With thanks to Mark Rast Juan Fontenla Stéphane Béland

1. Solar-Stellar Variability WorkshopSORCE PhotometryJerald Harderjerry.harder@lasp.colorado.edu, 1-303-492-1891 With thanks to Mark Rast Juan Fontenla Stéphane Béland

2. 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

3. Instrument overview • Instrument Type: Féry Prism Spectrometer • Wavelength Range: 200-2400 nm • Wavelength Resolution: 0.24-34 nm • Detector: ESR, n-p silicon, InGaAs • Absolute Accuracy: 2-8% • Relative Accuracy : ~0.5-0.02% (240-2400 nm) • 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 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 vis-IR irradiance over several solar rotational timescales’, A&A, 528, A67 (2011), doi: 10.1051/0004- 6361/201015632, 2011.

4. 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.

5. 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.

6. SIM degradation correction and long-term accuracy • Long-term degradation corrections in SIM are based solely on measured instrument quantities. • 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.

7. 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

8. 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 Topka et al., ApJ 1997 Moran et al., Sol. Phys., 1992 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

9. Ca IIK 393.45 nm, 0.273 FWHM Identify Active Regions PSPT feature identification & time series • 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 Red Continuum 607.095 nm, 0.458 FWHM Identify Umbra & Penumbra SRPM Mask Image Identify 7 solar Features

10. SIM & PSPT Facula + Plage • SIM 280 nm irradiance is proportional to the measured facular area in PSPT

11. SIM & PSPTsunspot umbra & penumbra #1 • Further corrections needed to account for wavelength stability later in mission. • 2 arc-sec error in prism rotation angle ≈ 0.145nm ≈ 8% of a prism step

12. SIM & PSPT sunspot umbra and penumbra #2 • Decreased irradiance is observed even when sunspot blocking is not indicated by PSPT 607 nm images.

13. 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.

14. Solar spectral irradiance variability in NRL SSI Sunspot Case: 04/30/2005 Facula-Plage: 08/29/2005 Solar Min Ref: 11/09/2007 J. Lean, GRL., vol. 27, pp 2425, 2000.

15. Strömgren Filters wrt Brightness Temperature

16. SIM integrated over Strömgren bands

17. 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

18. 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.” • 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

19. 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

20. 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

21. TSIS SIM derives heritage from SORCE SIM • TSIS SIM designed for long-term spectral irradiance measurements (climate research) • 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 • NIST SI-traceable Unit and Instrument level pre-launch spectral calibrations (SIMRF-SIRCUS) SORCE SIM TSIS SIM

22. Conclusions • The SIM observations indicate solar cycle trends both in- and out of phase with the TSI. • 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: • 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

23. EXTRAS

24. 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

25. 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

26. 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