Grzegorz Kowal.

A turbulência magnetizada é onipresente nos plasmas astrofísicos e crucial para a formação de estruturas densas e estrelas, a estabilidade de nuvens moleculares, a amplificação de campos magnéticos, a reconexão magnética e a produção e difusão de raios cósmicos. Durante a última década, ficou claro que a cascata de turbulência magnetizada que transfere energia dos movimentos em grande escala para a escala de dissipação não pode ser completamente compreendida sem levar em conta a reconexão magnética, um processo fundamental em plasmas magnetizados responsável por mudanças topológicas de campos e associado à liberação de energia magnética em regiões de aniquilação do campo magnético. Por outro lado, a reconexão magnética na presença de turbulência externa ou autogerada torna-se rápida. Esses fatos indicam uma relação forte e íntima entre esses dois fenômenos e sua importância para a dinâmica dos sistemas astrofísicos. Além disso, eles podem ser responsáveis pela radiação não térmica, atuando como processo de Fermi de primeira ordem, um mecanismo de aceleração capaz de produzir raios cósmicos de alta energia a partir de partículas térmicas em sistemas não relativísticos e relativísticos sem a necessidade da presença de fortes choques. Portanto, uma compreensão profunda desses três fenômenos: turbulência magnetizada, reconexão magnética e aceleração dos raios cósmicos e a interligação entre eles é a chave para entender a dinâmica dos plasmas astrofísicos. Por várias décadas, um esforço significativo foi colocado na compreensão desses fenômenos de forma independente. Nos últimos anos, porém, a comunidade científica tem percebido a importância de se entender a relação entre esses processos. O objetivo deste projeto é realizar um estudo teórico sistemático apoiado em modelagem numérica da interligação entre turbulência, reconexão magnética e produção de raios cósmicos em plasmas não relativísticos e relativísticos, com foco em regimes fortemente e fracamente colisionais, cobrindo a grande maioria de plasmas astrofísicos. Pretendemos investigar o papel das instabilidades na geração de turbulência local, a força e propriedades desta turbulência, seus efeitos na taxa de reconexão global, a eficiência do mecanismo de aceleração baseado na reconexão magnética turbulenta, bem como, as dependências destes processos do plasma-$\beta$, o número de Lundquist e o grau de anisotropia de pressão. As conclusões desses estudos serão aplicadas para entender as medições de satélite das interações entre o vento solar e a magnetosfera, o papel da reconexão turbulenta na dinâmica da coroa solar, o papel da difusão da reconexão magnética no processo de formação estelar, as observações de radiação não térmica em estrelas binárias com ventos em colisão, erupções de raios gama e núcleos galácticos ativos.

Nowadays in the astrophysical community, the problem of magnetic reconnection is usually studied in the non-relativistic case and approached using two different points of view, on microscopic scales using collisionless kinetic numerical modeling, and on macroscopic scales using magnetohydrodynamical (MHD) fluid approximation, which describes well collisional media. Both approximations have their strong points, but also significant limits, sometimes resulting in separate parameter spaces or different applications. The aim of this project is to extend the magnetic reconnection studies into the relativistic case in both regimes, collisional and collisionless. This requires an extension of our numerical tools, which work well in the classical regime, to include relativistic effects. In the case of MHD we usually apply special relativity, however, for collisionless fluid approximation based on the Chew-Goldberg-Low MHD approximation (CGL-MHD) allowing for the presence of the kinetic instabilities, relativistic version has to be derived. The results obtained with the collisionless approximation can be directly compared with the relativistic Particle-in-Cell (PIC) simulations, using for example a hybrid kinetic-MHD models, which dominate the studies in this regime up to date. As a consequence, this project will allow for getting a deeper insight into the reconnection problem and particle acceleration in most of the physical regimes found in the astrophysical objects, broadening application of magnetic reconnection and related processes in the relativistic regime. These applications in plasma physics cover turbulence generation and its feedback on the reconnection process, magnetic field amplification mechanism, high energetic particle acceleration and propagation, applications in highly relativistic environments, such as active galaxy nuclei and gamma ray burst, where the process of reconnection together with turbulence is expected to be very important in the acceleration of TeV particles. Moreover, the properties of turbulence and reconnection in relativistic environments can also significantly benefit from our studies.

The High Energy and Plasma Astrophysics group of E. M. de Gouveia Dal Pino of the Instituto de Astronomia, Geologia e Ciências Atmosféricas da Universidade de São Paulo (IAG-USP), A. B. M. Valio of the Centro de Radioastronomia e Astrofísica da Universidade Mackenzie (CRAAM), and Diego Falceta Goncalves of EACH-USP, have been developing for more than a decade several correlated projects using theoretical and numerical magneto-hydrodynamics (MHD) fluid description, as well as observational tools, to investigate astrophysical plasmas and high energy phenomena. In the recent years, our group has been also involved in the Cherenkov Telescope Array (CTA) Project, an international collaboration (involving 26 countries) aiming the construction of a new generation of ground-based gamma-ray telescopes, between 2016-2020. CTA will provide the deepest ever insight into the non-thermal, high-energy universe and allow significant contributions to cosmology, astroparticle physics and, in particular, to cosmic ray accelerators, cosmic magnetic fields and gamma-ray burst (GRB) investigation. In this proposal we got funding from FAPESP to support the construction of 3 Cherenkov telescopes to complete the ASTRI-CTA Mini-Array, in partnership with INAF- Italy and South-Africa. This Mini-Array of 7 telescopes will be the precursor and the core of the full CTA. The participation in the Mini-Array construction will ensure to the Brazilian members a solid involvement with the CTA Besides, in view of the extensive number of related projects being developed by our group, we also obtained funding to upgrade our computational facilities with a clustar of over 700 cores.

Nowadays in the astrophysical community, the problem of magnetic reconnection is approached using two different points of view, from the micro-physical scale using collisionless kinetic numerical studies, and from macroscopic magnetohydrodynamical fluid approximation studies, describing well the collisional media. Both approximations have their strong points, but also significant limits, sometimes resulting in separate parameter spaces. There is no real afford taken to understand the bridge between those two approximations applied to the magnetic reconnection problem. The aim of this project is to fill this gap and perform studies which allow to understand the precess of reconnection in both regimes, collisional and collisionless. The collisionless studies, using the Chew-Goldberg-Low magnetohydrodynamics fluid approximation (CGL-MHD) allowing for the presence of the kinetic instabilities, can be directly compared with the Particle-in-Cell (PIC) simulations, using for example a hybrid kinetic-MHD models, which dominate the studies in this regime up to date. As a consequence, the second part of this project will cover the applications of the magnetic reconnection and related processes in both regimes. These applications in plasma physics cover turbulence generation and feedback on the reconnection process, magnetic field amplification mechanism, high energetic particle acceleration. The results have applications in the physics of solar flares, the importance of reconnection in space weather (e.g. interactions with magnetosphere and processes in magnetotail), the interactions of solar magnetic field with the interstellar field in the heliopause, going further up to the scales of active galaxy nuclei and gamma ray burst, where the process of reconnection together with turbulence is expected to be very important in the acceleration of TeV particles. Finally, the properties of turbulence and reconnection in collisionless environments, such intracluster medium, can benefit significantly from our studies.

Joint Proposal FAPESP-HEBREW University of Jerusalem (HUJ) (process: 2011/51275-4): Supersonic jets are ubiquitous in the Universe as they are associated to a wide variety of astrophysical objects - from young stars to gamma-ray-bursts (GRBs). In particular, GRBs are the most energetic and probably most mysterious phenomena in the cosmos and in this joint Proposal with Prof. T. Piran of HUJ, one of the most prominent experts in the world in GRBs theory, we aim to investigate several aspects of the physics of the ultrarelativistic jets associated to GRBs. In this context, we plan to study: magnetic field reconnection, magnetic turbulence, MHD dynamics of jets, and particle acceleration by collisionless magnetized shocks and by magnetic reconnection within the relativistic regime. This project will maximize the synergy between the two groups. The research will involve both analytic techniques and numerical simulations. It will also involve analysis of astronomical satellite data concerning GRBs and the possibility to make theoretical predictions for the coming CTA (Cherenkov Telescope Array) gamma-ray observatory.

O projeto enquadra-se no Programa Equipamentos Multiusuários da FAPESP e destina-se à aquisição de um super-cluster para o Departamento de Astronomia e também para o Núcleo de Astrfíscia da UnicSul destinado sobretudo à realização de modelagem numérica de fontes e meios astrofísicos.

One of the fundamental properties of astrophysical magnetic fields is an ability to change their topology through a process known as magnetic reconnection. There is no possible understanding of the origin and evolution of the large-scale magnetic field without understanding the way magnetic field lines change. In a typical astrophysical plasma, magnetic diffusion of the medium is negligible and the so called ?frozen-in? condition is responsible for the change of magnetic topology. Reconnection is essential for understanding a wide variety of astrophysical processes, including stellar and galactic dynamos, astrophysical turbulence and the structure of accretion disks. To account for solar or star flares, the solar cycle, and the structure of the galactic magnetic field, reconnection must be fast, propagating with a speed close to the Alfv ́en speed. Earlier attempts to deal with magnetic reconnection assumed that the magnetized fluids are laminar and as a result they led to slow reconnection rates. The presence of a random magnetic field component substantially enhances the reconnection rate and enables fast reconnection, i.e., reconnection that does not depend on fluid resistivity. The enhancement of the reconnection rate is achieved via a combination of two effects. First of all, only small segments of magnetic field lines are subject to direct Ohmic annihilation. Thus the fraction of magnetic energy that
goes directly into fluid heating goes to zero as fluid resistivity vanishes. However, the most important enhancement comes from the fact that unlike the laminar fluid case where reconnection is constrained to proceed line by line, the presence of turbulence enables many magnetic field lines to enter the reconnection zone simultaneously. A significant fraction of magnetic energy goes into MHD turbulence and this enhances reconnection rates through an increase in the field stochasticity. In this way magnetic reconnection becomes fast when field stochasticityaccounted for. As a consequence solar and galactic dynamos are also fast, i.e., do not dependfluid resistivity. Another important issue in reconnection theory is on how the released energy in this process is transformed in heating and particle acceleration.
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