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