The outermost layer of the Sun, known as the solar corona, extends far beyond the Sun’s surface, which astronomers refer to as the photosphere. The solar corona is also the source of solar winds, which cause spectacular auroras, and can sometimes disrupt communication across the globe. However, the corona is more than a hundred times hotter than the photosphere, unlike what one could imagine for a ball of fire. Solar physicists call this mystery the ‘coronal heating problem’. What makes it more difficult for astronomers to study the corona is that it is much dimmer than the photosphere.
The coronal heating problem continues to baffle researchers studying the Sun as well as other stars, which also have scorching coronas. Superflares, or sudden dumping of vast amounts of energy in the solar corona, have been observed since the dawn of astronomy. Some earlier observations of the Sun have also shown that magnetic phenomena in the photosphere are ultimately responsible for heating the corona. However, little is understood about the detailed mechanism of how this happens. A recent study has shown that the conversion of magnetic energy to heat is somewhat random. The study, by researchers from the National Centre for Radio Astrophysics (NCRA) in Pune, India, was published in the journal The Astrophysical Journal Letters, and funded by the Department of Atomic Energy, Government of India.
Astronomers study the universe in all wavelengths of the electromagnetic spectrum, ranging from X-ray and gamma rays that have a smaller wavelength, to the long-wavelength radio waves. Visible light is only a tiny part of this broad spectrum.
“Since the solar surface is very bright in the optical wavelengths, they cannot be used to study the solar disc,” explained Mr Surajit Mondal.
He is a research scholar at NCRA and the lead author of the study.
On the other hand, radio wavelength electromagnetic waves are directly emitted by energetic particles created when the corona is heated. Hence, the researchers focussed on this part of the spectrum. This required them to observe the Sun through special telescopes, called radio telescopes, for which they turned to the Murchison Widefield Array (MWA) in western Australia.
The Sun is known to produce sudden and spectacular bursts of energy periodically. However, the researchers of the current study focused on what they call the ‘quiet Sun’, which is devoid of such activities. They observed the Sun for about 70 minutes in total. At the chosen wavelengths, snapshots of the Sun were obtained at every half a second, resulting in a large amount of data that was impossible for humans to analyse individually. Hence, they built software that automated the analysis and could create as many as 33,000 images of the quiet Sun, spanning all the wavelengths and times.
From each analysed image, the authors constructed the distribution of energy at different regions of the Sun at different wavelengths. Studying this variation helped them understand the relative importance of the massive energy output via superflares, as compared to continuous energy releases through smaller flares.
“Due to their high energy content, a single big flare can dump a much larger amount of energy compared to a weaker flare. However, the number of big flares is much smaller than the number of weaker flares. So only if the latter is much more frequent, can they make up for their energy required to heat the corona,” explains Mr Mondal.
The researchers concluded through the analysis that the solar corona is continuously bombarded with what they call ‘nanoflares’. Since the data had remarkable precision across time, they could further cross-check how strongly the energy deposition depended on time. They found that the nanoflares are impulsive and last for such small durations that they are barely detectable by the MWA. Hence, previous studies could not detect them. Besides, a significant number of such impulsive emissions were found to be present all over the Sun’s surface.
A preliminary calculation of the energy deposited from magnetic fields originating in the solar photosphere, done by the researchers, seems to confirm their findings. Since the amount of energy transferred from the photosphere to the corona cannot be directly measured, they hope that the broader community of solar physicists will turn to it next.
As a next step, the researchers plan to look into the characteristics of individual nanoflares in the corona. “To do that, we need observations at finely sampled wavelengths,” shares Mr Mondal, adding that they are collecting new observations of the Sun with the MWA.
This article has been run past the researchers, whose work is covered, to ensure accuracy.