Observations are reported of magnetospheric hydromagnetic (Alfvén) waves excited by rapid changes in ionospheric conductivity induced by solar flare x-ray and extreme-ultra-violet fluxes. The observed wave frequencies (∼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 Alfvén wave generation in other cosmic plasmas.

• Received 20 July 1981

DOI:https://doi.org/10.1103/PhysRevLett.47.1343