Study of solar chromosphere: Variation of Calcium K line profiles with solar cycle G. Sindhuja [Thesis]

Doctor of Philosophy Mangalore University, Mangalore 2016

The various features on the solar surface like sunspots, plages, network, active
network play a main role in the solar variability. This thesis uses observations
which were started in an effort to delineate the role of various features such as
plages and networks in the variations of Sun with solar cycle. During 1970s, a
new programme to study long term variability of solar chromosphere was started

by several researchers. They used Ca-K line profiles of Sun as a star as diagnos-
tic tool to study the long term variability of solar chromosphere. Skumanich et al.

(1984) proposed a three component model using cell, network and plage areas
and using extant laws of limb darkening. During minimum phase of the Sun they
were able to reconstruct the observed Ca-K line profile with the contribution from
cell and network. But during active phase, the addition of plage component was

found to be insufficient to fit the model with observed profiles. Therefore, exis-
tence of excess component called as ‘Active network’ component was introduced

during the active phase. Whereas White and Livingston (1978) found no variation
in the centre of solar disc with respect to solar cycle phase. Hence, this excess
component may be from higher latitudes. In view of the differences in the results
of model for the variation of solar activity and observations, a new technique of
observing the Sun was planned to take Ca-K line profiles as a function of latitude

and integrated over visible longitude since 1986 at kodaikanal Solar Tunnel Tele-
scope on day to day basis for the long period. Which means at each latitude we

get the integrated flux over the 180 degree longitudes from all the features like
cell, network, plage and active network. The spectra obtained on 807 days were
found to be useful and rest of the observations for about 400 days were discarded
due to existence of passing clouds which effected the profiles due to scattering
of light. On some days spectra of Sun as a star have also been obtained. The
variations in sky transparency are compensated by normalizing the profiles. The
K1 and K2 widths of the Sun as a star, derived for the kodaikanal data agree well

with those of Kitt Peak and NSO/Sac Peak on day to day basis with small scat-
ter due to time difference of ∼ 12 hours in observation due to location of these

observatories. Further, we found that the average values of K1 width of Sun as a star during
the minimum period of solar cycle 23 are smaller than those during 22 and the
K2 width appear to show an opposite trend. The lower values of K1 width during
the period of 2010 -11 indicating the lower chromospheric emission in Ca-K line
during the extended minimum period of solar cycle 23.

The plot of day to day variations in the K1 and K2 widths versus plage areas
determined from the Ca-K spectrohelio-grams shows 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 that has not
been considered at present due to observational limitations.

Further, We have derived Ca-K line parameters such as K1 and K2 widths and
K-index averaged over regions of 10◦

latitude and 180◦

longitudes from the spec-
troscopic observations and plage areas in these regions from the images of the

Sun in Ca-K line. The comparison of Ca-K line widths with plage areas in respec-
tive latitude belts shows that small-scale activity due to network areas is very

important in the study of irradiance variation with solar cycle phase.

The Ca-K line spectra as a function of solar latitude, indicates that K1 width attains
maximum amplitude at various latitude belts at different phases of the solar cycle.

The FWHM of K1 distribution at different latitudes shows that its width varies by
about 30% for the equatorial belts (<30◦)and 11% for the polar regions (>70◦)
latitude. Interestingly, K1 width varies by ∼6% only in the 40 - 60◦latitude belts
during the solar cycle. The analysis of cross-correlation coefficients of K1 width
between 35◦

latitude and other latitude belts as a function of phase differences
indicates that the activity representing toroidal field shifted at a uniform rate of
about 5.1 m s−1in northern hemisphere from mid latitudes towards equator.