An analysis of Alfvén radius based on sunspot number from 1749 to today

The Solar Probe Plus mission now under construction will provide the first in situ measurements from inside the orbit of Mercury. The most critical part of that mission will be measurements from inside the Alfven radius where the Alfven speed exceeds the wind speed and the physics of the solar wind changes fundamentally due, in part, to the multidirectionality of wave propagation. In this region waves from both sunward and antisunward of the observation point can effect the local dynamics including the turbulent evolution, heating, and acceleration of the plasma. While the location of this point can change with solar wind conditions, we ask the question of whether there is a systematic dependence on the solar cycle that moves the average Alfven radius to different locations depending upon solar activity. We show that the average Alfven radius is correlated with the sunspot number and moves systematically from ∼ 15 at solar minimum to 30 RS at solar maximum. The analysis shown here does not predict movement of the Alfven radius during the recent protracted solar minimum. We project the average Alfven radius backward and forward in time using the monthly sunspot record to attempt a better understanding of the historical record and predict the behavior of this point during the coming solar cycle.