Simultaneous observations of field-aligned beams and gyrating ions in the terrestrial foreshock
Dandouras, I.; Klecker, B.; Lucek, E. A.; Rème, H.; Bosqued, J. M.; Parks, G. K.; McCarthy, M.; Korth, A.; Kucharek, H.; Mazelle, C.; Bavassano-Cattaneo, M. B.; Wilber, M.; Sauvaud, J. A.; Meziane, K.; Hamza, A. M.; Le Quéau, D.; Lundin, R. N.
Canada, United States, France, United Kingdom, Germany, Italy, Sweden
Abstract
We examine an energetic (2-30 keV) upstream ion event presenting a clear double-peak spectrum observed ∼1 RE upstream from the bow shock. The lower-energy (E ∼ 3.5 keV) peak is associated with an ion beam propagating along the magnetic field direction, while the higher-energy (E ∼ 13 keV) peak is associated with gyrating ions having pitch angles ∼30°. The latter population progressively extends to lower energies over the span of the event. During times when the field-aligned beams were observed, the interplanetary magnetic field was remarkably steady, while the appearance of the 30° pitch angle gyrating ions was accompanied by the onset of large-amplitude ultralow frequency fluctuations of the magnetic field. Our analysis indicates that the gyrating ions had guiding centers on field lines downstream of the field-aligned component but that both populations could be sampled simultaneously because of the orbits of the former. We find that the downstream limit of the field-aligned beams is populated with protons having a speed 1.68 times the solar wind velocity, which is inconsistent with any known shock-related emission mechanisms. This boundary makes an angle of 77° with respect to the Sun-Earth line in agreement with theoretical predictions. Just downstream of this rapid transition, gyrating ions having a flow speed of 1.52 times the solar wind speed are observed in association with ULF waves. Like the field-aligned beams, the gyrating ions reported here have streaming speeds inconsistent with any known shock emission mechanisms. While the simultaneous observation of field-aligned and gyrating components is possible because of the large gyration orbits of the latter, the observational sequence is consistent with a very sharp (≲1 gyroradius) boundary separating the guiding centers of each. Explicit observations of such a sharp demarcation between these populations have not been reported before, and they place a significant constraint on the production mechanisms of the two populations. Our interpretation of these observations provides a refinement of the usual framework for foreshock morphology.