Techniques for Measuring Coronal Magnetic Fields

High Altitude Observatory (HAO) – National Center for Atmospheric Research (NCAR)

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Techniques for Measuring Coronal Magnetic Fields

Steven Tomczyk Solar in Sonoma 11/28/12

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


Approach

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

Need Systems Approach to Study the Coupled Solar Atmosphere


Review Methods to Measure Coronal Magnetic FieldsIllustrate with Example Data Discuss Future Prospects

In situ UV / EUV, Visible / IR

Zeeman, Hanle Effects Radio Gyroresonance Seismology

Strengths and Limitations

Talk Outline


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 directmeans 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


Circular Polarization (Stokes V) Determines BLOS

Stokes V scales as R

Wavelength dependence of Zeeman Effect favorsLong Wavelengths - Visible and Especially IR

Zeeman signals difficult to measure for UV and Shorter Wavelengths - Need 100-1000 G for UV lines

Zeeman Effect

2

Zeeman eff Bg Bhc Zeeman

Doppler

BRT


Judge et al. (2001) FeXIII 1074.7 nm has the best expected Zeeman S/N based on line intensity, magnetic sensitivity and sky background

Visible / IR

Coronal Zeeman Candidate Emission Lines

UV / EUVC IV 155 nm, Mg II 280 nm


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

Visible / IR Emission Lines


UV Zeeman Effect Future Prospects

Peter (2012)


Near IR Coronal Zeeman Example

Solar-C OFIS, FeXIII 1074.7 nm (Lin, et al., 2004)

46 cm aperture, integration time 70 mins, 20 arcsec fiber

V/I ~10-4 / G


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


Future Prospects for Zeeman Measurements

ATST 4-m aperture5 arcminute field-of-viewHigh Spatial ResolutionOperation into Far-IR

Photon Noise from Corona and Background


COSMO Large Coronagraph

Future Prospects for Zeeman Measurements

1.5-m refractive coronagraph1º field-of-view< 5ppm scattered lightSynoptic operation


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

Hanle Effect


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


Hanle Effect Range

Peter, 2011

Fineschi, 2001


SUMER Observation of Linear Polarization in O IV 103.2 nm (Raouafi, et al., A&A, 1999)

Hanle Effect Example

Interpreted by (Raouafi, et al., A&A, 2002) to yield B 3-6 G


Hanle Measurement Prospects

Ly-αTrujillo Bueno (2011)


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)


Future Radio Assets

Frequency Agile Solar Radiotelescope

Factor of 420 in frequencyand spatial resolution

High Time Resolution

www.fasr.org


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


Near Future Radio Assets

Owens Valley Solar Array Upgrade

www.ovsa.njit.edu

Underway now

Many fewer dishes than FASR - reduced imaging capability

Very high time resolution


TRACE July 14, 1998Oscillation Amplitude ~100 km/s

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 frome.g. Trace

Can use to infer strength of coronal magnetic field

Application is limited


Ubiquitous Waves

Velocity Amplitude ~0.3 km/s rms

Perturbations seen in velocity, not intensity

Wave propagation is aligned with magnetic field

Phase speeds 0.3-2 Mm/s

CoMP Instrument


Phase Speed Map

Phase Speeds 0.3-2 Mm/sPotential for Coronal Seismology


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

Coronal Vector Magnetic Field


Potential for Coronal Seismology

(G)cm10n

km/s60σ

σ1/2

39ev

BA

(km/s)cm10n

G20B1210Bv

1/2

39e

A

4(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


On the solar disk:Gyroresonance and Zeeman Effect in UV/EUV provide BLOS

Strong Fields > 200 G onlyHanle Effect of UV/EUV permitted lines

Weak Fields in restricted B range

Above the limb:Visible/IR ZeemanUV/EUV HanleRadio Bremsstrahlung

All Sensitive to Weak and Strong Fields but have LOS integration issues

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

Summary and Prospects


Inversion techniques developmentHanle EffectLOS Integration Issues - Tomographic ReconstructionRadio 3d Coronal Image Synthesis

Need density measurements to exploit seismology

All techniques will have significant systematic errors - probably larger than random errors

Required for Progress


SolMex - 5 Polarimeters!

Techniques for Measuring Coronal Magnetic Fields

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