Energy transfer in the solar wind-magnetosphere: Long-term fluctuations and intermittency

In this paper, we investigate the transfer of energy from the solar wind to the terrestrial magnetosphere. We make a selected brief review of the links between long-term fluctuations of the solar wind variables (velocity, magnetic field) and several multiscale magnetospheric processes. We emphasize those aspects that describe the non-linear magnetospheric response to solar wind changes. The variations of the flux of relativistic electrons with solar wind velocity over a solar cycle are discussed. Experimental evaluation of the substorm energy budget at solar maximum and minimum is also reviewed. The paper provides also a brief review on the intermittent fluctuations observed in the solar wind and the magnetosphere. The solar wind intermittency may be viewed as the hallmark of a turbulent process that sometimes departs from the traditional fluid or MHD turbulence. Recent analysis of geomagnetic indices and their correlation with the solar wind show that large amplitude, intermittent fluctuations have probabilities larger than for a Gaussian distribution, at smaller temporal scales. In the magnetosphere, particularly in the plasma sheet and the cusp the magnetic field fluctuations also show signs of intermittency that could be associated to a multifractal structure of the energy transfer. The dynamical aspects reviewed in this paper contribute to a picture of the magnetosphere as a non-linear system, driven out of equilibrium and responding to solar forcing on a broad range of spatio-temporal scales.