Irradiance Variations and pMode Frequency Changes Abstract on
Irradiance Variations and p-Mode Frequency Changes Abstract on p. 80 Claus Fröhlich PMOD/WRC, CH-7260 Davos Dorf, Switzerland § § VIRGO Radiometry Construction of a composite TSI 1978 -2002 Proxy model for TSI variability 1978 -2002 P-mode frequency changes 1991 -2002 and correlation with TSI § Conclusions In collaboration with: Thierry Appourchaux, William Chaplin, Yvonne Elsworth, Judith Lean, Richard Wachter 10/31/2021 SOHO 12/GONG 2002, Big Bear, Oct 28 -Nov 1, 2002 1
VIRGO Radiometry (1) VIRGO is an experiment on SOHO – an ESA/NASA mission launched in December 1995. For TSI measurements has 2 types of electrically calibrated cavity radiometers (ECR): PMO 6 V and DIARAD with different cavity geometry and paint. This allows for a detailed study of the temporal behavior, as e. g. degradation due to exposure to the sun. Level-1 data of the two operational and back -up radiometers are corrected for all a priori known influences, including the reduction to 1 AU. These time-series are a combination of the solar irradiance variation and long-term changes of the radiometers, such as due to degradation (exposure or non-exposure dependent). By comparison of the operational and less exposed back-up radio-meter exposure dependent changes can be determined. 10/31/2021 SOHO 12/GONG 2002, Big Bear, Oct 28 -Nov 1, 2002 2
VIRGO Radiometry (2) A model of the exposure dependent changes has been developed based on the effect of ‘sun-burning’ quartz. The sensitivity change is expressed as a hyperbolic function which includes the dose determined from the Mg. II index. . The behavior of PMO 6 V can be described with an early increase, a short-term and long-term decrease. It has to be noted that also PMO 6 V-B needs a correction for the early increase as its exposure time reached 10 days at MD 190. For DIARAD the degradation is somewhat masked by other effects. Hence, the correction is determined from the ratio to DIARAD-L/R. 10/31/2021 SOHO 12/GONG 2002, Big Bear, Oct 28 -Nov 1, 2002 3
VIRGO Radiometry (3) Level 1. 8 is all you can do with only one type of radiometer. From the ratio PMO 6 V/DIARAD it is suggested that exponential functions might be used to account for changes after initial and subsequent switch-on of the radiometer. A necessary condition for such a procedure is that the time constants involved are different enough in order to allocate the correction uniquely to each radiometer. Fortunately, this is the case and with the corrections applied we reduce the standard deviation of the difference by a factor of 2 -3. An interesting feature is the repetition of the difference pattern after 1012 days ( =0. 925). The share of the residuals is determined by comparing with 55 -day filtered ACRIM-2/3 data. 10/31/2021 Available from ftp. pmodwrc. ch/data/irradiance/virgo/TSI/ SOHO 12/GONG 2002, Big Bear, Oct 28 -Nov 1, 2002 4
Construction of the composite TSI The composite TSI is based radiometrically on ACRIM 1 and ACRIM 2. However, ACRIM 2 has to be related to ACRIM 1 which is done by comparison with HF and ERBE. First we have to correct HF for degradation (early increase and degradation). The most controversial and also most important correction is due to the two glitches in October 1989 and May 1990, originally detected by Lee et al. and Chapman et al. The second one was difficult to locate, but remained in earlier attempts to determine the total amount of the corrections. The similarity of the slope with the one determined for the earlier period by comparison with ACRIM 1 is striking. Recently it became clear that a combination of a slip after a switch-off in late September 1989 and a trend would better represent the situation. ACRIM 1 needs some correction of the degradation as applied originally. Corrections to ACRIM 2/3 are also needed. 10/31/2021 SOHO 12/GONG 2002, Big Bear, Oct 28 -Nov 1, 2002 5
Uncertainty of the composite The composite is now constructed around ACRIM 1 and the adjusted ACRIM 2. For the rest, the corrected HF data are shifted to fit in, as well as the VIRGO data. We may estimate the uncertainty of a possible trend to be about 3 ppm/a for periods longer than 10– 15 years; this implies a possible change of 70 ppm over the 23 years of the observations. If we add (rms) the uncertainties related to the tracing of ACRIM-II to I and of the HF correction ( 60 ppm) we get a total of 92 ppm. The observed change of the composite TSI as difference between two successive minima amounts to 52 ppm. (-5 ppm/a) which is not significantly different from zero. Willson (1997) neglects the HF correction and concludes that the 1996 minimum is 400 ppm higher. Available from ftp. pmodwrc. ch/data/irradiance/composite/ 10/31/2021 SOHO 12/GONG 2002, Big Bear, Oct 28 -Nov 1, 2002 6
Proxy model for TSI variability § § 10/31/2021 A model for TSI variability due to solar activity needs to account for the darkening due to sunspots and brightening due to faculae and network The influence of sunspots can be determined from observations of their position (distance from the disk center) and area and assuming a (size dependent) contrast CS. The result is called the photometric sunspot index PSI: PS = ASPOT[CS-1](3 +2)/2 A similar index can be determined for faculae, however, data for position and area are difficult to obtain. Thus the Mg. II index (core to wing ratio of the Mg. II H and K lines at 284 nm) is used as a proxy. In order to account for a possible difference between faculae and network the Mg. II data are separated into a short-term (PFs) and long-term part (PFl). The sunspot data are from NGDC, Boulder, and the Mg. II index is a composite from corresponding experiments on NIMBUS 7, NOAA 9 and 10, UARS and GOME. SOHO 12/GONG 2002, Big Bear, Oct 28 -Nov 1, 2002 7
Comparison of the proxy model with TSI The separation of the Mg. II index into a short- and long-term part is performed by determining a lower envelope of the facular variation. As the Mg. II line originates in the chromosphere this index does not represent the CLV of faculae which show limb brightening and thus their influence on irradiance is several days before and after the central meridian passage of the active region. This is taken into account by filtering the TSI+PS with a double peaked function (camel) and the short-term Mg. II with a single peaked one (Dromedary). The result of this procedure yields a correlation coefficient of 0. 918 which means that more than 83 % of the variance is ‘explained’ by the model. The drift of the residuals of 161 ppm/23 a (– 80 ppm between the two minima) compares well with 52 92 ppm of the composite. 10/31/2021 SOHO 12/GONG 2002, Big Bear, Oct 28 -Nov 1, 2002 8
Comparison by bi-variate analysis Another way to compare time series is in frequency space by bi-variate spectral analysis. This consists in calculating a linear filter (gain and phase) which transforms one time series into the other. The coherence squared times 100 gives the amount explained in one time series by the other. It is interesting to note that the model does a pretty good job at periods adjacent to the rotational period, but not at it. 10/31/2021 SOHO 12/GONG 2002, Big Bear, Oct 28 -Nov 1, 2002 9
Low-order p-mode frequency changes and correlation with TSI Data from LOI and the 3 SPM channels (402, 500, 862 nm) of VIRGO and from the BISON network are used to determine the p-mode frequencies changes of modes of low degree (0… 2). The period covered starts in 1991 for BISON and in 1996 for LOI and SPM and ends in January 2002. Each data set is analyzed in contiguous segments of length of 108 days. Shifts are averaged over n=18 to 24 for l=0 and 1 and n=17 to 23 for l=2. The indicated errors come from weighted scatter of data used to make each average (weights fixed by the formal uncertainties from the frequency fitting procedure). Correlation with TSI, TSI+PS –PFs and PFl, is performed by linear regression. TSI+PS –PFs may be regarded as a proxy for luminosity. 10/31/2021 SOHO 12/GONG 2002, Big Bear, Oct 28 -Nov 1, 2002 10
Degree dependence of correlation The slope of the correlation varies with the degree of mode. Moreover, the correlation increases with degree, indicating the importance of higher degrees in the expansion of the perturbation responsible for the correlation. The dashed lines are the result from the regression without weighting with errors. Thierry Appourchaux inverted the degree dependence to get information about latitudinal distribution of the perturbation. The result is still preliminary, but it looks interesting. 10/31/2021 SOHO 12/GONG 2002, Big Bear, Oct 28 -Nov 1, 2002 11
Conclusions § § § 10/31/2021 The analysis of the VIRGO radiometry provides not only a reliable time series, but also important knowledge about long-term behavior of solar radiometers in space The composite TSI is a reliable time-series with a long-term uncertainty of about 100 ppm. The observed trend of – 50 ppm is not significant. Thus the background TSI has no trend during the last 23 years. A proxy model is based on PSI and Mg. II index, separated in a shortand long-term part. The model explains 83 % of TSI. Bi-variate spectral analysis shows interesting features of the coherence as a function of frequency (e. g. at the 27 -rotational period), not revealed by the simple regression analysis. Frequency changes of low-degree p modes are highly correlated with a proxy for luminosity (TSI with the superficial effects – sunspots and faculae – removed) with =0. 85… 0. 95 for the SPM, and =0. 75. . 0. 85 for LOI and BISON. A preliminary inversion of the degree-dependent slopes for the latitudinal distribution of the perturbation is very interesting: the poles seem to play a more important role than the activity band around the equator. SOHO 12/GONG 2002, Big Bear, Oct 28 -Nov 1, 2002 12
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