Radio Measurements of the Height of Strong Coronal Magnetic Fields Above Spots at the Limb

1. Radio Measurements of the Height of Strong Coronal Magnetic Fields Above Spots at the Limb Jeff Brosius (Catholic Univ.) Stephen White (Univ. of MD)

2. Introductory Comments • Magnetic fields drive solar phenomena • Coronal magnetic fields are difficult to measure • JOP 100: Use coordinated EUV and radio observations to measure coronal magnetic field • Focus here on sunspot at limb on 2004 July 29: SOHO (CDS, EIT, MDI), TRACE, VLA • Key results: B = 1750 G at h = 8000 km; B = 960 G at h = 12,000 km; scale height = 6900 km

3. JOP 100 Target: AR 10652

4. Radio Emission: Thermal Bremsstrahlung • Emitted when free electrons collide with protons. • Depends on CEM (e.g., Brosius & Landi 2005) and T. • Minimum radio intensity emitted by a plasma.

5. Radio Emission: Thermal Gyroemission • Due to thermal electrons spiraling along magnetic field lines. • Depends on T, B, angle (e.g., White & Kundu 1997). • Occurs where observing frequency (f) is a harmonic (2, 3, 4,…) of gyrofrequency (B/357 GHz): B = 357f/n.

6. 15 GHz Radio Source Above Sunspot • 70x90 arcsec^2 TRACE white light image. • Peak brightness temperature 6.9x10^5 K. • First analysis of 15 GHz coronal T_B above limb. • Calculated free-free brightness temperature is inadequate (3x10^4 K). • Zero circular polarization. • Must be due to 3rd harmonic gyroemission: B=1750 G, at height of 8000 km.

7. Which Harmonic? • 2nd harmonic at 15 GHz requires B = 2600 G, and is surrounded by 3rd harmonic (also optically thick) layer at 1750 G. • 4th harmonic at 15 GHz requires B = 1300 G, but produces polarized source since O-mode is thin.

8. 8 GHz Radio Source Above Sunspot • 70x90 arcsec^2 TRACE white light image. • Peak brightness temperature 1.3x10^6 K. • Calculated free-free brightness temperature is inadequate (6 x 10^4 K). • Zero circular polarization. • Must be due to 3rd harmonic gyroemission: B = 960 G, at height of 12,000 km.

9. Bright Plume Above Spot at Limb • O V emission at 629.7 A (left), formed at log T = 5.4. • Ne VI emission at 562.8 A (right), formed at log T = 5.6. • Brightest plume emission above penumbra, but not necessarily above umbra. • Structure extending south and west is a transequatorial loop (Brosius 2006).

10. EUV and Radio Observations of Plume at Limb • Brightest radio emission is located away from brightest EUV plume emission. • 8 GHz emission from plume is due to thermal gyroemission. • 15 GHz emission from plume is weak, noisy, probably free-free.

11. Coordinating FASR (Frequency-Agile Solar Radiotelescope) with AIA • 100-antenna imaging array • Frequency range 0.03 – 30 GHz • Spatial resolution ~ 20/f(GHz) arcsec • Time resolution 10 – 100 ms • Nominal timeline: - Phase B Study 2005-2006 - Construction 2007-2010 - First Science 2009-2010 • However, FASR’s NSF funding profile has been slowed down considerably

12. Summary • Coordinated EUV, white light, radio observations of sunspot at limb yield direct measure of height of coronal magnetic field: B = 1750 G at h = 8000 km, and B = 960 G at h = 12,000 km. • Radio observations provide “coronal boundary conditions” against which to compare field extrapolations. • FASR will provide vast improvement over VLA.