James F. Drake
Professor of Physics
University of Maryland, College Park, MD
B. S., M. S. and Ph. D., Physics, University of California, Los Angeles
Professor Drake earned his bachelors degree from UCLA and remained at
UCLA to complete his doctorate in theoretical physics in 1975. After
completing his doctorate, Professor Drake remained at UCLA for a brief
time as a post-doctoral scholar and then moved to the University of
Maryland first as a post-doctoral scholar and then as a member of the
teaching faculty in the Department of Physics and the Institute for
Physical Science and Technology.
Professor Drake has worked on a very broad range of topics in the
general area of theoretical plasma physics using both analytical and
numerical techniques. His work has applications spanning a variety of
physical systems, including the solar corona, the earth's
magnetosphere and ionosphere, magnetically confined plasma, and the
interaction of intense lasers with plasma. His present focus is on
magnetic reconnection with space physics applications and turbulence
and transport with applications to the magnetic fusion program. In
recognition for his contributions to the field of plasma physics, he
was granted fellowship status in the American Physical Society and was
awarded a Humboldt Senior Scientist Research Award.
Magnetic Reconnection
Solar flares, storms in the earth's magnetosphere and disruptions in
laboratory fusion experiments are examples of large scale explosive
events which occur in plasma systems. The magnetic field is the source
of free energy driving these phenomena. This magnetic energy can be
released in locations where the magnetic field reverses
direction. That is, the magnetic field can self-annihilate in these
regions and transfer its energy to plasma flows and intense, high
energy beams. A major scientific challenge has been to explain the
short time scales of this energy release. It is well established that
the magnetic energy is released by magnetic reconnection, in which
magnetic field lines in opposing directions cross link, forming a
topological x-line. The topological change in the magnetic field
required to form the x-line requires a breakdown in the ideal
"frozen-in" flux condition, which occurs at small scales. As a result,
magnetic reconnection occurs in narrow boundary layers. This topic
is
considered to be one of the two or three most important topics in
plasma physics over the past thirty years because it occurs in so many
varied environments and because the dependence of a large-scale,
explosive phenomenon on the kinetic behavior at small scale has
intrinsic interest.
Key discoveries in magnetic reconnection made by Dr. Drake and his
colleagues center around the role of whistler waves in driving and
controlling magnetic reconnection. Traditionally it was believed that
the Alfven wave played the key role in driving reconnection. At small
scales, however, electron and ion motion decouple and the dynamics is
controlled by whistler waves. The whistler waves fundamentally alter
the reconnection process, causing the release rate of magnetic energy
to be insensitive to the mechanism which breaks the frozen-in
condition. The figure on the right shows the results of a computer
simulation of magnetic reconnection. From top to bottom are the
magnetic field lines, the plasma flows, and the ion and electron
currents. You can watch a computer
generated movie of fast
collisionless reconnection. The black lines are magnetic field lines,
which move toward each other and reconnect. The slingshot-like
behavior after reconnecting enables the magnetic field to release its
energy.
Key papers dealing with collisionless magnetic reconnection and associated particle acceleration are:
- Transition to
whistler mediated magnetic reconnection, written with Mark
Mandt and Richard Denton. First paper to demonstrate that the
whistler wave drives magnetic reconnection at small spatial scales and
identified the out-of-plane magnetic field as the signature of
whistler dynamics.
- The role
of electron dissipation on the rate of collisionless magnetic
reconnection, written with Michael Shay.
Demonstrates that the whistler
wave renders the reconnection rate insensitive to the mechanism which breaks
the frozen-in condition using analytical calculations backed by particle
simulations.
- The scaling of collisionless
magnetic reconnection for large systems , with Michael Shay,
Barrett Rogers and Richard Denton. Demonstrates that the dynamics of
the whistler wave leads to a scale invariant rate (and therefore
Afvenic rate) of magnetic reconnection even at very large scales.
- GEM magnetic
reconnection challenge, overview paper written with authors
from a broad range of institutions nationwide. This paper summarizes
the results of the GEM Reconnection Challenge project in which
magnetic reconnection was explored with a broad range of computational
models and the results compared to understand the key physics
controlling collisionless magnetic reconnection. The paper
demonstrated that the whistler wave was a necessary ingredient to
achieving fast reconnection in collisionless plasma.
- Formation of
electron holes and particle energization during magnetic
reconnection, written with Marc Swisdak, Michael Shay,
Barrett Rogers, Cynthia Cattell and Andreas Zeiler. Shows that the
intense current layers that develop during magnetic reconnection drive
instabilities that collapse into localized depletions of the electron
density, "electron holes", that compare well with those measured with
the Polar spacecraft in the Earth's magnetosphere.
- Catastrophe
model for fast reconnection onset, written with Paul Cassak
and Michael Shay. Addresses the question of why magnetic reconnection
occurs as an explosion. Shows that both fast Hall and slow Sweet-Parker
reconnection are simultaneously valid reconnection solutions over a
large range of resistivity. Below a critical resistivity the
Sweet-Parker solution disappears and the rate of reconnection abruptly
jumps by many orders of magnitude.
- Electron
acceleration from contracting magnetic islands during
reconnection, written with M. Swisdak, H. Che and
M. A. Shay. Shows that the magnetic islands that form during magnetic
reconnection efficiently transfer the released magnetic energy to
electrons through a Fermi-like mechanism. The model produces powerlaw
particle energy spectra that are consistent with both solar and
magnetospheric observations.
- Two-scale structure of the electron dissipation region during collisionless magnetic reconnection, written with M. A. Shay and M. Swisdak. Explores collisionless reconnection in very large systems using kinetic simulations. Shows that reconnection remains fast even in very large systems and that the electron dissipation region develops a two-scale structure along the outflow direction.
- Evidence for an
elongated (>60 ion skin depths) electron diffusion region during fast magnetic
reconnection, written with Tai D. Phan, M. A. Shay and
Jonathan Eastwood. The first estimates of the length of the electron
dissipation region from satellite observations in space.
Some recent news articles which have appeared discussing fast magnetic reconnection:
- Unlocking Secrets of Magnetic Fields' Power. An article by James Glanz of the New York Times reporting on the new theoretical ideas on reconnection related to whistlers.
- Magnetic Explosions in
Space by James Drake. Paper published as the feature News and
Views article in Nature. The paper presents background material on
magnetic reconnection and the role of whistlers as a preface to a
paper reporting the first observations of whistlers in a natural
environment reported by Xiao Deng and Hiroshi Matsumoto. Written for
the general science audience.
- Inside the Northern
Lights. Science Magazine article by Dennis Normile reporting
article by X. Deng and H. Matsumoto on the first observations of
whistlers as the driver of magnetic reconnection.
- Researchers uncover piece of puzzle
of magnetic explosions. Article in the University of Maryland
Outlook on the 2003 Science article on the formation of electrons
holes during magnetic reconnection.
- Triggering Solar
Mayhem. Physical Review Focus piece on the 2005 Cassak
Physical Review Letter article on a catastrophe model of magnetic
reconnection onset.
- Solar
Mayhem. Article in Softpedia on the 2005 Cassak Physical
Review Letter article on a catastrophe model of magnetic reconnection
onset.
- Space Magnetism May Hold Secret to
Fusion Power. Article by Jermeny Hsu in Space.com on the 2007 Phan et al. Physical Review Letter observations of the structure of the electron
dissipation region during reconnection in the Earth's magnetosheath using
data from the Cluster satellites.
Plasma Turbulence, Transport and Nonlinear Instability
The transport of plasma across magnetic fields plays a critical role
in most plasma systems, including boundary layers and shocks in space
and in laboratory magnetic fusion experiments. For example, energy
confined in laboratory fusion experiments leaks across the magnetic
field much faster than can be explained from predictions based on
classical Coulomb collisions. The free energy associated with the
large gradients of plasma pressure drives very small scale vortices
which cause the plasma wander across the magnetic field to the cold
walls of the container. Basic questions such as the explicit mechanism
which drives these vortices, what controls the level of fluctuations
and how the resulting plasma transport scales with the plasma parameters
remain poorly understood. Dr. Drake and his colleagues at Maryland
have developed a powerful 3-D numerical model for exploring the
nonlinear development of turbulence and transport in magnetized
plasma. This tool has been used to explore a range of fundamental
issues related to the nonlinear dynamics of plasma turbulence and to
answer very practical questions such as the scaling of energy and particle
transport.
A major surprise in this work was the discovery that linear stability
is not always an accurate indicator of the behavior of a system. The
classical approach for studying the dynamics of a system is to first
assume that the system is in equilibrium and explore the stability
around the equilibrium using linear perturbation theory. The
assumption is that if linear disturbances are stable, the system will
remain in a quiescent state. This whole approach has been questioned
in the case of pipe flow, where turbulence develops at Reynolds
numbers well below the predictions of simple linear stability theory,
but has remained largely unquestioned in other areas. In inhomogeneous
magnetically confined plasma transport can be dominated
by a nonlinear instability which has no counterpart in linear
perturbation theory. The linear properties of the system are
essentially irrelevant. A simple picture of the nature of the
nonlinear instability was formulated and a truncated set of equations
have been derived to produce the behavior, which was first observed in
a complex 3-D simulation of plasma turbulence. This work has broad
implications for understanding the dynamics of complex systems.
Recent papers dealing with turbulence and transport are:
