Study of sunspot Penumbra to Umbra area ratio using Kodaikanal white-light digitised data

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.