Evidence from Voyager 1/2 that Anomalous Cosmic Rays are Accelerated in the Heliosheath "Reservoir"

The heliosheath (at least that which has been sampled by the pair of Voyager spacecraft, VGR1 and VG2), is a "reservoir" filled with energetic particles, from 0.04-4.0 MeV ions up to and including anomalous cosmic ray (ACR) ions (H, He, C, O) in the 100 MeV-range of total energy. The rapid jump of the 0.04-4.0 MeV ion intensities indicated that they were clearly accelerated by the solar wind termination shock (TS), but their intensities then remained relatively constant thereafter inside the heliopause (HP). However, the ACR intensities rose gradually before the TS and continued to increase after the Voyagers crossed TS. The VGR1 ACR intensities maximized just before they essentially disappeared upon crossing the heliopause (at 121 AU) into the interstellar medium (whereafter only galactic cosmic rays of these species were detected at much lower intensities). Evidence, both observational and theoretical, is presented that the ACRs are being accelerated within the heliosheath itself. As observed by Hill et al. (2006) and later predicted by "reservoir" propagation theory (Roelof, 2012), a striking ordering of the ACR composition is obtained when computed in terms of total ion energy/charge (or equivalently, total energy, since almost all ACRs have charge +1 because of their origins as interstellar pickup ions). All four ACR species with common total energy ranges exhibited proportional intensity histories throughout VGR1's 8-year (94-121 AU) transit of the heliosheath. The signature (and site) of the ACR acceleration mechanism is identified with the nearly linear decrease of the radial component of the VGR1 plasma velocity over 2.5 years (104.0-113.2 AU). The absence of an increase of the 0.04-1.0 MeV ions is also in quantitative agreement with the predictions of "reservoir" transport. Although the ACR intensities at VGR2 have been similarly well-ordered in total energy, the corresponding signature of a gradient in the radial plasma velocity of the acceleration mechanism has not yet been obvious in the VGR2 data, perhaps because VGR2 (now 114 AU from the Sun) is only part way to the heliopause, and because the signature may be masked by the greater temporal ion intensity variations (compared to those measured by VGR1).