Technical Article

A center-median filtering method for detection of temporal variation in coronal images

High Altitude Observatory, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307-3000, USA

^* Corresponding author: plowman.joseph@gmail.com

Received: 30 January 2015
Accepted: 29 December 2015

Abstract

Events in the solar corona are often widely separated in their timescales, which can allow them to be identified when they would otherwise be confused with emission from other sources in the corona. Methods for cleanly separating such events based on their timescales are thus desirable for research in the field. This paper develops a technique for identifying time-varying signals in solar coronal image sequences which is based on a per-pixel running median filter and an understanding of photon-counting statistics. Example applications to “EIT waves” (named after EIT, the EUV Imaging Telescope on the Solar and Heliospheric Observatory) and small-scale dynamics are shown, both using 193 Å data from the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory. The technique is found to discriminate EIT waves more cleanly than the running and base difference techniques most commonly used. It is also demonstrated that there is more signal in the data than is commonly appreciated, finding that the waves can be traced to the edge of the AIA field of view when the data are rebinned to increase the signal-to-noise ratio.