The Dynamo Effect

All of the most luminous objects in the Universe, such as quasars, supernovae, and gamma-ray bursts, are powered by accretion, that is the infall of matter onto a central compact object such as a black hole.

In order for matter to accrete inwards, the angular momentum of the material must be transported outwards to slow its rotation and allow infall.

It is now understood that the mechanism that produces angular momentum transport in a plasma is in most cases plasma turbulence driven by the magneto-rotational instability (MRI).   Members of the center are trying to understand both turbulence and the MRI through a variety of studies.

Computational Studies of the MRI

One of the most powerful ways to study turbulence and the MRI is to use advanced numerical method to solve the equations of motion, and to use the resulting "numerical simulation" to understand the properties of the plasma in an accretion flow.   These simulations using a variety of different numerical algorithms, and exploit the computational power of some of the largest computers in the world.   Members of the center in both Germany and the USA are develoing a variety of state-of-the-art simulation codes to study the MRI.   Questions being addressed through numerical simulations are: (1) what is the effect of the net magnetic flux on the saturation amplitude and rate of momentum transport by the MRI?, and (2) how does microphysical dissipation such as viscosity and resistivity affect the MRI?

The PPPL MRI Experiment

In addition to numerical studies of the nonlinear regime and turbulence driven by the MRI, current effort at the center includes dedicated experiments like the MRI liquid Gallium experiment at PPPL.   This novel table-top experiment uses rotating co-axial cylinders to approximate the rotation of an astrophysical accretion disk.   An external magnetic field is imposed using current coils, and this makes the rotating flow unstable to the MRI.   The experiment not only provides new insight into the growth and saturation of the MRI under controlled conditions, but it also allows scientists to validate numerical codes used the study the MRI in astrophysics, by using these codes to model the experiment and comparing the results with actual data.   This interaction between experiment and simulation/theory and between two labs in different counties exemplifies the collaborative and international nature of the center.

The MRI in collisionless plasmas

When the density of particles in a plasma is very small, particles must travel a large distance before they collide with other particles.   If this distance is larger than or comparable to the size of the system (for example, the radius of the accretion flow in the case of the MRI), then the plasma becomes effectively collisionless.   A variety of new effects become important in collisionless plasmas, such as anisotropic transport (viscosity and thermal conduction), as well we kinetic instabilities (firehose and mirror modes) which can tangle any magnetic field in the plasma on very small scales.

There are many astrophysical systems in which the accreting plasma is thought to be in the nearly collisionless regime (such as the black hole at the center of our own Milky Way galaxy).   However, the properties of nearly collisionless accretion flows are poorly understand, and have only begun to be investigated.   Members of the center are playing a leading role in such studies by developing new numerical methods for studying collisionless plasmas, and applying these methods to studies of the MRI.

The MRI in supernovae

Members of the center are also studying the role of magnetic fields in supernova explosions, gamma ray bursts, and pulsars.   Supernovas are dramatic explosions that also lead to interesting objects such as rapidly spinning pulsars, black holes, and large nebulae, and they are used as standard candles in cosmological studies involving the expansion of the universe and dark energy. Gamma Ray Bursts (GRB), the most luminous objects in the sky, are thought to be radiation beamed from supernovas. The mechanisms that drive supernova explosions have been an active area of research recently. The rapid differential rotation that occurs during a supernova would seem to be an efficient mechanism for generating strong magnetic fields through dynamo action and/or the MRI.

Magnetic fields may play an important dynamic role in the explosion process in many cases, and even when they are dynamically weak, understanding the generation and thus the origin of magnetic fields in pulsars is still an important question. MPA and Princeton groups are both modeling the extremely complex physical processes causing core-collapse supernovae and GRBs by means of large-scale supercomputing simulations.