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Techniques for Measuring Coronal Magnetic Fields

Techniques for Measuring Coronal Magnetic Fields . Motivation. The Corona is a Magnetically Dominated System Coronal Magnetism is the Source of Space Weather Coronal Mass Ejections Energetic Particle Acceleration Coronal Heating Solar Wind Acceleration

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Techniques for Measuring Coronal Magnetic Fields

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  1. Techniques for Measuring Coronal Magnetic Fields

  2. Motivation The Corona is a Magnetically Dominated System Coronal Magnetism is the Source of Space Weather • Coronal Mass Ejections • Energetic Particle Acceleration • Coronal Heating • Solar Wind Acceleration are not currently understood and will remain so until we are able to obtain routine measurements of coronal magnetic fields LASCO

  3. Approach Need Systems Approach to Study the Coupled Solar Atmosphere • Must Measure Coronal Magnetic Field Strength and Direction - On Solar Disk and Above Limb • Need Complementary Observations of Coronal Plasma • Chromosphere / Transition Region Magnetic Field Observations • Observations Over a Large Range of Spatial and Temporal Scales >> Large Field-of-View and Synoptic

  4. Talk Outline • Review Methods to Measure Coronal Magnetic Fields • Illustrate with Example Data • Discuss Future Prospects • In situ • UV / EUV, Visible / IR • Zeeman, Hanle Effects • Radio • Gyroresonance • Seismology • Strengths and Limitations

  5. In Situ Measurements “Radio observations provide the most direct means of measuring coronal magnetic fields” D. Gary, 2012 S&H Decadal Survey White Paper In situ measurements provide the ONLY direct means of measuring the coronal magnetic field Pioneer, Mariner, Helios, ACE, Messenger spacecraft (r ≥ 0.29 AU) (Mariani & Neubauer, 1990) Solar Probe Plus / Fields (r ≥ 8.5 Rsun) All other methods are indirect

  6. Zeeman Effect • Circular Polarization (Stokes V) Determines BLOS • Stokes V scales as R • Wavelength dependence of Zeeman Effect favors • Long Wavelengths - Visible and Especially IR • Zeeman signals difficult to measure for UV and Shorter Wavelengths - Need 100-1000 G for UV lines

  7. Coronal Zeeman Candidate Emission Lines Visible / IR Judge et al. (2001) FeXIII 1074.7 nm has the best expected Zeeman S/N based on line intensity, magnetic sensitivity and sky background UV / EUV C IV 155 nm, Mg II 280 nm

  8. Visible / IR Emission Lines UV/EUV and Vis/IR methods are the same, except that At visible and IR wavelengths, the solar disk is MUCH brighter than the coronal emission Observations are confined to above the limb only For all techniques talked about today, the corona above the limb is optically thin Line of sight integration issues

  9. UV Zeeman Effect Future Prospects • Peter (2012)

  10. Near IR Coronal Zeeman Example • Solar-C OFIS, FeXIII 1074.7 nm (Lin, et al., 2004) • V/I ~10-4 / G • 46 cm aperture, integration time 70 mins, 20 arcsec fiber

  11. Near IR Coronal Zeeman Example • CoMP, FeXIII 1074.7 nm (Tomczyk et al., 2007) Intensity, LOS velocity, Field Direction, LOS Field Strength, from 10/20/05, 2.5 hours integration, 10 arcsec resolution Errors of Several Gauss in Bright Corona

  12. Future Prospects for Zeeman Measurements • ATST • 4-m aperture • 5 arcminute field-of-view • High Spatial Resolution • Operation into Far-IR • Photon Noise from Corona and Background

  13. Future Prospects for Zeeman Measurements • COSMO Large Coronagraph 1.5-m refractive coronagraph 1º field-of-view < 5ppm scattered light Synoptic operation

  14. Hanle Effect • Reduction of Linear Polarization by Magnetic Field • (Depolarization) • Linear Polarization produced by anisotropy of Radiation Field • Works on disk as well as above limb • Lines to use: • Lyman series (TR; Bommier & Sahal-Brechot,1982) • O VI 103.2 nm (Raouafi, et al., A&A, 1999) • Interpretation depends on atmospheric model, scattering geometry, velocity field, LOS integration

  15. Hanle Effect • Hanle Effect is due to Quantum coherences between atomic states • Effective over a restricted range of field strength - where Zeeman Splitting is approximately equal to Natural Line Width • A[107 s-1] ~ 0.88 g B[G] (Fineschi, 2001) • Works with UV/EUV permitted lines • For Forbidden lines (Vis/IR) - Hanle effect is saturated for very small field strengths and linear polarization contains no information on magnetic field strength - POS direction only

  16. Hanle Effect Range • Fineschi, 2001 • Peter, 2011

  17. Hanle Effect Example • SUMER Observation of Linear Polarization in O IV 103.2 nm (Raouafi, et al., A&A, 1999) • Interpreted by (Raouafi, et al., A&A, 2002) to yield B 3-6 G

  18. Hanle Measurement Prospects • Ly-α • Trujillo Bueno (2011)

  19. Radio Methods Gyroresonance: Opacity formed in thin layer - Maps at a given frequency provide iso-gauss surface Circular Polarization proportional to B (not Blos) Observed on solar disk, B > 200 G Physical height not known; need to assume harmonic order AR6615 observed with VLA (5, 8 15 GHz) Lee (2007)

  20. Future Radio Assets Frequency Agile Solar Radiotelescope Factor of 420 in frequency and spatial resolution High Time Resolution www.fasr.org

  21. Gyroresonance Uncertainty Difficuly to quantify Gyrofrequency: f(MHz) = 2.8 B(G) Then, σf(MHz) = 2.8 σB(G) For FASR, Frequency Resolution is 5 MHz Setting σf = 5 MHz, gives σB ~ 2 G

  22. Near Future Radio Assets Owens Valley Solar Array Upgrade Underway now Many fewer dishes than FASR - reduced imaging capability Very high time resolution www.ovsa.njit.edu

  23. Coronal Waves and Seismology Developed over the past decade (Aschwanden, Nakariakov, Vervichte, Schrijver, deMoortel and others Impulsively excited, strongly damped oscillations Seen in intensity images from e.g. Trace Can use to infer strength of coronal magnetic field TRACE July 14, 1998 Oscillation Amplitude ~100 km/s Application is limited

  24. Ubiquitous Waves • Perturbations seen in velocity, • not intensity • Wave propagation is aligned • with magnetic field • Phase speeds 0.3-2 Mm/s Velocity Amplitude ~0.3 km/s rms CoMP Instrument

  25. Phase Speed Map Phase Speeds 0.3-2 Mm/s Potential for Coronal Seismology

  26. Coronal Vector Magnetic Field Waves provide the POS component of the phase speed - Transverse Component of Coronal Magnetic Field Zeeman Effect provides LOS component Which can be combined to give the Vector Magnetic Field

  27. Potential for Coronal Seismology (Aschwanden, 2004) Then, An uncertainty in the phase speed of 60 km/s, and an electron density of 109 cm-3 results in a 1 G magnetic field uncertainty Sensitive Method – But Need Coronal Density Need ne/σne > ~3

  28. Summary and Prospects On the solar disk: Gyroresonance and Zeeman Effect in UV/EUV provide BLOS Strong Fields > 200 G only Hanle Effect of UV/EUV permitted lines Weak Fields in restricted B range Above the limb: Visible/IR Zeeman UV/EUV Hanle Radio Bremsstrahlung All Sensitive to Weak and Strong Fields but have LOS integration issues

  29. Summary and Prospects Radio Measurements offer the best prospects for High Time Resolution (1 s) - Flare Observations Visible/IR Zeeman Blos combined with Wave Seismology Btrans offer the possibility of Coronal Vector Magnetic Field Measurement

  30. Required for Progress Inversion techniques development Hanle Effect LOS Integration Issues - Tomographic Reconstruction Radio 3d Coronal Image Synthesis Need density measurements to exploit seismology All techniques will have significant systematic errors - probably larger than random errors

  31. SolMex - 5 Polarimeters!

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