument's
resolution. The spectrograph was able to clearly and unambiguously
distinguish the observations representing the extremely hot material.
"The fact that we were able to resolve this emission line so clearly
from its neighbors is what makes spectroscopists like me stay awake at
night with excitement," said Brosius. "This weak line observed over such
a large fraction of an active region really gives us the strongest
evidence yet for the presence of nanoflares."
There are a variety of theories for what mechanisms power these
impulsive bursts of heat, the nanoflares. Moreover, other explanations
have been offered for what is heating the corona. Scientists will
continue to explore these ideas further, gathering additional
observations as their tools and instruments improve. However, no other
theory predicts material of this temperature in the corona, so this is a
strong piece of evidence in favor of the nanoflare theory.
"This is a real smoking gun for nanoflares," said Adrian Daw, the
current principal investigator for EUNIS at Goddard. "And it shows that
these smaller, less expensive sounding rockets can produce truly robust
science."
NASA's
Solar Dynamics Observatory captured this image of what the sun looked
like on April 23, 2013, at 1:30 p.m. EDT when the EUNIS mission
launched. EUNIS focused on an active region of the sun, seen as bright
loops in the upper right in this picture. Image Credit: NASA/SDO
In
addition to having a lower cost, sounding rockets offer a valuable test
bed for new technology that may subsequently be flown on longer-term
space missions. Another advantage of sounding rockets is that the
instruments parachute back to the ground so they can be recovered and
re-used. The EUNIS mission will be re-tuned to focus on a different set
of solar wavelengths – ones that can also spot the extremely high
temperature material representative of nanoflares -- and fly again
sometime in 2016.
EUNIS was supported through NASA’s Sounding Rocket Program at the
Goddard Space Flight Center’s Wallops Flight Facility in Virginia.
NASA’s Heliophysics Division manages the sounding rocket program. EUNIS
launched from the White Sands Missile Range in New Mexico. At the time
of flight, the principal investigator for EUNIS was Doug Rabin at
Goddard.
