An Analysis of the Impact of Dynamic Small-Scale Flux-Rope Acceleration on Particle Acceleration Behind Shocks in the Inner Heliosphere

Traditionally, energetic particle flux enhancements in the large-scale solar wind have predominantly been explained in terms of diffusive shock acceleration (DSA). Recent research suggests that acceleration processes related to magnetic reconnection behind traveling shocks occurring in the vicinity of interplanetary coronal mass ejections (ICMEs), which are a major factor in space weather, can explain observed flux enhancements that contradict predictions by standard steady state DSA theory. A better way of interpreting flux enhancements in the vicinity of traveling shocks might be to consider a combination of DSA and acceleration by dynamic small-scale flux-rope (SMFR) downstream. To build up support for this new paradigm, data from the Helios A spacecraft has been analyzed to reveal two new SMFR flux enhancement events closer to the Sun than in any previous study. We developed a new comprehensive analytical solution of our Parker transport equation for energetic particle acceleration by dynamic SMFRs in spherical geometry that include all SMFR mechanisms in the transport theory. By fitting the spherical solution to the flux enhancement data with the Metropolis-Hastings algorithm, combined with using Grad-Shafranov reconstruction to determine SMFR properties, we identified the most probable dominant SMFR acceleration mechanism for these two new events. Our results indicate that 2nd order Fermi acceleration by the turbulent ideal MHD electric field parallel to the SMFR guide field is most likely responsible for the observed enhanced energetic particle fluxes.