Investigating the Heliosphere with Low‐energy Anomalous Cosmic Rays Available to Purchase

Two aspects of low‐energy (∼0.5–30 MeV/nucleon) anomalous cosmic ray (ACR) phenomena are unique. First, low‐rigidity (≲ 2 GV) ACRs are less affected by particle drifts than higher rigidity particles. Second, outer‐heliospheric ACRs having energies below the energy at the peak of the modulated spectrum, but above the adiabatic range, are governed by a different limit of the transport equation than higher‐energy ACRs, namely, the convection‐diffusion limit. It is therefore possible to uncover features of energetic particle transport in the heliosphere that are not readily apprehended using higher‐energy ACR measurements. We study the first property, in the context of outer‐heliospheric Voyager 1 (V1), Voyager 2 (V2), and Pioneer 10 (P10) particle intensity measurements made during the 1991–1999 cosmic ray recovery phase. In particular, we show that the effective “drift/convection pattern” of low‐rigidity particles during a period of positive heliomagnetic polarity (A > 0), such as this, is qualitatively different than the drift pattern usually discussed. The disagreement between radial and latitudinal intensity gradients determined using the new “quasi‐local” gradient (QLG) method and the standard non‐local gradient (NLG) method is discussed in light of models of the heliosphere showing longitudinal asymmetry. Earlier results regarding diminished high‐latitude transport suggest that the near‐equatorial region will have an enhanced role in the ACR transport. Absent this effect, the fact that the detected low energy particles are actually cooled products of higher energy populations would lead to the expectation that the cooled particles should show residual evidence of the drift undergone before the energy loss took place. The lack of such evidence suggests low latitudes are the more significant region. We will discuss these topics with the primary goal of highlighting the unique and necessary role low‐energy ACR measurements have in studying the heliosphere.