1. Probing the Solar Corona with Radioastronomical Observations Steven R. Spangler

2. Physics of Faraday Rotation: the cartoon

3. Scope of Talk: Observations of Extragalactic Radio Sources with Radio Interferometers

4. Very Large Array Radio interferometer 27 antennas B or A array Observations taken at 1465 and 1665 MHz

5. Advantages of Interferometric Observations of Extragalactic Radio Sources Simultaneous measurements on a set of lines of sight (pharetra) through the corona Use of “constellations” of radio sources for tomographic-like analyses

6. Plasma Contributions to the Faraday Rotation Integral We need enough observations to sort out various contributions to coronal density and magnetic field

7. New Coronal Faraday Rotation Results from the VLA Project AS764: August, 2003; 2X10 hour sessions on source 3C228 Project AS826: March-April 2005; 4X10 hour sessions (plus reference) for coronal “tomography”

8. Project AS826 o Observations: March 12 March 19 March 28 April 1 May 29 (reference) o 19 sources, 20 lines of sight

9. Measurements in AS826 Reference observation March 12 – reference observation

10. AS826 Preliminary Results SourceRM AverageError 2323-03361.0680.9976 2325-0496.9520.7295 2326-020-2.4180.9832 2328-049-4.1630.65297 2331-015-6.0872.564 2335-015-13.6710.305 2337-025-12.4580.4514 2338-042-4.9930.2619 2351-012 -27.35 0.693 2352-016-24.1751.428 2357-0243.1450.202 0006-0012.2550.207 0023+0455.180.125 0029+052-2.5150.196 0030+0580.5820.685 0034+0131.920.58 0039+0332.2721.27 0039+033-0.9050.79 0041+070-0.3710.613 0046+067 -14.067 1.509

11. How Can Faraday Rotation Observations Probe the Overall Structure of the Coronal Plasma?

12. Global RM Models: How well can synoptic coronal models account for FR Measurements? Mancuso and Spangler 2000 Residuals of ~ 2-3 rad/sq-m

13. Modeling the Observed Rotation Measure

19. AS826: Use of Synoptic Model for Corona These measurements of magnetic field and density at ~ 3R can be used to estimate B and n at greater distances probed by Faraday Rotation

20. Model Calculations (cont) o We will use different theoretical models of the coronal magnetic field to try to reproduce the measured rotation measures. o We will also examine whether the coronal magnetic field at 3.25 R accurately describes the field at 5-10 R.

21. Observed and Model RM Comparison

24. Adjustments to Synoptic Models for Magnetic Field and Plasma Density Product of density and magnetic field must be multiplied by a factor of 24.8 Densities must be multiplied by a factor between 2.2 * and 5 Magnetic field values must be multiplied by factor between 4.4 * and 11

25. Conclusions Simple synoptic models of the corona roughly reproduce “Pharetrae” of Faraday Rotation measurements, but large residuals. Measurements consistent with coronal field of 30-80mG at r=6R. (Paetzold et al 1987) Future observations could more effectively constrain the functional form of the coronal magnetic field. Rotation measure changes substantially on timescales of a few hours; too slow to be turbulence. Thus “Mesoscale Plasma Structures”. Smaller, faster fluctuations attributable to waves seen in spacecraft beacon data. Spatial variations in RM (differential Faraday Rotation) are small; constraints on coronal turbulence are reasonable but not decisive

26. Future Developments EVLA (Expanded VLA): Enormous increase in sensitivity of the VLA, in progress. But only if feed design prevents system temperature increase due to Sun. VLA at 5 GHz: Could make measurements closer to the Sun, observations have more impact.