Excitation of Magnetospheric Hydromagnetic Waves by Solar-Flare-Induced Change in Ionospheric Conductivity

Abstract

Observations are reported of magnetospheric hydromagnetic (Alfv\'en) waves excited by rapid changes in ionospheric conductivity induced by solar flare x-ray and extreme-ultra-violet fluxes. The observed wave frequencies (\ensuremath{\sim}0.01-0.02 Hz) are those expected for fundamental eigenoscillations of the geomagnetic field at middle latitudes. The measurements confirm a natural hydromagnetic wave generation mechanism not associated with the solar wind or geomagnetic storms. These results have implications for Alfv\'en wave generation in other cosmic plasmas.