Experimental evidence of direct penetration of upstream ULF waves from the solar wind into the magnetosphere during the strong magnetic storm of November 9, 2004

Upstream ULF waves were recorded by the ACE satellite, while the Pc5 ULF waves (geomagnetic pulsations) were simultaneously observed by the GOES12 satellite in the magnetosphere during the big magnetic storm on November 9, 2004. Their spectra showed similar structures with the maximum frequency band around 2.20–2.44 mHz. The upstream ULF waves were highly correlated with the Pc5 waves with a correlation coefficient of 0.82 for a time lag of 28.5 min. Both the interplanetary and the magnetospheric waves had intense radial components with very large-amplitude δBr/B>40%, which indicated characteristic of the compressional waves. Furthermore, variations of the plasma density were in phase with those in the magnetic field, suggesting that they were essentially magnetosonic waves. The results demonstrated that upstream magnetosonic waves in the solar wind can directly penetrate into the magnetosphere and cause the Pc5 ULF waves during a big magnetic storm. The penetration of the magnetosonic waves seems to be occurred merely around the noon sector.