Solar Angular Momentum Loss Over the Past Several Millennia

Abstract

The Sun and Sun-like stars lose angular momentum to their magnetised stellar winds. This braking torque is coupled to the stellar magnetic field, such that changes in the strength and/or geometry of the field modifies the efficiency of this process. Since the space-age, we have been able to directly measure solar wind properties using in-situ spacecraft. Furthermore, indirect proxies such as sunspot number, geomagnetic indices, and cosmogenic radionuclides, constrain the variation of solar wind properties on centennial, and millennial timescales. We use near-Earth measurements of the solar wind plasma and magnetic field to calculate the torque on the Sun throughout the space-age. Then, reconstructions of the solar open magnetic flux are used to estimate the time-varying braking torque during the last nine millennia. We assume a relationship for the solar mass loss rate based on observations during the space-age which, due to the weak dependence of the torque on mass loss rate, does not strongly affect our predicted torque. The average torque during the last nine millennia is found to be 2.2 × 1030erg, which is comparable to the average value from the last two decades. Our dataset includes grand minima (such as the Maunder Minimum), and maxima in solar activity, where the torque varies from ∼ 1 − 5 × 1030erg (averaged on decadal timescales), respectively. We find no evidence for any secular variation of the torque on timescales of less than 9000 years.