Abstract/Summary

We study the contribution of the solar wind Poynting flux Ssw, to the total power input into the magnetosphere. The dominant power delivered by the solar wind is the kinetic energy flux of the particles which is larger than Ssw by a factor of order Ma^2, where Ma is the Alfvén Mach number The currents J flowing in the bow shock and magnetosheath and the electric field E of the solar wind give regions where J.E <0, which are sources of Poynting flux, generated from the kinetic energy flux. For southward IMF, E is duskward and the currents in the high-latitude tail magnetopause are also sources of Poynting flux. We show transfer of kinetic energy into the magnetosphere is less efficient than direct entry of Ssw by a factor Ma. Because Ma is typically of order 10, this means that although the power density in the solar wind due to Ssw is typically only 1%, it is responsible for of order 10% of the energy input to the magnetosphere. To investigate the effect of this, we add a term to the solar wind-magnetosphere energy coupling function that allows for Ssw which increases the correlation with the geomagnetic am index for 1995-2017 (inclusive) from 0.908 to 0.924 for 1-day averages and from 0.978 to 0.979 for annual means. The increase for means on daily or smaller timescales is a small improvement but is significant (at over the 3-sigma level), whereas the improvement for annual or Carrington-rotation means is not significant.