Energetic interplanetary nucleon flux anisotropies: The effect of Earth's bow shock and magnetosheath on sunward flow

Abstract

Recent statistical surveys of interplanetary MeV energy nucleon flux anisotropies observed between prompt solar particle events during solar activity minimum have included time blocks of data obtained when the interplanetary magnetic field (IMF) connects the near earth spacecraft to earth's bow shock. The ensemble average nucleon flux anisotropy in the solar wind reference frame <ξD> obtained on these ‘connected’ field lines does not accurately represent the unperturbed <ξD> obtained on field lines free of this magnetic connection. Hourly average observations of interplanetary 0.5‐ to 1.8‐MeV proton fluxes, obtained near earth from 1972 to 1977, are correlated herein with simultaneous measurements of the IMF and solar wind plasma. At moderate flux levels during solar activity minimum, <ξD> indicates that nucleon flow in the solar wind frame is toward the sun and primarily along the IMF, although <ξD> is directed more sunward than strict field‐aligned propagation requires. Cross‐field transport is statistically significant and in the direction expected from the large‐scale MeV energy nucleon flux distributions throughout the heliosphere. The unperturbed nucleon flow direction relative to the IMF is used to demonstrate and characterize the interaction of MeV energy nuclei with the earth's bow shock and magnetosheath. The result of this interaction is that the mean value of <ξD> perpendicular to connected field lines is consistent with zero and therefore is not statistically significant. Obstruction of sunward nucleon flow on connected field lines is indicated by the variation of <ξD> with spacecraft position.