Mysteries of solar corona

Description

GS PAPER III: Science and Technology- developments and their applications and effects in everyday life.

Context: An instrument on board India’s Chandrayaan-2 mission has provided outstanding science results on the solar corona and heliophysics.

Sun corona

The Sun’s corona is the outermost part of the Sun’s atmosphere.
The corona is usually hidden by the bright light of the Sun's surface.
That makes it difficult to see without using special instruments. However, the corona can be viewed during a total solar eclipse.
It emits ultra-violet and X-ray wavelengths of the electromagnetic spectrum.
Corona consists of ionised gas at temperatures exceeding one million Kelvin, which is much higher than photospheric temperature of 6000K, the visible surface temperature of the Sun.

Why is the corona so dim?

The corona reaches extremely high temperatures. However, the corona is very dim. As it is about 10 million times less dense than the Sun’s surface.
This low density makes the corona much less bright than the surface of the Sun.

Why is the corona so hot?

The corona’s high temperatures are a bit of a mystery. Astronomers have been trying to solve this mystery for a long time.
The corona is in the outer layer of the Sun’s atmosphere—far from its surface. Yet the corona is hundreds of times hotter than the Sun’s surface.
A NASA mission called IRIS discovered packets of very hot material called "heat bombs" that travel from the Sun into the corona.
In the corona, the heat bombs explode and release their energy as heat. But astronomers think that this is only one of many ways in which the corona is heated.
The surface of the Sun is covered in magnetic fields. This is the force that makes magnets stick to metal, like the door of your refrigerator.
The Sun's magnetic fields affect charged particles in the corona to form beautiful features. These include streamers, loops, and plumes.

How does the corona cause solar winds?

The corona extends far out into space. From it comes the solar wind that travels through our solar system.

The corona's temperature causes its particles to move at very high speeds. These speeds are so high that the particles can escape the Sun's gravity.

Sunspots help understand life around other stars

This observation is against the natural expectation that the temperatures should reduce as we go away from the source of energy, and this is known as the ‘coronal heating problem.’
From observations, such as the presence of even hotter corona, called active regions above the Sunspots (dark patches seen in visible images of the Sun) where the magnetic fields are known to be stronger, it is suggested that the magnetic fields have an important role in the coronal heating.
While there are different theories regarding the actual mechanism, one of these relies on the occurrence of a large number of small solar flares called

Global magnetic field of Sun’s atmosphere measured for the first time

Observations of the Sun in soft X-rays with Solar X-ray Monitor (XSM) on board ISRO’s Chandrayaan-2 mission during the deepest solar minimum of the past hundred years give exciting details about the solar corona.
“For the first time, absolute abundances of elemental Mg, Al, Si in the quiet solar corona are derived.
The XSM also supports the quantitative measurements of elemental abundances of the lunar surface using the companion payload CLASS (Chandrayaan-2 Large Area Soft X-ray Spectrometer) developed by URSC (U R Rao Satellite Centre), an ISRO centre, which measures the X-ray fluorescence spectrum from the lunar surface.

Solar Orbiter’s first view of the Sun

At present, XSM is the only instrument that provides soft X-ray spectral measurements of the Sun, i.e., measures the intensity of X-ray in different energies over the 1 to 15 keV.
XSM provides such measurements with very good energy resolution at every second.
The solar minimum of 2019-2020 was even more peculiar as the Sun was extremely quiet, and its activity was at the lowest level over the past century.
This provided a unique opportunity for XSM to observe the quiet corona without active regions for long periods.
This was 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 be responsible for the coronal heating.
The abundances of the low FIP elements Mg, Al, and Si were estimated and found to be lower than the abundances seen in active region corona but higher than that in the photosphere.