Download - SHINE 2008
SHINE 2008
Vector Magnetic Fields from
the Helioseismic and
Magnetic ImagerSteven 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.
Historically there have been two classes
of instruments.
Magnetographs and
Spectropolarimeters…
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)
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
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
SHINE 2008
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
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
SHINE 2008
Observations
Initial guess model Spectral synthesisSynthetic Stokes
spectrum
Comparison
Final model
Model parameter modification
Yes
No
Stokes
Inversion
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
SHINE 2008
How precisely will HMI and VFISV
infer the properties of the vector
magnetic field?Simulations by Juan Borrero
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
SHINE 2008
SHINE 2008
180º Ambiguity
Resolution
Is a fundamental problem
Growing confidence in
our ability to resolve it
HMI will provide resolved
vector field