Extending the Sunspot Area Series from Kodaikanal Solar Observatory Bibhuti Kumar Jha123 Manjunath Hegde12 Aditya Priyadarshi12

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Extending the Sunspot Area Series from

Kodaikanal Solar Observatory

Bibhuti Kumar Jha 1,2,3,∗, Manjunath Hegde 1,2, Aditya Priyadarshi 1,2,

Sudip Mandal 4, B Ravindra 1and Dipankar Banerjee 1,2,5,∗

1Aryabhatta Research Institute of Observational Sciences, Nainital, Uttarakhand,

India

2Indian Institute of Astrophysics, Bangalore, Karnataka, India

3Pondicherry University, Kalapet, Puducherry, India

4Max Planck Institute for Solar System Research, G ¨

ottingen, Germany

5Center of Excellence in Space Sciences India, IISER Kolkata, Mohanpur, West

Bengal, India

Correspondence*:

Bibhuti Kumar Jha

bibhuti@aries.res.in; maitraibibhu@gmail.com

Dipankar Banerjee

dipu@aries.res.in

ABSTRACT

Kodaikanal Solar Observatory (KoSO) possesses one of world’s longest and homogeneous

records of sunspot observations that span more than a century (1904 – 2017). Interestingly, these

observations (originally recorded in photographic plates/ﬁlms) were taken with the same setup

over this entire time period which makes this data unique and best suitable for long-term solar

variability studies. A large part of this data, between 1921 – 2011, were digitized earlier and a

catalog containing the detected sunspot parameters (e.g., area and location) was published

in Mandal et al. (2017). In this article, we extend the earlier catalog by including new sets of

data between 1904 – 1921 and 2011 – 2017. To this end, we digitize and calibrate these new

datasets which include resolving the issue of random image orientation. We ﬁx this by comparing

the KoSO images with co-temporal data from Royal Greenwich Observatory. Following that, a

semi-automated sunspot detection and automated umbra detection algorithm are implemented

onto these calibrated images to detect sunspots and umbra. Additionally, during this catalog

updation, we also ﬁlled data gaps in the existing KoSO sunspot catalog (1921 – 2011) by virtue

of re-calibrating the ‘rouge’ plates. This updated sunspot area series covering nearly 115 years

(1904 – 2017) are being made available to the community and will be a unique source to study

the long term variability of the Sun.

Keywords: Sun, Sunspots, Solar Cycle, Kodaikanal Solar Observatory, white-light, sunspot area, umbra area

1 INTRODUCTION

Sunspots have always been a central part of our understanding of the Sun and its long-term variability

(Solanki, 2003). The systematic and methodical observation of these spots has revealed that their appearance

is periodic, with a periodicity of around 11 years, known as the solar cycle or solar activity cycle (Schwabe,

1844; Hathaway, 2015). Furthermore, the magnetic nature of these spots (Hale, 1908; Parker, 1955b) makes

them an ideal proxy for understanding solar magnetism and its complex variability (Tlatov and Pevtsov,

arXiv:2210.06922v1 [astro-ph.SR] 13 Oct 2022

B K Jha et al. Extended Sunspot Area Series from KoSO

2014; Nagovitsyn et al., 2017). Today, it is well established that the solar activity cycle is governed by the

solar dynamo process operating in the convection zone of the Sun (Parker, 1955a, 1975; Charbonneau,

2010). In the dynamo process, the solar activity cycle is the manifestation of the periodic nature of the

large-scale solar magnetic ﬁeld (poloidal



toroidal). As a consequence, the number of sunspots and the

corresponding area covered by these spots on the solar surface is dictated by the strength of the toroidal

ﬁeld generated in the dynamo process. Hence, the historical observation of these spots carries a vital

information about the nature of toroidal ﬁeld in the past and it will be crucial for the reconstruction of

historical global solar magnetic ﬁeld (Jiang et al., 2011, 2014). Apart from that, sunspots and solar activity

are also intrinsically linked to solar transient events such as solar ﬂares and coronal mass ejections (CMEs)

and their frequency of occurrence. Since these transients are the primary drivers for the space weather

condition, the historical observation will have crucial role for the understanding of space weather condition

in past.

Over the last 150 years, many observatories around the world have begun regular observations of the Sun.

The Royal Observatory of Greenwich (RGO; Willis et al., 2013) had been the leader in such a campaign

having the record of white-light observations from 1874 to 1976, which was later continued using the

Solar Optical Observing Network (SOON) by the US Air Force (USAF). After a few decades, in 1904,

Kodaikanal Solar Observatory (KoSO; Hasan et al., 2010) also joined this campaign and started regular

observation of the Sun on photographic plates/ﬁlms in multi-wavelengths (white-light since 1904, Ca-K

since 1904 and H-

since 1912), independently in India. In particular, the white-light observation taken at

KoSO provides one of the most homogeneous data series for over 100 years. Since these observations are

taken from the same location and using the same telescope (since 1918) for such an extended period, KoSO

provides a unique data series ideal for the long-term study of the Sun. Although the white-light data had

been digitized (Ravindra et al., 2013) since 1904; owing to calibration issues, it has not been utilized for

the period 1904 – 1920. Therefore, the area series (Mandal et al., 2017), the study of penumbra to umbra

area ratio (1923 – 2011; Jha et al., 2018; Jha et al., 2019) and solar differential rotation (1923– 2011; Jha

et al., 2021) were limited to the period 1921 – 2011.

In-spite of the fact that RGO is the only observatory in the world with white-light data for 1904– 1920

(Cycle-14 and Cycle-15), these images are available either in low resolution or in the form of drawings

. In

this regard, the high-resolution white-light data available from KoSO serves as the only data set for this

time frame. It is almost impossible for the ground based observatories to have a uniform and homogeneous

data because of the varying atmospheric conditions and bad weather. Hence, in the last 30 years, there has

been considerable effort to make the homogeneous sunspot area series by cross-correlating the sunspot

area from various observatories, e.g., Fligge and Solanki (1997), Baranyi et al. (2001, 2013), Balmaceda

et al. (2009) and recently Mandal et al. (2020). In all these studies, RGO data has been used primarily as a

reference to cross-correlate other data because of its outstanding data coverage. Now, the availability of

KoSO data will complement the existing data series and will be helpful in cross-correlating the RGO and

KoSO data in these initial overlapping periods. In this article, we present the extension of the KoSO sunspot

area series reported in Mandal et al. (2017) for 1904 – 2017 along with the umbral area after resolving the

calibration issues for the initial 17 years of data. We have also included data for the period of 2012 – 2017,

which was previously not reported due to ongoing digitization. In this article we present the updated data

statistics, the issues with the calibration and the resolution of these issues in Section 2. In Section 3, we

will discuss the updated area series and its comparison with the earlier ones, and ﬁnally, in Section 4 we

will summarize our ﬁndings.

1http://fenyi.solarobs.csfk.mta.hu/GPR/index.html

Frontiers 2

B K Jha et al. Extended Sunspot Area Series from KoSO

2 DATA

The white-light observation at Kodaikanal started in 1904 using a 10 cm objective lens telescope which

was later replaced in 1912 by a better quality lens while maintaining same size. A few years later, a

15 cm achromatic lens was installed, and the same setup has been used to take observations since June

13th, 1918 (Sivaraman et al., 1993), and is still in use. Since 2017, the unavailability of photographic

ﬁlms has interrupted regular observation, but it is still taken whenever the ﬁlms are available. These

observations at KoSO, initially taken on the photographic plates/ﬁlms, have been digitized using 4k

CCD at Kodaikanal and made available for the community (Ravindra et al., 2013; Mandal et al., 2017).

Here, we use the white-light digitized data for the period of 1904 – 1920 and 2012 – 2018, and will extend

the sunspot-area series reported earlier (1921 – 2011; Mandal et al., 2017) for the period of 1904 – 2017

(114 years , covering

≈

11 solar cycles). An example of a digitized white-light image from the very

initial period, is shown in Figure 1A. Furthermore, we discovered that there are a few observations during

1921 – 2011, that were missed in earlier area series (Mandal et al., 2017), so we included them in the

updated and revised series.

1904-01-16T08:20

(A)

1904-01-16T08:20

(B)

Figure 1.

A representative example of a very ﬁrst observation at KoSO, dated 16th January 1904 (A) raw

digitised negative image and (B) calibrated image with the contour of sunspot regions detected using a

semi-automated method. A zoomed-in view of the detected sunspot is shown in the inset.

2.1 Updated Data Statistics

In Figure 2(A), we present the number of observations per year from 1904 – 2017, with grey histograms

representing the observations used in area-series as reported by Mandal et al. (2017) and blue ones

representing the additional observations included in this new series. Since, Figure 2(A) includes the

multiple observation from the same day hence, to get an idea about the data coverage, we counted the

number of observing days in each year and plotted it against the years in Figure 2(B). From Figure 2(B),

we infer that the KoSO provides continuous white-light observations in the last 114 years with coverage of

Frontiers 3

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ExtendingtheSunspotAreaSeriesfromKodaikanalSolarObservatoryBibhutiKumarJha1;2;3;,ManjunathHegde1;2,AdityaPriyadarshi1;2,SudipMandal4,BRavindra1andDipankarBanerjee1;2;5;1AryabhattaResearchInstituteofObservationalSciences,Nainital,Uttarakhand,India2IndianInstituteofAstrophysics,Bangalore,Karnataka,I...

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