Contact Information Office: 333 White Hall Mailing Address: Department of Physics White Hall Box 6315 West Virginia University Morgantown, WV 26506 Phone: (304) 293 - 5102 Fax: (304) 293 - 5732 Email: Cassak Group Website

"Learn what is to be taken seriously and laugh at the rest." Hermann Hesse, Steppenwolf

Teaching

Research Overview

Biography

Paul Cassak earned his Bachelors of Science in Mathematics and Physics from the University of Arizona and his Masters of Science in Physics at the University of Wisconsin, Madison. In December of 2006, he completed his Ph.D. in Physics at the University of Maryland, College Park. He was a postdoctoral researcher at the University of Delaware until July 2007. Since then, he has been an assistant professor in the Department of Physics at West Virginia University.

Research

Paul Cassak studies magnetic reconnection and its applications using analytical techniques, large scale numerical simulations, and observational data as appropriate. Applications of reconnection are many; 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.

Magnetic reconnection is a fundamental plasma physics phenomenon involved in the large scale conversion of energy stored in magnetic fields into flow and heat of the surrounding plasma. Applications include solar flares and substorms in the Earth's magnetosphere, among many others.

Below is a link to a talk Paul gave at the AGU 2008 Joint Assembly meeting. It was an SPD Parker Lecture which celebrated the contributions of Eugene Parker in honor of the fiftieth anniversary of his paper predicting a solar wind. This talk covered the early history of the theory of magnetic reconnection, a more recent history through the 20th century, and a look at the questions people are working on in the 21st century. Please contact Paul with any comments or questions.

A similar presentation covered the history of magnetic reconnection and applications to the sun as a SHINE Plenary Talk in 2009.

Paul's research has focused on the following main topics:

In work carried out with Jim Drake and Michael Shay, Paul's work addressed the long-standing "Onset Problem" of magnetic reconnection. One must explain why reconnection events observed in Nature, such as solar flares, sawtooth crashes in fusion devices, and magnetospheric substorms, begin explosively. In addition to understanding the trigger mechanism which begins the reconnection process, one must also explain what prevents the trigger from occurring until a substantial buildup of free energy has taken place.

A notable publication shows that magnetic reconnection is bistable: the slow Sweet-Parker and fast Hall reconnection solutions are both accessible for a wide range of collisionalities. The edge of the bistable regime is catastrophic: as the thickness of the dissipation region is decreased, the Sweet-Parker solution abruptly ceases to exist. This provides a potential explanation to the Onset Problem: for a system undergoing Sweet-Parker reconnection, free magnetic energy accumulates because reconnection releases energy only very slowly. As the dissipation region thins, a critical threshold is passed, where the Sweet-Parker solution disappears and Hall reconnection begins abruptly. The stored magnetic energy is rapidly released during Hall reconnection, manifesting itself as a solar flare. See below for links to the above publication.

The following three papers describe further work on this topic, including 1) what causes the dissipation region to become thinner, 2) an extension of the previous work to include a guide field, and 3) how the existence of an intermediate unstable reconnection solution provides evidence that the onset of fast reconnection occurs due to physics locally near the X-line as opposed to at the boundaries.

In addition, a recent paper with Dermott Mullan shows that this model is consistent with data from solar and stellar flares and argues that the dynamics of magnetic reconnection plays a fundamental role in setting the conditions in solar and stellar coronae.

A model of the nonlinear dynamics was recently presented.

In collaboration with Michael Shay and others, Paul has studied the scaling and properties of asymmetric magnetic reconnection, that is, reconnection between plasmas of different density and with different magnetic field strengths. Much work has been done on the shock structure, but scaling with ambient system parameters had not been performed. We performed an analytic scaling analysis (in 2D with anti-parallel fields) to discern how reconnection scales. Further, we found that the location of the X-line and stagnation point need not be colocated in general. We verified the results using Resistive MHD and Hall-MHD simulations. A link to the papers are below.

Recent work has focussed on how the properties of collisional (Sweet-Parker) reconnection change when there are secondary islands. It was shown that the speed up of the reconnection rate is consistent with a model by Daughton et al. for reasonably large Lundquist number and that secondary islands are suppressed during embedded reconnection.

Another set of studies addressed the properties of reconnection in two dimensional magnetohydrodynamic turbulence, research headed by Sergio Servidio and Bill Matthaeus. It was determined that the most robust reconnection sites undergo Sweet-Parker reconnection, which shows how turbulence and reconnection work together to produce the cascade. References are below.

It has long been known that sawtooth events in fusion devices do not always reconnect all of the available flux. With graduate student Matt Beidler, we presented a model that may explain why.

Anyone interested in joining Paul's research group as a graduate or undergraduate student researcher should contact him at the email address above.

Outreach