Understanding how magnetic field lines will dance over the Sun is crucial to cracking a puzzling four-decade old problem as well as predicting space weather.
T.V. Venkateswaran writes for India Science Wire and tweets at @TVVen.
New Delhi: A team of Indian and British astronomers have predicted the likely shape of the Sun’s corona during the total solar eclipse on August 21. If the prediction comes true, it will mark a major scientific achievement and could facilitate more accurate forecasting of space weather in the near future.
Using computer simulations, a team led by led by Dibyendu Nandi of the Centre of Excellence in Space Sciences India (CESSI), Kolkata, has predicted the shape and structure of the solar corona, a superhot layer of gases in the star’s upper atmosphere that consists of ionised gases.
“Using data-driven solar surface flux transport model developed by our team and existing coronal models, we predict two lotus petal-like closed magnetic field structures, called helmet streamers, prominently visible in the eastern and western limb on the southern solar hemisphere,” Nandi told India Science Wire. “We also find two somewhat diffused structures in the northern hemisphere solar limbs in our simulations, with a possibility of a narrow streamer developing on the western limb. Our prediction also shows higher density of magnetic field lines in the south pole of the Sun.” (Emphasis added.)
The prediction is expected to help better forecast space weather, which impacts satellite operations, telecommunications, GPS networks and electric power grids on Earth. It could also provide clues to resolving the ‘coronal heating problem’. While the Sun’s surface has a temperature of 5,500 ºC, the corona, which begins about 2,000 km above the surface, blazes at over 1 million ºC.
Due to the Sun’s intense glare, the corona is normally not visible. But during the few minutes of a total eclipse when the Moon perfectly hides the Sun’s disc, the corona shows up in all its glory. Solar physicists travel across the world to witness total solar eclipses mainly to glimpse the corona (Latin for ‘crown’).
The shape and state of magnetic fields on the Sun determine the shape of the corona. These fields can’t be observed directly; however, the super-hot ionised gases present in the star’s atmosphere align themselves along these fields and give away their arrangement in space. Studies in the past have established faint straight structures protruding from the Sun’s north and south poles, called polar plumes, and petal like structures called helmet streamers.
“During the magnetically ‘calm’ phase, with very few sunspots on the surface, the corona has a relatively simple structure. Seen from Earth, the magnetic lines look like a pair of wings extending outward from the equatorial region of the Sun,” said Niruj Mohan Ramanujam, Chairperson of the Astronomical Society of India’s Public Outreach and Education Committee. “On the other hand, during the ‘stormy’, magnetically active phase like in 2012, with lot more active sunspots, the corona is complex and chaotic, with dense regions called streamers bursting forth in a haphazard fashion from all latitudes on the Sun.”
Predicting the coronal structure is crucial because it could help understand the different aspects of the solar wind, a steady stream of radiation and energetic particles being blown out by the Sun into the Solar System. When relatively fewer sunspots are on the surface, the solar wind blows from the star’s equatorial region at 400 km/s and from the polar regions at nearly 700 km/sec. When the sunspot activity is more pronounced, the magnetic fields become scrambled and the solar wind blows more energetically from all parts of the Sun. “If we can predict the size and shape of the solar corona, then we can infer [more details] about the solar wind, its strength and so on,” says Ramanujam.
The team came up with a computer-simulated prediction for the corona’s shape expected during the August 21 eclipse. To do so, they used older data collected during earlier eclipses, applied new knowledge of solar physics and, finally, factored the current sunspot activity in.
“This is the first time we are attempting this grand challenge and there is only another group that has attempted this before,” Nandi said. “If the prediction comes true, we will have to start moving forward and push our solar computer models to undertake other challenging tasks, like predicting the occurrence of solar storms and severe space weather.”
On March 10, 1989, a powerful explosion on the Sun had released a billion-tonne-cloud of solar plasma into space. A portion of this struck Earth’s magnetic field three days later, setting off a violent geomagnetic storm. This disturbed the electrical grid in Canada and caused a partial blackout in that country. “Just as a fall in the barometer indicates an approaching storm, predicting the shape of the corona could lead us to improve the forecasting of such intense geomagnetic storms,” Ramanujam explains.
CESSI is hosted by the Indian Institute of Science Education and Research, Kolkata. The CESSI group was joined by researchers from Durham University, the UK.
This article was originally published by India Science Wire.