Study of solar chromosphere: Variation of Calcium K line profiles with solar cycle

Abstract

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−1

in northern hemisphere from mid latitudes towards equator. In

the southern hemisphere activity shifted at faster rate 14 m s−1

in the beginning of the cycle and the speed decreased with time, yielding an average speed of 7.5 m s −1 , towards the equator. The shift of activity in the higher latitude belts showed a complex behaviour indicating pole-ward and equator-ward migration.

The K1 and K2 widths around 60◦

latitude belt show far less variation, almost negligible, during the solar cycle 22 and 23 as compared to other latitude belts and polar regions. These variations with latitude and time indicate the presence of two types of activity or flows. We infer the presence of counter flows in the polar regions and probably the existence of multi-cells for the meridional flow.

These findings, less variations in mid latitude belts as compared to polar regions, asymmetry in speed of shift in activity in both hemispheres and complex variation in the direction of shift in the activity representing poloidal fields in mid latitude belts will have important implications on the modelling of solar dynamo. We,

recommend that such type of analysis with better resolution and frequent obser- vations will be very useful for solar cycle studies.

Ca-K line profile modelling is one of the method to understand the Ca-K observa- tions quantitatively. We have used RH code by Uitenbroek to model the observed

line profiles. The observed line profiles are taken with high resolution spectro- graph mainly to study the core of the line profiles and it covers only about 2 Å

from the line centre. We tried different atmospheric models (these models were

used in solar irradiance modelling to compute the Ca-K line profile). We have de- termined the Ca-K line parameters from the model Ca-K line profile to compare

with the observed. The models FALXCO (cell interior) and FALF (bright network) with assumed contribution were used to model quiet Sun line profiles. For active Sun line profile plage model FALP is used additionally, with contribution of plages measured from Ca-K images taken from kodaikanal observatory. The quiet Sun line profile matches with the model profile [FALXCO + FALF]. But active Sun line

profile does not match with the model profile [FALXCO + FALF + FALP]. So the ex- cess component, that is the model profile subtracted from observed profile gives

the active network contribution. This measurement of active network contribution will help in solar irradiance modelling.