Using Electric Fields to Drive Simulations of the Solar Coronal Magnetic Field
Techniques for Measuring Coronal Magnetic Fields
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Transcript of 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
Steven Tomczyk Solar in Sonoma 11/28/12
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
Steven Tomczyk Solar in Sonoma 11/28/12
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
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 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
Steven Tomczyk Solar in Sonoma 11/28/12
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
Steven Tomczyk Solar in Sonoma 11/28/12
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
Steven Tomczyk Solar in Sonoma 11/28/12
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
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)
46 cm aperture, integration time 70 mins, 20 arcsec fiber
V/I ~10-4 / G
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
ATST 4-m aperture5 arcminute field-of-viewHigh Spatial ResolutionOperation into Far-IR
Photon Noise from Corona and Background
Steven Tomczyk Solar in Sonoma 11/28/12
COSMO Large Coronagraph
Future Prospects for Zeeman Measurements
1.5-m refractive coronagraph1º field-of-view< 5ppm scattered lightSynoptic operation
Steven Tomczyk Solar in Sonoma 11/28/12
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
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
Peter, 2011
Fineschi, 2001
Steven Tomczyk Solar in Sonoma 11/28/12
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
Steven Tomczyk Solar in Sonoma 11/28/12
Hanle Measurement Prospects
Ly-αTrujillo 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 frequencyand 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
www.ovsa.njit.edu
Underway now
Many fewer dishes than FASR - reduced imaging capability
Very high time resolution
Steven Tomczyk Solar in Sonoma 11/28/12
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
Steven Tomczyk Solar in Sonoma 11/28/12
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
Steven Tomczyk Solar in Sonoma 11/28/12
Phase Speed Map
Phase Speeds 0.3-2 Mm/sPotential for Coronal Seismology
Steven Tomczyk Solar in Sonoma 11/28/12
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
Steven Tomczyk Solar in Sonoma 11/28/12
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
Steven Tomczyk Solar in Sonoma 11/28/12
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
Steven Tomczyk Solar in Sonoma 11/28/12
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
Steven Tomczyk Solar in Sonoma 11/28/12
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
Steven Tomczyk Solar in Sonoma 11/28/12
SolMex - 5 Polarimeters!
Steven Tomczyk Solar in Sonoma 11/28/12