Solar corona puzzle: Indian scientists crack a long-pending mystery

A solar mystery known for decades may have finally been cracked by three Indian astrophysicists, who solve the puzzle with the help from a radio telescope located Down Under.

Proposing a new explanation for the long-pending coronal heating problem, the trio discovered tiny flashes of radio light from all over the Sun and identified the smoking guns behind these small magnetic explosions that, in their opinion, led to such an incredible rise in the temperature of the solar corona.

The Sun is extremely hot with a surface temperature of about 5,500 degrees Celsius. But surprisingly on top of this sits a layer of gas with a temperature of almost two million degrees - over 300 times hotter than the surface of the Sun.

This is known as the corona, which looks like a crown and is visible during a total solar eclipse.

What heats up the corona to two million degrees is one of the most challenging puzzles about the Sun and no one found a satisfactory answer to this till date.

In their journey to unravel this mystery, the scientists at the National Centre for Radio Astronomy, Pune have figured out that the extra energy heating up the corona must be coming from the magnetic fields. But exactly how the process works, remains unknown.

The strength of the magnetic fields varies a lot from one spot on the solar surface to another by more than a factor of 1,000, but the corona is hot everywhere. This means the heating process has to work all over the corona, even in regions of weak magnetic fields.

Until now the process of how this magnetic energy is deposited in the corona remained a mystery.

“What made this breakthrough possible is the availability of data from a new instrument, the Murchison Widefield Array (in Western Australia). The very weak radio flashes we have discovered are about 100 times weaker than the weakest bursts reported till now,” said Divya Oberoi, the principal investigator.

Such flashes were present all the time everywhere on the Sun including in the regions of weak magnetic fields, the so-called ‘quiet Sun’ regions, said Surajit Mondal, lead author of the paper that will appear in the June issue of the Astrophysical Journal Letters.

The preliminary estimates suggested that these tiny magnetic explosions should collectively have enough energy to heat the corona, which was exactly what was needed for solving the coronal heating problem, noted Atul Mohan, third member of the team.

The theory behind such tiny explosions were proposed by the American solar physicist Eugene Parker, who earned a rare distinction from NASA in 2017 when the US space agency named its latest solar probe after him. This was probably for the first time, NASA named a space probe after a living scientist.

“With this work, we have the strongest evidence till date that these tiny explosions, originally referred to as nano-flares by Parker in his theory in 1988, can indeed be heating up the corona. It is just that they have turned out to be more than 1,000 times weaker than his prediction and it took us a long time to find them,” said Oberoi.

“There are two theories to explain the coronal heating problem. But the observational evidence in support of the other theory (wave heating theory of corona) is limited,” he told DH.