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Giant pulsations are nearly monochromatic\udULF-pulsations of the Earth's magnetic field with periods of about 100 s and\udamplitudes of up to 40 nT. For one such event ground-magnetic observations as\udwell as simultaneous GEOS-2 magnetic and electric field data and proton flux\udmeasurements made in the geostationary orbit have been analysed. The\udobservations of the electromagnetic field indicate the excitation of an odd-mode\udtype fundamental field line oscillation. A clear correlation between variations\udof the proton flux in the energy range 30-90 keV with the giant pulsation event\udobserved at the ground is found. Furthermore, the proton phase space density\udexhibits a bump-on-the-tail signature at about 60 keV. Assuming a drift-bounce\udresonance instability as a possible generation mechanism, the azimuthal wave\udnumber of the pulsation wave field may be determined using a generalized\udresonance condition. The value determined in this way,  \udm = - 21 ± 4, is in accord with the value m = - 27 ± 6 determined\udfrom ground-magnetic measurements. A more detailed examination of the observed\udring current plasma distribution function f shows that odd-mode type\udeigenoscillations are expected for the case ∂f / ∂W > 0,\udmuch as observed. This result is different from previous theoretical studies as\udwe not only consider local gradients of the distribution function in real space,\udbut also in velocity space. It is therefore concluded that the observed giant\udpulsation is the result of a drift-bounce resonance instability of the ring\udcurrent plasma coupling to an odd-mode fundamental standing wave. The generation\udof the bump-on-the-tail distribution causing ∂f / ∂W > 0\udcan be explained due to velocity dispersion of protons injected into the ring\udcurrent. Both this velocity dispersion and the necessary substorm activity\udcausing the injection of protons into the nightside magnetosphere are observed. Key words. Magnetospheric physics (energetic particles\ud, trapped; MHD waves and instabilities) · Space plasma physics (wave-particle\udinteractions)

Concerning the generation of geomagnetic giant pulsations by drift-bounce resonance ring current instabilities