Solar Soft X-ray Irradiance Variability III: Magnetic Field Variations of Coronal X-ray Features

Abstract

The magnetic field changes the radiative output of the Sun and is the main source for all the solar surface features. To study the role of the underlying photospheric magnetic field in relation to emission features observed in the solar corona, we have used the full-disk soft X-ray images from Hinode/X-Ray Telescope (Hinode/XRT) and the magnetograms obtained from the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) for a period of about 13 years (May 2010 – June 2023), which covers Solar Cycle 24 and the ascending phase of Solar Cycle 25. A sophisticated and established algorithm developed in Python is applied to the X-ray observations from Hinode/XRT to segment the different coronal features by creating segmentation maps of the active regions (ARs), coronal holes (CHs), background regions (BGs), and X-ray bright points (XBPs). Further, these maps have been applied to the full-disk (FD) line-of-sight (LOS) magnetograms from HMI to isolate the X-ray coronal features and photospheric magnetic counterparts, respectively. We computed full-disk and featurewise averages of X-ray intensity and LOS magnetic field (MF) over ARs, CHs, BGs, XBPs, and FD regions. Variations in the quantities resulting from the segmentation, namely the mean intensity, temperature from the filter ratio method, and the unsigned magnetic field of ARs, CHs, BGs, XBPs, and FD regions, are intercompared and compared with the sunspot number (SSN). We find that the X-ray intensity and temperature over ARs, CHs, BGs, XBPs, and FD regions are well correlated with the underlying magnetic field. We discuss the intensity, temperature, and magnetic field variations of the full-disk corona and of all the features. The time series plots of the unsigned magnetic field of the full disk and all the features show magnetic field fluctuations synchronized with the solar cycle (sunspot number). Although the magnetic field of all features varies, the mean, spatially smoothed magnitude of the magnetic field values estimated for the whole observed period of the full disk is around 8.9 ± 2.60 G, active regions (ARs) are around 34.4 ± 18.42 G, whereas BGs, CHs, and XBPs are 7.7 ± 1.72 G, 6.6 ± 1.04 G, and 15.62 ± 8.76 G, respectively. In addition, we find that the mean magnetic field contribution of the background regions (BGs) is around 85\(\%\), whereas ARs, CHs, and XBPs are 11\(\%\), 2\(\%\), and 2\(\%\), respectively, to the average magnetic field of the full disk. The magnetic field time series of all the features suggest that the features show a high variability in their magnetic field and the fluctuations in magnetic field are correlated to fluctuations in intensity and temperature, suggesting that the magnetic field is important in producing different emission features, which are associated with different intensity and temperature values. The magnetic field is responsible for the heating rate of the emission features, which are highly variable on solar cycle timescales. We conclude from the full-disk intensity-temperature-magnetogram analysis that the magnetic field plays a crucial role in driving the different brightenings, emissions, and temperature and heating of the corona at the sites of these magnetic features. In this study, we demonstrate that the segmented coronal features observed in the soft X-ray wavelength can be used as proxies to isolate the corresponding underlying magnetic structures.