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Abstract. The enhancement of Pcl wave activity after magnetic sudden impulses (Sis) is well established by ground-based and satellite measurements. This phenomenon is explained by compressions of the magnetosphere under the action of interplanetary shocks. In this paper we use long-term ground-based observations of Pcl to study the variation of the Pcl wave activity before Sis. We conclude that there is a tendency for an increase of the rate of Pcl occurrence as the time gets closer to the SI. We interpret this effect as an impact of the interplanetary foreshocks on the Earth's magnetosphere. IntroductionIt is known that a magnetic sudden impulse ( In this work we analyze long-term ground-based observations of Pcl magnetic pulsations to study the probability of the Pcl occurrence within the 1.5-hour time interval preceding the SI. We conclude that an increase in Pcl activity prior to Sis takes place. This new observation is discussed in terms of the We see that the distribution of n (k) is nonuniform with the maximum value n(1) -38 for the first interval 0-5 min. 25,185

We have analyzed short-period geomagnetic pulsations measured by the north-south chain of pulsation magnetometers in Finland in association with the storm sudden commencement (SSC) recorded on September 22, 1999 at 12:22 UT. We observe that structured Pc 1 pulsations with amplitude modulation (modulation period is about 1000 seconds) were excited before the SSC whereas intense wave elements with rising frequency appear soon after SSC. It is concluded that the first effect is due to the interaction of the Earth's magnetosphere with the interplanetary foreshock region. The second one is likely the result of modification of Pc 1 waves due to interaction between the magnetosphere and the shock front structure leading to compression of the magnetosphere during the initial phase of the magnetic storm.

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Geomagnetic pulsations—the groundbased signals of magnetospheric hydromagnetic waves—have been studied for well over 100 years. Some of the first observations were reported in 1861 by B. Stewart, who studied the recordings of the great magnetic storm of 1859. In Helsinki in the 1840s, J. J. Nervander observed magnetic “undulations” of constant period of about 30 s in his declination variometer. At the turn of the century, more sensitive magnetic instruments and faster registration systems were developed and the basis was laid for the systematic classification of long period pulsations, which are now called Pc 3‐Pc 5 and Pi 2 pulsations.

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