A mechanism for the generation of cusp region hydromagnetic waves

Abstract

It is suggested that the existence of a sporadic multiple X line reconnection (MXR) process at the dayside magnetopause could be a source of energy to produce ULF hydromagnetic waves (ƒ ∼ 1–10 mHz) in the dayside magnetopause cusp region. The MXR process is characterized by the repeated formation and convection of magnetic islands and elongated plasma clouds, which compress and distort the adjacent closed geomagnetic field lines. This leads in a natural way to the generation of ULF waves with frequencies in the range 1–10 mHz along cusp geomagnetic field lines. These waves are expected to be appoximately linearly polarized, dominated by variations in the radial magnetic field component near the dayside magnetopause and by variations in the azimuthal magnetic field component near the resonant closed field lines in the magnetosphere. In addition, the field‐aligned currents associated with the MXR process may generate the impulsive magnetic field variations often observed to accompany the continuous variations in the ULF band. Hence the reconnection process may be an additional mechanism, together with the Kelvin‐Helmholtz instability and direct penetration of upstream waves, for the production of hydromagnetic energy in the dayside magnetosphere.