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Measuring the Magnetic Field in the Solar Corona

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Title: Measuring the Magnetic Field in the Solar Corona


1
Measuring the Magnetic Field in the Solar Corona
Steven R. Spangler University of Iowa
2
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

3
We know the magnetic field both below and above
the corona
4
Below the photosphere. Measurement of the
Zeeman Effect
5
Above the corona direct magnetometer
measurements in the solar wind
6
(No Transcript)
7
How do we measure B in the corona itself?
Direct measurements out here
Zeeman measurements here
8
Radioastronomical propagation measurements
Technique discussed here Faraday rotation
9
(No Transcript)
10
Physics of Faraday Rotation the cartoon
11
Physics of Faraday Rotation equations
12
Physics of Faraday Rotation II
13
The Physics of Faraday Rotation
Demonstration
14
The Instrument The Very Large Array
Radiotelescope
Operated by the National Radio Astronomy
Observatory (NRAO)
15
The Very Large Array
16
The North Liberty (Iowa) Radio Telescope
17
The background sources (signal generators for
propagation expmts)
Extragalactic radio sources
EG sources provide many drillholes through
corona
18
Extragalactic sources provide constellations of
background objects
Mancuso Spangler, Astrophys. J. 539, 480, 2000
19
Illustration of coronal Faraday rotation 3C79
20
Coronal Faraday Rotation
Mancuso and Spangler, Astrophys. J. 525, 195, 1999
21
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

22
Coronal MHD Model
Mancuso Spangler, Astrophys. J. 539, 480, 2000
23
Agreement of model and observed rotation measures
24
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

25
Observed fluctuations are small, but may be
dynamically significant
26
Future workanalysis of observations in August ,
2003
Approximate tracks of source 3C228 on August 16
August 18
27
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
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