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

Giant pulsations are nearly monochromatic ULF-pulsations of the Earth's magnetic ®eld 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 ®eld data and proton ̄ux measurements made in the geostationary orbit have been analysed. The observations of the electromagnetic ®eld indicate the excitation of an odd-mode type fundamental ®eld line oscillation. A clear correlation between variations of the proton ̄ux 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 ®eld may be determined using a generalized resonance condition. The value determined in this way, m ˆ ÿ21 4, is in accord with the value m ˆ ÿ27 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 di€erent 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 driftbounce 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.