Articles | Volume 29, issue 10
Ann. Geophys., 29, 1967–1975, 2011

Special issue: Cluster 10th anniversary workshop

Ann. Geophys., 29, 1967–1975, 2011

Regular paper 31 Oct 2011

Regular paper | 31 Oct 2011

Anomalous foreshock field-aligned beams observed by Cluster

K. Meziane1, A. M. Hamza1, M. Wilber2, C. Mazelle3, and M. A. Lee4 K. Meziane et al.
  • 1Physics Department, University of New Brunswick, Fredericton, Canada
  • 2Space Sciences Laboratory, University of California, Berkeley, USA
  • 3Centre d'Etude Spatiale des Rayonnements, Toulouse, France
  • 4Space Science Center, University of New Hampshire, Durham, USA

Abstract. We report occasional observations of two simultaneously distinct ion foreshock components recorded by the Cluster spacecraft upstream of the Earth's bow shock. In most occurrences, the lower-energy population originates as a field-aligned beam (FAB) associated with quasi-perpendicular regions, which loses energy as the IMF rotates into oblique geometries. A second beam, with energies in excess of ~10 keV, appears sometimes in association with the onset of ultra-low frequency (ULF) waves, and sometimes ahead of the appearance of the latter. Measurements from the mass spectrometer indicate that both beams consist of protons. While the lower-speed beam is well-accounted for by a known reflection mechanism, the non-radial IMF orientations as well as other arguments seem to rule out magnetosheath or magnetospheric sources for the higher energy component. The wave characteristics are typical of the oblique foreshock and we have found that they are in cyclotron-resonance with the low speed beam (FAB). These observations constitute a theoretical challenge since conventional mechanisms described in the literature cannot account for the production of beams at two different energies.