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Measuring the Magnetic Field in the Sun and the Interstellar Medium. Steven R. Spangler… University of Iowa. The solar corona and the interstellar medium…two astrophysical plasmas. Why is the coronal B field of interest?. Temperature of corona is 1-2 X 10 6 K

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Measuring the magnetic field in the sun and the interstellar medium
Measuring the Magnetic Field in the Sun and the Interstellar Medium

Steven R. Spangler… University of Iowa



Why is the coronal b field of interest
Why is the coronal B field of interest? astrophysical plasmas

  • Temperature of corona is 1-2 X 106 K

  • Magnetic fields probably involved via DC currents or MHD waves

  • Assessment of theories requires measurements





How do we measure b in the corona itself
How do we measure B in the corona itself? solar wind

Direct measurements out here

Zeeman measurements here


Radioastronomical propagation measurements
Radioastronomical propagation measurements solar wind

Technique discussed here: Faraday rotation



Physics of faraday rotation
Physics of Faraday Rotation solar wind

Phase speed of R&L waves

Phase shift (cm) after prop.

Phase shift (radians)

Rotation of polarization position angle


The physics of faraday rotation
The Physics of Faraday Rotation solar wind

Demonstration


The instrument the very large array radiotelescope
The Instrument: The Very Large Array Radiotelescope solar wind

Operated by the National Radio Astronomy Observatory (NRAO)



How one measures polarization position angles and faraday rotation with the vla
How one measures polarization position angles and Faraday rotation with the VLA

  • Polarization map of a radio galaxy at 1465 MHz



The background sources signal generators for propagation expmts
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
Extragalactic sources provide “constellations” of background objects

Mancuso & Spangler, Astrophys. J. 539, 480, 2000


Measurements in as826
Measurements in AS826 background objects

March 12 – reference observation

Observation through corona


Measuring the coronal magnetic field from a set of faraday rotation measurements
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


Plasma contributions to the faraday rotation integral
Plasma Contributions to the Faraday Rotation Integral Rotation Measurements

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


Conclusions
Conclusions Rotation Measurements

  • 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.


The interstellar medium another magnetized plasma
The interstellar medium: another magnetized plasma Rotation Measurements

Line of sight out of galaxy

You are here


Faraday rotation through the ism
Faraday rotation through the ISM Rotation Measurements




Conclusions1
Conclusions plasma of the interstellar medium

  • Faraday rotation observations with the VLA can measure the magnetic field in two quite different astrophysical plasmas.

  • These measurements can illuminate the dynamics and thermodynamics of the corona and the interstellar medium


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