The original version on this story appeared in Quanta Magazine.
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 that dot its glowing face, while in recent decades telescopes in space and on Earth have scrutinized the sun’s rays at wavelengths spanning the electromagnetic spectrum. The experiments also sniffed the Sun’s atmosphere, captured the solar wind clouds, collected solar neutrinos and high-energy particles, and mapped our star’s magnetic field—or tried to, since we have yet to actually observe the polar regions, which are key to learning about the interior. magnetic structure of the sun.
Despite all this research, however, one crucial question remained embarrassingly unresolved. The sun has a hot 6000 degrees Celsius on its surface. But the outer layers of its atmosphere, called the corona, can be a blistering — and confusing — 1 million degrees hotter.
You can see this burning gas shell during a total solar eclipse, as it did on April 8 over part of North America. If you were in the path of totality, you could see the corona as a glowing halo around the moon-shadowed sun.
This year, that halo looked different than it did during the last eclipse in North America in 2017. Not only is the Sun more active now, but you were looking at a structure that we—the scientists who study our home star—have to finally understand. Observing the sun from afar was not good enough to know what was heating the corona. To solve this and other mysteries, we needed a space probe that grazed the sun.
This spacecraft – NASA’s Parker Solar Probe – was launched in 2018. As it orbits the sun, dipping in and out of the solar corona, it has collected data that shows us how small-scale magnetic activity in the solar atmosphere makes the solar corona almost unimaginable hot.
From the surface to the shell
To begin to understand this burning corona, we need to look at magnetic fields.
The sun’s magnetic engine, called the solar dynamo, is located about 200,000 kilometers below the sun’s surface. As it develops, this engine drives solar activity, which waxes and wanes over periods of about 11 years. When the sun is more active, solar flares, sunspots, and flares increase in intensity and frequency (as they do now, near solar maximum).
On the sun’s surface, magnetic fields build up along the boundaries of shaking convective cells known as supergranules, which look like bubbles in a pan of boiling oil on the stove. The constantly boiling solar surface concentrates and strengthens these magnetic fields at the edges of the cells. These enhanced fields then fire transient jets and nanobursts as they interact with the solar plasma.
Magnetic fields can also erupt through the solar surface and cause larger-scale phenomena. In regions where the field is strong, you see dark sunspots and giant magnetic loops. In most places, especially in the lower solar corona and near sunspots, these magnetic arcs are “closed”, with both ends attached to the sun. These closed circles come in a variety of sizes, from miniature to the dramatic, blazing arcs seen during eclipses.
In other places, such loops are torn. The scorching solar corona is the source of the supersonic solar wind – streams of charged particles that form a massive protective bubble around the solar system called the heliosphere that extends far beyond the known planets. These particles carry magnetic fields with them, sometimes as far as deep space. When this happens, the magnetic loop extends to the edge of the heliosphere, forming a so-called “open” magnetic field.
