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

Introductory Comments Magnetic fields drive solar phenomena Magnetic fields drive solar phenomena Coronal magnetic fields are difficult to measure Coronal magnetic fields are difficult to measure JOP 100: Use coordinated EUV and radio observations to measure coronal magnetic field 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 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 Key results: B = 1750 G at h = 8000 km; B = 960 G at h = 12,000 km; scale height = 6900 km

JOP 100 Target: AR 10652

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.

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.

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 3 rd harmonic gyroemission: B=1750 G, at height of 8000 km.

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

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 3 rd harmonic gyroemission: B = 960 G, at height of 12,000 km.

Bright Plume Above Spot at Limb O V emission at A (left), formed at log T = 5.4. Ne VI emission at 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).

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.

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

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