Magnetised stellar winds are one of the main forms of magnetically driven stellar activity on cool stars (M<1.3M). They can protect planets from cosmic radiation, are responsible for mass- and angular momentum loss and carve bubbles out of the interstellar medium. They also interact with the atmospheres and magnetospheres of exoplanets with repercussions for planetary habitability.
Much of what we know about magnetised winds originates from the solar research community. In situ measurements have allowed for the study of the speed, composition and magnetic properties of the solar wind as well as possible heating and driving mechanisms. New missions like Parker Solar Probe and Solar Orbiter are only set to improve our understanding on these topics.
Studying other cool stars lets us examine how wind properties and driving physics may depend on parameters such as stellar mass, rotation or age. However, the winds of other stars are extremely difficult to probe. It is usually the global wind properties, e.g. total mass-loss rate, which are investigated since in situ measurements are not possible. However, even global properties are difficult to ascertain; direct detection of the free-free radio emission associated with stellar winds are still below the detection limits of current instruments and other methods for estimating mass-loss rates are still uncertain.
In this session, we aim to improve our understanding of magnetised winds using both theory and observation. We are especially keen to incorporate solar and stellar perspectives. The key questions we aim to address are
Much is still unknown about how winds are heated and accelerated. This is true even for the solar wind. However, Parker Solar Probe and Solar Orbiter, have both been launched in recent years and will help tackle this problem. Parker Solar Probe has already travelled closer to the Sun than any other previous spacecraft while Solar Orbiter has taken the closest images of the Sun to date. These missions are exploring the regions where solar wind acceleration is thought to occur in unprecedented detail and will shed light on the physical processes responsible for that acceleration. Theoretical and numerical studies will also be key to interpret and provide context to the observations.
To date, the most successful observation based method of determining mass-loss rates is the astrospheric absorption of lyman-alpha method. However, estimates from this method are model dependent and constrained to targets that are relatively close to Earth. Exploring other methods of observationally estimating stellar mass-loss rates, that can complement the lyman-alpha method, is therefore key. For instance, in recent years, new innovative methods of estimating mass-loss rates using exoplanet-wind interactions or prominence ejections as probes of the overall mass-loss rate have been developed. By having a broad range of methods to estimate mass-loss rates, more insight can be gained into how winds and mass-loss rates differ on stars of different masses and rotation rates.
One of the principal effects of stellar winds is to spin down the central star. However, the way in which stars lose angular momentum over their life times is still not well constrained due in part to the previously mentioned uncertainties in determining stellar mass-loss rates. An intriguing possibility that has emerged in recent years is the use of rotation period evolution to constrain spin-down torques, and hence mass-loss rates, over evolutionary time-scales. These types of estimates are useful since stellar rotation periods are comparatively easy to measure unlike mass-loss rates.