1. Strength and Structure of the Coronal Magnetic Field Steven R. Spangler University of Iowa

2. Strength and Structure of the Coronal Magnetic Field “Sol spectatorem, nisi deficit, non habet…” Seneca, De Cometis

3. Collaborators University of Iowa: Laura Ingleby Harvard-Smithsonian Center for Astrophysics: Mari Paz Miralles, Steven Cranmer, John Raymond

4. Importance and Role of the Coronal Magnetic Field Probably involved in the heating of the corona; many theoretical ideas Involved in support and expulsion of coronal plasma structures (prominences, CMEs) Need for knowledge of vector magnetic field throughout corona

5. Coronal Faraday Rotation

7. Instrument for Coronal Faraday Rotation: The NRAO Very Large Array (VLA) Observations at wavelengths of 21 and 18 cm

8. The Instrument: The Very Large Array Radiotelescope Operated by the National Radio Astronomy Observatory (NRAO)

9. Measuring the Coronal Magnetic Field from a set of Faraday Rotation Measurements Adopt “forward problem” approach Specify model density function n Specify model B field Iterate to obtain optimum agreement with observations

10. New Results: Observations of the Corona with the VLA and UVCS in August, 2003

11. Radio source 3C228 Image at left is from reference observations. Three source components were bright enough in polarized emission for a p.a. measurement in a single 10 minute scan. ABCABC

12. Measured Coronal Faraday Rotation 20 degrees = 8.3 rad/m 2 Solid symbols: 21cm; open 18cm BAC

14. Preliminary Results Large position angle rotation of 150d at 1465 MHz, RM=62.5 rad/m 2 No clear indication of differential RM between lines of sight separated by 40,000 km “Gross depolarization” does not seem to be present

15. Why was Rotation Measure so Large? Line of sight passed very close to Sun (6.1 Rs at end of session) “Proximate Point” (closest point to Sun along line of sight) occurred over the neutral line: condition for maximum Faraday rotation. This situation is also easiest to analyse

16. Line of sight to 3C228 Thanks to Bill Coles, UC San Diego

17. For conditions of maximum Faraday Rotation (Sakurai and Spangler 1994, ApJ 434, 773) RM=C R S (0.59 N 0 B 0 ) R -3.36 B(r) ~ r -2, n(r) ~ r -2.36 N 0 and B 0 are coefficients determining level of density and magnetic field in the corona.

18. Results on Magnitude of Coronal Magnetic Field With same model as Sakurai and Spangler (1994), RM=46.3 rad/m-sq (measured 62.5), B = 27 mG at 6.1 R s Above model scaled to reproduce observed RM give B(6.1)= 37 mG Coronal model from Mancuso and Spangler (2000) gives RM = 106 rad/m-sq, B(6.1) = 65 mG Above model scaled to reproduce observed RM gives B(6.1) = 38 mG

19. Coronal MHD Model Mancuso & Spangler, Astrophys. J. 539, 480, 2000

20. Conclusions New 3C228 polarimetric observations yield estimates of B at 6 solar radii of 27-65 mG. Future work will improve estimates. Observations provide constraints on differential RM in corona. Depolarization results will be forthcoming and will constrain properties of small scale turbulence.