The Earth's passage of the April 11, 1997 coronal ejecta: geomagnetic ®eld ̄uctuations at high and low latitude during northward interplanetary magnetic ®eld conditions

An analysis of the low frequency geomagnetic ®eld ̄uctuations at an Antarctic (Terra Nova Bay) and a low latitude (L'Aquila, Italy) station during the Earth's passage of a coronal ejecta on April 11, 1997 shows that major solar wind pressure variations were followed at both stations by a high ̄uctuation level. During northward interplanetary magnetic ®eld conditions and when Terra Nova Bay is close to the local geomagnetic noon, coherent ̄uctuations, at the same frequency (3.6 mHz) and with polarization characteristics indicating an antisunward propagation, were observed simultaneously at the two stations. An analysis of simultaneous measurements from geosynchronous satellites shows evidence for pulsations at approximately the same frequencies also in the magnetospheric ®eld. The observed waves might then be interpreted as oscillation modes, triggered by an external stimulation, extending to a major portion of the Earth's magnetosphere.