Faraday Rotation as a Diagnostic of
Cosmic Magnetic Fields
Steven R. Spangler
University of Iowa
Cosmic magnetic fields here means the
solar corona as well as that of the ISM
and elsewhere
Faraday Rotation in the corona and
elsewhere
Plasma Contributions to the
Faraday Rotation Integral
An illustration of Faraday rotation
Position angle rotation (1465 MHz) = 14 degrees
Coronal RM = 6 rad/sq-m
Ingleby, Spangler, Whiting 2007, ApJ 668, 520
Coronal Faraday Rotation
Expected level of coronal FR, given independent information
Faraday rotation in the Interstellar Medium
Minter and
Spangler 1996,
ApJ 458, 194
The measurement of currents in
astrophysical plasmas
“Do you have anything to say before we turn
on the current?”
Differential Faraday Rotation
A difference in Rotation Measure between
two closely-spaced lines of sight
Differential Faraday Rotation and Ampere’s
Law
Differential Faraday Rotation and
Ampere’s Law II
Method utilized in Wisconsin Reversed Field Pinch
(RFP) by Ding, Brower, et al (PRL 90, 035002, 2003)
Observations of Differential Faraday
Rotation on August 16, 18 2004
Inferred current of 2.5 GA between lines of sight
Total Faraday
Differential Faraday Rotation
separated
by ~ Rotation
35,000 km
Are differential Faraday rotation
measurements seen through the ISM
indications of interstellar currents?
A new age of opportunity for cosmic
Faraday rotation measurements; the
availability of the EVLA
• Lower noise receivers
• Larger bandwidth
• Continuous frequency
coverage
Why continuous frequency coverage will
be a great help in Faraday rotation
measurements
Old VLA… measurements in Lazio,
Spangler, Cordes 1990, ApJ 363, 515
EVLA… continuous frequency coverage
from 1.0 to 8.0 GHz (and beyond)
Conclusion: Faraday rotation is an
excellent diagnostic for magnetic fields
(large scale and turbulent) in tenuous,
collisionless cosmic plasmas
The technique will yield new
results in the future, with a new
instrument, the EVLA