Reconstruction of solar irradiance variations in cycles 21, 22 and 23 based on surface magnetic fields Thomas Wenzler Institute of Astronomy ETH Zurich

Outline I.MDI/SoHO and Kitt Peak Vacuum Tower (KPVT) observations of the National Solar Observatory (NSO) II.Comparison between KPVT/SPM and SoHO/MDI magnetograms with an application to solar irradiance reconstructions (Wenzler et al. 2004a) III.Can surface magnetic fields reproduce solar irradiance variations in cycles 22 and 23 (Wenzler et al. 2004b)? IV.Reconstruction of solar irradiance variations in cycles 21, 22 and 23 based on surface magnetic fields (Wenzler et al. 2005)

I.MDI/SoHO and Kitt Peak Vacuum Tower (KPVT) observations of the National Solar Observatory (NSO)  space-based observations MDI:1996 – today:high quality  ground-based observations KP:1992  today:Spectromagnetograph (SPM) good quality 1974  1992:512-channel magnetograph (512) noticeable lower quality  reconstruction over longer time period possible

II.Comparison between KPVT/SPM and SoHO/MDI magnetograms with an application to solar irradiance reconstructions (Wenzler et al. 2004a) The ground-based KP observations are unfortunately not as homogeneous as those recorded by MDI The KP instrument has been improved a few times, with the newest, the Spectromagnetograph (SPM; Jones et al. 1992), being in work since1992. Therefore, we started from a comparison of the performance of the SPM and the MDI and showed that SPM data can be employed to reconstruct TSI variations with almost the same accuracy as MDI.

The SPM data (magnetograms and continuum images) occasionally exhibit artifacts introduced by clouds, seeing and/or instrumental problems. If not identified and removed, magnetograms with artifacts would introduced a bias into the magnetic fluxes deduced from SPM data and lead to an inaccurate relation between SPM and MDI fluxes. They would introduce scatter into irradiance reconstructions and reduce the accuracy of other applications of these magnetograms. Therefore we have carefully examined the magnetograms and the appropriate intensity images for these possible artifacts. We identified regions with artifacts or obvious loss of data quality (often also missing parts of the image, when a scan was not complete) on 839 days, constituting 29% of the original SPM data record ( ). We stress that magnetograms were not removed on the basis of the irradiance reconstructions and its agreement or disagreement with the observations!

Example of a region affected by clouds in an SPM magnetogram recorded on April 13, 1999 (left). The same region in the corresponding MDI magnetogram (right).

Example of an SPM magnetogram with artifacts caused by instrumental problems (left) and the corresponding MDI magnetogram (right) for April 22, 1999.

Comparisons of the facular, umbral and penumbral fields as well as sunspot areas In the following, we compare histograms of the umbral, N u (  ; t), penumbral, N p (  ; t) and facular distributions, N f ( , B; t), obtained from MDI and SPM. The distributions are studied as a function of  or as a function of the magnetic flux.  = cos(  ), where  is the heliocentric angle B =  B LOS  / , where B LOS is the line-of-sight (LOS) component of the magnetic field. The factor 1/  in B takes into account that a magnetogram underestimates the true flux in pixels close to the limb since facular and network fields on the Sun are mainly vertical (Martínez Pillet et al. 1997; Bernasconi 1997).

Histogram of the umbral a) and penumbral b) mask vs.  for November 10, Solid lines represent SPM data, dashed lines MDI data.  = 1 is the disk centre;  = 0 is the limb.

Histograms of the facular distributions for November 10, a) The number of facular points vs. B LOS /  ; b) the same quantity vs. . Solide lines represent SPM data, dashed lines MDI data.

TSI reconstructions: Results for cycle 23 TSI measured by VIRGO (solid line) and reconstructed based on SPM data (filled circles) between 1996 and 2001, i.e. from minimum of cycles 23 to its maximum (top panel). The middle and lower panels show enlargements of four shorter intervals at different activity levels. The times corresponding to these zoom-ins are marked in the top panel by roman numerals. A reconstruction based on MDI data is represented (middle and lower panels only) by the plus signs.

Modeled TSI based on SPM data vs. VIRGO measurements. The correlation coefficient, r c, is indicated. The solid diagonal line represents the expectation values for a perfect model fit, a regression (dashed line) is hardly distinguishable. Modeled TSI between 1996 and 2001 based on SPM data vs. MDI-based reconstructions. The correlation coefficient is The diagonal line represents the expectation values for perfect models, the dashed line is a regression.

III.Can surface magnetic fields reproduce solar irradiance variations in cycles 22 and 23? The data we employ is the set of daily full-disk magnetograms and the corresponding continuum intensity images recorded by the SPM of the NSO at the KPVT from 1992 November 21 to 2003 September 21. Earlier magnetograms were obtained either in a different spectral line or with a different instrument. Due to the difficulty of obtaining reliable cross-calibrations, the accuracy of reconstructions based on multiple instruments is perforce lower, so that for the next step we consider only the time span mentioned above.

PMOD composite record of TSI (solid line) and reconstructed daily TSI based on SPM data (filled circles) for 2055 individual days between 1992 and 2003, i.e. from near the maximum of cycle 22 to the declining phase of cycle 23 (top panel). The middle and lower panels show enlargements of four shorter intervals at different activity levels. The times corresponding to these zoom-ins are marked in the top panel by double headed arrows under the roman numerals.

Modeled total solar irradiance based on SPM data vs. PMOD composite of measurements between 1992 and The correlation coefficient, r c, is indicated. The solid diagonal line represents the expectation values for a perfect model fit, a regression (dashed line) is hardly distinguishable from it.

IV.Reconstructions of solar variations in cycles 21, 22 and 23 based on surface magnetic fields Finally, the SATIRE (Spectral And Total Irradiance REconstructions; Solanki et al. 2004) model (Unruh et al. 1999; Fligge et al. 2000a,b; Krivova et al. 2003) can be extend to cover the whole period over which NSO/KP data are available. Before 1992 the older KP 512-channel solar magnetograph (Livingston et al. 1976) was in operation, with a noticeably lower quality of the data. In particular, the continuum images are not good enough to distinguish between the umbrae and penumbrae. Therefore only entire sunspots could be identified and the umbra/penumbra ratio had to be kept. This ratio was taken from the results for the SPM period.

PMOD composite record of TSI (solid line) and reconstructed daily TSI based on 512 data (filled circles) for 1734 individual days between 1974 and 1992, i.e. from the minimum of cycle 21 to the maximum of cycle 22 (top panel). The middle and lower panels show enlargements of four shorter intervals at different activity levels. The times corresponding to these zoom-ins are marked in the top panel by double headed arrows under the roman numerals.

PMOD composite record of TSI (solid line) and reconstructed daily TSI based on 512 and SPM data (filled circles) for 3789 individual days between 1974 and 2003, i.e. from the minimum of cycle 21 to the declining phase of cycle 23 (top panel). The lower panels show enlargements of three shorter intervals at the minimum of the cycles 21, 22 and 23. The times corresponding to these zoom-ins are marked in the top panel by double headed arrows under the roman numerals.

Modeled total solar irradiance based on 512 and SPM data vs. PMOD composite of measurements between 1978 and The correlation coefficient, r c, is indicated. The solid diagonal line represents the expectation values for a perfect model fit, the dashed line is a regression.

Reconstructed TSI vs. three different composite records of TSI We have compared the reconstructed TSI between 1978 and 2003 with the following three different composite records of TSI, which are compiled from detailed cross-calibrations of various daily radiometric measurements. PMOD composite (version 30) of TSI from PMOD/WRC (Fröhlich and Lean 1998; Fröhlich 2000; Fröhlich 2003) ACRIM composite of TSI (Willson and Mordinov 2003) ROB composite of TSI (Dewitte et al. 2004)

3-month running means of the PMOD (solid line), ACRIM (dotted line) and ROB (long dashed line) composite records of total solar irradiance compiled from detailed cross-calibrations of various daily radiometric measurements (top panel). The three lower panels show the deviations between the different composite records. The grey lines show daily averaged values and the black lines are 1-month running means.

PMOD composite record of TSI (grey line) and reconstructed daily TSI based on KP data (filled circles) between 1978 and 2003 (top panel). The middle panel shows the difference between the reconstructed TSI and the PMOD composite measurements. The solid line indicates difference = 0., a regression (dashed line) is hardly distinguishable. The lower panel shows the modeled TSI vs. the PMOD composite of measurements. The correlation coefficient, r c, is indicated. The solid diagonal line represents the expectation values for a perfect model fit, the dashed line is a regression.

ACRIM composite record of TSI (grey line) and reconstructed daily TSI based on KP data (filled circles) between 1978 and 2003 (top panel). The middle panel shows the difference between the reconstructed TSI and the ACRIM composite measurements. The solid line indicates difference = 0., a regression (dashed line) is hardly distinguishable. The lower panel shows the modeled TSI vs. the ACRIM composite of measurements. The correlation coefficient, r c, is indicated. The solid diagonal line represents the expectation values for a perfect model fit, the dashed line is a regression.

ROB composite record of TSI (grey line) and reconstructed daily TSI based on KP data (filled circles) between 1978 and 2003 (top panel). The middle panel shows the difference between the reconstructed TSI and the ROB composite measurements. The solid line indicates difference = 0., a regression (dashed line) is hardly distinguishable. The lower panel shows the modeled TSI vs. the ROB composite of measurements. The correlation coefficient, r c, is indicated. The solid diagonal line represents the expectation values for a perfect model fit, a regression (dashed line) is hardly distinguishable from it.

List of publications Wenzler, T., Solanki, S. K., Krivova, N. A., Fluri, D. M., 2004a, Comparison between KPVT/SPM and SoHO/MDI magnetograms with an application to solar irradiance reconstructions, A&A, in press Wenzler, T., Solanki, S. K., Krivova, N. A., 2004b, Can surface magnetic fields reproduce solar irradiance variations in cycles 22 and 23?, A&A, submitted Solanki, S. K., Krivova, N. A., Wenzler, T., 2004, Irradiance model, Adv. Space Res., in press Wenzler, T., Solanki, S. K., Krivova, N. A., 2005, Reconstruction of solar irradiance variations in cycles 21, 22 and 23 based on surface magnetic fields, A&A, in preparation