Suppression of Non-linear Saturation in Collisionless, High-Beta Slow Modes

With the support of hybrid-kinetic simulations and analytical theory, we demonstrate that large-amplitude (δB_‖/B0 ~ 1/2), long-wavelength slow modes in a high-beta collisionless plasma can suppress non-linear saturation of their transit-time damping. Due to their polarization, collisionless slow modes (non-propagating modes) of sufficient amplitude induce significant positive pressure anisotropy (p_⊥> p_‖) in regions where the plasma beta is enhanced, facilitating rapid growth of the mirror instability. Conversely, regions of negative anisotropy are accompanied by a decreased plasma beta, impeding activation of the firehose instability. Once the mirrors are of sufficient amplitude, they pitch-angle scatter the plasma ions, eroding the non-linear plateau and allowing the slow mode to resume its decay at the linear damping rate. The mirror-induced effective collisionality is investigated with respect to the slow-mode amplitude and the scale separation between the slow and mirror modes. These results provide yet another simple example, alongside self-interrupting Alfvén waves (Squire et al. 2016, 2017) and self-sustaining sound (Kunz et al. 2020), of how energetically weak magnetic fields fundamentally change the transport properties of a low-collisionality, magnetized plasma.