8 Sky brightness vs. solar activity

As we have mentioned in Sect. 4.3, during the time covered by the data presented here, the solar activity had probably reached its maximum. To be more precise, since the current solar cycle (No. 23) has a double peak structure (see Fig. 6), our measurements cover the descent from the first maximum and the abrupt increase to the second maximum. Mainly due to the latter transition, the solar density flux at 10.7 cm in our data set ranges from 1.2 MJy to 2.4 MJy, the median value being 1.8 MJy. Even though this is almost half of the full range expected on a typical complete 11 years solar cycle (0.8-2.5 MJy), a clear variation is seen in the same solar density flux range from similar analysis performed by other authors (see for example Mattila et al. 1996, their Fig. 6). In Fig. 13 we show the case of the R passband, where we have plotted the nightly average sky brightness vs. the solar density flux measured during the day immediately preceeding the observations. A linear least squares fit to the data (solid line) gives a slope of 0.14 $\pm$ 0.01 mag arcsec^-2 MJy^-1, which turns into a variation of 0.24 $\pm$ 0.11 mag arcsec^-2 during a full solar cycle. This value is a factor two smaller than what has been reported for B, V (Walker 1988b; Krisciunas 1990) and uvgyr (Leinert et al. 1995; Mattila et al. 1996) for yearly averages and it is consistent with a null variation at the 2 sigma level. Moreover, since the correlation factor computed for the data in Fig. 13 is only 0.19, we think there is no clear indication for a real dependency.

This impression is confirmed by the fact that a similar analysis for the B and V passbands gives an extrapolated variation of 0.08 $\pm$ 0.13 and 0.07 $\pm$ 0.11 mag arcsec^-2 respectively. These numbers, which are consistent with zero, and the low correlation coefficients (0.08 and 0.11 respectively) seem to indicate no short-term dependency from the 10.7 cm solar flux. Similar values are found for the I passband ($\Delta m=$ 0.22 $\pm$ 0.15 mag arcsec^-2). These results agree with the findings by Leinert et al. (1995) and Mattila et al. (1996) and the early work of Rosenberg & Zimmermann (1967), who have shown that the [OI]5577 Å line intensity correlates with the 2800 MHz solar flux much more strongly using the monthly averages than the nightly averages. For all these reasons, we agree with Mattila et al. (1996) in saying that no firm prediction on the night sky brightness can be made on the basis of the solar flux measured during the day preceeding the observations, as it was initially suggested by Walker (1988b). A possible physical explanation for this effect is that there is some inertia in the energy release from the layers ionised by the solar UV radiation, such that what counts is the integral over some typical time scale rather than the instantaneous energy input.