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:
- Alignment of interstellar, circumstellar and interplanetary dust, which provides a way to reliable studies of magnetic fields and turbulence.
- Acceleration of dust particles with implications for coagulation and fragmentation of them in different environments.
- 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.
Run by Alex Lazarian.
Outcome: A new model of reconnection has been successfully tested. The tested model is unique as it predicts fast reconnection for a wide variety of astrophysical parameters.
Transformative: The testing opens wide avenues of application of the original Lazarian & Vishniac model of reconnection to solving fundamental astrophysical problems from solar flares and star formation to gamma ray bursts.
Scientific problem: Magnetic reconnection is a fundamental process that governs how magnetic fields of different polarity interact and annihilate in plasma. This process is accepted to drive solar flares and speculated to affect many key astrophysical processes, but how magnetic reconnection can proceed fast in low resistivity cosmic plasma has been a long standing mystery.
Breakthrough: The paper provided successful numerical testing of the analytical quantitative predictions of the model of fast magnetic reconnection proposed by Lazarian & Vishniac 1999. Unlike earlier discussed solutions of the problem the Lazarian & Vishniac provides a universal solution applicable to various astrophysical environments and has very broad impact from more accurate predictions within Space Weather project to explaining mysterious gamma ray bursts.
Potential benefits: Understanding magnetic reconnection allows to better predict the occurrence of solar flares accelerating copious amounts of energetic particles which destroy sensitive equipment of space satellites and causes a lot of economic damage. In terms of scientific impact, it allows to evaluate the degree to which sophisticated numerical modeling of astrophysical processes represent astrophysical reality and suggest new ways to improve numerical modeling. In addition, it predicts new important process (see items 2, 3, and 4). NSF role for funding this research as well as the NSF funded Centers for Supercomputing where the high end computations were performed was absolutely essential for the success.
Outcome: A source of high energy particle acceleration was proposed to be within our Solar system.
Transformative: The model provides explanation for the puzzling observational data and may be a direct evidence of the acceleration of cosmic rays by magnetic reconnection.
Scientific problem: Cosmic rays are energetic particles that are accelerated to high energies by processes that are still hotly debated. Due to scattering by magnetic fields between us and their enigmatic sources the cosmic rays come to the Earth with high degree of uniformity. However, from a particular direction a significant excess of the cosmic ray arrivals has been detected.
Solution: We identified this direction with the magnetic wake produced by Solar system and identified the mechanism of acceleration with magnetic reconnection that happens as magnetic fields produced by the Sun and alternating due to magnetic field changes induced by Solar cycle reconnect and annihilate in the turbulent solar system wake.
Significance: This may be the first identification of an efficient high energy cosmic ray accelerator. Similar acceleration can happen in other astrophysical systems explaining a significant part of the observed cosmic rays. The results have been discussed by New Scientist magazine.
Outcome: Electron density fluctuations in our Galaxy represent a power spectrum of turbulence over many orders of magnitude. We extended this power law and it now spans for more than 11 orders of magnitude of scales.
Transformative: The results clearly support turbulence being the formative agent of the interstellar medium with energy being injected at scale of parsecs and going to scales less than an astronomical unit. This extension is essential for understanding processes from star formation to those of cosmic ray propagation. This is one of largest power laws of nature!
Scientific problem: It has been realized in the last decade that turbulence is one of the major agents affecting most of the processes in Astrophysics. For instance, turbulence has been accepted as the major process shaping interstellar medium. The sources of interstellar turbulence are being debated, however. The scale of stirring of turbulent motions or in other words, the scale of the energy injection, is an important scale on which many very different for galactic processes depend. For instance, the propagation of cosmic rays is very different if interstellar turbulence has injection scale less than a parsec. Similarly, heating of the medium by turbulence also very much depends on the scale of turbulence stirring. The issue of turbulence stirring is a hotly debated issue.
Solution: Using the publicly available WHAM (Wisconsin Halpha Mapper) data we extended the known so-called Big Power Law in the Sky (Armstrong et al. 1994) from 7 to 11 orders of magnitude. It is clear now that turbulence is being stirred at much larger scales than it was tested earlier and the many theories that assumed small scales of turbulence injection should be discarded. Our Extended Big Law in the Sky has been already reproduced in a couple of books, including the interstellar medium graduate textbook written by B. Draine.
Making Starbirth Easier
Outcome: A new model of star formation which uses the advances in understanding of magnetic reconnection has been tested with numerical simulations.
Transformative: The traditional paradigm of star formation based on ambipolar diffusion process faces severe problems explaining observational data. New model can explain new observations and opens ways to better modeling star formation.
Scientific problem: All stars including our Sun were born from clouds of gas as gravitational forces collected the matter from the scales of light years into relatively small dense hot luminous objects. Gas from which stars are born exhibits chaotic turbulent motion and it carries magnetic fields that counteract the gravitational collapse. For years the problem of removing of magnetic field from star forming regions has the focus of intense studies by astrophysicists. Magnetic fields couple with the gas in molecular clouds through their interactions with the minute fraction of ions present in the gas and, traditionally, it is assumed that the slippage of the ions and gas atoms, which is called ambipolar diffusion is responsible for the loss of magnetic field.
Breakthrough: The paper challenges the above accepted paradigm and identifies a process of magnetic reconnection in turbulent media as the major process of magnetic field removal from collapsing molecular clouds. This finding is based on the model of turbulent reconnection by Lazarian & Vishniac 1999. Simulations within the paper exhibit fast removal of magnetic flux without ambipolar diffusion.
Significance: Star formation is one of the most fundamental problems of astrophysics with large interest of general public. Simulations in the paper explain the observational data that cannot be explained with the existing paradigm of ambipolar diffusion.
International collaboration: This work is a result of international collaboration. A student from Brazil came to Madison to work with the PI to test numerically the PI’s idea of new scenario of star formation.
Anomalous Cosmic Rays
Outcome: A new origin of anomalous cosmic rays measured by Voyagers was proposed. It is related to acceleration of particles in sites of reconnection.
Transformative: This may be the first indication that the process of cosmic ray acceleration in reconnection sites works. Reconnection is expected to be widely spread and such processes can be very common explaining the origin of a substantial part of the cosmic ray population.
Scientific problem: When Voyagers passed the shock surrounding our Solar system they did not detect the change in the spectrum of the cosmic rays that was expected. Most of cosmic rays come from our entire galaxy, but a population of the low energy cosmic rays was always related to the acceleration in the termination shock produced by the interaction of the solar wind and the ambient interstellar medium. This explanation became problematic in view of the discovery by Voyagers.
Solution: The paper identifies magnetic reconnection as the source of anomalous cosmic rays.
Significance: This was the first paper suggesting the new mechanisms of the anomalous cosmic rays. The mechanism is applicable beyond the Solar system and corresponds to the PI’s long claim on the crucial importance of reconnection processes for cosmic ray acceleration (the PI was the first who identified in 2003 the reconnection cites with a very efficient first order Fermi acceleration process, claiming that the reconnection cites as efficient in cosmic ray acceleration as shocks (see more de Gouveia dal Pino & Lazarian 2003, astro-ph/0307054).