Session E7: Astrophysicists' Perspectives on Current Problems in Plasma Astrophysics I

Plasma Physics in Clusters of Galaxies

Clusters of galaxies are the largest self-gravitating structures in the universe. Each cluster is filled with a large-scale plasma atmosphere, in which primordial matter is mixed with matter that has been processed inside stars. This magnetized cluster plasma contains both thermal and relativistic species, and is a wonderful laboratory for applying ideas and tools developed in other arenas of plasma physics. Although clusters formed many Gyr ago, the cluster plasma is still being energized today -- but we are not sure by what. The plasma will clearly be affected by ongoing evolution of the gravitating matter in the cluster. Jets driven out from massive black holes in cluster-member galaxies will also impact the cluster plasma. Understanding the importance of these two drivers is one of today's major questions. Radio and X-ray observations have told us a great deal about the plasma atmosphere in the cluster, but the data alone cannot answer the big questions. We need to understand the physical and dynamic state of the cluster plasma; to get there from the data we must use tools and knowledge from MHD and plasma physics. Questions which need to be answered range from plasma mixing across magnetic surfaces, to acceleration of relativistic particles, to the nature of MHD turbulence and dynamos in the cluster environment.   

Astrophysical problems for which high-energy-density physics can matter

The physical scope of astrophysics is vast, spanning all of physics and more. High-energy-density physics (HEDP), concerned with dense and/or high-pressure systems corresponding roughly to energy densities above 10$^{12}$ ergs/cc, connects with a variety of problems in astrophysics. Astrophysical problems to which HEDP can contribute connect with either the physical properties or the nondimensional dynamics now accessible in the laboratory. In assembling a model of planetary structure one must know the relation of pressure and density in the HEDP range; this is being explored in ongoing experiments. In stellar structure the situation is similar with regard to x-ray opacities. Dynamic astrophysical systems are often approximately hydrodynamic, from clump destruction by shock waves to supernova remnant evolution to post-collapse stellar explosions. These systems typically are at high Reynolds number and involve very strong shock waves, which creates the ability to undertake very-well-scaled HEDP experiments aimed at specific problems. Such experiments are now beginning to show results that are not anticipated in computer simulations, and to prove useful in working with astrophysical data. Systems having a dynamically important magnetic field are more difficult. Understanding radiating systems in astrophysics poses substantial challenges, from the atomic physics involved in photoionization to the structure of radiative shocks in several regimes to the challenge of doing accurate simulations involving both radiation and hydrodynamics. Laboratory work in these areas is much less mature, but there is progress in the study of photoionized plasmas and radiative shocks, and in related simulations.   

The interstellar magnetic field: plasma problems

I will discuss a number of unsolved plasma problems whose solution will help in understanding the origin and evolution of the interstellar magnetic field. The prevailing theory of the origin of this field from a weak primordial seed field is the alpha-omega dynamo driven by interstellar turbulence. During the operation of this dynamo it is necessary for some flux to be expelled from the disc, and this has to happen without removing the interstellar medium as well. How this happens is an important astrophysical problem involving plasma physics. Furthermore, the turbulence initially produces small scale fields on scales below the inner scale of the turbulence. When the interstellar magnetic field is extremely weak the bulk plasma is still controlled by the orbits of the particles in the weak field. During this phase there are strong parallel and perpendicular pressure instabilities whose saturation has not been successfully worked out, and whose consequences are not understood. These instabilities can control the initial buildup of the interstellar magnetic field and impact its origin. The role of magnetic reconnection, in these early phases as well as later times when the field is strong, is not understood. Finally, the actual physics of magnetic reconnection itself is not understood. I will discuss the astrophysical importance of all these plasma problems.