Tuesday, August 3, 2021

New observations from Chandrayaan-2 unravels the mysteries of the Solar Corona

We used to think that life was constrained to a narrow warm band around a star but recent discoveries have shown that there are still potentially life-changing phenomena in our solar system which still remain a mystery. Diving deeper into these mysteries not only reveals the existence of our planet but also allows us to learn about other distant stars better.

Indian Space Research Organisation (ISRO) recently received outstanding results from an instrument placed aboard the Chandrayaan-2, which was hovering over the moon. The Sun has always been a source of research for astrophysicists across the world. The mystery unravelled, revolves around the hot outer atmosphere of the Sun, known as Corona which emits profusely in ultraviolet and X-ray wavelengths of the electromagnetic spectrum.

A team of scientists from the Physical Research Laboratory (PRL), Ahmedabad, used observations of the Sun on board the Chandrayaan-2 mission to find abundances of elements like magnesium, aluminium, and silicon and in addition to this, scientists also discovered and characterized around 100 “sub-A class” microflares providing new insight into the coronal heating puzzle.

What is Solar Corona & Coronal Heating?

The mysterious outer region of the Sun consists of hot and highly diffuse plasma, ionized gas at temperatures that exceed one million Kelvin, much higher than the photospheric temperature of 6,000 Kelvin, the visible surface temperature of the Sun.

The most mysterious and crucial puzzle related to the sun is its temperature. The ionized gas temperature at the core of the Sun reaches up to 15 million degrees Celsius, which drops to mere 5,700 degrees at the solar surface (photosphere). But the temperature starts to increase again above the photosphere with a height reaching one million degrees or more in the corona. The increase or rise of temperature in Corona despite moving away from the source of energy is against the natural expectation that the temperature should reduce. Scientists have termed this phenomenon as a ‘Coronal Heating Problem’.

Also, the Sun is a magnetized star and the active regions above the Sunspots have stronger magnetic fields. It is suggested that the magnetic fields play a crucial role in coronal heating. There are different theories governing this mechanism but one of the theories relies on the occurrence of a large number of small solar flares which are known as ‘nanoflares’. Another observation about the corona suggests that certain elements are found to have abundances three to four times higher in active regions than in the photosphere. This happens for elements that are easier to ionize.

ISRO Chandrayaan-2 beams back the mysteries of the Solar Corona

After landing on the dark side of the moon, the Chandrayaan-2 mission remained active in the form of its orbiter which was hovering over the Moon. Scientists used the Solar X-ray Monitor (XSM), designed and developed by PRL with support from various ISRO centers to study the spectrum of the Sun. The observations were started back in September 2019 during the period when the Sun was at a minimum when typically there were very few Sunspots and active regions on the Sun.

The solar minimum of 2019-2020 was characterized by an extremely quiet Sun and its activity was at the lowest level over the past century. ISRO stated that this provided a unique opportunity for XSM to observe the quiet corona without active regions for long periods. The scientists during the observation detected a remarkably large number (98) of extremely small flares in the quiet corona. These flares are so small that their intensity is well below the standard scale to classify solar flares and hence these are termed as “sub-A class microflares.”

This became the first observation and statistical study of such a large sample of microflares in the quiet Sun, supporting the hypothesis of the presence of even smaller scale flares everywhere on the solar corona that could, in turn, lead to coronal heating.

ISRO stated that since space-borne observations began, this is the lowest intensity of X-ray emission observed from the Sun. The X-ray observations lasted over 76 days, excluding the duration of the microflares, and provided qualitative measurements of abundances of various elements. Isro also stated that both the Chandrayaan-2 orbiter and the XSM instrument are performing extremely well, and expected to provide many more exciting and new results.

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