Plasma and Turbulence Studies

Magnetic reconnection and its implications

The problem of what happens with magnetic fields in highly conductive astrophysical plasmas is of major importance. If magnetic fields are nearly perfectly frozen in the magnetized fluid, as it follows from the textbook Sweet-Parker solution, then magnetized fluids are very different from fluids and resemble more felt or Jello. Attempts to appeal to collisionless effects do not produce a good explanation, as, first of all, many astrophysical fluids (e.g. most phases of the ISM) do not satisfy the criterion of being collisionless. A promising solution for the problem of reconnection has been suggested in Lazarian & Vishniac (1999, henceforth LV99), where it was shown that magnetic reconnection gets fast, i.e. independent of resistivity, in the presence of turbulence. This potentially provides a universal solution of the magnetic reconnection in astrophysics as turbulence is really ubiquitous in astrophysical environment.

Analytical predictions of the LV99 model of magnetic reconnection have been tested successfully numerically in Kowal et al. (2009) (see also higher resolution runs in Lazarian et al. 2010). More recently, the LV99 model was related to the well-known concepts of Richardson diffusion in magnetized fluids. The corresponding paper by Eyink, Lazarian & Vishniac (2011) has attracted the attention of the community by showing how LV99 reconnection model fits within the modern understanding of properties of turbulent fluids.

This calls for searching for consequences of the magnetic reconnection. The directions explored so far include cosmic ray acceleration and star formation. For the acceleration processes the LV99 reconnection induces first order Fermi reconnection, which can explain cosmic rays accelerated by different astrophysical objects. For star formation, the LV99 model predicts much faster removal of magnetic flux compared to the accepted paradigm based on ambipolar diffusion.

MHD turbulence and its implications

Properties of magnetic turbulence are important for many astrophysical processes, including star formation, acceleration of cosmic rays, transport of matter etc. We have studied this properties combining analytical and numerical tools.

Important progress has been achieved in both understanding of the imbalanced and balanced MHD turbulence. Our simulations got results consistent with the Beresnyak & Lazarian (2008) model  of turbulence, at the same time providing gross inconsistency with the predictions of all other existing  models. In terms of the balanced MHD turbulence, we showed that the turbulence shows diffuse non-locality, meaning that it is less local than MHD turbulence. As a result, we proved that present-day numerical simulation can not reveal the true spectral slope of turbulence making meaningless the tests of MHD turbulence theories based on the deviations of the spectral index from the Kolmogorov one.

We used our kinetic MHD code to show that in the presence of collisionless instabilities the turbulence loses its self-similarity accummulating fluctuations at the small scales. This sends a signal of warning to naïve modeling of intracluster medium and gives additional support to the Brunetti & Lazarian (2011) model of turbulence-particle interaction in intracluster plasmas (see below).

Interactions of MHD turbulence with energetic particles

Substantial improvement of understanding of turbulence-particle interactions has been achieved by Brunetti & Lazarian (2011a,b). In these papers, first of all, the constraints on the models of cosmic ray reacceleration were derived, more importantly, we argued that the damping of fast modes which were shown in Brunetti & Lazarian (2007) to dominate cosmic ray acceleration (see also Yan & Lazarian 2004), is much less than it follows from the naïve application of textbook plasma physics formulae. In fact, due to gyroresonance instability developing at the Larmor scale of plasma ions, the effective mean free path of particles decreases and the fluids become effectively collisional. We showed that as a result of this effect the efficiency of the acceleration of particles by turbulence increases.

Studies of Interstellar Turbulence Statistics from Observations

The major advance in this field has been a development of a new technique of studying magnetic turbulence from synchrotron fluctuations in Lazarian & Pogosyan (2011). There the description of synchrotron fluctuations for the arbitrary index of cosmic ray spectrum and models of axisymmetric turbulence corresponding to the models numerically proven in Cho & Lazarian (2003) has been performed. The former problem was a long standing one and cracking it opens new wide avenues for both better describing synchrotron fluctuations, including the fluctuations of synchrotrong polarization, which quantitative description is essential for studying illusive CMB B-modes and for bringing the studies of magnetic turbulence in our and nearby galaxies to a new stage. The achieved theoretical progress is very timely in view of the advancements in the SKA and LOFAR projects. 

Tsallis statistics to simulated maps of column desity, studies of ISM magnetization via studies of anisotropy of observer-measurable velocity centroids fluctuations. Important papers related to the studies of Big Power Law (in terms of electron density fluctuations) in the sky and velocity fluctuations for HI at high galactic altituted have been published. We also used our understanding of fluctuations arising from turbulence for proposing a new technique of separating foregrounds from CMB (see Cho & Lazarian 2010).

Properties of dusty plasmas and implications

Astrophysical environments are dusty and dust plays an important role for many astrophysical processes. Our study is focused on three directions:

1. Alignment of interstellar, circumstellar and interplanetary dust, which provides a way to reliable studies of magnetic fields and turbulence.
2. Acceleration of dust particles with implications for coagulation and fragmentation of them in different environments.
3. Microwave emission from the smallest population of particles, which is also known as spinning dust emission.

All these directions of dust-related research are of high astrophysical significance, as, for instance, spinning dust emission is an important component of foreground interfering with the CMB measurements.