The original version by this story appeared in Quanta magazine.
Our sun is the best observed star in the entire universe.
We see the light every day. For centuries, scientists have followed the dark spots on the shining face, while telescopes in space and on Earth have in recent decades closely examined the sun’s rays in wavelengths spanning the electromagnetic spectrum. Experiments have also sniffed the Sun’s atmosphere, captured clouds of solar wind, collected solar neutrinos and high-energy particles, and mapped our star’s magnetic field—or tried to, since we haven’t yet discovered the polar regions that are critical to learning. really observed. about the inner magnetic structure of the sun.
Despite all that research, however, one crucial question remained embarrassingly unresolved. On the surface the sun is a whopping 6000 degrees Celsius. But the outer layers of the atmosphere, called the corona, can be a blistering – and mind-boggling – 1 million degrees hotter.
You can see that scorching gas envelope during a total solar eclipse, as happened over part of North America on April 8. If you were in the path of totality, you might see the corona as a glowing halo around the moon-shadowed sun.
This year, that halo looked different than the one that appeared during the last North American solar eclipse, in 2017. Not only is the Sun more active now, but you were also looking at a structure that we – the scientists who study our home star – have finally discovered. come to understand. Observing the sun from afar was not good enough for us to understand what heats the corona. To solve this and other mysteries we needed a sun-grazing space probe.
That spacecraft – NASA’s Parker Solar Probe – was launched in 2018. As it orbits the sun and dips in and out of the solar corona, it has collected data that shows us how small-scale magnetic activity in the solar atmosphere brings the solar corona to a near standstill. unbelievably hot.
From surface to sheath
To understand that burning corona, we must take magnetic fields into account.
The sun’s magnetic engine, called the solar dynamo, is located about 125,000 miles below the sun’s surface. As it spins, that engine powers the sun’s activity, which waxes and wanes in periods of about eleven years. When the Sun is more active, solar flares, sunspots, and eruptions increase in intensity and frequency (as is happening now, around solar maximum).
On the Sun’s surface, magnetic fields build up at the boundaries of swirling convection cells known as supergranules, which look like bubbles in a pan of boiling oil on the stove. The constantly boiling solar surface concentrates and amplifies the magnetic fields at the edges of the cells. These enhanced fields then launch short-lived jets and nanoflares as they interact with solar plasma.
Magnetic fields can also erupt through the sun’s surface and cause larger-scale phenomena. In areas 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 loops come in different sizes, from tiny to the dramatic, flaming arcs seen during eclipses.
In other places, such loops are torn open. The Sun’s scorching corona is the source of a supersonic solar wind: streams of charged particles that form a huge protective bubble around the solar system called the heliosphere, extending far beyond the known planets. These particles carry magnetic fields, sometimes all the way into space. When that happens, the magnetic loop extends to the edge of the heliosphere, forming a so-called ‘open’ magnetic field.