Summary
Using in-situ spacecraft observations, we are able to track the rate of angular momentum loss of the Sun (i.e. the average torque) on the decennial timescale. The average torque we measure from the solar wind is lower than the torque required to model stellar evolution, but due to stellar spindown, the value must have been higher in the past. How far back do we have to go to find a match?
Sunspot numbers and geomagnetic indices let us reach the centennial timescale. We use these measurements to construct a relation between the torque and the open magnetic flux, which can be measured from the solar wind and reproduced from stellar models. Finally, we are able to use cosmogenic radionuclides formed from interactions between the solar wind and galactic cosmic rays to map out the solar magnetic activity, and hence the torque, over the past several millennia. We find that the average torque during the last nine millennia is surprisingly similar to the average value over the last two decades, meaning the low value seen today is due to even longer term changes.
Published in The Astrophysical Journal, Volume 883, Number 1. Adam J. Finley, Siddhant A. Deshmukh, Sean P. Matt, Mathew Owens, and Chi-Ju Wu. (2019) Solar Angular Momentum Loss over the Past Several Millennia