Tiny Solar Flares Could Be Heating Sun’s Corona To Millions Of Degrees

Solar flares that are too small to be detected from Earth could be causing the temperature of the sun’s corona to be as high as it is. Researchers made the suggestion in a paper published Oct. 9 that used data captured by the Focusing Optics X-ray Solar Imager (FOXSI) instrument.

The temperature of the sun’s atmosphere, which is in the millions of degrees, has long been a mystery for astrophysicists. Compared to the photosphere — the sun’s visible surface — whose temperature is about 6,000 degrees, the corona is hundreds to thousands of times hotter. Since the core of the sun is where the star generates its heat, the fact that the outer atmosphere — farther from the core than the photosphere — is hotter than the surface requires an explanation.

Nanoflares. That’s what researchers led by Shin-nosuke Ishikawa, project research fellow at the Japanese Aerospace Exploration Agency (JAXA), are calling the tiny flares they suggest are causing the extreme heating of the sun’s corona. These small but possibly frequently occurring flares were detected by combining the “hard X-ray data from FOXSI… and data from Hinode solar observatory,” according to a JAXA statement Tuesday.

“The key to this result is the sensitivity in hard X-ray measurements. Past hard X-ray instruments could not detect quiet active regions, and the combination of new technologies enables us to investigate quiet active regions by hard X-rays for the first time,” Ishikawa said in a NASA-issued statement Friday.

Hard X-rays, for the purpose of the paper, were defined as those above about 3 KeV, while soft X-rays were below 3 KeV.

FOXSI, a NASA-funded solar sounding rocket built by University of California, Berkeley, recorded the X-ray data during its second flight on Dec. 11, 2014. The 6-minute observation window during the 15-minute flight was sufficient for researchers to gather evidence for nanoflares which have been hypothesized for years as possibly being the mechanism that causes coronal extreme heating.

“This is a proof of existence for these kinds of events. There’s basically no other way for these X-rays to be produced, except by plasma at around 10 million degrees Celsius [18 million degrees Fahrenheit]. This points to these small energy release happening all the time, and if they exist, they should be contributing to coronal heating,” Steve Christe, project scientist for FOXSI at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and an author on the study said in the NASA statement.

Thanks to FOXSI, the researchers observed weak X-ray signals that originated in very hot plasma in the corona. Hinode’s X-ray telescope allowed for detection of soft X-rays, and combining the two made it possible to create a “temperature structure” of the region the researchers were observing, one that “was not showing X-ray brightening nor flaring activity.”

Since the current findings relate to only one region in the corona and are based on only a single 6-minute observation, the researchers said more observations over a longer time-span need to be carried out to answer other questions.

“This particular observation doesn’t tell us exactly how much it contributes to coronal heating,” Christe said. “To fully solve the coronal heating problem, they would need to be happening everywhere, even outside of the region observed here.”

Titled Detection of nanoflare-heated plasma in the solar corona by the FOXSI-2 sounding rocket,” the paper appeared online in the journal Nature Astronomy.