The "Other" Centuries-Long Record of Solar Magnetic Activity Cycles: Lessons From the Stars

Abstract

Studies of space climate and solar magnetic activity and studies of stellar dynamos and magnetic cycles are separated by discipline and by the quality of the data we have to work with: the Sun gives us detailed knowledge of a single star, while the stars give us comparatively sparse data across a range of (sometimes poorly known) masses and ages. This review, a collaboration among scientists with expertise in stellar astronomy, space climate, and solar physics, seeks to provide a guide to what the Sun's and stars' magnetic activity records can tell us about how stellar dynamos operate on decadal, millennial, and stellar-evolutionary timescales. It reviews the historical Solar sunspot record, terrestrial cosmogenic isotope records, and decades-long chromospheric activity measurements of dozens of sun-like stars across a range of masses, ages, and metallicities near the solar value. Particular emphasis is placed on the Maunder minimum, and the hunt for similar events in the stellar record, especially the recently discovered grand minimum event ongoing in the star HD 166620. These records show that solar-like dynamos are a common feature of Sun-like stars, and magnetic grand minimum events akin to the Maunder minimum might be very rare, with only one identified in 3,000 star-years of magnetic activity records. Together, solar and stellar dynamo studies are bringing a still-fuzzy picture into focus in which stellar magnetic activity weakens as stars spin down over billions of years towards a critical Rossby number near the solar value, at which point stellar cycles begin to be frustrated. These periods of frustration manifest as magnetic grand minima, and increase in frequency until they become permanent and stars enter a period of very low and constant magnetic activity.