Solar-Stellar Variability Workshop
SORCE Photometry
Jerald Harder
[email protected], 1-303-492-1891
With thanks to
Mark Rast
Juan Fontenla
Stéphane Béland
Solar-Stellar Variability Workshop, HAO, March 19, 2014
- J. Harder
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Topic Outline
 Introduction to the SORCE SIM
• Instrument capabilities and limitations
 Comparisons with PSTP & SRPM
 SIM observations via Strömgren filter equivalents
 Moving forward – the Radiometric Solar Imager
& TSIS
Solar-Stellar Variability Workshop, HAO, March 19, 2014
- J. Harder
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Instrument overview
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Instrument Type: Féry Prism Spectrometer
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Wavelength Range: 200-2400 nm
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Wavelength Resolution: 0.24-34 nm
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Detector: ESR, n-p silicon, InGaAs
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Absolute Accuracy: 2-8%
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Relative Accuracy : ~0.5-0.02% (240-2400 nm)
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Long-term Accuracy: 0.3-0.025%/yr (240-2400 nm) •
Field of View: 1.5x2.5˚ total
Pointing Accuracy/Knowledge: 0.016˚/0.008˚
Mass: 21.9kg
Dimensions: 88 x 40 x 19 cm
Orbit Average Power : 17.5 W
Orbit Average Data Rate: 1.5 kbits/s
Redundancy: 2 Redundant Channels
l =2400
ò E ( l ) d l » 1318.6 Wm
-2
l =240
» 96.9% of TSI
ò 41.9 Wm -2 missing from TSI
Harder J. W., G. Thuillier, E.C. Richard, S.W. Brown,
K.R. Lykke, M. Snow, W.E. McClintock, J.M. Fontenla,
T.N. Woods, P. Pilewskie, 'The SORCE SIM Solar
Spectrum: Comparison with Recent Observations' ,
Solar Physics, 263, Issue 1 (2010), pp 3,
doi:10.1007/s11207-010-9555-y
Pagaran, J., J. W. Harder, M. Weber, L. E. Floyd, and J. P.
Burrows, ‘Intercomparison of SCIAMACHY and SIM visIR irradiance over several solar rotational timescales’, A&A,
528, A67 (2011), doi: 10.1051/00046361/201015632, 2011.
Solar-Stellar Variability Workshop, HAO, March 19, 2014
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A word about resolution…
 SIM measures the irradiance weighted by the bandpass.
 Low resolution instruments respond to the density of lines,
not to individual lines.
Solar-Stellar Variability Workshop, HAO, March 19, 2014
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SORCE spacecraft & thermal events
 Thermal events change instrument performance most typically through
wavelength shift & light path through prism. Degradation corrections
must account for these changes.
 Time period of Version 17 remains the most stable and reliable time
period of SIM operations.
Solar-Stellar Variability Workshop, HAO, March 19, 2014
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SIM degradation correction and long-term accuracy
 Long-term degradation corrections in SIM are based solely
on measured instrument quantities.
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Correction is based on the comparison of two identical (mirror image)
spectrometers that have been exposed at different rates.
Corrections for photodiode detectors in the same channel are made by
comparison with the spectrally flat ESR detector after correcting for the
different optical paths through the prism.
Solar-Stellar Variability Workshop, HAO, March 19, 2014
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Spectral variability nomogram
• SIM observations consistent with an overall decrease in the temperature gradient in the
active (magnetic) solar photosphere.
• The change in T-gradient occurs in solar atmospheric layers close to the Teff value.
Harder et al., GRL,, 2009
Solar-Stellar Variability Workshop, HAO, March 19, 2014
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Independent observations with anti-solar cycle trends
Features contrast varies with
wavelength and heliocentric
angle and corresponds to the
slope of T vs. P.
(5 ’s between 525 to 677 nm)
Sanchez Cuberes et al., ApJ,2002
Moran et al., Sol. Phys., 1992
Topka et al., ApJ 1997
The photometric sums exhibit similar
temporal patterns: they are negatively
correlated with solar activity, with
strong short-term variability and weak
solar-cycle variability.
Preminger et al.., ApJ,2011
Solar-Stellar Variability Workshop, HAO, March 19, 2014
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Ca IIK
393.45 nm, 0.273 FWHM
Identify Active Regions
PSPT feature identification & time series
Red Continuum
607.095 nm, 0.458 FWHM
Identify Umbra & Penumbra
SRPM Mask Image
Identify 7 solar Features
 Feature area determined from intensity analysis of PSPT images
 Analysis done as a function of disk position and time
 Full disk irradiance determined from disk position and emitted
intensity from each atmospheric model
Solar-Stellar Variability Workshop, HAO, March 19, 2014
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SIM & PSPT Facula + Plage
 SIM 280 nm irradiance is proportional to the measured facular area in PSPT
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SIM & PSPT sunspot umbra & penumbra #1
 Further corrections needed to account for wavelength stability later in mission.
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2 arc-sec error in prism rotation angle ≈ 0.145nm ≈ 8% of a prism step
Solar-Stellar Variability Workshop, HAO, March 19, 2014
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SIM & PSPT sunspot umbra and penumbra #2
 Decreased irradiance is observed even when sunspot blocking is not
indicated by PSPT 607 nm images.
Solar-Stellar Variability Workshop, HAO, March 19, 2014
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Solar spectral irradiance variability in SRPM
Fontenla, J. M., et al. , JGR, 2011
SRPM analysis is able to capture offsetting trends observed by SIM, but the
magnitude of the effect are different.
Solar-Stellar Variability Workshop, HAO, March 19, 2014
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Solar spectral irradiance variability in NRL SSI
Sunspot
Case:
04/30/2005
FaculaPlage:
08/29/2005
Solar Min
Ref:
11/09/2007
J. Lean,
GRL., vol. 27,
pp 2425, 2000.
Solar-Stellar Variability Workshop, HAO, March 19, 2014
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Strömgren Filters wrt Brightness Temperature
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SIM integrated over Strömgren bands
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PSPT observations of facula
• Some facula and plage have negative contrast at
red continuum wavelengths
• Position of dark faculae on the disk is not a
simple function of heliocentric angle
• The fraction of dark Facula decreases into SC23
minimum and increases into rising phase of SC24
Solar-Stellar Variability Workshop, HAO, March 19, 2014
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A compelling need for the Radiometric Solar Imager (RSI)
Is the time dependence because the faculae (or unresolved underlying flux
distributions) are changing, or because the CLV against which their
contrast is measured is changing?
Ground based instrumentation can only measure photometric contrast
compared to some definition of the background “quiet-sun.”
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we do not know the center-to-limb variation of the “quiet-sun,” against
which these contrasts are measured
we do not know whether the structures, or the background against which
they are measured, or both, are changing with solar cycle
these differences are important in our interpretation of the solar spectral
irradiance
Solar-Stellar Variability Workshop, HAO, March 19, 2014
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RSI Concept
• Full disk photometric images with relative pixel-to-pixel precision of 1:103
• Separate radiometer which shares imager filter wheel and precision aperture
determines throughput of filter
• A filter transmission measurement prevents ambiguities in filter bandpass
Advantage:
Spectrometer does not require absolute calibration
Can have high resolution, but limited bandpass
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RSI Design
• 100 mm diameter entrance pupil and a 12.5mm aperture stop
where the filters are positioned
• Placing the filters at the aperture stop significantly reduces the
spatial uniformity requirements for the filters compared to placing
them just before the focal plane array, and make them much
smaller than placing them at the entrance
• Light path (ray angles) through filters is however slightly different
in telescope than into radiometer
Solar-Stellar Variability Workshop, HAO, March 19, 2014
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TSIS SIM derives heritage from SORCE SIM
TSIS SIM designed for long-term spectral irradiance
measurements (climate research)
SORCE SIM
Incorporate lessons learned from SORCE SIM (& other LASP
programs) into TSIS SIM to meet measurement requirements for
long-term JPSS SSI record
Specific required capabilities over SORCE SIM
 Reduce uncertainties in prism degradation correction to
meet long-term stability requirement
• Ultra-clean optical environment to mitigate contamination
• Addition of 3rd channel to reduce calibration uncertainties
 Improve noise characteristics of ESR and photodiode
detectors to meet measurement precision requirement
• Improved ESR thermal & electrical design
• Larger photodiode dynamic range integrating ADC’s (21 bits)
 Improve absolute accuracy pre-launch calibration
TSIS SIM
• NIST SI-traceable Unit and Instrument level pre-launch
spectral calibrations (SIMRF-SIRCUS)
Solar-Stellar Variability Workshop, HAO, March 19, 2014
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Conclusions
The SIM observations indicate solar cycle trends both in- and out of phase
with the TSI.
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Interpretable in terms of the solar brightness temperature and temperature gradients
with the solar atmosphere
SIM integrated over Strömgren v, b, & y filters reflect anti-solar cycle behavior with
the u-filter in phase.
The v, b, & y filters may not be adequate proxies of the TSI for sun-like stars
SIM ultraviolet observations show a high degree of consistency with:
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Facula and plage areas measured by PSPT
Calculated spectral irradiance estimated form SRPM
SIM irradiance at 607 nm tends to under-estimate the reported PSPT sunspot areas
and decreased irradiance is observed even when sunspot blocking is not indicated by
PSPT 607 nm images.
Plans
Continued analysis of SIM data to determine behavior in SC24 is essential
2017 deployment of TSIS SIM is mandatory to further this research – but
gaps are inevitable and difficult to resolve for SSI measurements
The development of radiometric imagery is the next logical step for
understanding solar variability and understanding the stellar connection
Solar-Stellar Variability Workshop, HAO, March 19, 2014
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EXTRAS
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Estimated trend uncertainties in the Visible
 Best observation for degradation corrections for SIM is 04/2004 to 05/2007, but
magnitude of uncertainties similar in the 2007-2011 period
 Uncertainty in the visible comparable to 2σ noise levels but reaches a minimum level at
≈2×10-4. Errors in the 2003-2004 and after 2011 time period require further refinements
• Improved wavelength registration will reduce uncertainties in the visible
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Improvements to be implemented in Version 20
(release planned for late spring 2014)
 Implement dynamical wavelength shifter based on instrument dispersion
equations to account for thermal/mechanical stresses induced by spacecraft
power cycling – Must be applied to every spectrum
 Particularly important for visible and infrared wavelengths from Sept 2011 to present
time
 Reanalysis of photodiode and prism glass refractive index temperature
coefficients due to decreased temperature stability
 Correct for non-exposure related photodiode degradation
 Not well represented in Version 19
 Perform AB comparisons and determine ray path through the prism
 Particularly important for first year of the mission and after full-time power cycling of
the instrument
 Version 20 analysis for UV and IR spectral regions has not started
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The RSI will:
• Elucidate the underlying causes of solar spectral variability by making first
radiometric measurements of the resolved solar disk
• First radiometric determination of center-to limb profiles of the quiet-sun and
solar magnetic elements as a function of solar cycle
• First determine of the photospheric temperature gradient both within and
outside of magnetic flux structures using opacity conjugate wavelengths
• Determine the veracity and cause of spectral irradiance trends for terrestrial
climate modeling
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