Mysteries of Sun: ISRO’s Chandrayaan 2 mission provides key insights into solar coronal heating, FIP bias

ISRO’s Chandrayaan 2: Outstanding results on the solar corona and heliophysics have been received by the Indian Space Research Organisation (ISRO) from an instrument placed aboard the Chandrayaan 2 mission. ISRO said that while scientists understood the aspect regarding the origin of energy of the Sun fairly well, along with many others of its features, there was still much to be known about the Sun, including some mysterious phenomena that could potentially change what we know so far. Some of these unknowns are the outer atmosphere of the Sun, called corona, as this is where emission of electromagnetic spectrum in ultraviolet and X-ray wavelengths takes place, according to a report in PTI.

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Scientists are aware of the fact that the solar corona has temperatures that exceed one million Kelvin due to the extensive presence of ionised gas. What’s more is the fact that the photospheric temperature of the Sun, that is the temperature of the surface of the Sun, is a mere fraction of this – 6,000 Kelvin. What has scientists stumped is that this is contrary to the natural expectation of temperature reduction as the distance from the source of energy increases. This phenomenon is referred to as the coronal heating problem.

ISRO said that based on the observations of active regions above the dark patches on the Sun having stronger magnetic fields, called Sunspots, experts are of the view that the magnetic fields have an important role to play in coronal heating. Many different theories have been devised to explain what might be actually happening, and the occurrence of a large number of small solar flares (or nanoflares) is the basis of one of these theories.

Scientists have also observed that certain elements in the corona are three to four times abundant in active regions as compared to the photosphere. This occurs in those elements that ionise easier or in lesser energy, i.e., elements having First Ionisation Potential (FIP) lower than 10 eV. This is the reason why the phenomenon is referred to as FIP bias. However, while the phenomenon is known, the reason behind its occurrence or its origin continue to remain a mystery.

Now, a team at Department of Space’s Physical Research Laboratory (PRL) in Ahmedabad used the Solar X-ray Monitor (XSR) onboard Chandrayaan 2 and its observations of the Sun in soft X-rays captured during the deepest solar minimum in the past century to understand new and exciting details of the corona of the Sun.

ISRO said that in the quiet solar corona, absolute abundances of elemental Al, Si and Mg have been derived for the first time, and the team found and characterised about 100 sub-A class microflares in quiet regions of the solar corona. This, ISRO said, would provide new insight regarding the problem of coronal heating.

The 98 microflares that have been observed have an intensity so low that they cannot fit in the standard scale to classify them. Currently, there are five classes of solar flares, with each one being 10 times more intense than the previous. These classes are A, B, C, M and X. This is why these microflares have been classified as sub-A class microflares.

ISRO said that this marked the first observation as well as statistical study of microflares in such a large number in the quiet part of the Sun. This, the agency said, could support the theory that even smaller flares could be present everywhere in the corona of the Sun, and that these could be the reason behind the coronal heating.

The observation by XSM also helped in providing measurements of the abundance of various elements, allowing the estimation of the abundance of low FIP elements Si, Al and Mg. It was found that these abundances were lower in quiet corona as compared to the active regions in corona, but were higher than the abundances in the photosphere. ISRO shared that this also marked the first report on measurement of abundances and the reduced FIP bias in the quieter regions of the Sun.