Abstract
Abstract. Giant pulsations are nearly monochromatic ULF-pulsations of the Earth's magnetic field with periods of about 100 s and amplitudes of up to 40 nT. For one such event ground-magnetic observations as well as simultaneous GEOS-2 magnetic and electric field data and proton flux measurements made in the geostationary orbit have been analysed. The observations of the electromagnetic field indicate the excitation of an odd-mode type fundamental field line oscillation. A clear correlation between variations of the proton flux in the energy range 30-90 keV with the giant pulsation event observed at the ground is found. Furthermore, the proton phase space density exhibits a bump-on-the-tail signature at about 60 keV. Assuming a drift-bounce resonance instability as a possible generation mechanism, the azimuthal wave number of the pulsation wave field may be determined using a generalized resonance condition. The value determined in this way, m = - 21 ± 4, is in accord with the value m = - 27 ± 6 determined from ground-magnetic measurements. A more detailed examination of the observed ring current plasma distribution function f shows that odd-mode type eigenoscillations are expected for the case ∂f / ∂W > 0, much as observed. This result is different from previous theoretical studies as we not only consider local gradients of the distribution function in real space, but also in velocity space. It is therefore concluded that the observed giant pulsation is the result of a drift-bounce resonance instability of the ring current plasma coupling to an odd-mode fundamental standing wave. The generation of the bump-on-the-tail distribution causing ∂f / ∂W > 0 can be explained due to velocity dispersion of protons injected into the ring current. Both this velocity dispersion and the necessary substorm activity causing the injection of protons into the nightside magnetosphere are observed.Key words. Magnetospheric physics (energetic particles , trapped; MHD waves and instabilities) · Space plasma physics (wave-particle interactions).
Highlights
Giant pulsations (Pg) are ULF-pulsations predominantly occurring in the morningside magnetosphere
Glassmeier et al.: Concerning the generation of geomagnetic giant pulsations azimuthal phase variation and indications toward an odd-mode wave structure led Green (1979), Takahashi et al (1992), and others to suggest that a drift-wave instability of the magnetospheric ring current
A ground-satellite coordinated case study of a giant pulsation event has been presented, which demonstrates that giant pulsations are associated with a drifting proton cloud injected into the nightside magnetospheric ring current due to substorm activity
Summary
Giant pulsations (Pg) are ULF-pulsations predominantly occurring in the morningside magnetosphere. Pgs are found to be characterised by rapid east-west phase variations with the azimuthal wave number, m, reaching values up to m ÿ40, where the negative sign indicates westward phase propagation These characteristics led several authors to conclude that giant pulsation wave ®elds are propagated in the guided poloidal mode, ®rst described in Dungey's (1954). Pg ground amplitudes are often smaller than those of Pc4-5 pulsations the ionospheric magnetic ®eld of Pgs is much larger due to the screening eect of the atmosphere which scales exponentially with the azimuthal wave number (e.g. Hughes and Southwood, 1976; Glassmeier, 1984). Glassmeier et al.: Concerning the generation of geomagnetic giant pulsations azimuthal phase variation and indications toward an odd-mode wave structure led Green (1979), Takahashi et al (1992), and others to suggest that a drift-wave instability of the magnetospheric ring current MLT), these mentioned problems will be tackled and a conjecture is formulated which explains many typical characteristics of giant pulsations
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