Coronal Faraday Rotation: Diagnostics of Current Sheets and MHD Waves

Steven R. Spangler, Laura D. Ingleby, Laura G. Spitler, Catherine A. Whiting

Scope of Talk: Observations of Extragalactic Radio Sources with Radio Interferometers

Very Large Array

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

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

Plasma Contributions to the Faraday Rotation Integral

We need enough observations to sort out various contributions to coronal density and magnetic field

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”

Project AS826

o Observations:•March 12

•March 19

•March 28

•April 1

•May 29 (reference)

o 19 sources, 20 lines of sight

Results from Project AS764: Continuous Observations of 3C228 on August 16 and 18, 2003

• Measurement of RM(t) to source as a whole

• Measurement of “Differential Faraday Rotation”, different rotation measures along different lines of sight (A,B, and C)

Faraday Rotation to 3C228 on August 16, 2003

Remarkably little differential Faraday rotation observed

Modeling the Observed Rotation Measure

Spatial Structure Function (no Taylor Hypothesis!)

Spatial Structure Function (no Taylor Hypothesis!)

Diff. RM feature

Preliminary and Tentative Rotation Measure Structure Function Results

• August 16, 15.77-18.62 UT: DRM < 0.38 rad/m2

• August 16, 20.53-23.22 UT: DRM < 4.8 rad/m2

• August 18, 20 -24 UT: DRM < 0.25 rad/m2

Meaning of Result in Terms of Coronal Plasma Parameters

From Minter and Spangler (1996), magnetic field fluctuations only

Turbulence properties

Quantitative Significance of Observed RM Structure Functions

August 16, 6.2R August 18, 5.2R

l=0.1R

L=1.0R

e=0.5

e=1.0

How Can Faraday Rotation Observations Probe the Overall Structure of the Coronal

Plasma? (AS826)

Global RM Models: How well can synoptic coronal models account for FR Measurements?

Mancuso and Spangler 2000

Residuals of ~ 2-3 rad/sq-m

AS826 Preliminary ResultsSource 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

0023+045 5.18 0.125

0029+052 -2.515 0.196

0030+058 0.582 0.685

0034+013 1.92 0.58

0039+033 2.272 1.27

0039+033 -0.905 0.79

0041+070 -0.371 0.613

0046+067 -14.067   1.509

2RM

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

Estimating the distant coronal plasma from measurements at ~3 R

An and AB are arbitrary adjustment factors

How radio line of sight probes the corona

Observed and Model RM Comparison-AS826

Observed and Model RM Comparison

Observed and Model RM Comparison

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 r=6R. (Paetzold 1987)

• 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

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.