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
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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
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
Zeeman  geff
2
hc
B B
Zeeman  B
R

Doppler
T
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) 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
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, FeXIII 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
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
Steven Tomczyk
Solar in Sonoma
11/28/12
Future Prospects for Zeeman Measurements
4-m aperture
5 arcminute field-of-view
High Spatial Resolution
Operation into Far-IR
ATST
Photon Noise from Corona
and Background
Steven Tomczyk
Solar in Sonoma
11/28/12
Future Prospects for Zeeman Measurements
COSMO Large Coronagraph
Steven Tomczyk
1.5-m refractive coronagraph
1º field-of-view
< 5ppm scattered light
Synoptic operation
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
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
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
AR6615 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, 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
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
CoMP 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
 B   ne
B

  9 3 
vA 
 1210 
4
 20 G   10 cm 
Then,
1/2
(km/s) (Aschwanden, 2004)
1/2
 σ v A   ne

  9 3  (G)
σ B  
 60 km/s   10 cm 
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 3d 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
SolMex - 5 Polarimeters!
Steven Tomczyk
Solar in Sonoma
11/28/12
Steven Tomczyk
Solar in Sonoma
11/28/12