Aditya-L1 solar mission: The Sun’s corona stands out for its extraordinary heat, with temperatures soaring to an astonishing 2 million degrees Celsius. This remarkable temperature disparity, when compared to the Sun’s surface, which maintains a relatively cooler temperature of approximately 5,000 degrees Celsius, continues to baffle solar scientists, said Somak Raychaudhury, an eminent astrophysicist currently serving as vice-chancellor at Ashoka University, explaining one of the major objectives of India’s first space-based solar observatory.

Watch: Aditya L1 Sun mission launch

Raychaudhury has served as the director of the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune from 2015-22 where the Solar Ultraviolet Imaging Telescope (SUIT) – one of the main payloads on the Aditya-L1 mission was developed. He explained to HT the importance of the Indian Space Research Organisation (ISRO) solar mission as it launched from Sriharikota.

Here are the edited excerpts from the interview:

Can you provide an overview of the mission’s objectives?

Our primary aim is to continuously observe the Sun for 24 hours a day, a feat not possible from Earth’s orbit, as the planet frequently obstructs the view. This uninterrupted observation is vital for monitoring the Sun’s activities closely. To accomplish this, we’ve equipped Aditya L1 with two major instruments, complemented by five smaller ones.

One of the key instruments, named SUIT (Ultraviolet Imaging Telescope), is an ultraviolet imager that captures continuous images of the Sun. The ultraviolet spectrum is essential for our observations, as objects at extremely high temperatures, like the Sun’s corona, emit predominantly ultraviolet and X-ray radiation.

The second major instrument, VELC (Visible Emission Line Coronagraph), is a spectrograph that focuses on the Sun’s corona—the outermost layer of the Sun’s atmosphere, which extends far beyond the Sun’s visible disk. VELC simultaneously monitors the corona alongside SUIT, enabling us to correlate changes in the Sun’s corona with events on the Sun’s surface.

One of our primary goals is to understand why the Sun’s corona is astonishingly hot, reaching temperatures of up to 2 million degrees, in stark contrast to the relatively cooler surface of the Sun at around 5,000 degrees. This temperature difference remains a baffling mystery in solar science. To unravel this mystery, we need to study the Sun’s high-energy phenomena, which emit ultraviolet and X-ray radiation.

We have noted a connection between the Sun’s surface and the emission of high-energy particles during solar storms, which are closely related to the Sun’s magnetic activity. These solar storms can significantly impact our technology, including satellites and communication systems, making it crucial to predict and understand them.

Aditya L1’s unique capability lies in its ability to simultaneously observe the Sun and its corona. This simultaneous monitoring will help us establish causal connections between the two, shedding light on the mysteries of the Sun’s behaviour.

Why was Lagrange Point 1 chosen for this mission?

In 2008, when the Aditya L1 mission was initially considered by ISRO, the plan was to place it in a near-Earth orbit for solar observations. However, this plan was reconsidered and altered because it became evident that an Earth orbit had a significant limitation: the spacecraft would be in Earth’s shadow for nearly half of its orbit, meaning it would miss crucial solar observations. Given our objective of continuous solar monitoring, this was a significant drawback.

This realization led us to choose Lagrangian 1, or L1, as our mission’s destination. L1 is a point in space where the gravitational forces of the Sun and Earth balance, allowing a spacecraft to essentially “park” there. It is located approximately 1.5 million kilometres from Earth, which is a mere 1% of the distance to the Sun. This proximity to Earth ensures that the spacecraft can constantly observe the Sun without being eclipsed by our planet.

While there are other Lagrangian points, such as L2, L1 offers the best vantage point for our mission. L2 is on the opposite side of Earth from the Sun, making it further away from the Sun and occasionally obstructed by Earth. In contrast, L1 provides an unobstructed view of the Sun and is the ideal location for our solar observations.

Could you provide a comparison between global solar missions and what we have learned about the Sun so far?

Solar observations are conducted worldwide, with various missions and observatories dedicated to studying the Sun’s activities. These missions have significantly enhanced our understanding of our nearest star. Observations from Earth-based solar telescopes, such as the Raipur Solar Observatory in India and planned national solar facilities, primarily focus on capturing optical light from the Sun.

These ground-based observatories specialise in helioseismology, which involves studying the Sun’s tiny vibrations by analyzing variations in its light. These vibrations reveal valuable insights into the Sun’s interior dynamics, including nuclear fusion processes and energy transport mechanisms.

However, to delve into high-energy solar phenomena and the Sun’s interactions with the solar system, we need to turn to space missions. These missions, including Solar Orbiter (SOLO) and Solar and Heliospheric Observatory (SOHO) from Europe, have historically concentrated on observing the Sun itself, primarily in the ultraviolet spectrum.

Aditya L1’s ultraviolet and X-ray observations are critical because such high-energy phenomena can only be observed from space due to the Earth’s ozone layer blocking these wavelengths. By studying these emissions, Aditya L1 aims to uncover the secrets of the corona’s extreme temperature and its influence on solar activity.