Measuring the Magnetic Field in the Sun and the Interstellar Medium
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Transcript of 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
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 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
Phase speed of R&L waves
Phase shift (cm) after prop.
Phase shift (radians)
Rotation of polarization position angle
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
How one measures polarization position angles and Faraday
rotation with the VLA
• Polarization map of a radio galaxy at 1465 MHz
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
Measurements in AS826
Observation through corona
March 12 – reference observation
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
We need enough observations to sort out various contributions to coronal density and magnetic field
Conclusions
• 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
You are here
Line of sight out of galaxy
Faraday rotation through the ISM
Variation of Faraday Rotation in the Interstellar Medium
What Faraday Rotation Observations have told us about the plasma of the interstellar medium
Conclusions
• 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