Dependence of hydromagnetic energy spectra on solar wind velocity and interplanetary magnetic field direction

We investigate the relationship of local daytime geomagnetic energy (measured in the period range ∼30–300 s at a mid‐latitude ground‐based station) to interplanetary parameters during two time intervals when the solar wind/interplanetary field conditions were quite different. We find that the geomagnetic energy levels in the entire period band are consistent with production by a Kelvin‐Helmholtz instability source at the earth's magnetopause. We further find that at the shorter periods (∼30–60 s) the energy may also have a source contribution from the mechanism proposed by Greenstadt, which involves wave energy transfer from the earth's bow shock to the magnetopause. This result represents the first evidence of frequency dependencies in the interplanetary source mechanisms for hydromagnetic energy production in the magnetosphere.