An interview with Ellen Zweibel

Interview by Bo Reipurth, SFN #355 - July 2022

Your first paper, in 1975 with Russell Kulsrud, dealt with an analysis of the Parker instability and its possible role in forming molecular clouds and OB associations. What motivated that study, and did it lead to your subsequent PhD thesis?

In my first year of grad school, I took Russell's course in plasma astrophysics. I was fascinated by the material, and asked Russell if I could work with him that summer. He suggested the project because he thought the role of the Parker instability in the ISM was overestimated and would be reduced by a more complete analysis, particularly of the compressibility of the ISM on the large length and time scales on which the instability operates. That summer I grew my theory wings but also had to grow a thick skin, because Gene Parker was very critical of our work. In the next academic year, I worked with Russell on a cosmic ray problem, but I had always wanted to do a thesis with Jerry Ostriker on stellar dynamics, and that is what I did. I returned to the problem of the Parker instability in 2016, focussing on the importance of cosmic ray physics for the onset of the instability and how it develops.

Among your early papers is a study with Mike Shull on confinement of cosmic rays in the interstellar medium.

That was fun. I knew Mike in grad school, and we both got faculty jobs at the University of Colorado in Boulder. He was working on supernova remnant evolution in molecular clouds, and we got the idea of combining my work with Russell with his expertise in molecular cloud and supernova astrophysics. I worked very hard on the plasma physics of cosmic rays in molecular clouds and whether the magnetic instabilities which confine cosmic rays in diffuse gas would also work in a dense, weakly ionized molecular medium. The plasma physics was interesting, but collisions between cosmic rays and thermal gas, which produce gamma-rays, neutrinos, and e+- pairs, turned out to be the dominant effect in limiting how far cosmic rays travel once they are accelerated. Our work is important for interpreting gamma-ray observations from molecular clouds.

Also in the early 1980s you wrote a highly cited study of hydromagnetic wave dissipation in molecular clouds. What were the key insights

At that time there was active discussion of whether the line broadening observed in molecular clouds represented supersonic hydrodynamic turbulence, and if so, what sustained it against dissipation. I thought that magnetic fields might provide some cushioning that would weaken shocks and reduce their dissipative effects as well as channel some of the turbulent energy into transverse motions that would dissipate more slowly than compressive ones. Although I was able to come up with parameters for which dissipation was weak, it seemed to require fine tuning. Numerical simulations of MHD turbulence have borne out this conclusion: magnetic turbulence is highly dissipative

In 1988 you published a paper on ambipolar diffusion and dynamos in turbulent gas. What did you learn, and what is the status of ambipolar diffusion today?

Turbulence can amplify magnetic fields, but without a mechanism for eliminating fields on small scales and for irreversibly changing the magnetic field topology, the resulting field will be a tangled mess lacking large scale coherence. I noticed that ambipolar diffusion formally does the job, but since it doesn't change magnetic topology I now think that by itself it isn't adequate. Although I considered this problem again in 2008 with indispensable help from Fabian Heitsch, the overall problem of creating fields with large scale structure under astrophysical conditions remains unsolved. As you know, ambipolar diffusion was introduced by Mestel and Spitzer as a way of removing enough magnetic flux from dense clouds to induce their gravitational collapse. Current measurements suggest that magnetic fields in molecular clouds are not strong enough to prevent their collapse, but it remains an interesting fact that the mass to flux ratio in stars is much higher than in clouds, and that magnetic braking is an important process in both clouds and stars.

Shortly after, you wrote a paper on magnetic reconnection in partially ionized gases.

This started a general thread on how magnetic fields reconnect in a weakly ionized gas and is somewhat related to the topic of the previous question. At that time, the main theories of reconnection, one of which went back to the late 1950's with Gene Parker's work on solar flares, predicted a rate which is the geometric mean of the Alfven crossing time and the Ohmic diffusion time, and is normally very slow. In a weakly ionized gas, there are really two Alfven speeds, one based on the inertia of the gas as a whole and the other on just the plasma - the latter is higher by a factor of (ρ/ρi)1/2. My paper showed which speed to use, but in the end it isn't a big effect. Later, in work with Axel Brandenburg, Fabian Heitsch, and others I looked at ambipolar diffusion based mechanisms for creating small scale currents with a short Ohmic dissipation time. I think this is more promising.

In 1998 you demonstrated that magnetic fields of clouds can be a source of turbulent energy which can be released through an instability driven by ambipolar drift.

The basic idea was that a self gravitating, magnetically supported cloud could lower its energy by collapsing, but is prevented from doing so as long as magnetic flux is conserved. Ambipolar diffusion carries magnetic flux outward, allowing the inner parts to collapse, but it is generally a rather slow process. I found an instability which redistributes magnetic flux on a timescale intermediate between the fast freefall time and slow ambipolar drift time which could potentially accelerate cloud collapse. The free energy released by contraction is converted to turbulent motion. Later, Remy Indebetouw performed nonlinear simulations of the instability which confirmed this. Although the instability hasn't been studied under fully realistic conditions, stellar energy sources are probably the main driver of turbulence in molecular clouds.

A few years later you showed that ion-neutral drift is significantly faster in a turbulent medium than in a quiescent one. What are the consequences of that?

At that time, I was puzzled by the apparently weak correlation between magnetic field strength and gas density, which had been reported by Tom Troland and Carl Heiles. We roughly expect B∝ρ1/2, but this was not seen, and I wondered if a breakdown of magnetic flux conservation could explain it. In a turbulent medium, large gradients develop locally on small scales, and the rate of ambipolar drift is proportional to the gradient in magnetic fields, so in a turbulent medium there will be many local regions of strong drift. My paper showed that these local sites of strong diffusion act jointly to transport magnetic flux down a global magnetic field gradient. One consequence of this is that ambipolar drift operates throughout the weakly ionized phases of the ISM, not just in star-forming regions.

In 2009 you and Masaaki Yamada wrote an Annual Reviews article on magnetic reconnection in astrophysical and laboratory plasmas. What do you consider the current major open questions in this area?

All branches of magnetic reconnection research - theory and simulation, laboratory experiment, space plasma, and remote astronomical observation - have just exploded in recent years, no pun intended, and there has been a great deal of progress in all of them. We're closing in on how magnetic reconnection is triggered, what processes actually break magnetic field lines, how reconnection interacts with turbulence, both pre-existing and self-generated, how relativistic and QED effects modifiy reconnection, and how electrons and ions are heated, and particles are accelerated to high energies in reconnection regions. All of these questions must be answered to develop reliable observational diagnostics of astrophysical reconnection.

What are you currently working on?

Much of my current work relates to the role of cosmic rays in galaxies and galaxy clusters - cosmic ray feedback problems. I'm fascinated by how the kinetic scale plasma processes by which cosmic rays interact with magnetic fields and thermal gas have consequences on global scales. (Of course, many other astrophysical processes display the same cross-scale coupling). As I mentioned, I've revisited Parker's instability using modern theories of cosmic ray transport. I also study the launching of galactic winds, acceleration of clouds, cosmic ray heating of the ISM and ICM, and propagation of cosmic rays within clouds. My star formation and molecular cloud studies have made me humble about asserting too much about how star formation works in other galaxies!

Separately, I've also been working for several years on shear flow turbulence, following up on some work that began with a study of Kelvin-Helmholtz instability in a weakly ionized gas (Watson et al. 2004). This time we assume fully ionized plasma, focusing on how the instability amplitude saturates, the amplification of magnetic fields, and what determines the turbulence spectrum in shear layers. I expect this work to be applicable to a variety of astrophysical problems.