Ca ii K Polar Network Index of the Sun: A Proxy for Historical Polar Magnetic Field
The Sun’s polar magnetic field is pivotal in understanding solar dynamo processes and forecasting future solar cycles. However, direct measurements of the polar field have only been available since the 1970s. The chromospheric Ca ii K polar network index (PNI; the fractional area of the chromospheri...
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2025-01-01
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| Online Access: | https://doi.org/10.3847/1538-4357/adb3a8 |
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| author | Dibya Kirti Mishra Bibhuti Kumar Jha Theodosios Chatzistergos Ilaria Ermolli Dipankar Banerjee Lisa A. Upton M. Saleem Khan |
| author_facet | Dibya Kirti Mishra Bibhuti Kumar Jha Theodosios Chatzistergos Ilaria Ermolli Dipankar Banerjee Lisa A. Upton M. Saleem Khan |
| author_sort | Dibya Kirti Mishra |
| collection | DOAJ |
| description | The Sun’s polar magnetic field is pivotal in understanding solar dynamo processes and forecasting future solar cycles. However, direct measurements of the polar field have only been available since the 1970s. The chromospheric Ca ii K polar network index (PNI; the fractional area of the chromospheric network regions above a certain latitude) has recently emerged as a reliable proxy for polar magnetic fields. In this study, we derive PNI estimates from newly calibrated, rotation-corrected Ca ii K observations from the Kodaikanal Solar Observatory (1904–2007) and modern data from the Rome Precision Solar Photometric Telescope (2000–2022). We use both of those Ca ii K archives to identify polar network regions with an automatic adaptive threshold segmentation technique and calculate the PNI. The PNI obtained from both the archives shows a significant correlation with the measured polar field from the Wilcox Solar Observatory (Pearson correlation coefficient r > 0.93) and the derived polar field based on an Advective Flux Transport Model ( r > 0.91). The PNI series also shows a significant correlation with faculae counts derived from Mount Wilson Observatory observations ( r > 0.87) for both Kodaikanal Solar Observatory and Rome Precision Solar Photometric Telescope data. Finally, we use the PNI series from both archives to reconstruct the polar magnetic field over a 119 yr long period, which includes the last 11 solar cycles (Cycles 14–24). We also obtain a relationship between the amplitude of solar cycles (in 13 month smoothed sunspot number) and the strength of the reconstructed polar field at the preceding solar cycle minimum to validate the prediction of the ongoing solar cycle, Cycle 25. |
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| id | doaj-art-31ba2b6fe0574f57bc9d1dcbda8b8640 |
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| issn | 1538-4357 |
| language | English |
| publishDate | 2025-01-01 |
| publisher | IOP Publishing |
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| series | The Astrophysical Journal |
| spelling | doaj-art-31ba2b6fe0574f57bc9d1dcbda8b86402025-08-20T02:07:13ZengIOP PublishingThe Astrophysical Journal1538-43572025-01-0198227810.3847/1538-4357/adb3a8Ca ii K Polar Network Index of the Sun: A Proxy for Historical Polar Magnetic FieldDibya Kirti Mishra0https://orcid.org/0009-0003-1377-0653Bibhuti Kumar Jha1https://orcid.org/0000-0003-3191-4625Theodosios Chatzistergos2https://orcid.org/0000-0002-0335-9831Ilaria Ermolli3https://orcid.org/0000-0003-2596-9523Dipankar Banerjee4https://orcid.org/0000-0003-4653-6823Lisa A. Upton5https://orcid.org/0000-0003-0621-4803M. Saleem Khan6Aryabhatta Research Institute of Observational Sciences , Nainital-263002, Uttarakhand, India; Mahatma Jyotiba Phule Rohilkhand University , Bareilly-243006, Uttar Pradesh, IndiaSouthwest Research Institute , Boulder, CO 80302, USA ; maitraibibhu@gmail.comMax Planck Institute for Solar System Research , Justus-von-Liebig-Weg 3, D-37077 Göttingen, GermanyINAF Osservatorio Astronomico di Roma , Via Frascati 33, 00078 Monte Porzio Catone, ItalyIndian Institute of Astrophysics , Koramangala, Bangalore 560034, India; Center of Excellence in Space Sciences India , IISER Kolkata, Mohanpur 741246, West Bengal, India; Indian Institute of Space Science and Technology , Thiruvananthapuram, 695 547, Kerala, IndiaSouthwest Research Institute , Boulder, CO 80302, USA ; maitraibibhu@gmail.comMahatma Jyotiba Phule Rohilkhand University , Bareilly-243006, Uttar Pradesh, IndiaThe Sun’s polar magnetic field is pivotal in understanding solar dynamo processes and forecasting future solar cycles. However, direct measurements of the polar field have only been available since the 1970s. The chromospheric Ca ii K polar network index (PNI; the fractional area of the chromospheric network regions above a certain latitude) has recently emerged as a reliable proxy for polar magnetic fields. In this study, we derive PNI estimates from newly calibrated, rotation-corrected Ca ii K observations from the Kodaikanal Solar Observatory (1904–2007) and modern data from the Rome Precision Solar Photometric Telescope (2000–2022). We use both of those Ca ii K archives to identify polar network regions with an automatic adaptive threshold segmentation technique and calculate the PNI. The PNI obtained from both the archives shows a significant correlation with the measured polar field from the Wilcox Solar Observatory (Pearson correlation coefficient r > 0.93) and the derived polar field based on an Advective Flux Transport Model ( r > 0.91). The PNI series also shows a significant correlation with faculae counts derived from Mount Wilson Observatory observations ( r > 0.87) for both Kodaikanal Solar Observatory and Rome Precision Solar Photometric Telescope data. Finally, we use the PNI series from both archives to reconstruct the polar magnetic field over a 119 yr long period, which includes the last 11 solar cycles (Cycles 14–24). We also obtain a relationship between the amplitude of solar cycles (in 13 month smoothed sunspot number) and the strength of the reconstructed polar field at the preceding solar cycle minimum to validate the prediction of the ongoing solar cycle, Cycle 25.https://doi.org/10.3847/1538-4357/adb3a8Magnetic fieldsSolar cycleSolar chromosphere |
| spellingShingle | Dibya Kirti Mishra Bibhuti Kumar Jha Theodosios Chatzistergos Ilaria Ermolli Dipankar Banerjee Lisa A. Upton M. Saleem Khan Ca ii K Polar Network Index of the Sun: A Proxy for Historical Polar Magnetic Field The Astrophysical Journal Magnetic fields Solar cycle Solar chromosphere |
| title | Ca ii K Polar Network Index of the Sun: A Proxy for Historical Polar Magnetic Field |
| title_full | Ca ii K Polar Network Index of the Sun: A Proxy for Historical Polar Magnetic Field |
| title_fullStr | Ca ii K Polar Network Index of the Sun: A Proxy for Historical Polar Magnetic Field |
| title_full_unstemmed | Ca ii K Polar Network Index of the Sun: A Proxy for Historical Polar Magnetic Field |
| title_short | Ca ii K Polar Network Index of the Sun: A Proxy for Historical Polar Magnetic Field |
| title_sort | ca ii k polar network index of the sun a proxy for historical polar magnetic field |
| topic | Magnetic fields Solar cycle Solar chromosphere |
| url | https://doi.org/10.3847/1538-4357/adb3a8 |
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