European Solar Orbiter Traces Origin of Solar Wind – Sky & Telescope

Tiny “picoflare” jets might be the origin of the solar wind.

That’s what a team led by Lakshmi Pradeep Chitta (Max Planck Institute for Solar System Research, Germany) found, after imaging these mini-jets in 2023 with the European Space Agency’s Solar Orbiter. Now, Chitta’s team presents new evidence in Astronomy & Astrophysics that these jets indeed seed the large-scale particle stream emanating from the Sun.

The solar wind, a constant stream of protons and electrons coming from the Sun, roams the solar system at speeds of up to 2 million miles per hour (800 km/s). At Earth, it can spark colorful auroras as well as harm satellites, disrupt power lines, and sever radio communications. Yet, even though it was discovered more than six decades ago and its effects on Earth are well-documented, the particle wind’s precise origin is still unknown.

Solar physicists know that the fastest wind particles emanate from coronal holes — huge gaps in the corona, the outer solar atmosphere, where magnetic field lines stretch outward into space instead of curling back toward the Sun. These field lines act like highways along which charged particles escape, forming the fast solar wind. Another, slower wind component has been harder to track down.

But what throws particles into these open field lines in the first place? The Solar Orbiter was launched in 2020 to find out. It comes as close to the Sun as 42 million kilometers (26 million miles), or one-third of the Earth-Sun distance. At closest approach in 2022 and 2023, the orbiter’s Extreme Ultraviolet Imager peered deep into coronal holes that showed up at the time. In these regions near the Sun, Chitta’s team found what they’ve previously termed picoflares, each one a twister of plasma only a few hundred kilometers wide and lasting hardly longer than a minute.

Picoflares’ energy output is 1,000 times weaker than even the faintest nanoflares, another type of small jet seen in the solar corona. (Hence the name, since pico indicates an amount 1,000 times smaller than nano.) Still, the observations showed that picoflares can accelerate particles up to 100 km/s, and the jets are so ubiquitous that combined they might provide the particles of the fast solar wind.

To test this, the scientists used the Solar Orbiter’s magnetic field maps to trace the connection between magnetic fields at the spacecraft back toward the Sun. Then they monitored the properties of the solar wind particles near the spacecraft, finding that those particles had taken magnetic routes tracing back to the same region where the jets are located.

Additionally, Chitta and his team found evidence that both fast and slow solar wind components came from the same areas. That’s surprising, as the connection of the slow wind and coronal holes hadn’t yet been fully established.

“[This] is very nice work,” comments Michael Hahn (Columbia University) who was not involved in the study. “The objective of Solar Orbiter was to measure the solar wind and the sources of the solar wind simultaneously, and this paper seems like a great example of that.” Hahn thinks the results align with recent findings from NASA’s Parker Solar probe, which found evidence that the fast solar wind streams form when open and closed magnetic field lines on the Sun snap and reconnect into new configurations.

“The . . . jets . . . are flows of material produced by reconnection,” Hahn adds. “So, these results are about different aspects of the same thing. In both cases, an emerging theme is that the solar wind is not a smooth continuous flow but instead appears to be emitted in small bursts.”

Parker and Solar Orbiter complement each other: While NASA’s spacecraft comes much closer to the Sun (only 6.2 million km from the Sun’s visible surface), ESA’s orbiter has a wider view from its Sun-facing cameras.

The complementary data sets will be helpful in explaining the full picture of the solar wind. Gregory Fleishman (New Jersey Insitute of Technology and Institute for Solar Physics, Germany) cautions that scientists still don’t understand how the observed small-scale dynamics of the jets determine the large-scale properties of the solar wind. “Nevertheless,” he says, “this study offers very important clues which will ultimately help to disentangle the mystery of the solar wind origin.”