Here's why Sun's outer atmosphere is hotter than its surface

The cause of Sun's counterintuitively high temperatures has been an outstanding question in solar physics for years and now, finally, it might have an answer.

Scientists have now discovered that tiny solar flares can shed some light on the matter as to why the Sun's atmosphere, the corona, is hundreds to thousands of times hotter than the star's visible surface, the photosphere.

As per the scientists, the Sun produces heat at its core, which means the situation should have been just the opposite. Normally, the layer that is closest to a source of heat, in this case, the Sun's surface, would have a higher temperature than the more distant layer, which is the Sun's atmosphere here.

"If you've got a stove and you take your hand farther away, you don't expect to feel hotter than when you were close," stated one of the study authors, Lindsay Glesener, from the University of Minnesota, US.

The cause of these counterintuitively high temperatures has been an outstanding question in solar physics for years.

Now, scientists have reached a possible conclusion for this coronal heating problem, which is the constant upsurge of tiny solar flares in the solar atmosphere. These flares are so small that they cannot be detected directly. Recently, scientists have claimed that a sounding rocket instrument of the space agency was able to spot signatures of the long-sought small solar flares, reported NASA.

The second flight of the FOXSI (Focusing Optics X-ray Solar Imager) instrument, on December 2014, on a suborbital sounding rocket, detected a particular type of light, dubbed hard X-rays, the wavelengths of which are much shorter than the light that humans can naturally see. These X-rays are a signature of extremely hot solar material and this is exactly what the scientists were seeking for years to explain the mysterious extreme heating of the Sun's outer atmosphere.

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These kinds of temperatures are generally produced in solar flares, which are powerful bursts of energy. However, in this case, there was no observable solar flare, which led to the conclusion that the hot material was possibly produced by a series of solar flares which are so small that they went undetected from Earth. They are termed nanoflares.

"The key to this result is the sensitivity in hard X-ray measurements," said Shin-nosuke Ishikawa, a solar physicist at the Japan Aerospace Exploration Agency, or JAXA, and lead author of the study. "Past hard X-ray instruments could not detect quiet active regions, and combination of new technologies enables us to investigate quiet active regions by hard X-rays for the first time," Ishikawa added.

Related topics : Nasa