Summary
Over the course of a star’s lifetime, solar winds strip away its angular momentum. The angular momentum loss rate (i.e. the torque) depends on the magnetic activity of the star as well the star’s mass loss rate. Normally, the surface magnetic field strength of the star is used to determine the torque exerted by the solar wind. However, the surface magnetic field strength is reliant on the multipolar configuration of the magnetic field, i.e. its dipolar, quadrupolar and higher order components. For stars other than our Sun, then, these quantities need to be derived to accurately determine the stellar spindown with this formulation.
Instead, the torque can be formulated using the open magnetic flux - a quantity invariant of the magnetic field geometry. It can be measured from the solar wind, but we do not have direct measurements of the open flux for other stars. But, using the open flux lets us avoid computationally expensive MHD simulations to determine the torque on a star. Through this, we are able to use the Sun to constrain open magnetic flux and determine the spindown of many stars in an efficient manner.