Dynamical Coupling between Anomalous Cosmic Rays and Solar Wind in Outer Heliosphere

Abstract

Voyager 2 (V2) observed that pickup ions (PUIs) and anomalous cosmic rays (ACRs) have a significant influence on the solar wind structures in the outer heliosphere. In particular, the maxima in energetic particle intensities often lagged behind the shock front, while the flow velocity in some cases featured a precursor in front of the shock. These two effects are believed to be caused by the cease of ACR injection from PUIs at large heliocentric distances, and the backward diffusion of ACRs, respectively. This paper investigates the dynamical coupling between the ACRs and the solar wind in the outer heliosphere by means of a time-dependent numerical MHD simulation, in which the ACRs are treated as a massless fluid that only contributes its pressure to the system. Two types of inner boundary conditions were used, namely a synthetic short-term shock event and an extended period of data-driven solar wind variability based on the OMNI database. The lag of the ACR pressure maximum relative to the shock front, and the extended shock precursor were reproduced by the numerical results. The increase rate of the lag is related to the corresponding diffusion coefficient and the injection efficiency from PUIs to ACRs. The model was also applied to the termination shock, where simulations likewise showed that the peak in the ACR distribution can be located a short distance downstream of the shock front, indicating that the time-dependent diffusive shock acceleration mechanism is a candidate to interpret the lag between the ACR pressure peak and the shock front observed by V2.

Effects of anomalous cosmic rays on the solar wind events in the outer heliosphere

“…In addition, the separation of the shock front from the peak of the ACRs count rate observed by Voyager 2 (Lazarus et al, 1999) may be due to an interruption in the formation of ACRs from PUIs via DSA, which leads to the peak convecting with the solar wind and separating from the shock front . After crossing the termination shock, Voyager 2 detected a short-term enhancement of the count rate of ACRs, which has been discussed through the magnetic reconnection, (e.g., Drake et al, 2010) and the related stochastic acceleration mechanism (Zank et al, 2015;Zhao et al, 2019); alternatively, it may also be related to the temporary increase in shock intensity caused by the passage of interplanetary shocks through the termination shock (Guo et al, 2022).…”