Abstract:
We obtained the Ca–K line profile of the Sun as a star since
1969 at the Kodaikanal Observatory (KO) and analysis of the data showed
the need to delineate the role of different chromospheric features to the
variations of solar irradiance. We, therefore, initiated a new methodology to make observations of Ca–K line profiles of the Sun as a function
of latitude and integrated over the longitude on a daily basis since 1986.
We have collected the data for about thousand days, spread over two solar
cycles. Earlier data (before 1997) were recorded on the photographic film
and later data using the CCD detector. The photographic film data were
digitized and analysed along with the data obtained from CCD camera.
From these data, we computed K1 and K2 widths for the Sun as a star,
using all the observed line profiles as a function of latitude. In addition,
we have analyzed the spectra of the whole Sun as a star obtained on some
days and compared it with the results obtained from latitude spectra of the
same day. The K1 and K2 widths of the Sun as a star derived from the KO
data are compared with values determined from the observations made
at other observatories to compare results from the new methodology of
observations adopted by us and the earlier techniques. The average values of K1 width during the minimum period. of solar cycle 23 are smaller
than those during the minimum period of cycle 22. Day-to-day variations
in the K1 and K2 widths and plage areas may imply that irradiance variations occur not only due to large-scale solar activity, but also because of
variations in some of the three types of network in quiet regions of the
Sun. The variation in intensity of the plages can also cause day-to-day
variations in widths.
