How Solar Soft X-Ray Irradiance Is Affected by the Size of Field of View

Abstract

Soft X-ray images of the Sun obtained with multiple broadband filters provide a simple and useful method to calculate averaged coronal temperature and emission measure, from which we can further calculate solar X-ray irradiance in physical units, e.g., watts per square meter. However, X-ray telescopes are often designed for pursuing high spatial resolution, and thus the field of view (FOV) of full-Sun images is often limited over the limb, extending to only ∼1.3 R ⊙. This indicates that the irradiance obtained from the limited FOV may underestimate the true full-Sun irradiance by failing to count the contribution from outside the FOV. This work uses Hinode/X-Ray Telescope (XRT) coronal images observed up to 1.7 R ⊙ to investigate the fraction of irradiance excluded from the FOVs limited in size. The analysis indicates that the irradiance obtained within 1.1 R ⊙, which is used for XRT irradiance study, excludes ∼3.5%/∼7% of irradiance relative to the value within 1.2/1.7 R ⊙, respectively, for the active corona observed in 2022 July. In contrast, the excluded fraction increases to ∼7%/∼13%, respectively, for the minimum corona observed in 2009 August. To further investigate the dependence of exclusion fraction on the Sun’s activity level, we process mission long Yohkoh/Soft X-ray Telescope full-Sun images to compare the irradiance within 1.1 and 0.9 R ⊙ with that obtained from the maximum FOV of 1.2 R ⊙. We confirm that the exclusion fraction is the largest in the period around solar minimum. We also find that the average value of exclusion fraction is slightly but measurably larger in the rising phase than the declining phase of the solar cycle.