Comparison of Solar Minima Using Solar Ultraviolet Irradiance Data

Transcription

1 Comparison of Solar Minima Using Solar Ultraviolet Irradiance Data Matthew DeLand and Richard Cebula (with thanks to Barry Schlesinger, Ernie Hilsenrath, Don Heath, Gary Rottman, Tom Woods, Linton Floyd, Gerard Thuillier, Guenther Brueckner, Dianne Prinz) Science Systems and Applications, Inc. (SSAI) SORCE Science Meeting, Keystone, CO 19 May 2010

2 Atmospheric Impact of Solar UV

3 Solar UV Measurement History

4 CHALLENGE: Instrument response changes are typically large and rapid relative to solar variations. Different techniques to characterize these changes have been used. 25% SOLSTICE: Stellar Observations SBUV/2: Reference Flights SUSIM: Onboard Calibration 25% 25%

5 Solar Minima Comparison NOAA-9 SBUV/2 data cover two minima ( , ). Ratio of average spectra shows ~1% difference for most wavelengths. Single Instrument Observed changes below 260 nm less than prediction based on ACRIM TSI trend. DeLand et al. [2004]

6 Public Solar UV Data Sets DeLand and Cebula [2008] (updated with SORCE data)

7 Creation of Composite Data Set Use 1 nm binned products (public release) for consistency. Normalize data to reference spectrum [Thuillier et al., 2004] to remove large-scale biases. Intercomparison of instruments during overlap periods to identify discrepancies. Compare each instrument s data to solar activity proxy index to help identify possible sensor issues. Generate synthetic values to fill data gaps. Create single product for full spectral/temporal range. M. T. DeLand and R. P. Cebula (2008), J. Geophys. Res., 113, A11103, doi: /2008ja

8 Normalize Irradiance Data Use Thuillier et al. spectrum as reference. Use coincident data for each instrument where available. ATLAS-1 mission (29 Mar 1992) overlaps NOAA-9, NOAA-11, SUSIM, SOLSTICE Use smoothing fit to minimize wavelength scale difference effects.

9 Intercomparison (1 of 387)

10 Comparison to Solar Proxy

11 Selection of Data (Thanks to Frank Eparvier for the design concept)

12 Content of Product Data Set Daily 1 nm spectra covering nm from November 1978 to June Source flag (e.g. instrument or proxy; also note if spectral and/or temporal interpolation performed). Absolute uncertainty (spectral values taken from individual instrument analysis). Relative uncertainty (time-dependent values taken from individual instrument analysis).

13 Composite Product: nm

14 Data vs. Proxy nm synthetic SME SUSIM SOLSTICE synthetic

15 Validation Comparisons

16 Solar Minima Comparison Use same dates as NOAA-9 comparison (average 27 days at minimum). Three instruments used in 1986, two different instruments used in Same Y-axis range, but endpoints shifted and nm data added. Composite Product

17 Extension of Composite Product Using SORCE Data SORCE data coverage begins May 2003 (SOLSTICE), April 2004 (SIM). All data (so far) at lower activity levels than ATLAS-1 reference spectrum normalization based on ATLAS-3 reference spectrum. Normalization dates with matching solar activity level not available until late 2005 (2 years after launch) accuracy of degradation correction comes into play. Variable SIM resolution introduces other issues for NUV data that are not treated at this time.

18 Extended Data Set Structure

19 Normalization (no overlap) Select date with comparable solar activity to reference spectrum (ΔMgII = ±0.3%, Δ<MgII> 81 = ±0.1%). Accuracy of normalization depends on spectral consistency of solar variation, long-term instrument characterization.

20 Normalization Ratio vs. ATLAS-3 SOLSTICE SIM

21 Solar Minima Comparison Choose most recent minimum in April Results look consistent with previous plots. SORCE uncertainty values estimated from on-line notes and discussion. However, this is only a snapshot. Extended Composite

22 Extended Composite Data #1

23 Extended Composite Data #2

24 Extended Composite Data #3

25 Conclusion and Status Comparison of solar UV irradiance between minima requires excellent long-term calibration accuracy. NOAA-9 SBUV/2 data show no change between Cycles 21/22 and 22/23 within long-term uncertainty. Composite UV irradiance data set covers (available on-line at LISIRD web site, lasp.colorado.edu/lisird/cssi/cssi.html) can be extended to latest minimum using SORCE data. Comparison of minima from different cycles is complicated by absolute calibration differences, normalization of data to create composite product, residual instrument response changes. No clear evidence for multi-decade trends in UV spectral irradiance data.