Abstract
It is of great interest and importance to study the variabilities of solar EUV, UV and X-ray irradiance in heliophysics, in Earth’s climate, and space weather applications. A careful study is required to identify, track, monitor and segment the different coronal features such as active regions (ARs), coronal holes (CHs), the background regions (BGs) and the X-ray bright points (XBPs) from spatially resolved full-disk images of the Sun. Variability of solar soft X-ray irradiance is studied for a period of 13 years (February 2007–March 2020, covers Solar Cycle 24), using the X-Ray Telescope on board the Hinode (Hinode/XRT) and GOES (1 – 8 Å). The full-disk X-ray images observed in Al_mesh filter from XRT are used, for the first time, to understand the solar X-ray irradiance variability measured, Sun as a star, by GOES instrument. An algorithm in Python has been developed and applied to identify and segment coronal X-ray features (ARs, CHs, BGs, and XBPs) from the full-disk soft X-ray observations of Hinode/XRT. The segmentation process has been carried out automatically based on the intensity level, morphology and sizes of the X-ray features. The total intensity, area, and contribution of ARs/CHs/BGs/XBPs features were estimated and compared with the full-disk integrated intensity (FDI) and GOES (1 – 8 Å) X-ray irradiance measurements. The XBPs have been identified and counted automatically over the full disk to investigate their relation to solar magnetic cycle. The total intensity of ARs/CHs/BGs/XBPs/FD regions are compared with the GOES (1 – 8 Å) X-ray irradiance variations. We present the results obtained from Hinode/XRT full-disk images (in Al_mesh filter) and compare the resulting integrated full-disk intensity (FDI) with GOES X-ray irradiance. The X-ray intensity measured over ARs/CHs/BGs/XBPs/FD is well correlated with GOES X-ray flux. The contributions of the segmented X-ray features to FDI and X-ray irradiance variations are determined. It is found that the background and active regions have a greater impact on the X-ray irradiance fluctuations. The mean contribution estimated for the whole observed period of the background regions (BGs) will be around \(65\pm10.97\%\), whereas the ARs, XBPs and CHs are \(30\pm11.82\%\), \(4\pm1.18\%\) and \(1\pm0.52\%\), respectively, to total solar X-ray flux. We observed that the area and contribution of ARs and CHs varies with the phase of the solar cycle, whereas the BGs and XBPs show an anti-correlation. We find that the area of the coronal features is highly variable suggesting that their area has to be taken into account in irradiance models, in addition to their intensity variations. The time series results of XBPs suggest for an existence of anti-correlation between the number of XBPs and the sunspot numbers. It is also important to consider both the number variation and the contribution of XBPs in the reconstruction of total solar X-ray irradiance variability.
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Acknowledgements
Hinode is a Japanese mission developed and launched by ISAS/JAXA, in collaboration with NAOJ as a domestic partner, NASA and STFC (UK) as international partners. The Hinode science team at ISAS /JAXA has conducted the Scientific operation of the Hinode mission. This team mainly consists of scientists from different institutes in the partner countries. JAXA and NAOJ (Japan), STFC (UK), NASA (USA), ESA, and NSC (Norway) have provided the support for the post-launch operation. The Hinode team had contributed all their efforts in the design, build, and operation of the mission. RK wishes to express his sincere thanks to all members of ISEE for the support provided under Joint International Research Program. A part of these results were presented by RK at the PSTEP-4 International Symposium held at ISEE/Nagoya University from January 28 – 30, 2020 and his visit to this symposium has been supported by MEXT/JSPS KAKENHI Grant Number JP15H05816. The authors are thankful to the referee for the valuable comments and suggestions that improved the manuscript considerably.
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Adithya, H.N., Kariyappa, R., Shinsuke, I. et al. Solar Soft X-ray Irradiance Variability, I: Segmentation of Hinode/XRT Full-Disk Images and Comparison with GOES (1 – 8 Å) X-Ray Flux. Sol Phys 296, 71 (2021). https://doi.org/10.1007/s11207-021-01785-6
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DOI: https://doi.org/10.1007/s11207-021-01785-6