Probing the Solar Corona with Radioastronomical Observations Steven R. Spangler.

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Presentation transcript:

Probing the Solar Corona with Radioastronomical Observations Steven R. Spangler

Physics of Faraday Rotation: the cartoon

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

Measurements in AS826 Reference observation March 12 – reference observation

AS826 Preliminary Results SourceRM AverageError

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

Global RM Models: How well can synoptic coronal models account for FR Measurements? Mancuso and Spangler 2000 Residuals of ~ 2-3 rad/sq-m

Modeling the Observed Rotation Measure

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

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

Observed and Model RM Comparison

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

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 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

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.