About the Slow Wind

The origins and release mechanisms of stellar winds are long-lasting open challenges in astrophysics. Stellar winds play a fundamental role in the long-term evolution of stars and the habitability of their orbiting planets. In the solar case, the wind is observed in at least two states, fast and slow winds, that differ in their bulk properties and composition, pointing to different coronal origins. A theoretical explanation for the slow wind must explain both its variable bulk properties and its peculiar composition.

This includes the measured high charge states of minor ions, the abundance variation of Helium during the solar cycle and the high abundance of elements with low First Ionisation Potential (so-called FIP effect) reaching four times the photospheric abundance.

SLOW_SOURCE is a comprehensive research project that will use current and upcoming observations as well as completely novel models of the solar atmosphere to determine the origin of the slow wind.

Among other things we will develop plasma transport models coupling major and all known important minor constituents along realistic coronal magnetic field lines. This model will be the first of its kind producing synthetic observations (spectroscopy, imagery) and expected in situ signatures directly from the modelled minor constituents. Combined with data from space and ground-based observatories, our new multi-species, multi-temperature 3-dimensional modelling of coronal plasma will provide new ways to infer the properties of stellar winds and tools to study the fundamental transport and heating processes of stellar plasmas. Determining the enigmatic release mechanism(s) of the slow solar wind constitutes a key objective of the Parker Solar Probe and Solar Orbiter missions that are obtaining revolutionary observations of the solar wind.

The solar wind is an ionized gas called a plasma that has been measured in situ near 1 Astronomical Unit (AU) for several decades. It has also been detected more recently with remote-sensing instruments called heliospheric imagers. The speeds of the solar wind measured near 1 AU typically range from 300 to 800 km/s, proton temperatures take values between 100 000 K in the slow wind to typically twice that temperature in the fast wind.

Today there is no doubt that the fast solar wind measured in situ originates near the center of coronal holes at the Sun. The fast solar wind streams out along magnetic fields connecting regions deep inside coronal holes the interplanetary medium. The origin of the slow wind is still unknown and likely more complex, consisting of multiple source components.

The composition of the solar wind is likely to be a key to unlock the mystery of the slow solar wind. The solar wind is composed mainly (95%) of protons and electrons but it carries along heavier ions as well (iron, oxygen, carbon, etc...). This composition cannot change between the region where the solar wind forms and where it is measured in situ.

So changes in composition measured far away from the Sun reflect changes that have happened when the solar wind was born. Compositional measurements confirm that the fast solar wind is born in coronal holes because we find the same composition in situ than detected remotely in coronal holes. The slow solar wind on the other hand has a very different composition. It is enriched in heavy elements that ionize easily (elements with low First Ionisation Potential: FIP), this strange composition (FIP effect) is not typically measured inside coronal holes but rather along magnetic loops.

No model has yet explained at the same time the bulk and compositional properties of the SSW. A successful model should explain not only the basic bulk properties of the SSW but also the strong solar cycle variation in the abundance of alpha particles, the high ionisation states of heavy ions and the FIP effect.The ERC SLOW_SOURCE project aims at providing a first 3-D model of coronal ions solving for the coupled transport of important major and minor constituents along dynamic magnetic field lines.

The origin of the SSW is highly debated, however it is possible to trace back the SSW to broad coronal structures called streamer rays that are clearly visible during solar eclipses. Below these rays lie dome-shaped structures, called helmet streamers that enclose magnetic loops surrounded by open magnetic field lines along which the SSW can escape out to the interplanetary medium.

The presence of complex magnetic fields in the vicinity of streamers could explain why they are observed as highly dynamic structures. There are at least two possible sources for the SSW. One theory suggests that the wind forms, like the fast solar wind, from coronal holes. This is called the ‘quasi-steady state’ theory for the origin of the SSW. It assumes the wind escapes along the open magnetic flux tubes that originate on the boundary of coronal holes. This theory could explain why a fraction of the slow solar wind exhibits similar turbulence properties than the fast solar wind. Solar wind models testing that theorycan reproduce a broad range of remote-sensing observations and in situ measurements. The SLOW_SOURCE project will test if this theory could explain the different compositions of the fast and slow solar wind (see Fast or Slow part).

The alternative ‘dynamic theory’ suggests that the slow solar wind actually comes from magnetic loops in the solar atmosphere. However magnetic loops are supposed to confine the solar gas in the corona. So how could that plasma escape into the solar wind? One possibility is that solar plasma somehow manages to transit from closed to open magnetic field lines via magnetic reconnection. The formation of a wind via sporadic releases of plasma initially confined to loops through magnetic reconnection is radically different to the alternative picture of a ‘steady-state’ wind accelerated along quiescent open flux tubes. The SLOW_SOURCE project aims at testing these two theories of the slow solar wind using a combination of observations and novel numerical models.