The solar wind that streams from stars like our Sun can collide with itself, creating shock waves that batter planetary atmospheres with fast particles. On Earth, these “corotating interaction regions” cause geomagnetic storms, but their effects on the thousands of known exoplanets remain almost entirely unstudied. This project fills that gap by modelling how these stellar wind collisions vary with a star’s mass and age, using magnetic maps of over 100 stars to pinpoint where fast and slow wind streams emerge and crash together. A proof-of-concept simulation of the young Sun-like star kappa Ceti already showed pressure pulses 1,300 times stronger than the background stellar wind. The research will also track how the Sun’s own interaction regions evolved over its lifetime, comparing that timeline with key stages in Earth’s atmospheric history. This is fundamental science: it asks how common and how powerful these stellar wind collisions are across the galaxy. The immediate payoff is a deeper understanding of the space weather environments that shape exoplanet atmospheres—knowledge that could eventually inform which planets might retain habitable conditions.
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Circe: Co-rotating Interaction regions colliding with exoplanets On the present-day Earth, geomagnetic storms occur when fast and slow streams of the solar wind collide, generating shocks and producing showers of fast particles that penetrate into, and interact with, the Earth’s atmosphere. These corotating interaction regions permeate the solar wind and their cumulative effect on planetary atmospheres can exceed that of the more powerful, but less frequent, flare-related coronal mass ejections. Although these interaction regions are known to be important in the solar wind, and are well-studied in massive stars, there have been no large-scale studies of their frequency and power in solar-type stars. This project will extend our understanding of these features to the many other types of stars now known to host a range of exoplanets whose upper atmospheres may be vulnerable to the heating induced by this geomagnetic activity. We will develop semi-analytical models in a broad-based study that will characterise these corotating interaction regions as a function of stellar mass and age. We will build on the recent growth in space-based in-situ studies of the solar wind that provide a wealth of data on these interaction regions and the impact of their accelerated particles on the Earth, Mars and Venus. For other stars, we will use spectropolarimetric studies that provide magnetic maps of the surfaces of > 100 stars, many of which are known planet hosts. These maps show the locations from which fast and slow wind streams emerge and hence determine where they collide. Young, rapidly-rotating stars are likely to produce the most powerful interactions. Indeed, using one of these maps as an input, a proof-of-concept MHD simulation of the young Sun kappa Ceti predicted corotating interaction regions whose pressure pulses are 1300 times greater than the background stellar wind. We will also use rotational evolution models to evolve a solar-mass star in time from its pre-main sequence phase to the present day. This will allow us to determine at what point in its history the solar wind hosted the most powerful corotating interaction regions. We will compare this with the known stages in the evolution of the Earth’s atmosphere, and estimate the distribution of energies of the particles accelerated by these interaction regions.
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