Energetic protons accelerated at corotating shocks: Pioneer 10 and 11 observations from 1 to 6 AU
Authors: Tsurutani B. T. , Smith E. J. , Pyle K. R. , Simpson J. A. 
Published: February 2008

The Pioneer 10 and 11 Jet Propulsion Laboratory vector helium magnetometer and the University of Chicago 0.5‐ to 1.8‐MeV proton telescope data are used to examine the relationship between low‐energy proton increases and corotating interaction regions (CIR's) in the heliosphere between 1 and 6.3 AU. The 0.5‐ to 1.8‐MeV proton flux enhancements are correlated with the forward and reverse shocks bounding the CIR's. Fifty to sixty percent of all identified shocks are accompanied by time‐coincident proton events. Conversely, almost all proton events occurring at CIR boundaries are associated with shocks: 92% of all proton events detected at the leading boundary are accompanied by forward shocks, and 72% of the proton events near the trailing CIR edge are associated with reverse shocks. Proton intensities are highest when the shock normal angle with respect to the magnetic field direction is ≥80°, for both forward and reverse shocks. One long lasting CIR, which persisted for more than 14 rotations, is studied to determine detailed field‐particle relationships and their evolutionary changes with time (and distance from the sun). The CIR recurs with a sidereal period of 24.7±0.1 days, a value comparable to the average period of magnetic features near the sun's equator. Considerable variation in the CIR field intensity and associated proton count rate occurs from rotation to rotation, indicating a variability in the high‐speed stream interaction properties and in the overall efficiency of the proton acceleration mechanism(s). A general correlation between the maximum CIR field intensity and the maximum proton count rate is noted. The evolution of proton flux characteristics, with time and distance, is studied. Initially, the characteristic double peaks in proton intensity, associated with the forward and reverse shocks, are approximately equal in magnitude. With time, the second, trailing peak becomes significantly larger. The two peaks are not always coincident with the shocks but are often found inside the CIR. The minimum proton flux, located between the two proton maxima, appears to be correlated with the maximum field strength of the CIR. The proton flux in this minimum region increases with time, presumably due to cross‐field diffusion of protons from the neighboring maximum regions. Spectral softening and large proton‐to‐helium ratios are characteristic of the particles near the leading CIR edge, but not of the trailing edge. Proton anisotropies are often large and field aligned. The ‘flow’ is generally away from the forward and reverse shocks, indicating that the shocks are the source of particle acceleration. Upstream of forward shocks, the proton flow is in the solar direction. Protons are found to stream in the antisolar direction within the CIR. The distribution is isotropic upstream of the reverse shock. Intense magnetohydrodynamic fluctuations are present within the CIR, indicated by large field variances. Although the variances are orders of magnitude larger within the CIR than in the quiet regions outside CIR's, the relative field fluctuation, given by the value σ²/B², is generally less inside the CIR than outside. The evidence presented in this paper strongly supports shock acceleration as the primary source of the 1‐MeV protons. A schematic figure, which incorporates many of the features deduced in this report, is given to illustrate the relationship between the energetic protons, the forward and reverse shocks, and the interplanetary magnetic field structure. The predictions of various theories and mechanisms for interplanetary nucleon acceleration are discussed in light of the experimental results presented.