Abstract
Abstract. On 14 January 2001, the four Cluster spacecraft passed through the northern magnetospheric mantle in close conjunction to the EISCAT Svalbard Radar (ESR) and approached the post-noon dayside magnetopause over Green-land between 13:00 and 14:00 UT. During that interval, a sudden reorganisation of the high-latitude dayside convection pattern occurred after 13:20 UT, most likely caused by a direction change of the Solar wind magnetic field. The result was an eastward and poleward directed flow-channel, as monitored by the SuperDARN radar network and also by arrays of ground-based magnetometers in Canada, Greenland and Scandinavia. After an initial eastward and later poleward expansion of the flow-channel between 13:20 and 13:40 UT, the four Cluster spacecraft, and the field line footprints covered by the eastward looking scan cycle of the Söndre Strömfjord incoherent scatter radar were engulfed by cusp-like precipitation with transient magnetic and electric field signatures. In addition, the EISCAT Svalbard Radar detected strong transient effects of the convection reorganisation, a poleward moving precipitation, and a fast ion flow-channel in association with the auroral structures that suddenly formed to the west and north of the radar. From a detailed analysis of the coordinated Cluster and ground-based data, it was found that this extraordinary transient convection pattern, indeed, had moved the cusp precipitation from its former pre-noon position into the late post-noon sector, allowing for the first and quite unexpected encounter of the cusp by the Cluster spacecraft. Our findings illustrate the large amplitude of cusp dynamics even in response to moderate solar wind forcing. The global ground-based data proves to be an invaluable tool to monitor the dynamics and width of the affected magnetospheric regions.Key words. Magnetospheric cusp, ionosphere, reconnection, convection flow-channel, Cluster, ground-based observations
Highlights
The term “cusp” is today applied to a number of characteristics of the dayside magnetosphere: in particular, the magnetic cusp, the particle cusp, the cusp aurora and the cusp currents
For a given notional electric field imposed on the magnetosphere by the solar wind flow (ESW from dawn to dusk for purely southward interplanetary magnetic field (IMF)), the cusp can be regarded as the location where (J CF · ESW ) reverses from positive to negative
In order to support this conclusion, we present in the bottom panel of Fig. 13 an optical keogram from the Italian all-sky camera (ITACA) at Ny Alesund (i.e. 557.7 nm emissions along a meridional profile plotted versus latitude and time with the assumption of a central emission altitude at 110 km)
Summary
The term “cusp” is today applied to a number of characteristics of the dayside magnetosphere: in particular, the magnetic cusp, the particle cusp, the cusp aurora and the cusp currents. Various mechanisms for populating a closed LLBL with sheath plasma have been proposed, while others have argued that there is no need to invoke a closed LLBL at all It appears that all the features of the LLBL can be interpreted in terms of open field lines that have either been reconnected only very recently (alternatively have been re-closed), or which thread the magnetopause away from the subsolar point where magnetosheath number densities are lower (Lockwood and Smith, 1993; Fuselier et al, 1999). The number density of electrons in the cusp is always very close to that of the ions (Burch, 1985; Lockwood and Hapgood, 1998) How this “quasi-neutrality” is maintained on newly-opened field lines as they evolve is one of the main current problems in understanding the cusp. We will explain the main characteristic features of the utilised instrumentation in the text next to the presentation of the observations, and refer to more instrument-specific Cluster publications in this issue, or publications in the Satellite Ground-Based Coordination Sourcebook (ESA SP1198)
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