How solar eclipse will help study Sun, why Aditya L1’s view will not be blocked

Why does an eclipse offer a good opportunity to study the Sun?

A solar eclipse takes place due to the alignment of the Sun, the Earth, and the Moon with respect to each other. During a solar eclipse, which can last from a few minutes to several hours, the Moon is positioned between the Earth and the Sun. As the Moon casts its shadow, either partially or fully, over Earth, the Sun ceases to be visible in full or in part, and a partial or total solar eclipse occurs.

“Since the Moon plays the perfect occulter during total solar eclipses, these events provide a great opportunity to observe the solar atmosphere, especially the inner corona, where important dynamics are believed to occur,” Dipankar Banerjee, Director, Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital, told The Indian Express.

Banerjee will be in Dallas, Texas during the eclipse, along with his colleagues at ARIES, S Krishna Prasad and T S Kumar.

(From left to right) T S Kumar, S Krishna Prasad, and Dipankar Banerjee at the Cotton Bowl stadium, Dallas, with their experimental setup ahead of the total solar eclipse on Monday. (Photo courtesy ARIES)

The scientists have planned multiple experiments and solar observations during the eclipse from this location, which will offer images close to the totality central line, besides favourable weather for taking observations.

Will Aditya L1, India’s first dedicated solar mission, help in the study of the Sun during the eclipse?

Earlier this year, India successfully placed the Aditya L1 spacecraft in Lagrange Point 1 of the Earth-Sun system, which is about 1.5 million km from Earth. The satellite’s seven payloads have since been undergoing the calibration and performance verification phase.

“It (the eclipse) will be a very good opportunity to study the Sun’s corona during totality phase of the eclipse both from ground-based telescope and the coronagraph onboard the Aditya L1,” Banerjee said.

The satellite’s view will not be impacted by the eclipse because it has been placed at a vantage point from where continuous and uninterrupted observations are possible. “Aditya L1 spacecraft will not see the solar eclipse as the Moon is behind the spacecraft, at the Lagrange Point 1, the eclipse that is visible on Earth doesn’t have much significance at that location,” S Somanath, chairman of the Indian Space Research Organisation (ISRO) told NDTV.

However, the inner coronagraphs on board space platforms suffer from light scatterings, which makes it hard to see the inner corona in white light close to the limb. The Moon being a natural occulter, blocks the solar diskas and allows scientists to see the corona very close to the solar disk (inner corona).

Lagrange points. (Via ISRO)

Aditya-L1’s trajectory. (Via ISRO)

What experiments will be carried out by the Indian team during the eclipse?

The Indian scientists will carry out a high-cadence narrowband (0.5 nanometre) imaging observation of the solar corona in the green emission line (Fe XIV 5303 °A). In the past, a number of emission lines including the Fe X 6374 °A, Fe XI 7892 °A, Fe XIII 10747 °A and Fe XIV 5303 °A were identified and used to observe the corona in the visible and infrared wavelength ranges.

They will also conduct both on-band and off-band observations to capture the line emission. This setup, Banerjee said, could help investigate the high-frequency oscillatory and variable phenomena as observed in the solar atmosphere during the totality phase.

Any filter allows a certain wavelength passband centre to that specific wavelength to transmit. The filter to be deployed by the ARIES team (at 5303 A with 5 A passband) will allow scientists to see the Sun’s plasma with about a million Kelvin temperature.

The experiments will allow the Indian team to calibrate some of the instruments on the space platform. The cross calibration of similar set-ups on the ground and in space will provide better a quantitative measure of the parameters observed from space.

How is studying the solar atmosphere during a total solar eclipse different from studying the Sun at other times?

The solar corona is only visible during total solar eclipse; else, one needs to block the solar disk artificially using an instrument called coronagraph. Artificial coronagraphs suffer from scattered light and it becomes difficult to see the inner corona.

But during a solar eclipse, the Moon, which provides occultation perfectly and blocks the solar disk, makes the inner corona visible.

So, several countries have either been sending missions to study the Sun or have established solar observatories for performing continuous observations of the solar activity.

One solar cycle lasts for 11 years, during which the Sun’s activities can range from the solar minima (emergence of the lowest number of sunspots) to the solar maxima (emergence of the highest number of sunspots), corresponding to the Sun’s most quiet and the most intense sides respectively.

Currently, the solar cycle 25, which commenced in 2020, is underway. With the Sun fast approaching its maximum magnetic activity (solar maxima), it is expected that many structures in the inner corona will be seen.