Sunny side up

Here's what scientists aim to gain from Aditya-L1, India’s first mission to the Sun

In 2008, the Indian Space Research Organisation (ISRO) announced Aditya-1, India's first solar mission to study the Sun. This ambitious endeavour, with various indigenously-developed instruments on board, holds much promise for our scientific community as they expect to unravel the mysteries of our closest star, the Sun. Now renamed as Aditya L1, the data from the instruments on board are expected to be a treasure-trove of information on the dynamic processes on the Sun's surface and its atmosphere.

Scheduled to be launched in a year, Aditya-L1 would be placed about 800 kilometres from the surface of the Earth. It would sit at a point called the 'Lagrangian point 1 (L1)', hence the L1 in the name, with its position fixed relative to the Earth and facing the Sun. It is because, at this point in the Sun-Earth line of sight, the gravitational force of the Earth, the gravitational force of the Sun and the centrifugal force of the satellite cancel out each other.

"The Aditya-L1 mission is India's first mission to explore the Sun, and we are very excited about it," says Prof Dibyendu Nandi from the Indian Institute of Science Education and Research, Kolkata. He is the Principal Investigator at the Centre of Excellence in Space Sciences, India (CESSI)—the first-of-its-kind consortium of astrophysicists from various Indian institutes. CESSI is playing a pivotal role in the Aditya-L1 mission as affiliated researchers here are working with scientists and engineers at ISRO and other institutions in the country to design and develop some of the onboard instruments and provide computational simulation support for the mission.

While the Aditya mission is a breakthrough in the making, considering only NASA, the Japanese Aerospace Exploration Agency (JAXA) and the European Space Agency (ESA) have had solar probes in the past, why should India start to understand the dynamics of the Sun? What can Aditya tell us about other stars and the Universe? What kind of scientific progress can data from Aditya enable? Here are the answers.

Why study the Sun?

Humanity's interest in observing the Sun is as old as history. In about 800 BC, the Chinese seem to have made the first reports of sunspots. In the early 1600s, four astronomers in Europe, including Galileo Galilei, made independent observations of sunspots with telescopes. Observations of the Sun and investigations of what goes on in it have driven many advances over the years, like the discovery of helium, better understanding of nuclear physics and even the discovery of neutrino oscillations. In the twentieth century, as scientists discovered the outlines of how stars work, understanding the Sun better assumed even greater importance. As the closest star to us, observations and investigations of the Sun can provide an unparalleled insight into the inner workings of stars.

"The Sun, which controls the atmosphere of all planets in the Solar System, shows many variations in its radiations due to the activities in it," explains Prof Nandi. Hence, understanding these variations can provide clues on the origin of life, the atmosphere of exoplanets and insights into plasma—the state of matter abundant in the Universe.

A long-studied aspect of the Sun's activity is the sunspots—dark spots on the surface of the Sun that have high magnetic flux. So far, we know that these sunspots have an 11-year cycle, during which they first increase in number, and then decrease. During the first half of this period, sunspots frequently appear at the mid-latitudes in both the northern and southern hemispheres, and then move towards the equator, before decreasing during the second half.

"In the early 1900s, astronomers discovered that these sunspots are highly magnetised, and the magnetic field in these regions is about ten thousand times stronger than the Earth's," says Prof Nandi. The high magnetic field also results in solar storms and solar flares, when high energy photons are emitted by the Sun. Sometimes, the plasma in the corona also gets spewed into space because of the high energy, and this is called 'coronal mass ejection'. These solar events could interfere with the workings of our satellite communication systems, like GPS (Global Positioning System) and weather monitoring, resulting in chaos.

Hence, it is essential to know when such solar flares occur, and at what magnitude, so we can be better prepared. "Hence, it is important to understand how activities in the solar atmosphere influence the space weather in our Solar System. By doing so, we can predict, and have disaster management systems in place for sensitive applications," explains Prof Nandi. With Aditya, all these can be done indigenously, making India self-reliant in dealing with the vagaries of space weather.

Lastly, the mission hopes to answer a long-unsolved question in solar physics—the coronal heating. The Sun's corona, which is the outermost layer of the solar atmosphere, is abnormally hot with temperatures in the range of millions of degrees. On the contrary, the Sun's surface is about 5,600 degrees Celsius. The laws of thermodynamics do not explain this abnormally hot corona, and scientists believe that this is not the result of thermal heating, but magnetic heating. "When magnetic fields of opposite polarity come together, they lose their field lines due to a process called magnetic reconnection. It is believed that this loss of magnetic energy is converted into localised heating in the corona," explains Prof Nandi.

As the corona becomes extremely hot, it throws out plasma from the Sun and creates solar storms and winds. An understanding of this immense magnetic activity and how it restructures the shape of the corona and causes solar storms is fundamental to understanding plasma physics. Sun provides a unique laboratory to observe these activities, which is impossible with other distant stars. The models that we construct about how plasma flows, can help answer more significant questions on the structure of other objects in the Universe like active galactic nuclei— bright and dense regions at the centres of galaxies.

Instruments onboard Aditya L1

The instruments on the mission are aimed at studying the magnetic activity at the Sun's corona, and understanding how it influences space weather. The Visible Emission Line Coronagraph (VELC), built at the Indian Institute of Astrophysics, Bengaluru, is designed to understand coronal mass ejections and how they propagate in space. This sensitive instrument measures the density and strength of the magnetic field in the corona, which can then help us figure out the coronal temperature.

"Right now, no solar mission can measure the magnetic field of the Sun's corona as it is only observable during the eclipse. With Aditya, we can now collect this information in real-time," says Dr Nandi.

The Solar Ultraviolet Imaging Telescope (SUIT), being designed by the Inter-University Centre for Astronomy & Astrophysics (IUCAA), Pune, measures the Ultraviolet radiation emitted from the Sun, which is a primary determinant of life of Earth. “One of the main questions we are looking to address is how does the Sun manage to produce ultraviolet radiation from the upper surface of the atmosphere. We need to observe the radiation at different layers of the atmosphere and figure out how it is produced and why it varies,” says Dr Durgesh Tripathi from IUCAA, who is the Principal Investigator of SUIT. The data collected from this instrument would also be used as inputs to climate models that accurately predict the Earth's climate.

Onboard Aditya-L1 are two X-ray spectrometers that study the X-rays radiated during solar flares. The Solar Low Energy X-ray Spectrometer (SoLEXS) analyses low-energy X-rays, while the High Energy L1 Orbiting X-ray Spectrometer (HEL1OS) estimates the energy used to accelerate particles during eruptive events like solar flares. These two spectrometers are being built at the U R Rao Satellite Centre (URSC) - ISRO.

Two in-situ instruments measure the local space weather around the satellite. The Aditya Solar wind Particle Experiment (ASPEX), designed by the Physical Research Laboratory (PRL) at Ahmedabad, looks at the variations in the properties of the solar wind, and its distribution and spectral characteristics. The Plasma Analyser Package for Aditya (PAPA) helps to understand the composition of the solar wind and is being designed by ISRO’s Vikram Sarabhai Space Centre. An onboard magnetometer, called MAG and built by ISRO’s Laboratory for Electro-Optics Systems (LEOS), measures the enhancement in the magnetic field when a solar storm is passing.

Enabling better research in solar physics

With a suite of powerful instruments, Aditya is turning to be a darling for India's solar astrophysicists, who have big audacious research goals. "The data from Aditya will enable both fundamental and applied research," hopes Prof Nandi. The mission's data would help in deciphering the Sun's coronal magnetic field and understanding how they drive solar storms and power plasma heating in the solar corona. The understanding of the space weather is also going to add to India's might of indigenous technology in protecting its satellites and communication systems.

A better insight into the Sun's activities that enhance the Ultraviolet emissions has implications on the planetary atmosphere in the Solar System. With instruments like SUIT, we can get an image of various layers of the Sun’s atmosphere. “The data shall help us monitor the solar activity and their connections within the atmosphere. It would also be used to monitor the solar UV spectral irradiance, which is central to the chemistry of Ozone and Oxygen in the stratosphere of the Earth,” explains Dr Tripathi.

The coming together of various institutes of the country in providing the scientific prowess to Aditya is also remarkable. "It is probably one of the few times where we have taken a bottom-up approach in designing the instruments based on what science we want to enable eventually," opines Dr Nandi. The mission has seen multiple discussions between academic researchers in these institutes who will be the eventual users of the data that Aditya provides, and the engineers at ISRO who are vital in launching the mission.

The research community is preparing to use data from Aditya as soon as possible—within six months of the launch. For instance, Prof Nandi's research group is already building a framework of tools that can use data about the Sun's coronal observation and understand the magnetic fields in the corona. "We can use maps of the Sun's surface to perform data-driven computational models of the corona and constrain these models with observations," he says, explaining how his group intends to advance our understanding of the Sun. There have also been efforts to accurately calibrate the onboard instruments such that the data, when they come in, are usable and accurate.

"We are very excited about Aditya, and are looking forward to achieving new milestones in the area of solar physics," signs off an excited Prof Nandi.

(The author is with Gubbi Labs)