The Reappearance of Anomalous Cosmic Ray Hydrogen

New observations from the Cosmic Ray Subsystem (CRS) on the Voyager spacecraft clearly show that anomalous cosmic ray hydrogen has reappeared , clearing up a controversy that has existed since the previous solar minimum [1, 2]. The new observations are consistent with the evidence for anomalous cosmic ray hydrogen seen in the 1987 solar minimum. Comparison of the anomalous and galactic cosmic ray modulation for the two solar minima indicates that there is substantial modulation of the galactic cosmic rays beyond the solar wind termination shock. 1 Introduction It is thought that anomalous cosmic rays (ACRs) originate as interstellar neutrals that drift into the heliosphere, are either photo-ionized, or ionized by charge exchange with solar wind particles, and then are convected out to the solar wind termination shock, where they are accelerated [3, 4]. The presence of some ACR hydrogen is expected [5], and indeed, hydrogen "pick-up" ions have been seen in the solar wind [6], but until now, the evidence for ACR hydrogen has been circumstantial [1, 7]. Observations of ACR hydrogen are difficult because, whereas the other ACR components have a higher rigidity than galactic cosmic rays (GCR) at the same energy, because they are singly-ionized as opposed to the fully-ionized GCR, this is not true for ACR hydrogen. Therefore, the ACR and GCR protons undergo very similar modulation inside the termination shock. With each successive period of minimum solar modulation, the Voyager spacecraft move closer to the termination shock and the source of anomalous cosmic rays. Voyager 1, at about 57 AU radial distance and 247° heliospheric longitude, λ, 32° latitude, β, during 94, has observed anomalous cosmic ray fluxes which are higher than those observed at Voyager 2 (~43 AU, λ = 284°, β =-11° in 1994) or either Voyager during the 1987 solar minimum. Both Voyagers are moving roughly upstream relative to the inflowing interstel-lar neutrals, which are coming from λ = 255°, β =-8° [8]. 2 Observations Figure 1 shows the spectral evolution of the proton spectra at Voyager 1 and 2 between the beginning of 1993 and the end of 1994. Because the analysis uses events from several different telescopes and trigger conditions, and there is a question of the normalization of the fluxes between these different event types, a 10% systematic error has been added in quadrature with the statistical fluctuations for points with energies <150 MeV. The >150 MeV points are …