Professor and WVU Woodburn Fellow

Theoretical and Computational Plasma Physics

Department of Physics and Astronomy

West Virginia University

# Research

## Research Focus

Paul Cassak is a theoretical and computational plasma physicist. His research focus is on magnetic reconnection and its applications using analytical techniques, large scale numerical simulations, and observational data as appropriate. His other research interests include fundamental plasma physics processes as they occur in reconnection, plasma turbulence, and shocks. [Image from M. Hesse and P. A. Cassak, "Magnetic Reconnection in the Space Sciences: Past, Present, and Future," J. Geophys. Res., 125, e2018JA025935 (2020)]

## Research Applications

Paul Cassak's research has many applications, including solar eruptions (flares and CMEs) and similar eruptions on other sun-like stars, substorms and solar wind-magnetospheric coupling in the geomagnetic magnetic field (relevant to the field of space weather), disruptive events in fusion plasmas, and various astrophysical settings. Figure from M. Hesse and P. A. Cassak, "Magnetic Reconnection in the Space Sciences: Past, Present, and Future," J. Geophys. Res., 125, e2018JA025935 (2020)

## Selected Projects and Research Accomplishments

### The PHASMA Experiment

The group is part of a collaboration with WVU Profs. Scime, Tu, and Kobelski, to study reconnection in a laboratory experiment designed to measure kinetic-scale physics non-perturbatively. (Support from DOE, NSF, and NASA is gratefully acknowledged.) [Image taken by P. Cassak of the lab of E. Scime.]

### Kinetic Entropy

The group is investigating the use of entropy in the kinetic theory description as a means for identifying energy conversion and dissipation (with Haoming Liang and Vadim Roytershteyn). (Support from NSF is gratefully acknowledged.) [Image from H. Liang et al., "Decomposition of Plasma Kinetic Entropy into Position and Velocity Space and the Use of Kinetic Entropy in Particle-in-Cell Simulations"

Phys. Plasmas, 26, 082903 (2019)]

### Magnetospheric Multiscale (MMS) Mission Physics

Cassak is a collaborator on the Theory and Modeling Team of NASA's Magnetospheric Multiscale mission and has been involved with numerous studies of kinetic-scale reconnection physics. [Image from J. L. Burch et al., "Electron-Scale Measurements of Magnetic Reconnection in Space", Science, 352, 6290 (2016)]

### 3D Magnetic Reconnection

Early models of magnetic reconnection assumed it was 2D, but numerical simulations have fostered research into 3D reconnection. The group has studied the orientation of X-lines, reconnection in tokamaks, the spreading of localized reconnection, signatures of localized reconnection, how to locate magnetic separators (where reconnection can occur in 3D), supra-arcade downflows, reconnection jet fronts, and reconnection at Earth's dayside magnetopause (with Matt Beidler, Lucas Shepherd, Colin Komar, Jim Drake, Jack Gosling, Tai Phan, Jiong Qiu, Alex Glocer, John Dorelli, and Milton Arencibia). (Support from NSF and NASA is gratefully acknowledged.) [Image from C. M. Komar and P. A. Cassak, "The local dayside reconnection rate for oblique interplanetary magnetic fields", J. Geophys. Res., 121, 5105 (2016)]

### Signatures of Reconnection

The group has studied a number of properties of reconnection that can be used to identify reconnection in observations, including the "Larmor electric field" (with Michael Shay and Kittipat Malakit) and tripolar Hall magnetic field signatures (with Stefan Eriksson and Vitor Souza). (Support from NASA is gratefully acknowledged.) [Image from K. Malakit et al., "A New Electric Field in Asymmetric Magnetic Reconnection", Phys. Rev. Lett., 111, 135001 (2013)]

### Reconnection with Flow Shear

Reconnection in many settings is immersed in an external bulk flow; the canonical example is at Earth's dayside magnetopause. The group performed fluid and particle-in-cell simulations to study this process and develop a prediction of the efficiency of reconnection as a function of flow shear, and compared the theory to observations (with Antonius Otto, Chris Doss, and Rick Wilder). (Support from NSF and NASA is gratefully acknowledged.) [Image from C. E. Doss et al., "Particle-in-cell simulation study of the scaling of asymmetric magnetic reconnection with in-plane flow shear", Phys. Plasmas, 23, 082107 (2016)]

### Reconnection and Turbulence

Reconnection and turbulence often go together. The group has made fundamental contributions to understanding how reconnection happens in a turbulent medium (with Michael Shay, Sergio Servidio, Bill Mattheaus, and colleagues). [Image from S. Servidio et al., "Statistics of Magnetic Reconnection in Two-Dimensional Magnetohydrodynamic Turbulence", Phys. Plasmas, 17, 032315 (2010)]

### Secondary Islands

When large-scale reconnection occurs in weakly collisional systems, the current sheets are prone to breaking up into so-called secondary islands. The group has done fundamental research into the rate of reconnection in such systems (with Michael Shay), the effect of it taking place in thick current sheets (with Jim Drake), and how it impacts the transition to collisionless reconnection (with Lucas Shepherd). [Image from L. S. Shepherd and P. A. Cassak, "Comparison of secondary islands in collisional reconnection to Hall reconnection", Phys. Rev. Lett., 105, 015004 (2010)]

### Asymmetric Reconnection

Asymmetric reconnection occurs when the plasma parameters on either side of the reconnection site are different, which is typical of reconnection in many settings including the magnetopauses of Earth and other planets and in fusion devices. The group has done fundamental work on this problem, including the rate at which it occurs for arbitrary ambient conditions (with Michael Shay), the structure of the diffusion region, the role of diamagnetic effects when there is an out-of-plane (guide) magnetic field, and reconnection in the 3D magnetospheric geometry (with Matt Beidler, Marc Swisdak, Haoming Liang, Gabor Toth, Yuxi Chen, Binzheng Zhang, and Sanni Hoilijoki). (Support from NSF and NASA is gratefully acknowledged.) [Image from P. A. Cassak and M. A. Shay, "Scaling of Asymmetric Magnetic Reconnection: General Theory and Collisional Simulations", Phys. Plasmas, 14, 102114 (2007)]

### Nonlinear Dynamics of Reconnection and Onset

Going back to Cassak's dissertation work under Jim Drake, the group has done much work on the nonlinear dynamics of reconnection and reconnection onset. Projects include a saddle-node bifurcation model of reconnection onset, the role of the Hall effect and pressure anisotropies in setting the reconnection rate, and why the reconnection rate is 0.1 (with Jim Drake, Michael Shay, Yi-Hsin Liu, and Kittipat Malakit). [Image from P. A. Cassak et al., "Onset of Fast Magnetic Reconnection", Phys. Rev. Lett., 98, 215001 (2007)]