1. Faraday Rotation as a Diagnostic of Cosmic Magnetic Fields
Steven R. Spangler
University of Iowa

2. Cosmic magnetic fields here means the solar corona as well as that of the ISM and elsewhere

3. Faraday Rotation in the corona and elsewhere

4. Plasma Contributions to the Faraday Rotation Integral

5. 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

6. Coronal Faraday Rotation
Expected level of coronal FR, given independent information

7. Faraday rotation in the Interstellar Medium
Minter and Spangler 1996, ApJ 458, 194

8. The measurement of currents in astrophysical plasmas
“Do you have anything to say before we turn on the current?”

9. Differential Faraday Rotation
A difference in Rotation Measure between two closely-spaced lines of sight

10. Differential Faraday Rotation and Ampere’s Law

11. Differential Faraday Rotation and Ampere’s Law II
Method utilized in Wisconsin Reversed Field Pinch (RFP) by Ding, Brower, et al (PRL 90, , 2003)

12. Observations of Differential Faraday Rotation on August 16, 18 2004
Inferred current of 2.5 GA between lines of sight separated by ~ 35,000 km
Total Faraday Rotation
Differential Faraday Rotation

13. Are differential Faraday rotation measurements seen through the ISM indications of interstellar currents?

14. A new age of opportunity for cosmic Faraday rotation measurements; the availability of the EVLA
Lower noise receivers
Larger bandwidth
Continuous frequency coverage

15. Old VLA… measurements in Lazio, Spangler, Cordes 1990, ApJ 363, 515
Why continuous frequency coverage will be a great help in Faraday rotation measurements
Old VLA… measurements in Lazio, Spangler, Cordes 1990, ApJ 363, 515

16. EVLA… continuous frequency coverage from 1.0 to 8.0 GHz (and beyond)

17. 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