Anomalous cosmic ray acceleration and the Voyager paradox

Anomalous cosmic rays (ACRs) are charged particles (ions and electrons) accelerated to high energies in the outer heliosphere, at distances of 80-150 AU from the Sun. The two Voyager deep space probes have been monitoring ACR activity for over 30 years. Starting in 2002 Voyager 1 began to observe a new,
lower-energy particle population believed to the related to ACRs. This population became more intense as the spacecraft was approaching the solar-wind termination shock, long though to be the source of ACRs. However, as Voyager 1 crossed the shock near the end of 2004, energetic particle spectra did not
unfold into a power law, as was predicted by theories of shock acceleration. Instead, the two-component nature of the spectrum persisted some distance into the heliosheath, the region of shocked solar wind acting as a "buffer zone" between the solar and the interstellar plasmas.

Voyager 2 observations made during its crossing in mid-2007 painted a similar picture, with a few quantitative differences. Several new theories have been put forward to explain the ACR paradox. As an enormous evolving structure with a complex shape and history the termination shock is unlike most interplanetary shocks in that particle acceleration occurs intermittently and only at some locations. For example, acceleration may be more efficient near the ecliptic plane or in the "flanks" of the shocks, both regions not visited by either Voyager. Shock acceleration is also heavily influenced by the dynamics of the solar wind whereby large-scale transient structures disrupt the acceleration process. This talk will discuss competing new theories of ACR acceleration at a blunt and dynamically evolving termination shock in light of the recent Voyager results.

Vladimir Florinski

UAHuntsville