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Solar Secrets: How a professor from IITD led a team to understand the Sun’s corona

In conversation with Prof. Suprit Singh, Department of Physics, IIT Delhi

Can you tell us about the experiment in brief?

We, me along with Dr. Bharti Arora from University of Liege, and Paras Sharma, a recent graduate from the Physics Department of IIT Delhi and now at Stony Brook University, observed and gathered the polarisation data of the Sun’s corona (the outer atmosphere) during Total Solar eclipse at Dallas, Texas on April 8th 2024. 

What is the purpose of the study?

There are many open questions regarding the Sun’s corona. One of them is called the coronal heating problem. As we all know that the Sun’s surface temperature is around 6000K but as we move away from the surface, the temperature spikes suddenly and reaches more than a million degrees Kelvin. No one knows why. Similarly, the radio emission of corona is expected to be polarised and it is observed to be so. However, the optical brightness is also polarised but we don’t have any theoretical understanding of why that is so. 

The aim of this experiment was to gather data of the polarisation of the corona in the optical band in order to build theoretical models to explain it. The problem with observing the corona is that it is logarithmically fainter than the bright solar disc. Thus, it is only observable when we can somehow cover the solar disc, which does happen during the total solar eclipse (during a total solar eclipse, the sun’s disc is completely blocked by the moon, therefore making the corona and even stars appear for a few minutes called totality). That’s why these eclipses are of immense scientific significance. 

What was interesting about this experiment?

The centre line is the locus of all the points on the Earth’s surface where the eclipse will be observable for the maximum time (that is, maximum totality) and with nearly 100% coverage (percentage of solar disc blocked by the moon). As we move away from the line, both these factors decrease significantly. Thus, the April 8th eclipse was interesting to us because its centre line went across the continent and was not confined to the ocean as it can many times. There are other difficulties such as weather which might make us lose a good sighting so this condition increases our chances of not messing up as we can just relocate to a different location. 

Centre line of the solar eclipse, April 8th 2024

April 20th 2023

We also conducted an Australian preliminary expedition in April 2023 so that the team could go through the procedure and get themselves acquainted. That was a very short totality of merely 50 seconds over a very remote area, Exmouth in the northwestern region of Australia around 1400 km from Perth. Fun fact, if you pull up the AirBnB heat map of bookings around April 7th-8th in the USA, you can see a distinctive line along the eclipse’s path, evidently a large gathering of enthusiastic astronomers! Similarly, there was a population spike in that tiny peninsular region of Australia for a few days on 20 April 2023!

Can you go into the details and tell us more about the equipment and how you funded the excursion?

Centre line of the solar eclipse, April 20, 2023

There are two types of corona namely K and F coronas based on the brightness, elongation and source of light. There have been observations made by an American group but in monochrome, integrated over the whole optical bandwidth. We observed both these coronas in three channels: red, green, and blue. Thus, we got the polarisation data along with its spectral distribution. The composition of white light images has already been done. 

Diamond Ring from the Total Solar Eclipse,Apr 8th 2024

We will have to do computation on the channels data. What we are doing is spectropolarimetry, that is, studying the polarisation in spectral bins. The totality at Dallas lasted around 4 minutes, thus we had to be quick. The process involved moving with the Earth’s rotational movement so that we can cancel its effects. This involved aligning our equipment with the Polaris star, the celestial north pole of the Earth and using a guider to correct any errors in tracking. This process had to be done before the actual event. 

Most of our equipment was pretty standard astronomy equipment, refractor astrograph telescopes, a Nikon Z7II camera for high frame rate imaging, and an equatorial mount and guider for tracking. To get the polarisation data, we used an in-house-assembled special filter wheel with 3 polarisers aligned at angles 0, 60, -60 degrees with respect to the horizontal axis of the camera that will allow us to compute the Stokes parameters that characterise the polarisation of light.

Part of the equipment was provided by the institute research grant and some part of my travel was also borne from the funds given to us by the institute. 

What are some of the applications of this experiment?

Solar corona is a very less understood phenomenon even after seven decades of study. Coronal mass ejections are sudden spurts of charged particles from the Sun’s surface which interact with charged particles in the Earth’s ionosphere and affect communication satellites. By better understanding the corona, we will be able to prevent such damage. They do offer a great time for auroras!

Looking forward, how do you think the outcome of this experiment will be?

We have a large amount of computational work to do in order to interpret the data and eventually use them to build theoretical models based on magnetohydrodynamic principles (because one can model the corona as composed of plasma) and predict the shape of the corona and confirm our model’s validity during future eclipses.

Do you have any message for the student community of IITD to contribute to astrophysics and astronomy?

As you can see, such novel endeavours sit at the very intersection of physics, mathematics, computational numerical methods, instrumentation, and all aspects of engineering. So, students, no matter what their background, can always play an important role in uncovering the mysteries of the universe. After all, Paras, one of the team members, is also an IIT Delhi graduate. 

Prof. Suprit Singh (centre) along with Paras Sharma (right) and Dr. Bharti Arora (left) 

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