Narrow Results

Intermittency is an essential property of astrophysical fluids, which demonstrate an extended inertial range. As intermittency violates self-similarity of motions, it is impossible to naively extrapolate the properties of a fluid obtained computationally with relatively low resolution to the actual astrophysical situations. In terms of astrophysics, intermittency affects turbulent heating, momentum transfer, interaction with cosmic rays, radio waves and many more essential processes. Because of its significance, studies of intermittency call for coordinated efforts from both theorists and observers. In terms of theoretical understanding, we are still just scratching the surface of a very rich subject. We have some theoretically well-justified models that are poorly supported by experiments; we also have the She–Leveque model, which could be vulnerable on theoretical grounds, but, nevertheless, is well supported by experimental and laboratory data. We briefly discuss a rather mysterious property of turbulence called "extended self-similarity" and the possibilities that it opens up for the intermittency research. We then analyze simulations of MHD intermittency performed by different groups and show that their results do not contradict to each other. Finally, we discuss the intermittency of density, of turbulence in the viscosity-dominated regime as well as of polarization of Alfvenic modes. The latter provides an attractive solution to account for a slower cascading rate that is observed in some of the numerical experiments. We conclude by claiming that substantial progress in the field may be achieved by studies of turbulence intermittency via observations.

We study the problem of deceleration of an arbitrarily magnetized relativistic ejecta in a static unmagnetized medium and its connection to the physics of gamma-ray bursts (GRBs). By computing exact solutions of the Riemann problem describing this scenario, we find that with the same initial Lorentz factor, the reverse shock becomes progressively weaker with increasing magnetization parameter σ (the Poynting-to-kinetic flux ratio). The reverse shock becomes a rarefaction wave when σ exceeds a critical value defined by the balance between magnetic pressure in the ejecta and thermal pressure in the forward shock. In the rarefaction wave regime, the rarefied region is accelerated to a Lorentz factor that is significantly larger than the initial value due to the strong magnetic pressure in the ejecta. We discuss the implications for models of GRBs.

Initial discovery of Cosmic Rays (CRs) dates back to a century ago (1912). Their identification as particles rather than radiation dates to about 20 years later and in 20 more years also the first suggestion that they were associated with SNRs was in place. The basic mechanism behind their acceleration was suggested almost 40 years ago. Much work has been done since then with regard to the aim of proving that both the acceleration mechanism and site are well-understood, but no definite proof has been obtained: in spite of impressive progress of both theory and observations, the evidence in support of the commonly accepted interpretation is only circumstantial. In the following, I will try to make the point on where we stand in terms of how our theories confront with data. I will review recent progress on the subject and try pointing the avenues to pursue in order to gather new proofs, if not a smoking gun evidence of the origin of Galactic CRs.

The results of MHD simulations of the formation and development of magnetized jets on a NEODIM laser installation are presented. We simulated a plasma flow and chose the numerical method, boundary and initial conditions. We investigated the picture of the flow and compared it with the experiment.