Techniques for Measuring Coronal Magnetic Fields High Altitude

Techniques for Measuring Coronal Magnetic Fields High Altitude Observatory (HAO) – National Center for Atmospheric Research (NCAR) The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research under sponsorship of the National Science Foundation. An Equal Opportunity/Affirmative Action Employer.

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 LASCO are not currently understood and will remain so until we are able to obtain routine measurements of coronal magnetic fields Steven Tomczyk Solar in Sonoma 11/28/12

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 Over a Large Range of Spatial and Temporal Scales >> Large Field-of-View and Synoptic Steven Tomczyk Solar in Sonoma 11/28/12

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 Steven Tomczyk Solar in Sonoma 11/28/12

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 Steven Tomczyk Solar in Sonoma 11/28/12

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 Steven Tomczyk Solar in Sonoma 11/28/12

Coronal Zeeman Candidate Emission Lines Visible / IR Judge et al. (2001) Fe. XIII 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 Steven Tomczyk Solar in Sonoma 11/28/12

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 Steven Tomczyk Solar in Sonoma 11/28/12

UV Zeeman Effect Future Prospects Peter (2012) Steven Tomczyk Solar in Sonoma 11/28/12

Near IR Coronal Zeeman Example Solar-C OFIS, Fe. XIII 1074. 7 nm (Lin, et al. , 2004) V/I ~10 -4 / G 46 cm aperture, integration time 70 mins, 20 arcsec fiber Steven Tomczyk Solar in Sonoma 11/28/12

Near IR Coronal Zeeman Example Co. MP, Fe. XIII 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 Steven Tomczyk Solar in Sonoma 11/28/12

Future Prospects for Zeeman Measurements 4 -m aperture 5 arcminute field-of-view ATST High Spatial Resolution Operation into Far-IR Photon Noise from Corona and Background Steven Tomczyk Solar in Sonoma 11/28/12

Future Prospects for Zeeman Measurements COSMO Large Coronagraph 1. 5 -m refractive coronagraph 1º field-of-view < 5 ppm scattered light Synoptic operation Steven Tomczyk Solar in Sonoma 11/28/12

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 Steven Tomczyk Solar in Sonoma 11/28/12

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 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 Steven Tomczyk Solar in Sonoma 11/28/12

Hanle Effect Range Fineschi, 2001 Peter, 2011 Steven Tomczyk Solar in Sonoma 11/28/12

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 Steven Tomczyk Solar in Sonoma 11/28/12

Hanle Measurement Prospects Ly-α Trujillo Bueno (2011) Steven Tomczyk Solar in Sonoma 11/28/12

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 AR 6615 observed with VLA (5, 8 15 GHz) Lee (2007) Steven Tomczyk Solar in Sonoma 11/28/12

Future Radio Assets Frequency Agile Solar Radiotelescope Factor of 420 in frequency and spatial resolution High Time Resolution www. fasr. org Steven Tomczyk Solar in Sonoma 11/28/12

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 Steven Tomczyk Solar in Sonoma 11/28/12

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 Steven Tomczyk Solar in Sonoma 11/28/12

Coronal Waves and Seismology Developed over the past decade (Aschwanden, Nakariakov, Vervichte, Schrijver, de. Moortel and others Impulsively excited, strongly damped oscillations Seen in intensity images from e. g. Trace Can use to infer strength of coronal magnetic field Application is limited Steven Tomczyk TRACE July 14, 1998 Oscillation Amplitude ~100 km/s Solar in Sonoma 11/28/12

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 Co. MP Instrument Steven Tomczyk Solar in Sonoma 11/28/12

Phase Speed Map Phase Speeds 0. 3 -2 Mm/s Potential for Coronal Seismology Steven Tomczyk Solar in Sonoma 11/28/12

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 Steven Tomczyk Solar in Sonoma 11/28/12

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 Steven Tomczyk Solar in Sonoma 11/28/12

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 Steven Tomczyk Solar in Sonoma 11/28/12

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 Steven Tomczyk Solar in Sonoma 11/28/12

Required for Progress Inversion techniques development Hanle Effect LOS Integration Issues - Tomographic Reconstruction Radio 3 d Coronal Image Synthesis Need density measurements to exploit seismology All techniques will have significant systematic errors - probably larger than random errors Steven Tomczyk Solar in Sonoma 11/28/12

Sol. Mex - 5 Polarimeters! Steven Tomczyk Solar in Sonoma 11/28/12

Steven Tomczyk Solar in Sonoma 11/28/12