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

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Presentation transcript:

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

The solar corona and the interstellar medium…two astrophysical plasmas

Why is the coronal B field of interest? Temperature of corona is 1-2 X 10 6 K Magnetic fields probably involved via DC currents or MHD waves Assessment of theories requires measurements

We know the magnetic field both below and above the corona

Below: the photosphere. Measurement of the Zeeman Effect

Above the corona: direct magnetometer measurements in the solar wind

How do we measure B in the corona itself? Zeeman measure ments here Direct measure ments out here

Radioastronomical propagation measurements Technique discussed here: Faraday rotation

Physics of Faraday Rotation: the cartoon

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

The Physics of Faraday Rotation Demonstration

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

The Very Large Array

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

The North Liberty (Iowa) Radio Telescope

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

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

Measurements in AS826 Observation through corona March 12 – reference observation

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

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

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.

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

Faraday rotation through the ISM

Variation of Faraday Rotation in the Interstellar Medium

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

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