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

by Joseph K. Clark

ISRO said that while scientists understood the aspect regarding the origin of energy of the Sun reasonably well, along with many other 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 the corona, as this is where the emission of the electromagnetic spectrum in ultraviolet and X-ray wavelengths takes place, according to a report in PTI. ISRO said that in the quiet solar corona, absolute abundances of elemental Al, Si, and Mg had been derived for the first time 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.

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Scientists know that the solar corona has temperatures exceeding one million Kelvin due to the extensive presence of ionized 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. Scientists stumped 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.

Chandrayaan 2

Experts believe that magnetic fields have an essential role in coronal heating. ISRO said that based on the observations of active regions above, the dark patches on the Sun had more required magnetic fields, called Sunspots. Many different theories have been devised to explain what might be happening. 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 some corona aspects are three to four times abundant inactive regions compared to the photosphere. This occurs in those elements that ionize easier or with 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 origin remains a mystery.

Now, a team at the 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 absolute abundances of elemental Al, Si, and Mg in the quiet solar corona had been derived for the first time. The team found and characterized about 100 sub-A class microflares in dead regions of the solar corona. ISRO said this would provide new insight regarding the coronal heating problem.

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

ISRO said that this marked the first observation and 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 more minor 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 provide measurements of the abundance of various elements, allowing the estimation of the lot of low FIP elements Si, Al, and Mg. It was found that these abundances were lower in the quiet corona than the active regions in the halo but were higher than the abundances in the photosphere. ISRO shared that this also marked the first report on the measurement of quantities and the reduced FIP bias in the quieter regions of the Sun.

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