Paul Cassak

Assistant Professor
Theoretical and Computational Plasma Physics
Department of Physics
West Virginia University

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:
email

Cassak Group Website

CISM Retreat 2003
CISM Retreat 2003
"Learn what is to be taken seriously
and laugh at the rest."
Hermann Hesse, Steppenwolf

Teaching

Spring 2012
Fall 2011
Fall 2010
Spring 2010
Fall 2009
Spring 2009
Fall 2008
Fall 2008
Phys 784, Advanced Magnetohydrodynamic Theory
Phys 112, General Physics II
Phys 783, Plasma Kinetic Theory
Phys 784, Advanced Magnetohydrodynamic Theory
Phys 333, Electricity and Magnetism I
Phys 783, Plasma Kinetic Theory
Phys 333, Electricity and Magnetism I
Phys 782, Computational Plasma Physics - Guest Lecture on Numerical MHD (December 1, 2008)


Research Overview

Curriculum Vitae (last updated December 31, 2011)

A Complete List of Publications and Presentations (last updated December 13, 2011)

Research Interests - Magnetic reconnection, nonlinear dynamics, physics of the solar corona, magnetospheric physics, fusion physics, space physics, plasma theory, physics education and outreach.


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.


Background to Reconnection

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.

The Theory of Magnetic Reconnection: Past, Present, and Future, PDF (4.7 MB)

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

Theory of Magnetic Reconnection for Solar Applications, PDF (6.3 MB)

Research Topics

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

1. Reconnection Onset

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.

Catastrophe Model for Fast Magnetic Reconnection Onset,
P. A. Cassak, M. A. Shay, and J. F. Drake, Phys. Rev. Lett., 95, 235002 (2005)

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.

A Model for Spontaneous Onset of Fast Magnetic Reconnection
P. A. Cassak, J. F. Drake, and M. A. Shay, Ap. J., 644, L145-L148, 2006 June 20

Catastrophic Onset of Fast Magnetic Reconnection with a Guide Field
P. A. Cassak, J. F. Drake, and M. A. Shay, Phys. Plasmas, 14, 054502 (2007)

Onset of Fast Magnetic Reconnection
P. A. Cassak, J. F. Drake, M. A. Shay and B. Eckhardt, Phys. Rev. Lett., 98, 215001 (2007)

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.

From Solar and Stellar Flares to Coronal Heating: Theory and Observations of How Magnetic Reconnection Regulates Coronal Conditions
P. A. Cassak, D. J. Mullan, and M. A. Shay, Ap. J. Lett., 676, L69-L72, 2008, March 20

A model of the nonlinear dynamics was recently presented.

A saddle-node bifurcation model of magnetic reconnection onset
P. A. Cassak, M. A. Shay, and J. F. Drake, Phys. Plasmas, 17, 062105 (2010)

2. Asymmetric Reconnection and Reconnection with Shear Flows

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.

Scaling of Asymmetric Magnetic Reconnection: General Theory and Collisional Simulations
P. A. Cassak and M. A. Shay, Phys. Plasmas, 14, 102114 (2007)

The Scaling of Asymmetric Hall Magnetic Reconnection
P. A. Cassak and M. A. Shay, Geophys. Res. Lett., 35, L19102 (2008)

Structure of the Dissipation Region in Fluid Simulations of Asymmetric Magnetic Reconnection
Invited Paper for the Division of Plasma Physics of the American Physical Society
P. A. Cassak and M. A. Shay, Phys. Plasmas, 16, 055704 (2009)

Scaling of asymmetric magnetic reconnection: Kinetic particle-in-cell simulations
K. Malakit, M. A. Shay, P. A. Cassak and C. Bard, J. Geophys. Res., 115, A10223, 2010

Magnetic Reconnection with Asymmetry in the Outflow Direction
N. A. Murphy, C. R. Sovinec, and P. A. Cassak, J. Geophys. Res., 115, A09206, 2010

Scaling of the Magnetic Reconnection Rate with Symmetric Shear Flow
P. A. Cassak and A. Otto, Phys. Plasmas, 18, 074501, 2011

Theory and Simulations of the Scaling of Magnetic Reconnection with Symmetric Shear Flow
P. A. Cassak, Phys. Plasmas, 18, 072106, 2011

3. Secondary Islands in Magnetic Reconnection and Reconnection in Turbulence

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.

Scaling of Sweet-Parker Reconnection with Secondary Islands
P. A. Cassak, M. A. Shay, and J. F. Drake, Phys. Plasmas, 16, 120702 (2009)

The Impact of Microscopic Magnetic Reconnection on Pre-Flare Energy Storage
P. A. Cassak and J. F. Drake, Ap. J. Lett., 707, L158-162, 2009 Dec 20

Comparison of secondary islands in collisional reconnection to Hall reconnection
L. S. Shepherd and P. A. Cassak, Phys. Rev. Lett., 105, 015004 (2010)

Magnetic Reconnection for Coronal Conditions: Reconnection Rates, Secondary Islands and Onset
P. A. Cassak and M. A. Shay, Space Sci. Rev., doi:10.1007/s11214-011-9755-2

Magnetic Reconnection in Two-Dimensional Magnetohydrodynamic Turbulence
S. Servidio, W. H. Matthaeus, M. A. Shay, P. A. Cassak and P. Dmitruk, Phys. Rev. Lett., 102, 115003 (2009)

Statistics of Magnetic Reconnection in Two-Dimensional Magnetohydrodynamic Turbulence
S. Servidio, W. H. Matthaeus, M. A. Shay, P. Dmitruk, P. A. Cassak, and M. Wan, Phys. Plasmas, 17, 032315 (2010)

Magnetic reconnection as an element of turbulence
S. Servidio, P. Dmitruk, A. Greco, M. Wan, S. Donato, P. A. Cassak, M. A. Shay, V. Carbone, and W. H. Matthaeus Nonlin. Processes Geophys., 18, 675 (2011)

4. Incomplete Reconnection in Sawteeth

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.

Model for Incomplete Reconnection in Sawtooth Crashes
M. T. Beidler and P. A. Cassak, Phys. Rev. Lett., 107, 255002 (2011)

In the Media

The following two articles are about the 2005 paper with Shay and Drake:

Triggering Solar Mayhem
J. Andrew, Physical Review Focus, December 2005

Solar Mayhem
V. Tarko, Softpedia, January 2006

The following two articles are about recent funding Paul received:

WVU physics dept wins additional stimulus funds
Ben Adducchio, West Virginia Public Broadcasting, August 2009

CAREER Making Research
Lauren Peretti, Eberly College of Arts and Sciences Newsletter, December 2010

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


Outreach

Outreach talk called "The Sun and Space Weather" delivered to the Pulsar Search Collaboratory, May 2010.



This page last updated on January 7, 2012.