Corotating Interaction Regions (CIRs): impacts with exoplanets

Abstract

Corotating interaction regions (CIRs) are compressions that form in stellar winds when streams of different speeds collide. They form an Archimedean spiral around the star and can compress any exoplanetary magnetospheres they impact. They may also steepen into shocks capable of accelerating particles to high energies. We model the frequency and strength of these CIRS for stars of spectral types F-M. We show that the minimum radius, $r_{CIR}=\Delta \phi u_{slow}/\Omega$, at which CIRs form varies strongly with the rotation rate (and hence age) of the star. For some exoplanets, such as those in Earth or Mars orbits, CIRs can form within the exoplanet's orbit at all stellar rotation rates, depending on the angular size of the fast wind segment ($\Delta \phi$). These exoplanets will experience CIR impacts at all stellar ages. However, for closer-in orbits such as Mercury or Venus, this may only be the case at higher stellar rotation rates. Both the frequency and impact of CIRs depend on the stellar rotation rate. For exoplanets with $P_{orbit}\gg P_*$, CIR impacts lasting for a time $\Delta t$ raise the exoplanetary outflow rate by a factor $R$. If $P_*\leq N\Delta t$ the CIR pulses overlap in time, whereas if $N\Delta t < P_* \leq N\Delta t(R+1)$, the planet experiences discrete pulses of compression and relaxation and the CIR-related outflow is more than 50$\%$ of the total. For $P_* > N\Delta t(R+1)$ the pulses are less frequent, and contribute less than $50\%$ of the total outflow.