Large Increases of Low-Energy Ion Intensities at Voyager 1 (85 AU) in Mid-2002: Association with Solar Activity in Late-2001

We report on observations of relatively large intensity increases of low-energy ions (~30 keV to ~10 MeV) that began at Voyager 1 in July 2002. At this time Voyagers 1 and 2 were at respective helioradii 85 AU and 68 AU, and at heliographic latitudes 34°N and 24°S. We use data from the Low Energy Charged Particle (LECP) instruments on each spacecraft. Thus far during the mid-2002 event (which was still evolving when this abstract was written), peak intensities of protons 0.6-1.8 MeV and 3-17 MeV have reached ~10% and ~30%, respectively, those observed by Voyager 1 in association with the powerful GMIR-driven shock in late-1991, when the spacecraft was at 45 AU (i.e., half as far from the Sun). The intensity increases at Voyager 1 are relatively rapid (~few days) and nearly coincident for ion energies from at least 30 keV to several MeV. This is strong evidence that the intensity increases arose from local acceleration at a heliospheric shock, probably that driven by an MIR formed by coalescence of ejecta from enhanced solar activity during the period October-November 2001. An estimate of the disturbance's average radial speed during the ~8-9 months it took to reach 85 AU is then ~600-550 km/s. In contrast to the situation at Voyager 1, energetic ion intensities at Voyager 2 have remained at relatively low levels during 2002. There is no evidence that the disturbance that began passage by Voyager 1 in July passed Voyager 2 ~1-2 months earlier, as one might expect for a uniformly expanding spherical disturbance. The disturbance is evidently non-spherical and is confined mainly to latitudes well north of Voyager 2. This is consistent with observations made in the inner heliosphere. The October-November 2001 solar activity produced high intensities of energetic ions not only near the ecliptic at 1 AU, but also at Ulysses, which was at ~2 AU and above ~70°N heliographic latitude during this period. We will discuss the Voyager 1 low-energy ion data, including angular distributions. We will also describe the effects, if any, of the disturbance on higher energy ions (e.g., Forbush decreases); however, such effects have yet to appear in the evolving intensity profiles.