Resonant Alfvén waves excited by a sudden impulse

Abstract

On March 6, 1979, long‐period hydromagnetic waves were excited in the forenoon magnetosphere by a sudden impulse (SI). The plasma drift velocity and magnetic field oscillations associated with these waves were observed by the electron gun experiment and the magnetometer, respectively, onboard the GEOS 2 satellite in the equatorial plane. The waves had both compressional and transverse components and had their probable source in a single, tailward traveling, large‐scale magnetopause surface “wavelet” (i.e., a single rarefaction/compression pulse) caused by the passage of an interplanetary shock front. This surface wavelet apparently coupled into the inner magnetosphere via the field line resonance mechanism. The satellite observations and the Poynting vectors calculated from these are consistent with a location of the resonance region earthward of GEOS 2 during the first 5 min after the SI while the satellite was located just inside the resonance region at later times. This shift in the relative location of the resonance region was probably caused by an increase of the Alfvén velocity as a result of the compression of the magnetosphere associated with the sudden impulse. The stronger‐than‐usual compressional magnetic field component most likely had its origin in the relatively larger wavelength of the SI‐excited surface wavelet (compared with that of Kelvin‐Helmholtz‐excited surface waves).