Extreme energetic particle events by superflare-asssociated CMEs from solar-like stars.

Discovery of frequent superflares on active cool stars opened a new avenue in understanding the properties of eruptive events and their impact on exoplanetary environments. Solar data suggest that coronal mass ejections (CMEs) should be associated with superflares on active solar-like planet hosts and produce solar/stellar energetic particle (SEP/StEP) events. Here, we apply the 2D Particle Acceleration and Transport in the Heliosphere model to simulate the SEPs accelerated via CME-driven shocks from the Sun and young solar-like stars. We derive the scaling of SEP fluence and hardness of energy spectra with CME speed and associated flare energy. These results have crucial implications for the prebiotic chemistry and expected atmospheric biosignatures from young rocky exoplanets as well as the chemistry and isotopic composition of circumstellar disks around infant solar-like stars.

Collisional magnetized shock waves: One-dimensional full particle-in-cell simulations.

Although collisional electrostatic shock waves have been investigated extensively via theory, simulations, and experiments, there are comparatively few studies about collisional magnetized shock waves. We investigate collisional magnetized shocks by performing one-dimensional full particle-in-cell simulations that incorporate ion-ion, electron-electron, and ion-electron Coulomb collisions, for perpendicular and quasiparallel shock waves. The effect of Coulomb collisions is to drive a shock wave into a more laminar state. For a perpendicular shock, the magnetic overshoot becomes small because the electron pressure perpendicular to the magnetic field is isotropized and decreases due to electron-electron collisions. For the quasiparallel case, we find that ion-electron collisions severely suppress the standing whistler wave, which is present in the form of large amplitude waves in a collisionless shock wave.

Transition to turbulence in a five-mode Galerkin truncation of two-dimensional magnetohydrodynamics.

The chaotic dynamics of a low-order Galerkin truncation of the two-dimensional magnetohydrodynamic system, which reproduces the dynamics of fluctuations described by nearly incompressible magnetohydrodynamic in the plane perpendicular to a background magnetic field, is investigated by increasing the external forcing terms. Although this is the case closest to two-dimensional hydrodynamics, which shares some aspects with the classical Feigenbaum scenario of transition to chaos, the presence of magnetic fluctuations yields a very complex interesting route to chaos, characterized by the splitting into multiharmonic structures of the field amplitudes, and a mixing of phase-locking and free phase precession acting intermittently. When the background magnetic field lies in the plane, the system supports the presence of Alfvén waves thus lowering the nonlinear interactions. Interestingly enough, the dynamics critically depends on the angle between the direction of the magnetic field and the reference system of the wave vectors. Above a certain critical angle, independently from the external forcing, a breakdown of the phase locking appears, accompanied with a suppression of the chaotic dynamics, replaced by a simple periodic motion.

Energy dissipation and entropy in collisionless plasma.

It is well known that collisionless systems are dissipation free from the perspective of particle collision and thus conserve entropy. However, processes such as magnetic reconnection and turbulence appear to convert large-scale magnetic energy into heat. In this paper, we investigate the energization and heating of collisionless plasma. The dissipation process is discussed in terms of fluid entropy in both isotropic and gyrotropic forms. Evolution equations for the entropy are derived and they reveal mechanisms that lead to changes in fluid entropy. These equations are verified by a collisionless particle-in-cell simulation of multiple reconnecting current sheets. In addition to previous findings regarding the pressure tensor, we emphasize the role of heat flux in the dissipation process.