Title: Measuring the Magnetic Field in the Solar Corona
1Measuring the Magnetic Field in the Solar Corona
Steven R. Spangler University of Iowa
2Why 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
3We know the magnetic field both below and above
the corona
4Below the photosphere. Measurement of the
Zeeman Effect
5Above the corona direct magnetometer
measurements in the solar wind
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7How do we measure B in the corona itself?
Direct measurements out here
Zeeman measurements here
8Radioastronomical propagation measurements
Technique discussed here Faraday rotation
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10Physics of Faraday Rotation the cartoon
11Physics of Faraday Rotation equations
12Physics of Faraday Rotation II
13The Physics of Faraday Rotation
Demonstration
14The Instrument The Very Large Array
Radiotelescope
Operated by the National Radio Astronomy
Observatory (NRAO)
15The Very Large Array
16The North Liberty (Iowa) Radio Telescope
17The background sources (signal generators for
propagation expmts)
Extragalactic radio sources
EG sources provide many drillholes through
corona
18Extragalactic sources provide constellations of
background objects
Mancuso Spangler, Astrophys. J. 539, 480, 2000
19Illustration of coronal Faraday rotation 3C79
20Coronal Faraday Rotation
Mancuso and Spangler, Astrophys. J. 525, 195, 1999
21Measuring 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
22Coronal MHD Model
Mancuso Spangler, Astrophys. J. 539, 480, 2000
23Agreement of model and observed rotation measures
24Turbulence
- 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
25Observed fluctuations are small, but may be
dynamically significant
26Future workanalysis of observations in August ,
2003
Approximate tracks of source 3C228 on August 16
August 18
27Conclusions 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