Strength and Structure of the Coronal Magnetic Field
- Slides: 20
Strength and Structure of the Coronal Magnetic Field Steven R. Spangler University of Iowa
Strength and Structure of the Coronal Magnetic Field “Sol spectatorem, nisi deficit, non habet…” Seneca, De Cometis
Collaborators • University of Iowa: Laura Ingleby • Harvard-Smithsonian Center for Astrophysics: Mari Paz Miralles, Steven Cranmer, John Raymond
Importance and Role of the Coronal Magnetic Field • Probably involved in the heating of the corona; many theoretical ideas • Involved in support and expulsion of coronal plasma structures (prominences, CMEs) • Need for knowledge of vector magnetic field throughout corona
Coronal Faraday Rotation
Instrument for Coronal Faraday Rotation: The NRAO Very Large Array (VLA) Observations at wavelengths of 21 and 18 cm
The Instrument: The Very Large Array Radiotelescope Operated by the National Radio Astronomy Observatory (NRAO)
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
New Results: Observations of the Corona with the VLA and UVCS in August, 2003
A B C Radio source 3 C 228 Image at left is from reference observations. Three source components were bright enough in polarized emission for a p. a. measurement in a single 10 minute scan.
Measured Coronal Faraday Rotation Solid symbols: 21 cm; open 18 cm 20 degrees = 8. 3 rad/m 2 B A C
Preliminary Results • Large position angle rotation of 150 d at 1465 MHz, RM=62. 5 rad/m 2 • No clear indication of differential RM between lines of sight separated by 40, 000 km • “Gross depolarization” does not seem to be present
Why was Rotation Measure so Large? • Line of sight passed very close to Sun (6. 1 Rs at end of session) • “Proximate Point” (closest point to Sun along line of sight) occurred over the neutral line: condition for maximum Faraday rotation. This situation is also easiest to analyse
Line of sight to 3 C 228 Thanks to Bill Coles, UC San Diego
For conditions of maximum Faraday Rotation (Sakurai and Spangler 1994, Ap. J 434, 773) RM=C RS (0. 59 N 0 B 0) R-3. 36 B(r) ~ r -2 , n(r) ~ r -2. 36 N 0 and B 0 are coefficients determining level of density and magnetic field in the corona.
Results on Magnitude of Coronal Magnetic Field • With same model as Sakurai and Spangler (1994), RM=46. 3 rad/m-sq (measured 62. 5), B = 27 m. G at 6. 1 Rs • Above model scaled to reproduce observed RM give B(6. 1)= 37 m. G • Coronal model from Mancuso and Spangler (2000) gives RM = 106 rad/m-sq, B(6. 1) = 65 m. G • Above model scaled to reproduce observed RM gives B(6. 1) = 38 m. G
Coronal MHD Model Mancuso & Spangler, Astrophys. J. 539, 480, 2000
Conclusions • New 3 C 228 polarimetric observations yield estimates of B at 6 solar radii of 27 -65 m. G. • Future work will improve estimates. • Observations provide constraints on differential RM in corona. • Depolarization results will be forthcoming and will constrain properties of small scale turbulence.
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- Right hand grip rule
- Paramagnetic diamagnetic ferromagnetic
- Magnetic field strength
- Farade laws
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