The sun’s corona, or outer atmosphere, is a few hundred times as hot as the solar surface. Astronomers aren’t sure why. But many suspected that spicules may play a role. These flame-like tendrils undulate across the sun’s surface. What causes them had been a mystery, too — until now. New data suggest those spicules develop as the sun’s magnetic fields realign themselves.
The wispy spicules rise thousands of kilometers up from the sun’s surface. They carry plasma — ionized gas — up into the corona. And that appears to pump heat into the corona.
Astronomers had suspected the sun’s turbulent magnetic field might trigger the spicules. But they weren’t sure. So one team decided to explore the idea.
Solar physicist Tanmoy Samanta works in China at Peking University in Beijing. He and his colleagues pointed toward the sun a telescope at Big Bear Solar Observatory in California. It snapped images as spicules formed. At the same time, they measured the sun’s magnetic field in the same area. The team discovered that pockets of the local magnetic field often reversed course and pointed in the opposite direction from the prevailing field. Within minutes of this happening, thickets of spicules often emerged.
Those plasma filaments lasted for just minutes.
Opposing magnetic fields create a tension. But it gets resolved when the fields break and realign. Samanta’s group suggests that that the energy released in this “magnetic reconnection” creates the spicules.
“The magnetic field energy is converted to kinetic and thermal energy,” says study coauthor Hui Tian. He’s another solar physicist at Peking University. Thermal energy is heat. Kinetic refers to the energy of motion. Here, Tian says, “The kinetic energy is in the form of fast plasma motion — jets, or spicules.”
The researchers shared their new findings November 15 in Science.
The team also probed whether that energy released by the realigned magnetic fields made it into the corona. For this, they pored over images taken at the same time by NASA’s orbiting Solar Dynamics Observatory. The images revealed a glow from charged iron atoms. These were directly over the spicules. That glow, Tian says, means the plasma reached roughly 1 million degrees Celsius. Whether that’s enough to sustain scorching temps throughout the corona, however, remains unknown.
“Their observations are amazing,” says Juan Martínez-Sykora. He’s a solar physicist at the Lockheed Martin Solar & Astrophysics Laboratory. It’s in Palo Alto, Calif. Capturing this level of detail is difficult, he notes. After all, individual spicules are relatively small. They also come and go quickly. Martínez-Sykora does caution, though, that the magnetic-reconnection story needs to be checked with computer models or with more observations. As it stands, it remains only a reasonable guess, he says.