Turbulence and dynamo

Collisionless Alfvén waves

Shear-Alfvén waves are probably the most fundamental low-frequency plasma wave, and form the basis for magnetized turbulence phenomenologies. Our recent paper presents a stringent (and fascinating) limit on their amplitude in collisionless plasmas at high beta, where the gas pressure dominates the magnetic pressure. The limit comes about because the decreasing field of the wave creates a pressure anisotropy that causes its own restoring force to disappear. When this happens, you get bizarre square zig-zags in the magnetic field lines as the plasma self organizes to remove the magnetic tension, and very strong damping of the velocity field. We now have some preliminary evidence for this effect in solar wind data from the WIND spacecraft! We’ve also been running some hybrid-PIC simulations, see here for a video of the perturbed magnetic field. (Collaborators: Eliot QuataertAlex Schekochihin, Matt KunzStuart Bale, and Chris Chen)

Kinetic dynamos and turbulence

I’ve recently been thinking about how turbulence will change when you have a plasma that is effectively collisionless. An important difference, compared to a normal magnetized fluid, is that a changing magnetic field induces a pressure anisotropy (a different pressure parallel and perpendicular to the field). If this grows too large, the plasma becomes wildly unstable to the firehose and mirror instabilities, which act to limit the anisotropy to its marginal level, as well as scattering particles and generally causing havoc. In some ways, such plasmas behave similarly to a standard fluid, in others (such as for high-beta Alfvén waves – see above) they can be completely different. Our recent simulations of turbulence where the driving amplitude is above the Alfvén-wave amplitude (see above) show that high-beta collisionless turbulence may be very different from MHD. (Collaborators: Eliot Quataert and Alex Schekochihin)

The magnetic shear-current effect

The main result of my thesis is the suggestion of a new way that large-scale fields can be created by small-scale turbulence, which we termed the magnetic shear-current effect. The idea is that the large-scale fields arise as a consequence of the interaction between small-scale tangled fields and a velocity shear. This is interesting because small-scale fields are always amplified in plasma turbulence (the small-scale dynamo), so large-scale fields must grow on a bath of magnetic, as well as velocity fluctuations. The mechanism is particularly promising for the large-scale order seen in simulations of the magnetorotational turbulence in accretion disks. (Collaborators: Amitava Bhattacharjee)

Supersonic turbulence and dust dynamics

Supersonic turbulence can be very different to the standard Kolmogorov paradigm, being dominated by strong shocks. It is also very important in the interstellar medium (ISM), since its properties control many aspects of star formation. We’ve been applying intermittency models to understand the density distribution in such turbulence. Another related problem is the dynamics of dust in such turbulence. Dust behaves differently to the gas itself because it has inertia, and thus will take time to slow down or speed up upon hitting a region of gas with different velocity. Its interaction with supersonic turbulence has hardly been studied (see here), despite the importance for the physics of the ISM. (Collaborator: Phil Hopkins)

Dynamos in proto-neutron stars

In a core-collapse supernovae with strong rotation and magnetic fields, the explosion and jet launching may be helped by magnetorotational turbulence and a dynamo, creating enormously powerful “hypernovae.” Philipp Moesta and collaborators recently found that a powerful dynamo is excited in the turbulence following collapse. We’re looking to understand how this works and how it might scale to astrophysically relevant parameters. (Collaborators: Philipp Moesta)