Origin of Fast and Slow Solar Wind Discovered
Data from the ESA’S Solar Orbiter show tiny plasma jets in the source regions of the solar wind.
Tiny plasma jets on the Sun drive both the fast and the slow solar wind, as can be seen from data collected by ESA’s Solar Orbiter spacecraft. An international team of researchers led by the Max Planck Institute for Solar System Research (MPS) in Germany has succeeded in relating images of tiny plasma jets in coronal holes, dark regions in the hot atmosphere of the Sun, to in situ measurements of the solar wind. The jets are approximately 100 kilometers wide, last for approximately a minute and hurl charged particles into space at speeds of about 100 kilometers per second. Until now, it was assumed that different phenomena were responsible for both types of solar wind. Furthermore, source regions of the slow solar wind could not be determined precisely. The new results are published today in the journal Astronomy & Astrophysics.
Tiny Plasma Jets as Seen by Solar Orbiter
In 2023, a group of researchers led by the MPS discovered tiny plasma jets near the Sun's south pole. They were visible in images taken by the Extreme-Ultraviolet Imager (EUI) telescope on board the Solar Orbiter spacecraft. The data analysis suggested that our star does not eject the solar wind as a uniform stream of particles. Rather, it fluctuates on the smallest scales. However, the researchers initially linked the newly discovered phenomenon only to the fastest solar particles. The new study now shows that the tiny jets also drive slower particles.
Into space at hypersonic speed
The solar wind is an unceasing stream of charged particles from the Sun that travels to the distant edges of the Solar System. It consists of two components: the fast solar wind and the slow solar wind. The particles of the fast solar wind race through space at hypersonic speeds of up to more than 500 kilometers per second. Their source regions appear as dark areas, so-called coronal holes, in images of the solar corona. At these locations, the field lines of the solar magnetic field extend far into interplanetary space. The slow solar wind reaches speeds of about 300 kilometers per second. Until now, its origin had been more elusive.
For the current study, the researchers now evaluated EUI images of coronal holes together with in situ measurements of the solar wind and magnetic fields taken by Solar Orbiter's Solar Wind Analyzer (SWA) and Magnetometer (MAG). The data were collected in October 2022 and April 2023, when the spacecraft reached its closest point to the Sun on its elliptical orbit. At both times, Solar Orbiter was approximately 50 million kilometers from our star. This is roughly a third of the distance between Earth and the Sun. This proximity allows for particularly high-resolution images of the solar corona.
When analyzing the data, the researchers discovered that not only the fast solar wind, but also parts of the slow solar wind originate in coronal holes. “We were very surprised to see that the same tiny plasma jets appear to be driving both the fast and the slow solar wind,” says Lakshmi Pradeep Chitta [KDB2]from MPS, who led the new study. “Previously, we had assumed that different processes are at work,” he adds.
During its next close flybys of the Sun in March and September of this year, Solar Orbiter will collect further data. These will help to understand more precisely how the small plasma flows hurl the solar wind particles into space.
