Geomagnetic micropulsations in the Pc 4 frequency band which are highly monochromatic and have amplitude‐modulated wave forms have been termed ‘giant pulsations’ and have been observed to reach amplitudes of several tens of nanoteslas. In this paper, four such events are analyzed by using meridian line and two‐dimensional magnetometer networks. It is found that these pulsations are highly localized in latitude and have a large azimuthal wave number k⊥, although the large magnitude of k⊥ does not necessarily reflect the longitudinal extent of the disturbed region. While such events are extremely rare, they may occur on 2 or more consecutive days. The pulsations occur on quiet days and normally after a sustained period of low magnetospheric activity. The initiation of substorm activity appears to quench the giant pulsations, which may return after the substorm has subsided. We suggest that the pulsations accompany suddenly reduced levels of magnetospheric convection, and indeed the source region of the pulsations moves poleward during the course of the pulsational activity, suggesting a recovery phase relationship. It is suggested that the Pc 4 giant pulsations may be due to the occurrence of field line resonances at the plasmapause in the region where the electric field changes its azimuthal direction from westward to eastward.

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Data from the IMP-I satellite are used to determine the lag time between the response of the index of geomagnetic activity AE to fluctuations in the azimuthal component of the interplanetary electric field as measured in the solar ecliptic coordinate system. The lag time is found to lie in the range 30-50 min. The magnitude of the lag time is related to the average energy content of the magnetosphere.On utilise des donnees provenant du satellite IMP-I pour determiner le delai entrela rCponse de I'indice d'activiti giomagnetique AE et les fluctuations de la composante azimuthale du champ Clectrique interplanktaire tel que mesure dans le systtme de coordonntes de I'Ccliptique solaire. On trouve que la longueur de ce dtlai est dans I'intervalle 30-50 min. On relie la valeur de ce dClai I'tnergie moyenne contenu dans la magnCtosphtre.

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A statistical study of relativistic electron (>2 MeV) fluence derived from geosynchronous satellites and Pc5 ultralow frequency (ULF) wave power computed from a ground magnetic observatory data located in Canada's auroral zone has been carried out. The ground observations were made near the foot points of field lines passing through the GOESs from 1987 to 2009 (cycles 22 and 23). We determine statistical relationships between the two quantities for different phases of a solar cycle and validate these relationships in two different cycles. There is a positive linear relationship between log fluence and log Pc5 power for all solar phases; however, the power law indices vary for different phases of the cycle. High index values existed during the descending phase. The Pearson's cross correlation between electron fluence and Pc5 power indicates fluence enhancement 2–3 days after strong Pc5 wave activity for all solar phases. The lag between the two quantities is shorter for extremely high fluence (due to high Pc5 power), which tends to occur during the declining phases of both cycles. Most occurrences of extremely low fluence were observed during the extended solar minimum of cycle 23. The precursory attribute of Pc5 power with respect to fluence and the enhancement of fluence due to rising Pc5 power both support the notion of an electron acceleration mechanism by Pc5 ULF waves. This precursor behavior establishes the potential of using Pc5 power to predict relativistic electron fluence.