Advances in Space Research 34 (2004) 265–268

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Variation in the network flux as derived from the CalciumK-line profiles as function of latitude and solar cycle phase

Jagdev Singh *, Iraj Gholami, S. Muneer

Indian Institute of Astrophysics, Bangalore 560 034, India

Received 19 October 2002; received in revised form 15 April 2003; accepted 15 April 2003

Abstract

We have obtained the Calcium K-line profiles, on daily basis, at all latitudes, while integrating the spectrum over the visible

longitudes of the Sun by moving a �340 mm solar image at a uniform speed in the E–W direction of the Sun. The spectra were

recorded at a dispersion of 9.34 mm/�A on 35 mm Kodak 103-a0 film for the time period of 1986–95. Then we started to obtain the

spectra using 1k� 1k liquid cooled CCD Camera. We plan to study possible long-term variations in the chromospheric rotation

rate, differential rotation rate, and changes in Polar Regions with the phase of the solar cycle using various parameters of this line.

Here we discuss the variation in K-line flux, K1-width and K2-width in terms of the contribution from plages and variation in the

network flux with the phase of the solar cycle.

� 2004 Published by Elsevier Ltd on behalf of COSPAR.

Keywords: Calcium K-line; Solar physics; Solar spectrum

1. Introduction

One of the methods to study the long-term variability

of the Sun is to monitor CaþK line profiles of the Sun as

a star (White and Livingston, 1978). Skumanich et al.

(1984) proposed a three-component model of the solar

cycle variability of the Calcium K emission using extent

contrast and fractional area parameters for (1) cell, (2)

network and (3) plage components. They also intro-duced an additional network component, ‘Active net-

work’, in excess of the quiet Sun value. Comparison of

CaþK line data with space based irradiance measure-

ments has shown the importance of sunspots, plage and

network faculae as sources for solar cycle variability

(Pap et al., 1991). Recently, plage and enhanced net-

work indices have been derived from CaþK spectrohe-

liograms and filtergrams by Worden et al. (1998) andCaccin et al. (1998) to study the variation in these in-

dices with the solar cycle phase, and to correlate these

* Corresponding author. Tel.: +91-80-668-1386; fax: +91-80-553-

4043.

E-mail address: jsingh@iiap.ernet.in (J. Singh).

0273-1177/$30 � 2004 Published by Elsevier Ltd on behalf of COSPAR.

doi:10.1016/j.asr.2003.04.063

parameters with the VUV and UV irradiances. Theobservations of the Sun as a star, and various indices

derived from the CaþK filtergrams and spectrohelio-

grams, lack information about the variation in quiet

network flux. To determine if this variation exists, we

started a program at Kodaikanal observatory, on a daily

basis, in 1986, to monitor the CaþK line profiles as a

function of latitude and integrated over the visible lon-

gitudes. Also, the rotation modulation characteristic ofthe CaþK line profile will permit us to study the varia-

tion in the rotation rate with the phase of solar cycle, if

any, and chromospheric differential rotation. The basic

purpose of this paper is to report the method of obser-

vations and type of data, which we have been taking

since 1986 on a daily basis and to describe the plan of

our study.

In addition, we present the methodology adopted tocompute the plage and network contributions to the

parameters of Calcium K-line profiles as a function of

latitude. Here we report the analysis of the data ob-

tained during 1986–87. The analysis of the remaining

data will yield results with better accuracy as these have

been obtained with CCD camera.

Fig. 1. Plot of Ca-K index (0.5 �A) versus plage area for 1986.

266 J. Singh et al. / Advances in Space Research 34 (2004) 265–268

2. Observations and data analysis

A 38 cm objective of 36.6 m focal length forms a 34

cm image of the Sun at the slit of the spectrograph. A

Sun chart, corresponding to the image size and helio-graphic latitude of disc centre ‘B’ on that day, is made

on a thick paper sheet with latitude lines at an interval of

10� drawn on it. This is kept near the focal plane of the

Sun’s image in such a way that N–S axis marked on Sun

chart becomes parallel to the axis of rotation of image.

To obtain the spectrum at a selected latitude and inte-

grate over the 180� longitudes at that latitude, the image

is moved in the E–W direction of the Sun along thegiven latitude line at a uniform speed with the help of

second mirror of the coelostat. The spectra were re-

corded on 35 mm Kodak 103-a0 film till 1995. A set of

18 spectra have been obtained on a daily basis. This

means at 17 different latitudes of the Sun, with an in-

terval of 10� each and the 18th is a spectrum of Sun as a

star. These spectra are recorded using a 18.3 m focus

spectrograph with 600 lines per mm grating blazed at 2.5lm in the first order. The spectrograph provided a dis-

persion of 9.34 mm/�A in sixth order at the CaþKwavelength. All the profiles have been normalized at an

intensity value of 13% at 3935.16 �A in the red wing of

CaþK line (White and Suemoto, 1968).

We now record the spectra using a 1k� 1k peltier

cooled CCD camera from Photometries Co., Tucson,

USA. To cover a sufficient portion of the spectrumaround the CaþK line, we are making the observations

in fifth order, which provides a spectral resolution of

about 16 m�A. Using a binned pixel size of 48 lm pro-

vides a spectral scale of 7.07 m�A/pixel. The slit width of

100 lm, kept during the observations, gives a spectral

resolution of 14.7 m�A.

Fig. 2. K1-width of Ca-K line versus plage area for 1986.

3. Results and future studies

We have computed K-index, K1, K2 and Wilson–

Bappu widths and K1V, K2V, K3, K2R and K1R intensities

for a number of CaþK line profiles taken in 1986–87.

There is a considerable scatter in the intensity values on a

day-to-day basis, but the variation in the values of widths

is small. The scatter in intensity values may be partly realand partly because of the photometric accuracy due to

the use of film. The average values for 2–3 days of data

reduce this scatter in intensity values.

To determine the average contribution of the network

to the K-line flux, K1-width and K2-width, on a yearly

basis, we have plotted these indices against the calcium

K-plage areas. These areas are measured from the K-line

spectroheliograms obtained at Kodaikanal observatoryfor all the latitudes separately. Fig. 1 shows the plot of

K-line flux (0.5 �A) for the (20–25�) N and (25–30�) Nlatitude belts versus K-plage area along with the linear

fits for the year 1986. We assume that the value of in-

tercept at zero plage area gives the contribution due to

network flux. We plan to compute these values for each

year and then study the variation in network flux at

different latitudes over an interval of 5�, and with the

phase of solar cycle.Figs. 2 and 3 show the plots of K1-width and

K2-width against K-plage area for the year 1986, re-

spectively. K1-width increases with plage area whereas

K2-width decreases with plage area. We computed these

parameters for zero plage area from the linear fits to the

data for each latitude belt.

Fig. 4 shows variation of K1- and K2-width as a

function of latitude. It is not clear why the value of K1

and K2-widths is less for (80�-limb) N latitude belt as

compared to those for (70–80�) N latitude belt. Our

main purpose is not to study these parameters as a

function of latitude but to find out the variation of these

parameters with time at all latitude belts.

Fig. 5 shows K-line flux (0.5 and 1.0 �A) derived from

the linear fits described earlier versus latitude. We as-

sume that this flux is due to the network component.The figures indicate that flux varies with latitude. It is

Fig. 4. Plot of K1 and K2 width of Ca-K line as a function of latitude.

Fig. 5. Ca-K line index due to network as a function of latitude.

Fig. 3. K2-width of Ca-K line versus plage area for 1986.

J. Singh et al. / Advances in Space Research 34 (2004) 265–268 267

minimum near the equator, larger at middle latitude

belts and again decreases at higher latitude belts. It is

surprising that a minimum occurs at about 10� N.

Analysis of more data especially data obtained with

CCD camera may give clues to this kind of behavior.

4. We plan to use these data to study the following

1. The sunspots and related features give information

only between 10� and 40� latitude belts whereas this

method will yield information about the Polar

Regions as well.

2. We would get plage free profiles at all latitude beltsduring the different phases of the solar cycle, which

would make it possible to study the changes in the

active network as a function of the solar cycle.

3. The power spectral analysis of the K-index (Singh

and Livingston, 1987) for different latitude belts

would provide information on the chromospheric ro-

tation as a function of latitude and hence would help

to establish the chromospheric differential rotationrate.

4. Singh and Prabhu (1985) have shown, from the anal-

ysis of plage areas, that chromospheric rotation rate

varies with time, but the analysis is restricted to only

few latitude belts due to the occurrence of plages in

those latitudes. The analysis of the present data

may yield the values of rotation rate with time in

all latitudes, and thus would help explain the slowand fast rotating bands on the solar surface.

268 J. Singh et al. / Advances in Space Research 34 (2004) 265–268

5. The data will also be used to correlate K-index of the

Sun as a star with the various UV, VUV and irradi-

ance in the visible wavelength region measured from

space experiments.

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