Solar wind energy transfer through the magnetopause of an open magnetosphere

An expression for the total power P T transferred from the solar wind to an open magnetosphere is derived. The power is transmitted through an ‘open’ magnetopause with a nonzero normal component of the magnetic field, which is identified as a rotational discontinuity. The total power P T consists of (1) the power P EM representing the electromagnetic energy transfer and (2) the power P KE representing the rate of kinetic energy carried by particles penetrating into the magnetosphere. It is found that P EM ≃ V SW B SW ψ, P KE ≃ (½ M A ‐ 1) P EM and P T ≃ ½ M A P EM , where V SW , B SW , and M A are the velocity, magnetic field, and the Alfvén‐Mach number in the solar wind, respectively, and Ψ is the open magnetic flux in the magnetosphere. The Alfvén‐Mach number of the flow at the magnetopause determines the nature of the local energy transfer; the power per unit area transferred from the solar wind to the magnetosphere consists mainly of kinetic energy. The electromagnetic energy rate P EM controls the near‐earth magnetospheric activity, whereas the kinetic energy rate P KE (≃ 3‐4 P EM ) should dominate the dynamics of the distant magnetotail.