NASA SDO: IRIS captures New information about sun's atmosphere

NASA’s Solar Dynamics Observatory provided the outer image of a coronal mass ejection on May 9, 2014. 

The IRIS mission views the interface region that lies between the sun’s photosphere and corona in unprecedented detail for researchers to study.

Credit: NASA, Lockheed Martin Solar & Astrophysics Laboratory

NASA's Interface Region Imaging Spectrograph (IRIS) has provided scientists with five new findings into how the sun's atmosphere, or corona, is heated far hotter than its surface, what causes the sun's constant outflow of particles called the solar wind, and what mechanisms accelerate particles that power solar flares.

The new information will help researchers better understand how our nearest star transfers energy through its atmosphere and track the dynamic solar activity that can impact technological infrastructure in space and on Earth.

Details of the findings appear in the current edition of Science "On the prevalence of small-scale twist in the solar chromosphere and transition region"DOI: 10.1126/science.1255732

"These findings reveal a region of the sun more complicated than previously thought," said Jeff Newmark, interim director for the Heliophysics Division at NASA Headquarters in Washington.

"Combining IRIS data with observations from other Heliophysics missions is enabling breakthroughs in our understanding of the sun and its interactions with the solar system."

The first result identified heat pockets of 200,000 degrees Fahrenheit, lower in the solar atmosphere than ever observed by previous spacecraft.

Scientists refer to the pockets as solar heat bombs because of the amount of energy they release in such a short time.

Identifying such sources of unexpected heat can offer deeper understanding of the heating mechanisms throughout the solar atmosphere.

For its second finding, IRIS observed numerous, small, low lying loops of solar material in the interface region for the first time.

The unprecedented resolution provided by IRIS will enable scientists to better understand how the solar atmosphere is energized.

A surprise to researchers was the third finding of IRIS observations showing structures resembling mini-tornadoes occurring in solar active regions for the first time.

These tornadoes move at speeds as fast as 12 miles per second and are scattered throughout the chromosphere, or the layer of the sun in the interface region just above the surface.

These tornados provide a mechanism for transferring energy to power the million-degree temperatures in the corona.

Another finding uncovers evidence of high-speed jets at the root of the solar wind. The jets are fountains of plasma that shoot out of coronal holes, areas of less dense material in the solar atmosphere and are typically thought to be a source of the solar wind.

The final result highlights the effects of nanoflares throughout the corona. Large solar flares are initiated by a mechanism called magnetic reconnection, whereby magnetic field lines cross and explosively realign.

These often send particles out into space at nearly the speed of light. Nanoflares are smaller versions that have long been thought to drive coronal heating.

IRIS observations show high energy particles generated by individual nanoflare events impacting the chromosphere for the first time.

"This research really delivers on the promise of IRIS, which has been looking at a region of the sun with a level of detail that has never been done before," said De Pontieu, IRIS science lead at Lockheed Martin in Palo Alto, California.

"The results focus on a lot of things that have been puzzling for a long time and they also offer some complete surprises."

More Information
Science "On the prevalence of small-scale twist in the solar chromosphere and transition region"DOI: 10.1126/science.1255732

NASA SDO: EUNIS mission - Coronal Heating theory detected

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. 

Credit: NASA/SDO

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

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 Extreme Ultraviolet Normal Incidence Spectrograph (EUNIS) mission, 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.

Jeff Brosius

"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.


NASA's EUNIS sounding rocket mission spotted evidence to explain why the sun's atmosphere is so much hotter than its surface. 

Credit: NASA/Goddard/Duberstein 

The EUNIS sounding 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.

The EUNIS team stands in front of the sounding rocket before its second launch on Nov. 6, 2007. 

The mission will launch again for a six-minute flight to observe the sun on December 15, 2012. 

Credit: U.S. Navy

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.

Unlike a conventional image, NASA's Extreme Ultraviolet Normal Incidence Spectrograph will provide what's known as "spectra" such as above, which show lines to highlight which wavelengths of light are brighter than others. 

That information, in turn, corresponds to which elements are present in the sun's atmosphere and at what temperature. 

Credit: NASA/EUNIS

Scientists have hypothesised 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 neighbours 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."

The EUNIS experiment undergoing tests before launch. 

Credit: NASA

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 favour of the nanoflare theory.

Adrian Daw

"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."

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.

More Information: Pervasive Faint Fe XIX Emission from a Solar Active Region Observed with EUNIS-13: Evidence for Nanoflare Heating - Jeffrey W. Brosius et al. 2014 ApJ 790 112. doi:10.1088/0004-637X/790/2/112