shine 2008
DESCRIPTION
Vector Magnetic Fields from the Helioseismic and Magnetic Imager. Steven Tomczyk (HAO/NCAR) Juan Borrero (HAO/NCAR and MPS). SHINE 2008. From Juan Borrero. SHINE 2008. Basic problem is to infer properties of magnetic field from observations of the Stokes profiles. - PowerPoint PPT PresentationTRANSCRIPT
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SHINE 2008
Vector Magnetic Fields from
the Helioseismic and
Magnetic ImagerSteven Tomczyk (HAO/NCAR)
Juan Borrero (HAO/NCAR and MPS)
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SHINE 2008
From Juan Borrero
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SHINE 2008
Basic problem is to infer properties of
magnetic field from observations of the
Stokes profiles.
Historically there have been two classes
of instruments.
Magnetographs and
Spectropolarimeters…
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SHINE 2008
Magnetograph
Instrument with typically poor spectral
resolution and good spatial resolution. Few
samples across line profile.
Usually only Stokes V - LOS Flux Density
Examples: BBSO, GONG, KPNO, MDI
Pro: Fast Temporal cadence
Con: LOS Flux density, saturation (systematic
errors)
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SHINE 2008
Spectropolarimeter
Instrument with typically good spectral resolution
and moderate spatial resolution. Many samples
across line profile. Usually spectrograph, I, Q, U,
V
Allow inference of vector field (B and geometry)
Examples: ASP, Hinode SP, IVM, SOLIS
Pro: Very precise
Con: Slow to acquire map (Hinode SP - 15
minutes, partial Sun), complicated and slow data
inversion
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SHINE 2008
HMI - Imaging Spectropolarimeter
Full Sun image - 1 arcsecond resolution (4096x4096)
Full Stokes I, Q, U, V with 6 samples across FeI 617.3
nm line
Allows inference of vector magnetic field
Full magnetic information every 90 seconds - 10
minute cadence for standard data product
Precision is not as good as Hinode SP but very fast
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SHINE 2008
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SHINE 2008
HMI will have:
– Full Disk Coverage
– High Temporal Cadence
– Continuous Observations
● Evolution of magnetic structure of sunspots and active regions
● Magnetic shear accumulations and impulsive release
● Evolution of helicity in active regions
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SHINE 2008
● Full Stokes I,Q,U,V allows inference of Vector Magnetic Fields
● More accurate velocities in the presence of magnetic fields through removal of Q, U & V to I crosstalk
– Flows around sunspots, Sunspot seismology
● More accurate flux determinations
– Total flux, as opposed to longitudinal
● True full disk magnetometry
– Limb fields, polar fields
Vector polarimetry with HMI
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SHINE 2008
Observations
Initial guess model Spectral synthesisSynthetic Stokes
spectrum
Comparison
Final model
Model parameter modification
Yes
No
Stokes
Inversion
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SHINE 2008
HMI inversion code
Very Fast Inversion of the Stokes Vector (VFISV) by
Juan Borrero
Provides inference of:Magnetic Field StrengthTotal Magnetic FluxInclinationAzimuth and ErrorsFilling FactorLine-Of-Sight VelocityThermodynamic Properties
Milne-Eddington radiative transfer
Initial guess with Neural Network and Weak Field
Approximation
Quick look and full inverted data product
Macroturbulence and damping not fit
Look up table for Voigt/Faraday functions
Works with high or low spectral sampling data
Very fast
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SHINE 2008
How precisely will HMI and VFISV
infer the properties of the vector
magnetic field?Simulations by Juan Borrero
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SHINE 2008
σB
(Gauss)
σInclination
(deg)
σAzimuth
(deg)
σVlos
(m/s)
Sunspot 4-5 0.2 0.2 20
Plage 10 1 1 30
Network 40-50 2-3 5-7 60-90
HMI Expected
Errors
We expect to see saturation starting at about 4000
Gauss
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SHINE 2008
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SHINE 2008
180º Ambiguity
Resolution
Is a fundamental problem
Growing confidence in
our ability to resolve it
HMI will provide resolved
vector field