Abstract
Abstract. During the early orbit phase, the Cluster spacecraft have repeatedly crossed the perpendicular Earth’s bow shock and provided the first multi-spacecraft measurements. We have analyzed data from the Cluster Ion Spectrometry experiment (CIS), which observes the 3D-ion distribution function of the major species in the energy range of 5 eV to 40 keV with a 4 s resolution. Beams of reflected ions were observed simultaneously at all spacecraft locations and could be tracked from upstream to the shock itself. They were found to originate from the same distribution of ions that constitutes the reflected gyrating ions, which form a ring distribution in the velocity space immediately upstream and downstream of the shock. This observation suggests a common origin of ring and beam populations at quasi-perpendicular shocks in the form of specular reflection and immediate pitch angle scattering. Generally, the spatial evolution across the shock is very similar on all spacecraft, but phased in time according to their relative location. However, a distinct temporal structure of the ion fluxes in the field-aligned beam is observed that varies simultaneously on all spacecraft. This is likely to reflect the variations in the reflection and scattering efficiencies.Key words. Interplanetary physics (planetary bow shocks; energetic particles; instruments and techniques)
Highlights
Observations of energetic ions upstream of the Earth’s bow shock were reported by Asbridge et al (1968), Lin et al (1974) and West and Buck (1976)
Due to the natural variations in the solar wind and the interplanetary magnetic field (IMF), and in the topology of the Earth’s bow shock, a wide dynamic range of the controlling parameters and different shock orientations can be studied during repetitive crossings of the Earth orbiting spacecraft
The reflected ions form a beam directed along the interplanetary magnetic field; these ions are accelerated in a single reflection off the quasi-perpendicular bow shock (e.g. Paschmann et al, 1980) with their energies rarely exceeding ≈ 10 keV
Summary
Observations of energetic ions upstream of the Earth’s bow shock were reported by Asbridge et al (1968), Lin et al (1974) and West and Buck (1976). The reflected ions form a beam directed along the interplanetary magnetic field; these ions are accelerated in a single reflection off the quasi-perpendicular bow shock Paschmann et al, 1980) with their energies rarely exceeding ≈ 10 keV They are more commonly called field-aligned beams to distinguish themselves from those reflected ions that are convected downstream as gyrating ions with the IMF. Diffuse ions upstream of the quasi-parallel shock form a much more isotropic distribution whose energy spectra extend to ≈ 200 keV/e We will follow the reflected ion populations, both field-aligned beam and gyrating ions, from the upstream region into the quasi-perpendicular shock. It is demonstrated that both beam and gyrating ions apparently emerge from the same process at the shock, whose effectiveness shows significant temporal variations on the scale of less than one minute
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