A new theory proposes that millions of tiny whirlpools of plasma on the Sun's surface could explain why the star's corona burns at temperatures millions of degrees hotter than its surface.
Katariina Nykyri of Embry-Riddle Aeronautical University published research in The Astrophysical Journal Letters arguing that Kelvin-Helmholtz vortices, structures a few dozen kilometers across, drive coronal heating. The Daniel K. Inouye Solar Telescope on Maui captured the first high-resolution images of these vortices in August.
Kelvin-Helmholtz waves form when fast-moving plasma passes slower, magnetized plasma. They contain vast amounts of mechanical energy. As the resulting vortices churn across the solar surface, their kinetic energy transfers into magnetic fields, converting mechanical motion into magnetic stress.
Nykyri calculated that each vortex carries around two septillion ergs of energy. Millions of these vortices spin up and dissipate every few minutes across the Sun's 1.4-million-kilometer surface.
When vortices dissipate, they squeeze nearby magnetic field lines together, forming ultra-thin current sheets a few centimeters to meters wide. Once squeezed past a critical point, the field lines snap and rearrange in a process called magnetic reconnection. This converts stored magnetic energy into heat and high-speed plasma jets that compose the corona.
Nykyri's calculations show that only five to eight percent of the mechanical energy stored in vortices needs to reach the corona to replenish heat otherwise radiated away. In darker coronal holes, at most 21 percent of vortex energy could maintain coronal temperatures.
The theory explains why the corona heats more efficiently than the surface. When magnetic reconnection occurs at the solar surface, energy distributes among trillions of neutral atoms, raising each atom's temperature only slightly. In the corona, where atoms are far less dense, the same energy release increases individual atom temperatures much more dramatically.
Scientists still must track wave transmission and polarization through the chromosphere to confirm whether sufficient energy reaches it to sustain coronal temperatures as Nykyri suggests.