Tracking granules on the Sun's surface and reconstructing horizontal velocity fields: I. the CST algorithm

TL;DR

This paper introduces the CST algorithm for tracking granules on the Sun's surface to reconstruct horizontal velocity fields, enhancing understanding of solar surface dynamics through advanced image segmentation and multi-resolution analysis.

Contribution

The paper develops the CST method, generalizes segmentation techniques, and applies wavelet-based multi-resolution analysis for improved solar surface velocity field reconstruction.

Findings

Improved granule tracking accuracy with shape-based segmentation.

Multi-resolution analysis reveals velocity field at different scales.

Algorithm for computing divergence and vorticity fields.

Abstract

Determination of horizontal velocity fields on the solar surface is crucial for understanding the dynamics of structures like mesogranulation or supergranulation or simply the distribution of magnetic fields. We pursue here the development of a method called CST for coherent structure tracking, which determines the horizontal motion of granules in the field of view. We first devise a generalization of Strous method for the segmentation of images and show that when segmentation follows the shape of granules more closely, granule tracking is less effective for large granules because of increased sensitivity to granule fragmentation. We then introduce the multi-resolution analysis on the velocity field, based on Daubechies wavelets, which provides a view of this field on different scales. An algorithm for computing the field derivatives, like the horizontal divergence and the vertical vorticity, is also devised. The effects from the lack of data or from terrestrial atmospheric distortion of the images are also briefly discussed.