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 106 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 measurements here

Direct measurements 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