Radial and Latitudinal Intensity Gradients of Anomalous Cosmic Rays During the Solar Cycle 22 Recovery Phase

The structure of the heliosphere can be probed by studying the spatial intensity gradients of anomalous cosmic ray (ACR) ions because the transport of these particles is influenced by heliospheric phenomena such as the solar wind, heliomagnetic turbulence, and large-scale drift processes. Moreover, the ACR source itself is thought to be within the heliosphere, namely, at the termination shock of the solar wind. Calculation of non-local radial and latitudinal intensity gradients requires three separate, simultaneous observation positions, such as the Voyager 1 and 2 spacecraft and the well-monitored near-Earth region of space, from which suitable ACR observations are available. Unfortunately, with such large spacecraft separations, the considerable assumptions needed concerning the form and spatial dependence of the intensity gradients place the meaningfulness of this sort of gradient determination in question. Although the non-local method is appropriate for widely-spaced spacecraft if the spatial gradients are small or well-understood, there is no a priori reason to expect low-energy ACRs, for example, to meet this restriction. Fortunately, with an alternative method, it is possible to calculate gradients by making primarily temporal rather than the spatial assumptions, utilizing fewer than three spacecraft. Since, during the most recent recovery phase, a single form for the temporal dependence, 1-exp(-t), is widely observed to successfully describe cosmic ray measurements, while knowledge of the local spatial structure between the inner and outer heliosphere is quite limited, use of the alternative gradient method is appropriate. We present this method, calculate radial and latitudinal intensity gradients using ACR measurements from the Voyager LECP instruments, and discuss the implications these results have on our understanding of ACR transport and the structure of the heliosphere.