Solar wind control of the radial distance of the magnetic reconnection site in the magnetotail

To understand magnetotail dynamics, it is essential to determine where magnetic reconnection takes place in the near‐Earth magnetotail during substorms. The Geotail spacecraft thoroughly surveyed the near‐Earth plasma sheet at radial distances of 10–31 RE during the years 1995–2003. Thirty‐four clear reconnection events were identified using the criterion of strong electron acceleration. Various solar wind parameters prior to each reconnection event were examined in order to find the factor controlling the location of the magnetic reconnection site in the magnetotail. The same analyses were carried out for fast tailward flow events. The most important factor was determined to be the solar wind energy input, which can be expressed by −Vx × Bs, where Vx is the x component of the solar wind velocity and Bs is the southward component of the interplanetary magnetic field. It is likely that higher efficiency of energy input, rather than the total amount of energy input, primarily controls the location of magnetic reconnection; magnetic reconnection takes place closer to the Earth when efficiency of energy input is higher. The effect of solar wind dynamic pressure is minor. The present result suggests that the tail magnetic reconnection location during substorms is controlled by solar cycle variations in the solar wind.