Simultaneous observations of field-aligned beams and gyrating ions in the terrestrial foreshock -: art. no. A05107

We examine an energetic (2-30 keV) upstream ion event presenting a clear double-peak spectrum observed similar to1 R-E upstream from the bow shock. The lower-energy (E similar to 3.5 keV) peak is associated with an ion beam propagating along the magnetic field direction, while the higher-energy (E similar to 13 keV) peak is associated with gyrating ions having pitch angles similar to30degrees. 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 30degrees 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 77degrees 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 (less than or similar to1 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.