Reaching for the star: A close look at our history of staring at the sun

We’ve never peered closer at our nearest star, and the view is dazzling. New probes and telescopes are capturing the dark hearts of sunspots, blasts of molten plasma, glints of magnetic fields.

The car-sized Parker probe sent out by NASA (the US’s National Aeronautics and Space Administration) has travelled into the corona or outer atmosphere of the Sun, and is expected to keep relaying images back from as little as 6 million km from the Sun’s surface (for perspective, Earth is about 149 million km from the star).

It isn’t just the view; we are also beginning to understand the sun — perhaps a little.

Advanced instruments on board the European Space Agency’s Solar Orbiter have revealed what are being called “campfires”, tiny flares that could help solve the mystery of why the corona, with temperatures of over 1 million degrees Celsius, is so much hotter than the surface of the Sun, which averages 5,500 degrees Celsius.

“That’s a mystery that has baffled scientists since the 1940s, since they first figured out just how hot the corona is,” says Jagdev Singh, consulting scientist with the Indian Institute of Astrophysics in Bengaluru and former head of the Kodaikanal solar observatory in Tamil Nadu.

The view is about to get better from India too. The Indian Space Research Organisation (ISRO) is readying to launch the Aditya L1 solar mission in 2023. While it will be stationed about 1.5 million km from Earth (crossing only 1% of the distance between Earth and the Sun), it will be the first Indian mission to offer an unfiltered look at the star.

Aditya L1 will also make ISRO only the fourth space agency in the world (after NASA, the European Space Agency or ESA and the Japan Aerospace Exploration Agency or JAXA) to launch a solar mission.

The spacecraft’s primary role will be to study solar activity and space weather. “Having indigenous data allows the Indian community to very quickly analyse and assess the impact of impending solar storms and solar activity, which has the power to disrupt technological infrastructure in space and on Earth,” says Dibyendu Nandi, head of the Center of Excellence in Space Sciences India (CESSI) at the Indian Institute of Science Education and Research (IISER) Kolkata, and chair of ISRO’s Aditya-L1 Space Weather Monitoring and Prediction Plan Committee.

Of course, there will be pictures too, and for most onlookers that’s the most exciting part. Because the Sun is essentially a massive nuclear fusion bomb, one that explodes every day. With a core temperature of 15 million degrees Celsius (Earth’s core, incidentally, is at about 6,000 degrees Celsius), the hydrogen gas that makes up most of the Sun’s mass is ionised, the electron stripped from the atom to form a superhot gaseous soup of charged particles called plasma.

As the plasma splashes to the surface, it generates magnetic fields so strong that they can prevent heat from escaping and light from entering. These are visible on the surface of the star as sunspots.

As they move across the Sun and intensify, sunspots spew plasma and radiation out into the solar system, in events called solar flares and coronal mass ejections (CMEs). The plasma can travel at up to 3,000 km per second, taking a few days to reach Earth. The radiation travels at the speed of light, and can reach Earth in less than 10 minutes.

“Really intense CMEs will travel faster than the solar wind, like a tsunami,” says Nandi. “The stronger geomagnetic storms can impact high-frequency radio communication, GPS networks and satellite communications. They can damage satellite sensors and take down ground-based electric power grids. Solar super storms can cause trillions of dollars worth of damage and can occur a few times in a century.”

Enter the dragon

In 1859, Earth was hit by the strongest geomagnetic storm on record. It damaged power grids and took down telegraph systems across Europe and North America. Auroras were seen in the night sky as far from the Poles as Cuba and Japan.

More recently, in February this year, a geomagnetic storm knocked 40 SpaceX satellites out of orbit and sent them hurtling back to Earth.

The opposite can also happen. One peculiar 70-year period, from 1645 to 1715, was a notably extended grand solar minimum. “This is a period when the Sun is said to have been asleep,” says Chitradeep Saha, a space scientist with CESSI. “It corresponds with a severe period called the Little Ice Age, when many lakes and rivers in the northern hemisphere, including the Thames in London, remained frozen for surprisingly long periods. Medieval artists have captured this in paintings of people skating on frozen water bodies.”

Saha is among those still studying this period; he was lead author of a recent paper on it. Fresh clues could come from new instruments digging deeper into the Sun’s behaviour and its anomalies. ESA’s Solar Orbiter, for instance, has 10 highly advanced instruments on board and one of these is the Extreme-Ultraviolet Imager that revealed the “campfires” or tiny flares.

Aditya L1 will have seven indigenously developed instruments on board, designed to study multiple wavelengths of solar activity and coronal heating. When the spacecraft was first commissioned in 2008, it was envisioned as a single-instrument low-orbit satellite fitted with a coronagraph to study the corona. As the scope of the mission was enhanced, ISRO invited scientific institutions across the country to submit proposals for instruments. In a rare collaborative space mission, contributions are being made by the Indian Institute of Astrophysics (IIA), Bengaluru; the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune; and the Physical Research Laboratory in Ahmedabad.

The most ambitious of these instruments is the Visible Emission Line Coronagraph, being built at IIA. It will examine the origins and behaviour of solar storms (essentially CMEs). “The instrument has been designed to capture images from really close to the solar surface and up to the outer solar corona,” says Nandi. The Solar Ultraviolet Imaging Telescope, another key instrument, being built at IUCAA in collaboration with institutes such as IISER Kolkata, will study the flow of energy from the Sun’s surface to the outer layers of its atmosphere.

Elsewhere, there is a plan for a National Large Solar Telescope (NLST) to be installed at Pangong Lake in Ladakh. With an aperture of 2 metres (the Daniel K Inouye Solar Telescope in Hawaii, the world’s largest, has a 4.24-metre aperture), NLST is being designed to study the origins and dynamics of the magnetic fields on the solar surface. Along with Aditya L1 and MAST (the Multi Application Solar Telescope at the Udaipur observatory, currently India’s largest, with an aperture of 50 cm), it could transform how India sees the Sun.

Together, they could also help unravel the mysteries of why the Sun and its corona behave as they do. Although, with an orb this inscrutable and hard to study, scientists are prepared for the implements to throw up all-new mysteries instead.

HOW CLOSELY ARE WE PEERING? A TOUR IN SIX SNAPSHOTS

Spot check

Sunspots are cooler regions that form on the surface of the Sun. They are characterised by magnetic fields so strong that they trap light and inhibit convection, making them appear darker than their surroundings. Because they inhibit convection or the release of heat energy, sunspots cause dramatic streaks to form around them, as the blazing plasma tries to find other ways out. A greater number of sunspots indicates a solar maximum, the period in the 11-year solar cycle when the Sun is at its most active. Few sunspots indicate that the Sun is at its solar minimum. The next solar maximum is expected in mid-2025.

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Surface tension

The dark borders surrounding each convection cell are cooler regions where solar particles are raining back down onto the Sun. Amid this chaos are bright glints that represent magnetic fields, thought to channel energy up to the outer layers of the corona or outer atmosphere.

The Daniel K Inouye Solar Telescope (DKIST), incidentally, has a 4.24-metre aperture and can capture solar surface features as small as 30 km wide. In comparison, India’s most powerful solar telescope, the Multi Application Solar Telescope (or MAST) at the Udaipur Solar Observatory, has an aperture of 50 cm. India may get one far larger. The proposed National Large Solar Telescope at Pangong Lake in Ladakh will have an aperture of 2 metres and will be designed to study magnetic fields on the solar surface, among other things.

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Close shave

The image shows a coronal streamer made up of jets of solar material. Such streamers usually extend outwards from regions of increased solar activity.

Parker, a spacecraft about the size of a small car, has since breached the outer coronal atmosphere many times, flying less than 10 million km from the Sun’s surface. Its mission is to provide data on solar winds and the heating mechanisms of the corona, and it is expected to continue relaying images to Earth from as little as 6 million km away. Its final orbit is scheduled for 2025, when it will lower itself onto the surface of the Sun.

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Star bursts

The phenomenon is called coronal mass ejection. CMEs typically take one to three days to intersect with Earth’s orbit. Fortunately, in its massive orbit around the Sun, Earth was not in the direct path of this one. The radiation still caused dramatic auroras at the Poles a few days later.

When a CME does hit Earth, it has the power to damage satellites, disrupt telecommunications networks and affect power grids. The largest such geomagnetic storm on record occurred in 1859 and caused blackouts across Europe and North America, with auroras visible as far from the Poles as Cuba and Japan.

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Campfires

In 2022, EUI sent back even more detailed images. The one above right, taken from 75 million km away, is a mosaic of 25 images taken over four hours. In its ultra-high-resolution original, it contains more than 83 million pixels.

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Home made

Titled A Year In The Sun, it won the Royal Observatory Greenwich’s 14th Astronomy Photographer of the Year competition, in the Sun category. Mukherjee, 28, is a linguistics researcher from Kolkata. He used an ordinary DSLR fitted with a telephoto lens and a solar filter, to take photos of the Sun every day over 365 days (December 25, 2020 to December 31, 2021, including six days that yielded no results due to cloud cover), to create the composite.

“June to September were particularly difficult,” he says. “The monsoon meant that I sometimes had to wait for five or six hours to get a glimpse of the sun. But I learnt to be patient, which is the key to doing something new. Even if the image hadn’t got any recognition, I would have been happy with my effort.”