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Irradiance variations of the Sun

The time series of total solar irradiance (TSI) and optical spectral irradiance at 402, 500 and 862 nm (SSIs) obtained by the VIRGO experiment on board the satellite SoHO were analysed by Lanza, Rodonò and Pagano [A28, D6 in press] in order to model their variability in the framework of a purely stellar-like approach. The different time scales of variability were estimated by means of the pooled variance method revealing the growth and decay of sunspots and faculae in active regions, as well as their rotationally modulated visibility. The determination of the rotation period of the Sun from the time modulation of the TSI and SSIs was made difficult by the short lifetimes of photospheric brightness inhomogeneities in comparison to the rotation period. Only during the phases with the lowest level of activity of solar cycle 23, when the variability is dominated by long-lived faculae, it was possible to determine the true solar synodic period. The simultaneous modelling of the rotational modulation of the TSI and SSIs was performed by means of a simple stellar-like approach which extended the model previously applied to the TSI modulation alone (Lanza et al. 2003, see also [D19]). The present model yielded residuals about 20-30 times smaller than the amplitudes of the TSI and SSI variations at all phases during the 11-yr activity cycle (see Figs. 1.2 and 1.3). The determination of the model parameters, including the temperature of the surface brightness inhomogeneities and the trade-off among them, were discussed and compared with the results obtained with different model approaches. The advantages and the drawbacks of applying the model to solar-like stars were also considered.

Figure 1.2: Two subsets of the TSI and SSI time series during the minimum of solar cycle 23 (open diamonds) with their respective best fits (solid lines) are plotted in panels (a), (c), (e) and (g) for the labelled passbands in the left and right columns, respectively. The model is usually embedded into the data sequence, except when data gaps are present. The residuals are plotted in the corresponding lower panels (b), (d), (f) and (h), respectively. The time is indicated in days from 1$^{\rm st}$ January 1996 on the lower scale and in years on the upper scale. The data subsets range from 13 July 1996 to 11 September 1996 on the left panels, including the intervals previously analysed by Eker et al. (2003) and Fligge et al. (2000, Fig. 6), and from 5 November 1996 to 4 January 1997 on the right panels, previously analysed by Fligge et al. (2000, Fig. 7), respectively.
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\psfig{file=sun/ms0028f7.ps,width=8cm}
Figure 1.3: The same as Fig. 1.2 for two data subsets close to the maximum of solar cycle 23, ranging from 29 January 2000 to 29 March 2000 in the left panels, and from 29 March 2000 to 28 May 2000 in the right panels, respectively. These time series were previously modelled by Krivova et al. (2003, Fig. 3 right panels).
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\psfig{file=sun/ms0028f8.ps,width=8cm}
\psfig{file=sun/ms0028f9.ps,width=8cm}

next up previous contents index
Next: Helioseismology, the Solar Standard Up: Solar Physics Previous: Spectroscopic diagnostic and modelling   Contents   Index
Innocenza Busa' 2005-11-14