Probing the Solar Corona with Radioastronomical Observations

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Probing the Solar Corona with Radioastronomical
Observations
Steven R. Spangler
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Physics of Faraday Rotation the cartoon
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Scope of Talk Observations of Extragalactic
Radio Sources with Radio Interferometers
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Very Large Array

• Radio interferometer
• 27 antennas
• B or A array
• Observations taken at 1465 and 1665 MHz

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Advantages of Interferometric Observations of
Extragalactic Radio Sources

• Simultaneous measurements on a set of lines of
  sight (pharetra) through the corona
• Use of constellations of radio sources for
  tomographic-like analyses

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Plasma Contributions to the Faraday Rotation
Integral
We need enough observations to sort out various
contributions to coronal density and magnetic
field
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New Coronal Faraday Rotation Results from the VLA

• Project AS764 August, 2003 2X10 hour sessions
  on source 3C228
• Project AS826 March-April 2005 4X10 hour
  sessions (plus reference) for coronal tomography

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Project AS826

• Observations

• March 12
• March 19
• March 28
• April 1
• May 29 (reference)

• 19 sources, 20 lines of sight

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Measurements in AS826
March 12 reference observation
Reference observation
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AS826 Preliminary Results
Source RM Average Error
2323-033 61.068 0.9976
2325-049 6.952 0.7295
2326-020 -2.418 0.9832
2328-049 -4.163 0.65297
2331-015 -6.087 2.564
2335-015 -13.671 0.305
2337-025 -12.458 0.4514
2338-042 -4.993 0.2619
2351-012   -27.35   0.693
2352-016 -24.175 1.428
2357-024 3.145 0.202
0006-001 2.255 0.207
0023045 5.18 0.125
0029052 -2.515 0.196
0030058 0.582 0.685
0034013 1.92 0.58
0039033 2.272 1.27
0039033 -0.905 0.79
0041070 -0.371 0.613
0046067 -14.067   1.509
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How Can Faraday Rotation Observations Probe the
Overall Structure of the Coronal Plasma?
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Global RM Models How well can synoptic coronal
models account for FR Measurements?
Mancuso and Spangler 2000
Residuals of 2-3 rad/sq-m
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Modeling the Observed Rotation Measure
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AS826 Use of Synoptic Model for Corona
These measurements of magnetic field and density
at 3R can be used to estimate B and n at
greater distances probed by Faraday Rotation
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Model Calculations (cont)

• We will use different theoretical models of the
  coronal magnetic field to try to reproduce the
  measured rotation measures.
• We will also examine whether the coronal
  magnetic field at 3.25 R accurately describes the
  field at 5-10 R.

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Observed and Model RM Comparison
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Observed and Model RM Comparison
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Observed and Model RM Comparison
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Adjustments to Synoptic Models for Magnetic Field
and Plasma Density

• Product of density and magnetic field must be
  multiplied by a factor of 24.8
• Densities must be multiplied by a factor between
  2.2 and 5
• Magnetic field values must be multiplied by
  factor between 4.4 and 11

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Conclusions

• Simple synoptic models of the corona roughly
  reproduce Pharetrae of Faraday Rotation
  measurements, but large residuals.
• Measurements consistent with coronal field of
  30-80mG at r6R. (Paetzold et al 1987)
• Future observations could more effectively
  constrain the functional form of the coronal
  magnetic field.
• Rotation measure changes substantially on
  timescales of a few hours too slow to be
  turbulence. Thus Mesoscale Plasma Structures.
• Smaller, faster fluctuations attributable to
  waves seen in spacecraft beacon data.
• Spatial variations in RM (differential Faraday
  Rotation) are small constraints on coronal
  turbulence are reasonable but not decisive

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Future Developments

• EVLA (Expanded VLA) Enormous increase in
  sensitivity of the VLA, in progress. But only if
  feed design prevents system temperature increase
  due to Sun.
• VLA at 5 GHz Could make measurements closer to
  the Sun, observations have more impact.