The origin of the "slow" solar wind: an enigma resolved!

How does the solar wind originate? Despite decades of observations and modeling, the causes of this phenomenon, which originates at the surface of the Sun, and the mechanisms that release it into space, are not well understood. Using data from ESA's Solar Orbiter probe, an international team, including an astronomer from the Observatoire de Paris - PSL, presents a lead that appears in the journal Nature Astronomy on May 28, 2024 (Yardley et al).

An international scientific team (United Kingdom, Spain, United States, Italy, Ireland, France, Netherlands, Switzerland, Germany) analyzed the corona and the solar wind, based on data sent by the Solar Orbiter probe following its first close pass to the Sun in March 2022.


By combining solar images and in situ measured data, the international team managed to more clearly identify the origin of the slow solar wind.

What is the solar wind? The solar wind is the continuous flow of charged particles (forming a plasma) from the Sun. It originates from a layer of the solar atmosphere called the solar corona. This plasma fills the entire heliosphere up to distances of 93 million miles (150 million km). Near Earth, it typically travels between 185 and 500 miles per second (300 and 800 km per second). Below 310 miles per second (500 km per second), the wind is classified as "slow."

Explanation in images

Researchers analyzed global views, taken in extreme ultraviolet, of the very hot solar corona (a few million degrees). The brightest regions are the hottest; they are called "active regions." The darkest regions of the solar corona are known as "coronal holes"; these are the sources of the fast solar wind.


Close satellite views (such as Figure 1b) show at the base of the corona, bright regions, pointed by dark gray arrows. Researchers observed upward movements of the plasma there (Figure 1c), identifying these active areas as the sources of the slow solar wind.

Based on these observations, scientists modeled, using calculations, the magnetic field lines that emanate from them. Represented in color (Figure 1a), the lines extend outward, far from the Sun; they are open.

Their calculations thus provided a theoretical prediction of the link between the "source" coronal region and the in situ measurements of the solar wind.

A prediction confirmed by in situ measurements

The chemical composition of the solar wind remaining unchanged during its journey can be used as a marker to determine the specific origin of a portion of the solar wind. Thus, in situ composition measurements were taken, such as the iron to oxygen abundance ratio.

Spectroscopic analysis results showed that the iron to oxygen ratio is different inside the coronal holes and active regions, consistent with in situ measurements in the associated regions of the solar wind.

Physical differences between the "source" regions are also confirmed with other in situ measurements, such as the ionization level (the number of electrons lost by an atom).

This indicates that the solar source of the slow wind is hotter than that of the faster wind. This is consistent with an origin associated with active regions. Such results allow constraining the physics involved in the acceleration of the solar wind.

What mechanisms are at work?

If this plasma is trapped by the closed magnetic field (a closed "magnetic bottle"), how could it escape to form the slow solar wind?

In fact, these active regions are surrounded by an open magnetic field (as shown in Figure 1a), where the "magnetic bottle" is open toward interplanetary space. The slow solar wind comes from a small region where open and closed magnetic fields coexist. There, through a process called "magnetic interchange reconnection," some plasma from the active region can escape and form the slow solar wind.

This notably explains the difference in composition and ionization levels between slow and fast winds.

About the Solar Orbiter mission In 2020, the European Space Agency (ESA), with support from NASA, launched the Solar Orbiter mission. This satellite is a complex scientific laboratory equipped with ten scientific instruments. These enable a wide range of data acquisition, from in situ (local) measurements of the solar wind plasma to images taken with imagers and spectrographs, to date the closest and most detailed of the Sun. The ESA Solar Orbiter mission is an international collaboration involving scientists and institutions from around the world working together.

Reference:
Multi-source connectivity as the driver of solar wind variability in the heliosphere,
Nature Astronomy,
Yardley S., Brooks, D., D'Amicis R., Owen C., Long D., Baker D., Démoulin, P., Owens, M.J. et al.