Measuring the Magnetic Field in the Solar Corona

Steven R. Spangler… University of Iowa

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: equations

Physics of Faraday Rotation II

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

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

Illustration of coronal Faraday

rotation: 3C79

Coronal Faraday Rotation

Mancuso and Spangler, Astrophys. J. 525, 195, 1999

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

Coronal MHD Model

Mancuso & Spangler, Astrophys. J. 539, 480, 2000

Agreement of model and observed rotation measures

Turbulence

A turbulent plasma will have spatially and temporally random fluctuations in plasma

density and magnetic field. These will generate random fluctuations in the Faraday

rotation of a source viewed through the corona

Observed fluctuations are small, but may be dynamically

significant

Future work…analysis of observations in August , 2003

Approximate “tracks” of source 3C228 on August 16 & August 18

Conclusions and Summary• Radio remote sensing observations can detect and

quantitatively estimate the magnetic field in the solar corona.

• These observations can constrain the radial dependence and form of the large scale, static field

• The observations can also measure or limit the properties of magnetohydrodynamic turbulence in the corona

• Future investigations (observations being analysed, or in planning) can improve on the above results