Observations of three‐dimensional anisotropies of 35‐ to 1000‐keV protons associated with interplanetary shocks

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Abstract
We present results of a detailed analysis of three‐dimensional anisotropies of protons in the energy range 35–1000 keV observed in association with interplanetary shocks on ISEE 3. We compare observations of high time resolution anisotropies made close to the shock in seven energy channels with theoretical predictions for Fermi acceleration and shock drift acceleration, and we find good evidence for both types of acceleration. We find a small number (six) of events exhibiting the signature of Fermi acceleration, and a somewhat larger number (20) exhibiting the signature of shock drift acceleration, the “Fermi” events being associated with strong, fast quasi‐parallel shocks and the “shock drift” events being associated with weaker, slower quasi‐perpendicular events. In the solar wind frame of reference the Fermi events have moderate upstream anisotropies, with flow away from the shock persisting for periods of one to two hours, the anisotropy decreasing with increasing energy, whereas downstream these events are isotropic. These events exhibit slow quasi‐exponential intensity increases of 1–2 orders of magnitude, peaking at the shock, and slowly decaying after the shock, often rising to a secondary peak some hours later. The shock drift events have large upstream first‐order anisotropies close to the shock, with flow away from the shock, and moderate downstream first‐order anisotropies, with flow toward the shock. The most notable feature of the shock drift events is a large negative second harmonic immediately downstream of the shock, signifying protons gyrating around the magnetic field at pitch angles of around 90°. These events have shock spike intensity increases lasting for a few minutes or tens of minutes. At all energies the largest intensity increases are observed with the Fermi events. Since the Fermi events are associated with the largest fluxes of solar flare protons, this may be due to a combination of solar particle background and protons accelerated in the vicinity of the shock.