Measuring the Magnetic Field in the Sun and the Interstellar Medium

1. Measuring the Magnetic Field in the Sun and the Interstellar Medium Steven R. Spangler… University of Iowa

2. The solar corona and the interstellar medium…two astrophysical plasmas

3. Why is the coronal B field of interest? • Temperature of corona is 1-2 X 106 K • Magnetic fields probably involved via DC currents or MHD waves • Assessment of theories requires measurements

4. We know the magnetic field both below and above the corona

5. Below: the photosphere. Measurement of the Zeeman Effect

6. Above the corona: direct magnetometer measurements in the solar wind

8. How do we measure B in the corona itself? Direct measurements out here Zeeman measurements here

9. Radioastronomical propagation measurements Technique discussed here: Faraday rotation

11. Physics of Faraday Rotation: the cartoon

12. Physics of Faraday Rotation Phase speed of R&L waves Phase shift (cm) after prop. Phase shift (radians) Rotation of polarization position angle

13. The Physics of Faraday Rotation Demonstration

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

15. The Very Large Array

16. How one measures polarization position angles and Faraday rotation with the VLA • Polarization map of a radio galaxy at 1465 MHz

17. The North Liberty (Iowa) Radio Telescope

18. The background sources (signal generators for propagation expmts) Extragalactic radio sources EG sources provide many “drillholes” through corona

19. Extragalactic sources provide “constellations” of background objects Mancuso & Spangler, Astrophys. J. 539, 480, 2000

20. Measurements in AS826 March 12 – reference observation Observation through corona

21. 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

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

23. Conclusions • 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.

24. The interstellar medium: another magnetized plasma Line of sight out of galaxy You are here

25. Faraday rotation through the ISM

26. Variation of Faraday Rotation in the Interstellar Medium

27. What Faraday Rotation Observations have told us about the plasma of the interstellar medium

28. Conclusions • Faraday rotation observations with the VLA can measure the magnetic field in two quite different astrophysical plasmas. • These measurements can illuminate the dynamics and thermodynamics of the corona and the interstellar medium