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An analysis of Alfvén radius based on sunspot number from 1749 to today
Author(s) -
Goelzer Molly L.,
Schwadron Nathan A.,
Smith Charles W.
Publication year - 2014
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
eISSN - 2169-9402
pISSN - 2169-9380
DOI - 10.1002/2013ja019420
Subject(s) - solar radius , sunspot , physics , solar wind , radius , solar minimum , astrophysics , solar maximum , solar cycle , plasma , computational physics , coronal mass ejection , meteorology , magnetic field , nuclear physics , computer security , quantum mechanics , computer science
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 Alfvén radius where the Alfvén 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 Alfvén radius to different locations depending upon solar activity. We show that the average Alfvén radius is correlated with the sunspot number and moves systematically from ∼ 15 at solar minimum to 30 R S at solar maximum. The analysis shown here does not predict movement of the Alfvén radius during the recent protracted solar minimum. We project the average Alfvén 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.