Thomas Zurbuchen in Quanta:
Our sun is the best-observed star in the entire universe.
We see its light every day. For centuries, scientists have tracked the dark spots dappling its radiant face, while in recent decades, telescopes in space and on Earth have scrutinized sunbeams in wavelengths spanning the electromagnetic spectrum. Experiments have also sniffed the sun’s atmosphere, captured puffs of the solar wind, collected solar neutrinos and high-energy particles, and mapped our star’s magnetic field — or tried to, since we have yet to really observe the polar regions that are key to learning about the sun’s inner magnetic structure.
For all that scrutiny, however, one crucial question remained embarrassingly unsolved. At its surface, the sun is a toasty 6,000 degrees Celsius. But the outer layers of its atmosphere, called the corona, can be a blistering — and perplexing — 1 million degrees hotter.
More here.