SST/CRISP observations of opposite polarity
field and convective flows in a sunspot penumbra
G.B. Scharmer*, J. de la Cruz Rodriguez, P. Sütterlin, V.M.J. Henriques
Based on:
”Opposite polarity field with convective downflow and its
relation to magnetic spines in a sunspot penumbra”, A&A 553,
A63 (2013).
*Institute for Solar Physics, Stockholm University
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Fundamental questions relating to sunspot penumbrae:
Broad-band Ca H movie (Henriques)
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•
How is the energy transported
to the visible surface?
•
Why do penumbrae have a
small-scale filamentary
structure?
•
Why are there strong azimuthal
variations in field strength and
inclination angle across
filaments?
•
Why do bright penumbral
filaments have dark cores?
•
What is the origin of the
Evershed flow?
Convective gap model (Spruit & Scharmer 2006)
Spine (more
vertical B)
Intra-spine
(more hor. B)
τ=1
Dark core
Evershed flow
Convection
Dark cores in penumbral filaments (2002)
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Convective gap model (2006)
hysical Journal, 750:62 (21pp), 2012 May 1
Results of simulations
(Rempel, 2012)
Side view
Central upflow
−rho (v grad) v
Bx, Bz
By
Vx, Vz
Vy
z
y
x
tau=1
jxB
−grad p + rho g
B
V
z
Lateral downflow
y
Penumbral filament in MHD simulation
up
down
Top view
y
Cross Section
x
iagram summarizing the basic field and flow structure of a penumbral filament as present in the numerical simulation. We present a sc
for the
field,
this is perfectly
consistent
with
tion view. x and z denote Except
the horizontal
and horizontal
vertical direction
perpendicular
to the filament,
y denotes
the direction along the filament a
central upflow regions, the
flow and field
aligned, while
flow submerges
horizontal
field lines in lateral downflow
predictions
of are
thewell
convective
gapthe
model
(Spruit &mostly
Scharmer
2006)
e a reduced field strength, but they contain a core with a non-vanishing vertical field component. Some of the associated flux continue
ns downward within the filament cavity. Depending on the position of the τ = 1 level the latter might become visible as inverse polari
hear of the Evershed flow induces a strong horizontal field component that is concentrated along the τ = 1 surface. This leads to str
fredag 9 augusti 13
The first missing observational link:
No evidence for convective downflows in Hinode data
(Franz & Schlichenmaier 2009).
But
Detection of convective dark downflows in CI 5380 with SST/CRISP
(Scharmer et al. 2011, Joshi et al. 2011).
...and in FeI 6301 with SST/CRISP
(Scharmer & Henriques 2012).
The penumbra flow-field shows a clear brightness - vertical velocity
correlation, in the sense expected for convection.
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The second missing observational link:
Opposite polarity field associated with convective downflows
G. Scharmer, J. de la Cruz Rodriguez, P. Sütterlin & V.M.J. Henriques
(A&A 553, A63, 2013)
The data
Sunspot at 15 deg heliocentric distance
SST/CRISP Stokes spectra in the 630.15 and 630.25 nm lines at 0.16 arcsec resolution,
using the SST 37-electrode (low-order) AO system
Data reduction
• MOMFBD image reconstruction (low-order, with 36 modes)
• Straylight compensation (40% straylight, FWHM 1.8”)
• Small-scale dewarping
• Demodulation for telescope polarization
• Determination of CRISP spectral transmission profile
• Inversions with NICOLE, including CRISP transmission
profile pixel by pixel
• Transformation of B to the solar coordinate system
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NICOLE inversions
3 nodes for temperature (assumed quadratic with log τc)
3 nodes for BLOS
2 nodes for Btx and Bty (assumed linear with log τc)
2 (3) nodes for vLOS
1 (0) node for micro-turbulence (depth independent)
⇒ 13 free parameters fitted by NICOLE
Full Stokes at 31 wavelengths ⇒ 124 observables!
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The sunspot vertical field
(in the solar frame)
Bz at τ = 1
2 Bz nodes
Bz at τ = 0.1
3 Bz nodes
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Bz at τ = 1
3 Bz nodes
Bz at τ = 0.01
3 Bz nodes
LOS velocities
Intra-spines (inclination of B > 60 deg)
-3
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-2
-1
0
+1
+2
Spines (inclination of B < 60 deg)
+3 (km/s)
The penumbra has 2 distinctly
different components, Evershed flow
exists only in intra-spines
Opposite polarity field (solar frame)
Green: inclination
less than 60 deg
Red: opposite polarity
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Opposite polarity field (solar frame)
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Opposite polarity field (observers frame)
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Validation of results
Is the opposite polarity field real?
Strong network
field, 2×2” subfield
Blos
τ=1
T
τ=1
Blos
τ = 0.1 vlos
τ=1
NCP
3-lobed (abnormal)
Stokes V profiles!
τ = 0.05
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Limb side penumbra
Opposite polarity field aligns
perfectly with downflow lanes
vlos
Bz
Red pixels
T
Bxy
Abnormal V profiles!
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Other pixels
Azimuthal fits of LOS velocities for opposite polarity field
Assume: vLOS =
vr cos ⇥ sin + vz cos ,
Make azimuthal fits separately for opposite polarity pixels and ”other” intra-spine pixels:
Opposite pol. pixels ”Other” pixels
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Opposite polarity field is associated with
downflows throughout the penumbra
hysical Journal, 750:62 (21pp), 2012 May 1
Results of simulations
(Rempel, 2012)
Side view
Central upflow
−rho (v grad) v
Bx, Bz
By
Vx, Vz
Vy
z
y
x
tau=1
jxB
−grad p + rho g
B
V
z
Lateral downflow
y
Penumbral filament in MHD simulation
up
down
Top view
y
Cross Section
x
iagram summarizing the basic field and flow structure of a penumbral filament as present in the numerical simulation. We present a sc
Observations
fully
support
convective
flow
patterns
predicted
tion view. x and z denote ==>
the horizontal
and vertical
direction
perpendicular
to the
filament,
y denotes
the direction along the filament a
central upflow regions, the
and field&
areSpruit
well aligned,
flow submerges
mostly
horizontal field lines in lateral downflow
byflow
Scharmer
2006,while
and the
shown
in Rempel’s
simulations.
e a reduced field strength, but they contain a core with a non-vanishing vertical field component. Some of the associated flux continue
ns downward within the filament cavity. Depending on the position of the τ = 1 level the latter might become visible as inverse polari
hear of the Evershed flow induces a strong horizontal field component that is concentrated along the τ = 1 surface. This leads to str
fredag 9 augusti 13
Spines and opposite polarity field at τ=1
from Rempel’s (2012) simulations
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Observed opposite polarity field and spines at τ=1
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Conclusions from Nicole inversions
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•
Opposite polarity field exists everywhere in the penumbra, also in its
inner parts
•
Opposite polarity field is located in the intra-spines, often close to
the boundaries to the spines (where the field is much more vertical)
•
Opposite polarity field is associated with downflows (~ 1 km/s), as
expected
•
Locations of downflows close to the spines is in agreement with
predictions of the convective gap model (Spruit & Scharmer 2006)
and recent simulations (Rempel 2012)
•
Convection (temperature correlated vertical flows) exists
everywhere in the penumbra
•
The RMS of the vertical component of the penumbral convective
velocity field corresponds to about 1.2 km/s, which is sufficient to
heat the penumbra
Coming back to the fundamental questions...
• How is the energy transported to the visible
surface?
• Why do penumbrae have a small-scale
filamentary structure?
• Why are there strong azimuthal variations in
field strength and inclination angle across
filaments?
• Why do bright penumbral filaments have
dark cores?
• What is the origin of the Evershed flow?
Broad-band Ca H image (Henriques)
fredag 9 augusti 13
Coming back to the fundamental questions....
• How is the energy transported to the visible
surface?
• Why do penumbrae have a small-scale
filamentary structure?
• Why are there strong azimuthal variations in
field strength and inclination angle across
filaments?
• Why do bright penumbral filaments have
dark cores?
• What is the origin of the Evershed flow?
Answer: convection (in an inclined field)
Broad-band Ca H image (Henriques)
fredag 9 augusti 13
Coming back to the fundamental questions....
• How is the energy transported to the visible
surface?
• Why do penumbrae have a small-scale
filamentary structure?
• Why are there strong azimuthal variations in
field strength and inclination angle across
filaments?
• Why do bright penumbral filaments have
dark cores?
• What is the origin of the Evershed flow?
Broad-band Ca H image (Henriques)
fredag 9 augusti 13
Answer: convection (in an inclined field)
(Scharmer et al. 2008)
Extra slides not shown during my presentation
fredag 9 augusti 13
Comments on straylight compensation of SST data
Constraints
1. Obs. granulation contrast low => Straylight must be strong (>40% @ 630 nm)
2. Min. umbra intensity low => FWHM of straylight PSF must be small (< 2”.5)
Conclusions
* RMS granulation contrast and umbra intensity powerful constraints
* Straylight PSF narrow (~1-2 arcsec) => Must be from aberrations
* Precise shape and width of straylight PSF of secondary importance
Note: Convective downflows evident also without straylight compensation
(Scharmer & Henriques, A&A 540, A19, 2012, Fig. 14.)
fredag 9 augusti 13
Contributions to SST straylight
Contributors
1. AO and MOMFBD residual high-order aberrations
(Scharmer et al. 2010)
2. Small-scale
aberrations from optics and AO calibration errors
(Löfdahl & Scharmer 2012)
3. High-altitude seeing (h~10 km, isoplanatic patch ~2” =>
not compensated by AO (FOV ~ 8”-12”) or MOMFBD (FOV ~5”).
(but tip-tilt compensated by short exposure times)
Magnitude of contributions
1. 0.5 rad rms (r0=20 cm, 40 corrected modes) => Strehl 0.78.
Note: r0=20 cm (0”.5 seeing) optimistic!
2. 0.4-0.5 rad rms => Strehl 0.85-0.78
(with 37-electrode AO, probably better with new 85-electrode AO)
3. 0.64 rad rms (r0=50 cm, tip-tilt corr, zenith dist 60 deg) =>
Strehl 0.66. (Note: r0=50 cm is optimistic!)
Combined Strehl: 0.40-0.44
=> More than 50% straylight from aberrations
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The ”AO-halo”
(From ”Adaptive optics for
Astronomical Telescopes” by
John Hardy)
* Core FWHM given by λ/D
* Core peak given by Strehl (~0.5 for SST)
* Halo FWHM given by λ/d (d<<D)
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The ”MFBD-halo”
MTF squared
* Core FWHM given by λ/D
* Core peak given by Strehl
* Halo FWHM given by λ/d (d<<D)
* Accounting for 90% encircled energy
when r0=15 cm requires a straylight
PSF with at least 1”.8 diameter
(Figures from Scharmer et al. 2010, A&A 521, A68)
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Comment on Hinode MTF, PSF and straylight
Full: diffr. limited 50 cm
Short dash: + central obsc.
Long dash: + spider
dash-dot: + focus error
Compare Danilovic et al. 2008
Hinode PSF gives significant
straylight (>35%) from within
the first 2 diffraction rings.
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