Automated Coronal Hole Detection using Local Intensity Thresholding Techniques

TL;DR

This paper presents a new local intensity thresholding method for automated coronal hole detection in EUV and X-ray solar images, and compares the properties of identified coronal holes with solar wind data at 1 AU.

Contribution

It introduces a histogram-based local thresholding technique for coronal hole detection that is consistent across multiple instruments and links coronal hole properties to solar wind characteristics.

Findings

The thresholding algorithm accurately identifies coronal boundaries across different instruments.

Coronal hole size correlates with the duration of high-speed solar wind streams.

Flux tubes in coronal holes expand super-radially within 1 AU.

Abstract

We identify coronal holes using a histogram-based intensity thresholding technique and compare their properties to fast solar wind streams at three different points in the heliosphere. The thresholding technique was tested on EUV and X-ray images obtained using instruments onboard STEREO, SOHO and Hinode. The full-disk images were transformed into Lambert equal-area projection maps and partitioned into a series of overlapping sub-images from which local histograms were extracted. The histograms were used to determine the threshold for the low intensity regions, which were then classified as coronal holes or filaments using magnetograms from the SOHO/MDI. For all three instruments, the local thresholding algorithm was found to successfully determine coronal hole boundaries in a consistent manner. Coronal hole properties extracted using the segmentation algorithm were then compared with in situ measurements of the solar wind at 1 AU from ACE and STEREO. Our results indicate that flux tubes rooted in coronal holes expand super-radially within 1 AU and that larger (smaller) coronal holes result in longer (shorter) duration high-speed solar wind streams.