High Resolution Views of Solar Faculae

Transcription

High Resolution Views of Solar Faculae
High Resolution Views of Solar Faculae
Tom Berger
Lockheed Martin Solar and Astrophysics Lab
[email protected]
Outline
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What the heck are “solar faculae” anyway??
Why do we care? Solar irradiance!
Measurements of center-to-limb variation
High resolution images of solar faculae
High resolution movies of solar faculae
Theoretical models of faculae
Introduction: what are “faculae”?
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Faculae = plural of facula - Latin for “small torch”
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History of observations:
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Originally discovered near the limb in full-disk images of
the Sun, faculae are the small, bright, patterns around dark
sunspots and in the “photospheric network”.
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First modern (photometric) study of faculae by Rogerson
(1961). Older studies by Waldmeier and Ten Bruggencate
(1939), Lockyer, Secchi, Galileo…
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First connection to magnetic field by Chapman & Sheeley
(1968) and Frazier (1970).
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First sub-arcsecond (sub 1000 km) resolution imaging by
Dunn & Zirker (1973), Mehltretter (1974), and Muller
(1980s).
Facular appearance and wavelength
BBSO Ca II Kline
MDI
Continuum
28-October-2003
28-October-2003
Why do we care?
Small-scale magnetic flux and irradiance
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The Total Solar Irradiance (TSI) increases by ~0.1% at sunspot
maximum: the more dark sunspots there are on the Sun, the brighter it
gets.
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Why? Because bright faculae “outnumber” and outlast dark sunspots
and overcompensate the sunspot irradiance deficit.
ACRIM Composite TSI
Wolf Sunspot Number
Courtesy J. Lean
Small-scale magnetic flux and irradiance
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Sunspot irradiance deficit (darkness): easy to model
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Only weakly dependent on size and wavelength in the visible
and UV - not dependent on disk position.
Sunspots are relatively large and easy to observe and measure.
Physical mechanisms of sunspot darkening are well understood
(magnetic suppression of convective heat flow).
Facular brightness: difficult to model
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Depends sensitively on wavelength, size, and disk position.
Small-scale magnetic flux ranges in angular size from 1--2
arcsecond (micropores) down to less than 0.1 arcsecond (flux
tubes”) – very hard to observe.
Hardest thing to measure/model: the Center-to-Limb Variation
(CLV) in brightness of faculae as a function of size and/or
magnetic field strength.
Measurements of CLV
Definitions:
ƒ θ = angle between observer (Earth) and solar surface normal.
ƒ µ = cosθ,
µ = 0 at the limb; µ = 1 at disk center.
ƒ Contrast = (Ifac – Iqs)/ Iqs
Libbrecht & Kuhn
ApJ, 1984
“Hot Wall” Model
1.0
0.8
0.6
0.4
Chapman & Klabunde, 525.0 nm
ApJ, 1982
0.2
0
⃝ Wang & Zirin, 525.0 nm
Sol Phys, 1987
Measurements of CLV
SVST 630.2 nm continuum data
Topka, Tarbell, et al.
ApJ, 1997
Spatial resolution = 0.5”
µ = 0.31 – 0.996
MDI 676.8 nm continuum data
Ortiz, Solanki, et al.
A&A, 2002
Spatial resolution = 2”
High resolution images of faculae
AR 10039 24-July-2002
TRACE white light image
µ ~ 0.45
Tickmarks = 10 Mm
High resolution images of faculae
AR 10039 24-July-2002,
G-band 430.5 nm, µ = 0.45
Tickmarks = 1 Mm
High resolution images of faculae
AR 10039 24-July-2002,
G-band 430.5 nm, µ = 0.45
Tickmarks = 1 Mm
High resolution images of faculae
AR 10039 24-July-2002,
G-band 430.5 nm, µ = 0.45
Tickmarks = 1 Mm
High resolution images of faculae
High resolution images of faculae
SST G-band, disk center 25-May-03
Disk center 13-May-2003
High resolution images of faculae
TRACE WL
AR 10377
06-June-2003
µ = 0.6
High resolution images of faculae
SST G-band
AR 10377
06-June-2003
µ = 0.6 θ = 53°
Tickmarks = 1 Mm
High resolution images of faculae
SST 630.25nm
AR 10377
06-June-2003
µ = 0.6 θ = 53°
Tickmarks = 1 Mm
High resolution images of faculae
SST G-band
AR 10377
06-June-2003
µ = 0.6 θ =53°
0.81Rs
0.80Rs
0.79Rs
0.78Rs
Tickmarks = 1 Mm
High resolution images of faculae
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1
5
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High resolution images of faculae
Cut 1
Cut 3
Cut 2
Cut 4
High resolution images of faculae
Cut 5
High resolution images of faculae
Tickmarks = 1 Mm
High resolution images of faculae
Tickmarks = 1 Mm
High resolution images of faculae
High resolution images of faculae
High resolution MOVIES of faculae
Swedish 1-meter Solar Telescope, La Palma
16-June-2003, G-band 430.5 nm
µ = 0.65
Courtesy B. de Pontieu, LMSAL
High resolution MOVIES of faculae
Swedish 1-meter Solar Telescope, La Palma
16-June-2003, G-band 430.5 nm
µ = 0.65
Courtesy B. de Pontieu, LMSAL
Theoretical models of faculae
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“Hot Cloud” or “Hot Hill” models
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Rogerson (1961), Muller (1975, “Facular Granules”),
Hirayama & Moriyama (1979), Schatten (1986, “Hillock &
Cloud”)
“Hot Wall” flux tube models
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Spruit (1976), Knölker & Schüssler (1988, 2D MHD models)
Theoretical models of faculae
67 – 117 km
Topka, et al. ApJ 1997
Zw = Wilson depression = Zw(B), function of magnetic field strength
D = diameter of magnetic “flux tube”
tau = optical depth. tau=1 is the “surface” from which light is emitted
Theoretical models of faculae
ƒ 3D compressible MHD models
ƒ Keller, Schussler, et al., ApJL 2004 May
ƒ Carlsson, Stein, et al., ApJL 2004 July
Theoretical models of faculae
arcsec
µ = 0.5
Conclusions
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Observations and models are converging.
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CLV of 3D MHD models replicates observations, including bright faculae
out to µ = 0.08 (θ = 88º).
Models still lack complexity & number density of bright faculae for
equivalent magnetic flux density.
Models have yet to demonstrate the dynamic effects seen in the movie
data: faculae are dynamic on time scales of minutes.
Lateral radiation transfer (“hot wall”) mechanism responsible for facular
brightening. But Wilson depression wall is not what we see as bright – it’s
an extended sample of the granule behind the flux tube.
TBD:
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Thorough CLV survey at 100 km resolution, including narrowband
continuum and other line region wavelengths.
Spectral line measurements of temperature, density, velocity, mag field for
comparison to models
Stokes polarimetry line diagnostics.
ab initio facular irradiance calculations using new MHD models.