XRT Picture of the Week (XPOW)

2025 December 02



Composite long exposure images revealing the temperature properties of a Coronal Hole on the Sun

Coronal Hole is Not so Cool!?

In the midst of the butterfly-shaped coronal hole's disk passage, we performed a special observation of this coronal hole aiming to measure its temperature. Coronal holes are a unipolar region (dominated by a single magnetic polarity) where their magnetic fields are open to space and known as a source of a fast stream of coronal plasma into space, called solar wind. Coronal holes look dark in soft X-ray because of its low density. It is therefore very challenging to derive the quantities there from the very low signal level emitted from the region. This time, we used extra long exposure time to observe the whole Sun with multiple filters including Al_mesh and Al_poly with a special observing sequence #1CE4, At this level of Sun's activity, we usually observe the quiet region of the Sun with exposures around 1 second (707ms for Al_mesh, 1.4sec for Al_poly, at 2x2 binning images). This time, however, we applied the total exposure time of 5.8 sec and 11.6 sec for Al_mesh and Al_poly filters, respectively. The top two images in the above are the composite images using those with special long exposures (note that the bright area of the Sun are replaced with those from shorter exposure images to avoid saturated pattern). We then applied the filter ratio method to derive the temperature and emission measure of the full-Sun area averaged over the line of sight, as shown in the bottom two images in the above. To those who are interested, here is the summary of the procedure: Creation of composite images Subtraction of stray light images Additional dust spot correction Additional binning to 8x8 for better S/N Applying xrt_teem.pro with the solar spectral model calculated with Chianti 11.02, coronal abundance The temperature map we derived shows that the coronal hole has the similar level of temperature to the ambient quiet sun, not significantly cooler than that. This is something new, but we need more careful analysis to conclude that. One thing we have not yet considered is the scattered light from active regions or nearby quiet region. In theory, the scattered light is corrected by decovolving the point spread function calculated from the mirror shape (via xrt_deconvolve.pro). However, it is known from eclipse observations and limb flare data that the actual point spread function has wider wing component and thus larger amount of scatter light should exist (see a poster by Kashima et al in 2023. More reliable evaluation of the scattered light component would enable us to conclude that the coronal hole is not so cool. Keywords: Coronal Hole, Full Disk Filters: Al-Mesh, Al-Poly

(Prepared by Aki Takeda)

The XRT instrument team is comprised of SAO, NASA, JAXA, and NAOJ.

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