Abstract
Abstract. We present magnetospheric observations of very large amplitude global scale ULF waves, from 9 and 10 December 2000 when the upstream solar wind speed exceeded 600 km/s. We characterise these ULF waves using ground-based magnetometer, radar and optical instrumentation on both the dawn and dusk flanks; we find evidence to support the hypothesis that discrete frequency field line resonances (FLRs) were being driven by magnetospheric waveguide modes. During the early part of this interval, Cluster was on an outbound pass from the northern dusk side magnetospheric lobe into the magnetosheath, local-time conjugate to the Canadian sector. In situ magnetic fluctuations, observed by Cluster FGM, show evidence of quasi-periodic motion of the magnetosheath boundary layer with the same period as the ULF waves seen on the ground. Our observations represent the first simultaneous magnetometer, radar and optical observations of the characteristics of FLRs, and confirm the potential importance of ULF waves for magnetosphere-ionosphere coupling, particularly via the generation and modulation of electron precipitation into the ionosphere. The in situ Cluster measurements support the hypothesis that, during intervals of fast solar wind speed, the Kelvin-Helmholtz instability (KHI) can excite magnetospheric waveguide modes which bathe the flank magnetosphere with discrete frequency ULF wave power and drive large amplitude FLRs. Paper submitted to the special issue devoted to "Cluster: First scientific results", Ann. Geophysicae, 19, 10/11/12, 2001.Key words. Magnetospheric physics (magnetopause, cusp and boundary layers; MHD waves and instabilities; solar wind-magnetosphere interactions)
Highlights
Global oscillations of magnetospheric field lines can be excited in the magnetosphere by the solar wind. Dungey (1955) was the first to consider the possibility that ultra-low frequency (ULF) waves might constitute Alfven waves which stand along geomagnetic field lines. Southwood (1974) and Chen and Hasegawa (1974) first considered how fast surface waves, excited by the Kelvin-Helmholtz instability (KHI) on the magnetopause, could drive standing Alfven field line resonances (FLRs) on field lines of matching eigenfrequency
We have presented co-ordinated ground-based and Cluster observations which demonstrate that very large amplitude Pc5 ULF waves can be excited on the flanks of the magnetosphere during fast solar wind speed intervals
These global scale ULF waves possess discrete frequency characteristics which are consistent with the energising of magnetospheric waveguide modes and subsequently FLRs
Summary
Global oscillations of magnetospheric field lines can be excited in the magnetosphere by the solar wind. Dungey (1955) was the first to consider the possibility that ultra-low frequency (ULF) waves might constitute Alfven waves which stand along geomagnetic field lines. Southwood (1974) and Chen and Hasegawa (1974) first considered how fast surface waves, excited by the Kelvin-Helmholtz instability (KHI) on the magnetopause, could drive standing Alfven field line resonances (FLRs) on field lines of matching eigenfrequency. The geographic pole is indicated with a cross; contours of geomagnetic latitude and longitude are plotted with increments of 10 and 15 degrees, respectively Later in this interval, from 01:30 UT until around 08:00 UT (and later), the morning local time sector showed clear evidence from European ground instrumentation of large amplitude pulsations and field line resonances. The Cluster data allow us to test the hypothesis that fluctuations close to the magnetopause, such as the KHI, may have energised large amplitude pulsations and field line resonances on the flanks of the magnetosphere
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