Visualization of High Dynamic Range Solar Imagery and the Radial Histogram Equalizing Filter

TL;DR

The paper introduces RHEF, a new contrast enhancement technique for high dynamic range solar images that reveals faint coronal structures more effectively than existing methods, without requiring parameter tuning.

Contribution

It presents the Radial Histogram Equalizing Filter (RHEF), a novel hybrid contrast enhancement method that combines radial spatial awareness with histogram equalization for solar imagery.

Findings

RHEF effectively reveals faint coronal structures.

RHEF outperforms established enhancement methods.

Implementation available in Python and IDL.

Abstract

Standard visualizations of Extreme Ultraviolet (EUV) solar imagery often fail to convey the full complexity of the Sun's corona, especially in faint off-limb regions. This can leave the misleading impression of the Sun as a bright ball in a dark void, rather than revealing it as the dynamic, structured source of the solar wind and space weather. A variety of enhancement algorithms have been developed to address this challenge, each with its own strengths and tradeoffs. We introduce the Radial Histogram Equalizing Filter (RHEF), a novel hybrid technique that optimizes contrast in high dynamic range solar images. By combining the spatial awareness of radial graded filters with the perceptual benefits of histogram equalization, RHEF reveals faint coronal structures and works out of the box -- without requiring careful parameter tuning or prior dataset characterization. RHEF operates independently on each frame, and it enhances on-disk and off-limb features uniformly across the field of view. For additional control, we also present the Upsilon redistribution function -- a symmetrized cousin of gamma correction -- as an optional post-processing step that provides intuitive programmatic tonal compression. We benchmark RHEF against established methods and offer guidance on filter selection across various applications, with examples from multiple solar instruments provided in an appendix. Implemented and available in both Python sunkit_image and IDL, RHEF enables immediate improvements in solar coronal visualization.