Segmentation of Loops from Coronal EUV Images

TL;DR

This paper introduces a new semi-automated method for extracting and analyzing coronal EUV loops from solar images, improving noise handling and loop identification for better magnetic field modeling.

Contribution

The authors develop a regularized derivative estimation technique and a spline-based curve fitting scheme for accurate loop segmentation in EUV images, with a semi-automated user interface.

Findings

Successfully extracted coronal loops from STEREO EUV images

Validated loop structures against magnetic field models

Demonstrated potential for studying loop oscillations and stereoscopy

Abstract

We present a procedure which extracts bright loop features from solar EUV images. In terms of image intensities, these features are elongated ridge-like intensity maxima. To discriminate the maxima, we need information about the spatial derivatives of the image intensity. Commonly, the derivative estimates are strongly affected by image noise. We therefore use a regularized estimation of the derivative which is then used to interpolate a discrete vector field of ridge points ``ridgels'' which are positioned on the ridge center and have the intrinsic orientation of the local ridge direction. A scheme is proposed to connect ridgels to smooth, spline-represented curves which fit the observed loops. Finally, a half-automated user interface allows one to merge or split, eliminate or select loop fits obtained form the above procedure. In this paper we apply our tool to one of the first EUV images observed by the SECCHI instrument onboard the recently launched STEREO spacecraft. We compare the extracted loops with projected field lines computed from almost-simultaneously-taken magnetograms measured by the SOHO/MDI Doppler imager. The field lines were calculated using a linear force-free field model. This comparison allows one to verify faint and spurious loop connections produced by our segmentation tool and it also helps to prove the quality of the magnetic-field model where well-identified loop structures comply with field-line projections. We also discuss further potential applications of our tool such as loop oscillations and stereoscopy.