MHD surface waves, fluctuations of the heliospheric current sheet and related acceleration of energetic particles in the solar wind

The occurrence of both large and small-scale MHD surface waves on the heliospheric current sheet has been known for decades (i.e., Musielak & Suess, 1988, 1989; Ruderman, 1990). Recent studies suggest that the waves can produce unexpected effects related to particle acceleration. Observations show that (i) the HCS often possesses a rippled form, with ripples of rather small scales ( 0.1 AU), (ii) there are signatures of fluctuations of the HCS with periods from minutes to hours, and (iii) HCS ripples are filled with small-scale magnetic islands presumably produced by magnetic reconnection (Cartwright&Moldwin, 2010; Eriksson et al. 2014; Khabarova et al. 2015, 2016). At the same time, HCS crossings are often associated with energetic particle flux enhancements, which may be explained by dynamical processes in magnetic islands (Zank et al. 2014, 2015; le Roux et al. 2015, 2016). We have developed an analytical model of the electromagnetic field in an electro-neutral periodically fluctuating magnetic island. The electric field is supposed to be purely inductive and has an essential longitudinal component almost in the entire island during the entire period of its fluctuations. Also, we have numerically simulated trajectories, energies and angle distribution function for velocity directions of protons accelerated within a fluctuating bubble, employing several sets of typical parameters of the solar wind observed near the HCS at 1 AU. Energetic particles are suggested to be pre-accelerated to tens keV by magnetic reconnection at the HCS. We show that MHD surface waves can produce resonant effects in magnetic islands, which leads to significant additional particle acceleration (from keV to MeV energies) near the HCS. Interestingly, we found that (i) particles with lesser initial energies experience larger relative acceleration than particles with higher initial energies, and (ii) acceleration occurs until all particles finally gain some limit energy. This may explain the energy threshold phenomenon observed in plasmas with magnetic islands: the amplification factor is largest for the smallest energy below the threshold and the order of amplification is inversed above the threshold, which suggests the existence of different mechanisms of particle acceleration associated with magnetic islands (Zank et al.2015).