A Critical Review of Measurements and Models of Composite Total Solar Irradiance II Posters
Presiding: R C Willson, Columbia University; J M Pap, University of Maryland Baltimore County
SH23B-01 1330h
A Critical Review of Measurements and Models of Composite Total Solar Irradiance
After nearly three decades of total solar irradiance monitoring there is still uncertainty about long-term trends, even whether there is a significant solar cycle dependence. The curret proxy data used in irradiance modeling are summarized and their limitations considered. The recent data obtained during solar cycle 23 provides an enlarged database and additional short-term irradiance variations associated with active region developments. New data about large-scale solar features reveal solar cycle variations and dramatic changes during episodes of extreme activity. These observations suggest potential mechanisms for irradiance variation independent of, or in addition to, sunspots and faculae.
SH23B-02 1330h
Comparison of ACRIM and PMOD Total Solar Irradiance composites time series during solar cycles 21 - 23
The set of contiguous, redundant and overlapping total solar irradiance (TSI) observations made by satellite experiments since late 1978 enables construction of continuous composite TSI time series. A multi-decadal TSI composite requires establishment of the relationship between ACRIM1 and ACRIM2 results across the two year gap between them using one of two overlapping data sets: the Nimbus7/ERB or ERBS/ERBE. Two TSI composites have been published, the ACRIM using the Nimbus7/ERB comparisons [Willson & Mordvinov, 2003] and the PMOD using the ERBS/ERBE comparisons [Frohlich & Lean, 1998]. The ACRIM composite uses unaltered published results from a subset of the satellite TSI data. The most significant feature of the ACRIM composite for climate change is an upward trend of 0.04 (+/- 0.01) % per decade between activity minima during solar cycles 21-23. The PMOD composite uses a different subset of the same satellite TSI data, modifying some results using TSI proxy models as a guide. There are a number of differences between the PMOD and ACRIM composites but the most important is the absence of a significant trend between minima in the PMOD. The trend difference is the direct result of the choice of Nimbus7/ERB or ERBS/ERBE to relate ACRIM1 and ACRIM2 results across the ACRIM gap. The lack of a minima-to-minima trend in composites based on the ERBS/ERBE can be shown to be an artifact of uncorrected degradation during the `gap'. The significance of the ACRIM composite trend will be explored in the context of the traceability of the constituent TSI data sets. Other differences between the ACRIM and PMOD composites result from PMOD's use of TSI proxy models to justify altering published results of ACRIM1 and Nimbus7/ERB. TSI regression (proxy) models based on chromospheric spectral features are not competitive in accuracy, precision or traceability with satellite TSI observations and are therefore may be expected to cause spurious effects when used in the construction of TSI composites. The TSI record has been sustained by overlapping, redundant experiments using their level of measurement precision to sustain longer term traceability. This TSI monitoring strategy is essential for continuity in the future because the uncertainty of current satellite sensors (~ 0.1 %) is an order of magnitude too large to detect subtle long term TSI variations of potential climate change significance. [Willson, R.C., A. V. Mordvinov, JGRL 30, pp. 1199-1202, 2003, Frohlich C., J. Lean, JGRL 25, pp. 4377-4380, 1998]
http://www.acrim.com
SH23B-03 1330h
Sunspot fragmentation and total solar irradiance modelling
Observational evidence suggests that sunspot decay is due, at least in part, to a fragmentation process. We have developed a model for this, where the sunspot fragmentation produces an ensemble of small scale magnetic flux tubes, which themselves can remain at the surface for many days. The number of sunspots emerging at the solar surface varies over the 11 year solar activity cycle. Consequently, the size distribution of magnetic structures is also modulated by the cycle. The simplicity of our model allows us to track the evolution of this distribution over very long time scales (many solar cycles). There are several applications for such a model. For example, to the total solar irradiance, which also varies over the solar cycle. Broadly speaking, the irradiance contribution from different magnetic features depends on their size (large features, such as sunspots, are dark, whereas small flux tubes tend to be bright). By combining this property with our sunspot fragmentation model, we show that the resultant system can produce an irradiance contribution that behaves very much like the observed total solar irradiance. Our model has several input parameters (one crucial example is the actual relationship between the size of a magnetic feature and its brightness). We have used a genetic algorithm to adjust the parameters in order to optimize the agreement between our model and the observations. Will we discuss those results and also comment on the contribution our model makes to the weak field component of solar magnetic flux budget.
SH23B-04 1330h
A Comparison of Total Solar Irradiance to the Mg II Index Based on SORCE Measurements
The reconstruction of the total solar irradiance (TSI) over the long-term often uses sunspot area and a faculae proxy such as the Mg II core-to-wing index (Mg index). With interest in validating this approach, we compare the Mg index to TSI measurements using data from NASA's SOlar Radiation and Climate Experiment (SORCE). The SORCE carries four solar irradiance instruments that have been monitoring the Sun since early 2003. The Total Irradiance Monitor (TIM) measures the TSI with unprecedented stability and low noise during the daytime portion of each spacecraft orbit with a time cadence of 100 seconds. The SOLar STellar Irradiance Comparison Experiment (SOLSTICE) measures solar spectral irradiance from 115 to 320 nm, having 0.1 nm spectral resolution at the Mg II lines near 280 nm. The Mg II lines are scanned approximately seven times each day, and the Mg index provides a good indicator of chromospheric activity. We compare the SOLSTICE Mg II core-to-wing index to TIM TSI measurements, giving advantages over previous comparisons including simultaneity, low noise, and high spectral resolution for the Mg II lines.
http://lasp.colorado.edu/sorce
SH23B-05 1330h
A Critical Review of Models of Composite Total Solar Irradiance
We compare ACRIM and PMOD total solar irradiance satellite composites during solar cycles 21-23. By assuming only random uncertainties of the TSI satellite data sets and their overlapping comparisons we determine the statistical uncertainty of two simple satellite composites, which are statistically equivalent to ACRIM and PMOD composites. We find that the secular upward trend of +0.047%/decade between the minima of solar cycles 21-22 and 22-23 presented by the ACRIM satellite composite is statistically equivalent to the -0.009%/decade trend between the same minima presented by the PMOD composite.
SH23B-06 1330h
Estimation of TSI Variability During the Declining Phase of Cycle 23
Our previous study gave an empirical model with only two solar activity indices that reproduced TSI measurements from 1996 to 2003 in Cycle 23 to within 100 ppm rms (de Toma et al., 2004). The indices used in the model are: a photometric index derived from full-disk images in the red continuum at 672.3nm taken at San Fernando Solar Observatory (the Sigma-red index) and a chromospheric index based on irradiance measurements in the MgII doublet at 280nm (the MgII core/wing index). Here, we recompute the empirical model using the latest VIRGO TSI measurements during the rising and maximum phase of Cycle 23 and compute TSI estimates from the maximum of Cycle 23 toward the next minimum. Comparison between extrapolations of the model and TSI observations gives insight on the predictive capability of the model.
SH23B-07 1330h
Comparisons between Ground-Based Photometry and Space-Based Measurements of the Total Solar Irradiance
We will review the usefulness of ground-based full-disk photometry in conjunction with space-based measurements of the Total Solar Irradiance (TSI). It is known that sunspots and faculae cause changes in the TSI. These features need to be modeled using ground-based photometry and their effects removed in order to search for possible other causes of TSI variation. Work to date has shown that approximately 94% of the variance in TSI can be explained by sunspots and faculae/network. Since ground-based photometry is carried out daily, it can help identify anomalies in space-based TSI measurements. Finally, ground-based photometry can help in tying together TSI measurements from different spacecraft that have different native irradiance scales. This work has been partially supported by grants from NASA and NSF.