SORCE Science meeting: Models of Irradiance Variability
18 – 19 September 2012
Precision Solar Photometric Telescope (PSPT):
Image Analysis of Solar Cycles 23 and 24
Mark Rast
Laboratory for Atmospheric and Space Physics
Department of Astrophysical and Planetary Sciences
University of Colorado, Boulder
Peter Fox
Nathan Goldbaum
Jeff Kuhn
Haosheng Lin
Randy Meisner
Ada Ortiz
Dick White
Students, observers, the HAO IG,
the HAO CSMT, LASP IT, LASP DS
• Brief description of the PSPT telescope, its capabilities and limitations
• Highlight some observed properties with emphasis on small the role of weak field
• Future instrumentation goals
Solar Radiative Output Variations workshop,
November 1987, Boulder, Colorado
Ground-based Photometric Observing Program
working group, October 1989, Tucson Arizona
NSF SunRISE Program:
February 1990 report:
• “design, construction and deployment of
two precision solar photometric telescopes
(PSPT’s)”
In the context of ozone and climate changes,
understand the causes of solar variability
• “optimized for accurate photometric
observations of sunspots, faculae, and other
photospheric brightness inhomogeneities”
• “data analysis and theoretical studies that
will be required for effective interpretation
of these data, and … their implication for
atmospheric changes”
TOTAL RECOMMENDED BUDGET:
$11.1M over 5 years (FY92 – FY96)
Goal: high precision (0.1%) full disk
photometry to determine magnetic structure
contributions to irradiance variations.
Osservatorio Astronomico di Roma (430m):
since February 1996
Mauna Loa Solar Observatory (3397m):
since April 1998
• 15cm refractor
• Simple optical design
(minimize scattered light)
• Active mirror
(image stabilization
to ~0.25 arc sec)
• 2048x248 detector
(~1 arc sec pixels)
• 3 + 2 filters
http://lasp.colorado.edu/pspt_access/
Algorithm to measure detector gain variation (flat-field):
(Kuhn, Lin, Loranz 1991)
• Assume the Sun is unchanged between (slightly defocused) offset images
• Pixel intensities at identical image locations should be identical
• Pixel gain values at identical detector locations should be identical
• Procedure is sensitive to any differences not stationary with respect to the solar image
[e.g. pixel to pixel gain variations and amplifier quadrants, but also differences due to
offset image light path (filter density variations) and image evolution (seeing)]
• Every PSPT image is examined, as a
histogram equalized image, for flat-field
defects.
• Images with quadrant defects are rejected.
• Residual defects below human visual acuity
are the largest know source of systematic error
in the data.
Center to limb variation (Limb-darkening):
I 0 (r,q ) = å
n
Typically use
å[ A
P (r)cos(nq ) + Bnm Pm (r)sin(nq )]
nm m
m
m = 0 ® 6 Legendre polynomials and n = 0 ® 2 Fourier components
I - I0
Ic =
I0
NOTE:
The phase of
the Fourier components
is free at each r.
Correlation between CaIIK intensity
and magnetic flux density
Extraction of areal coverage of differing magnetic components
Synthesis of
full-disk
spectra as a
function of
time
Modeling of
thermodynamic
structure of magnetize
atmospheres
Radiative intensity is a combination of “magnetic” and “thermal” contributions:
dI
Mask 2
I0
dI
I0
Mask 6
< 0.025
CaIIK
< -0.05
I - I0
I0
Bl
Mask 2
Latitudinal variation in the solar
photospheric intensity:
Plot mean intensity between
as a function of θ:
0.3 < m < 0.45
Mask 0
56 best image triplets from March 2005 to July 2006
aligned with heliographic north vertical (P0=B0=0) and averaged.
~0.1 – 0.2% (~2.5K) polar enhancement
Mask 2
What causes this enhancement?
equatorial
midlatitude polar
Radiative intensity is a
combination of “magnetic” and
“thermal” contributions even at
the smallest scales and weakest
field strengths
0.3 < m < 0.45
Supergranular intensity contrast:
20 March 2001 1731UT
Thermal signal
becomes apparent
after masking out 95%
of the pixels, all pixels
with magnetic flux
densities ≥ 0.7 G
Þ
~1K
supergranular
temperature
contrast
2 July 2005
27 August 2009
• SIM shows anti-correlation with cycle at PSPT
red continuum wavelengths (brighter with
decreasing activity)
• Some faculae and plage have negative contrast
at red continuum wavelengths
• The fraction of dark faculae changes with cycle
Is this a change in the facular intensity or the clv
against which the contrast is measured?
Spectral irradiance modeling to date is based on contrast measured against a stationary
internetwork CLV, but:
• we do not know the absolute CLV of the internetwork to 0.1%
• we do not know the absolute CLV of any magnetic structure to 0.1%
• we do not know the variance in any of these
• Most of the solar disk (70 – 80%) is internetwork
• Very small changes in the internetwork intensity can have dramatic effects on SSI.
• A change in the number density of very weak field elements can have important
contributions
• The radiative properties of flux
elements is not uniform for all
structures in a class
Perhaps the sunspot/faculae
decompositions is to coarse to
explain spectral variations.
High resolution observations and RMHD models
Statistical models of unresolved contributions
Full-disk radiometric and
synoptic photometric imaging
Ensemble of solar atmosphere
and transfer models
Spectral irradiance
monitoring
RSI-PIPB: Photometric Imager and Pass Band Monitor
Photometric images at select narrow pass-bands.
RSI-HSIM: High Resolution Spectral Irradiance Monitor
High spectral resolution irradiance measurements.