ALBERT

Description: Ultra-low frequency (ULF) waves play an important role in transferring energy by buffeting the magnetosphere with solar wind pressure impulses. The amplitudes of magnetospheric ULF waves, which are induced by solar wind dynamic pressure enhancements or shocks, are thought to damp in one half a wave cycle or an entire wave cycle. We report in situ observations of solar wind dynamic pressure impulse-induced magnetospheric ULF waves with increasing amplitudes. We found six ULF wave events induced by solar wind dynamic pressure enhancements with slow but clear wave amplitude increase. During three or four wave cycles, the amplitudes of ion velocities and electric field of these waves increased continuously by 1.3 ~ 4.4 times. Two significant events were selected to further study the characteristics of these ULF waves. We found that the wave amplitude growth is mainly contributed by the toroidal mode wave. Three possible mechanisms of causing the wave amplitude increase are discussed. Firstly, solar wind dynamic pressure perturbations, which are observed in a duration of 20 ~ 30 minutes, might transfer energy to the magnetospheric ULF waves continually. Secondly, the wave amplitude increase in the radial electric field may becaused by superposition of two wave modes, a standing wave excited by the solar wind dynamic impulse and a propagating compressional wave directly induced by solar wind oscillations. When superposed, the two wave modes fit observations as does a calculation that superposes electric fields from two wave sources. Thirdly, the normal of the solar wind discontinuity is at an angle to the Sun-Earth line. Thus, the discontinuity will affect the dayside magnetopause continuously for a long time.