Nonthermal ions and associated magnetic field behavior at a quasi-parallel Earth's bow shock


Ion and magnetic field measurements at Earth's bow shock from the AMPTE-UKS and -IRM spacecraft, are examined in high time resolution during a 45-min interval when the field remained closely aligned with the model bow shock normal (θBn ∼ 0°). Dense (> 1% of the solar wind phase space density) ion beams are detected almost exclusively in the midst of short-duration (≤ 30 s) periods of turbulent magnetic field wave activity. The maximum energy of the beams, which is comparable to the solar wind's energy, and their azimuthal location suggest that these ions may originate from specular reflection off the shock. However, we discover many examples of propagation at large elevation angles relative to the ecliptic plane, which is inconsistent with reflection in the standard model shock configuration. The associated waves span the frequency range between ∼ 0.3 and 3 Hz. They are elliptically polarized, are preferentially left-handed in the observer's frame of reference, yet are less confined to the maximum variance plane than other previously studied foreshock waves. The angles of propagation of the waves with respect to the solar wind magnetic field (and shock normal) span a wide range of values but are typically ∼ 30°. The considerable evolution in the appearance of the waves from one spacecraft to the other precludes us from estimating any of their properties in the plasma rest frame. Nevertheless, the association of the wave activity with the ion beams suggests that the former may be triggered by an ion-driven instability, and possible candidates are discussed.

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JGR: Space Physics



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