The Mystery of Nanoflares

When you attach the prefix "nano" to something, it usually means "very small." Solar flares appear to be the exception.

Researchers are studying a type of explosion on the sun called a 'nanoflare.' A billion times less energetic than ordinary flares, nanoflares have a power that belies their name.

"A typical 'nanoflare' has the same energy as 240 megatons of TNT," says physicist David Smith of UC Santa Cruz. "That would be something like 10,000 atomic fission bombs."

A new ScienceCast video explores the mystery of the sun's tiniest flares. Play it

The sun can go days, weeks or even months without producing an ordinary solar flare. Nanoflares, on the other hand, are crackling on the sun almost non-stop.

"They appear as little brightenings of the solar surface at extreme ultraviolet and X-ray wavelengths," continues Smith. "The first sightings go back to Skylab in the 1970s."

The relentless crackle of nanoflares might solve a long-standing mystery in solar physics: What causes the sun's corona to be so hot?

Imagine standing in front of a roaring fire. You feel the warmth of the flames. Now back away. You get cooler, right? 

That's not how it works on the sun.  The visible surface of the sun has a temperature of 5500 C.  Moving away from the surface should provide some relief.  Instead, the sun's upper atmosphere, known as the "solar corona," sizzles at a million degrees--a temperature almost 200 times higher than that of the roaring furnace below. 

For more than a half-century, astronomers have tried to figure out what causes the corona to be so hot.  Every year or so, a press release appears purporting to solve the mystery, only to be shot down by a competing theory a year or so later.  It is one of the most vexing problems in astrophysics. 

Smith thinks nanoflares might be involved. For one thing, they appear to be active throughout the solar cycle, which would explain why the corona remains hot during Solar Minimum.  And while each individual nanoflare falls short of the energy required to heat the sun's atmosphere, collectively they might have no trouble doing to job. 

To investigate this possibility, Smith turned to a telescope designed to study something completely different. 

X-rays stream off the sun in this image showing observations from by NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, overlaid on a picture taken by NASA's Solar Dynamics Observatory (SDO).  [more] 

Launched in 2012, NASA's NuSTAR X-ray telescope is on a mission to study black holes and other extreme objects in the distant cosmos. Solar scientists first thought of using NuSTAR to study the sun about seven years ago, after the space telescope's design and construction was underway. Smith contacted the principal investigator, Fiona Harrison of the California Institute of Technology in Pasadena, to see what she thought. 

"At first I thought the whole idea was crazy," says Harrison. "Why would we have the most sensitive high energy X-ray telescope ever built, designed to peer deep into the universe, look at something in our own back yard?" 

Eventually, she was convinced.  As Smith explained, NuSTAR has just the right combination of sensitivity and resolution to study the telltale X-ray flickers of nanoflares. A test image they took in late 2014 removed any doubt.  NuSTAR turned toward the sun and, working together with NASA's Solar Dynamics Observatory, captured one of the most beautiful images in the history of solar astronomy. 

The next step, says Smith, is to wait for Solar Minimum.  The current solar cycle will wind down in the years ahead, leaving the sun mostly free of sunspots and other magnetic clutter that can obscure nanoflares.  NuSTAR will be able to survey the stellar surface and gather data on these explosions like no telescope has done before. 

Will it solve the mystery of nanoflares and the solar corona?  "I don't know," says Smith, "but I cannot wait to try."

Credits:  

Author: Dr. Tony Phillips | Production editor: Dr. Tony Phillips | Credit: Science@NASA

Source:  NASA Science

Best Evidence Yet For Coronal Heating Theory Detected by NASA Sounding Rocket

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.

Related Links:

• More on EUNIS

• More on sounding rockets

Karen C. Fox
NASA's Goddard Space Flight Center, Greenbelt, Maryland