Radio Radio Measurements of Measurements of

the Height of the Height of Strong Coronal Strong Coronal

Magnetic Fields Magnetic Fields Above Spots at the Above Spots at the

LimbLimbJeff Brosius (Catholic Univ.)Jeff Brosius (Catholic Univ.)

Stephen White (Univ. of MD)Stephen White (Univ. of MD)

Introductory CommentsIntroductory Comments

Magnetic fields drive solar phenomenaMagnetic fields drive solar phenomena Coronal magnetic fields are difficult to measureCoronal magnetic fields are difficult to measure JOP 100: Use coordinated EUV and radio JOP 100: Use coordinated EUV and radio

observations to measure coronal magnetic fieldobservations to measure coronal magnetic field Focus here on sunspot at limb on 2004 July 29: Focus here on sunspot at limb on 2004 July 29:

SOHO (CDS, EIT, MDI), TRACE, VLASOHO (CDS, EIT, MDI), TRACE, VLA Key results: B = 1750 G at h = 8000 km; B = Key results: B = 1750 G at h = 8000 km; B =

960 G at h = 12,000 km; scale height = 6900 960 G at h = 12,000 km; scale height = 6900 kmkm

JOP 100 Target: AR JOP 100 Target: AR 1065210652

Radio Emission: Thermal Radio Emission: Thermal BremsstrahlungBremsstrahlung

Emitted when free Emitted when free electrons collide electrons collide with protons.with protons.

Depends on CEM Depends on CEM (e.g., Brosius & (e.g., Brosius & Landi 2005) and T.Landi 2005) and T.

Minimum radio Minimum radio intensity emitted intensity emitted by a plasma.by a plasma.

Radio Emission: Thermal Radio Emission: Thermal GyroemissionGyroemission

Due to thermal Due to thermal electrons spiraling electrons spiraling along magnetic field along magnetic field lines.lines.

Depends on T, B, Depends on T, B, angle (e.g., White & angle (e.g., White & Kundu 1997).Kundu 1997).

Occurs where Occurs where observing frequency observing frequency (f) is a harmonic (2, 3, (f) is a harmonic (2, 3, 4,…) of gyrofrequency 4,…) of gyrofrequency (B/357 GHz): B = (B/357 GHz): B = 357f/n.357f/n.

15 GHz Radio Source Above 15 GHz Radio Source Above SunspotSunspot

70x90 arcsec^2 TRACE 70x90 arcsec^2 TRACE white light image.white light image.

Peak brightness Peak brightness temperature 6.9x10^5 K.temperature 6.9x10^5 K.

First analysis of 15 GHz First analysis of 15 GHz coronal T_B above limb.coronal T_B above limb.

Calculated free-free Calculated free-free brightness temperature is brightness temperature is inadequate (3x10^4 K).inadequate (3x10^4 K).

Zero circular Zero circular polarization.polarization.

Must be due to 3Must be due to 3rdrd harmonic gyroemission: harmonic gyroemission: B=1750 G, at height of B=1750 G, at height of 8000 km.8000 km.

Which Harmonic?Which Harmonic?

22ndnd harmonic at 15 harmonic at 15 GHz requires B = GHz requires B = 2600 G, and is 2600 G, and is surrounded by 3surrounded by 3rdrd harmonic (also harmonic (also optically thick) layer optically thick) layer at 1750 G.at 1750 G.

44thth harmonic at 15 harmonic at 15 GHz requires B = GHz requires B = 1300 G, but produces 1300 G, but produces polarized source polarized source since O-mode is thin.since O-mode is thin.

8 GHz Radio Source Above 8 GHz Radio Source Above SunspotSunspot

70x90 arcsec^2 TRACE 70x90 arcsec^2 TRACE white light image.white light image.

Peak brightness Peak brightness temperature 1.3x10^6 K.temperature 1.3x10^6 K.

Calculated free-free Calculated free-free brightness temperature is brightness temperature is inadequate (6 x 10^4 K).inadequate (6 x 10^4 K).

Zero circular polarization.Zero circular polarization. Must be due to 3Must be due to 3rdrd

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

Bright Plume Above Spot Bright Plume Above Spot at Limbat Limb

O V emission at 629.7 A O V emission at 629.7 A (left), formed at log T = (left), formed at log T = 5.4.5.4.

Ne VI emission at 562.8 Ne VI emission at 562.8 A (right), formed at log T A (right), formed at log T = 5.6.= 5.6.

Brightest plume emission Brightest plume emission above penumbra, but not above penumbra, but not necessarily above umbra.necessarily above umbra.

Structure extending Structure extending south and west is a south and west is a transequatorial loop transequatorial loop (Brosius 2006).(Brosius 2006).

EUV and Radio EUV and Radio Observations of Plume at Observations of Plume at

LimbLimb Brightest radio Brightest radio

emission is located emission is located away from brightest away from brightest EUV plume emission.EUV plume emission.

8 GHz emission from 8 GHz emission from plume is due to plume is due to thermal thermal gyroemission.gyroemission.

15 GHz emission 15 GHz emission from plume is weak, from plume is weak, noisy, probably free-noisy, probably free-free.free.

Coordinating FASR Coordinating FASR (Frequency-Agile Solar (Frequency-Agile Solar

Radiotelescope) with AIARadiotelescope) with AIA 100-antenna imaging array100-antenna imaging array Frequency range 0.03 – 30 GHzFrequency range 0.03 – 30 GHz Spatial resolution ~ 20/f(GHz) arcsecSpatial resolution ~ 20/f(GHz) arcsec Time resolution 10 – 100 msTime resolution 10 – 100 ms Nominal timeline: Nominal timeline: - Phase B Study 2005-2006- Phase B Study 2005-2006 - Construction 2007-2010- Construction 2007-2010 - First Science 2009-2010- First Science 2009-2010 However, FASR’s NSF funding profile has been However, FASR’s NSF funding profile has been

slowed down considerablyslowed down considerably

SummarySummary Coordinated EUV, white light, radio Coordinated EUV, white light, radio

observations of sunspot at limb yield direct observations of sunspot at limb yield direct measure of height of coronal magnetic measure of height of coronal magnetic field:field:

B = 1750 G at h = 8000 km, andB = 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 Radio observations provide “coronal

boundary conditions” against which to boundary conditions” against which to compare field extrapolations.compare field extrapolations.

FASR will provide vast improvement over FASR will provide vast improvement over VLA.VLA.