Measuring the Magnetic Field in the Solar Corona Steven R. Spangler… University of Iowa.

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Presentation transcript:

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 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: 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