Abstract
We study the long-term behaviour of the sunspot penumbra to umbra area ratio by analysing recently digitised Kodaikanal white-light data (1923 – 2011). We implement an automatic umbra extraction method and compute the ratio over eight solar cycles (Cycles 16 – 23). Although the average ratio does not show any variation with spot latitudes, cycle phases and strengths, it increases from 5.5 to 6 as the sunspot size increases from 100 μhem to 2000 μhem. Interestingly, our analysis also reveals that this ratio for smaller sunspots (area \({<}\,100\) μhem) does not have any long-term systematic trend as was earlier reported from the photographic results of the Royal Observatory, Greenwich (RGO). To verify the same, we apply our automated extraction technique to Solar and Heliospheric Observatory (SOHO)/Michelson Doppler Imager (MDI) continuum images (1996 – 2010). Results from this data not only confirm our previous findings, but they also show the robustness of our analysis method.
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Notes
This catalogue is available online at https://kso.iiap.res.in/new/white_light.
Description is available at http://www.idlcoyote.com/idldoc/cg/cgotsu_threshold.html.
For more details, visit https://www.harrisgeospatial.com/Software-Technology/IDL.
This data is downloaded from http://fenyi.solarobs.csfk.mta.hu/ftp/pub/DPD/data/dailyDPD1974_2016.txt.
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Acknowledgements
Kodaikanal solar observatory is a facility of the Indian Institute of Astrophysics, Bangalore, India. The data utilised in this article is now available for public use at http://kso.iiap.res.in. We would like to thank Ravindra B. and Manjunath Hegde for their tireless support during the digitisation, calibration and sunspot detection processes. The authors also acknowledge Subhamoy Chatterjee for his useful suggestions during the entire process.
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Jha, B.K., Mandal, S. & Banerjee, D. Study of Sunspot Penumbra to Umbra Area Ratio Using Kodaikanal White-light Digitised Data. Sol Phys 294, 72 (2019). https://doi.org/10.1007/s11207-019-1462-2
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DOI: https://doi.org/10.1007/s11207-019-1462-2