Constructing Time-Dependent Coronal Hole Maps Using Synchronized Multi-Instrument EUV Data

Abstract:

Coronal holes are manifested as relatively low intensity regions of the corona seen in EUV and X-Ray images. They are usually associated with open magnetic field regions, and their evolution can help us understand how the Sun’s magnetic field evolves in time and how these changes propagate out into the heliosphere.

Here we discuss our method for detecting coronal holes in EUV imaging data. Our particular focus is on building instantaneous snapshots of hole boundaries by stitching together images from multiple vantage points (provided by STEREO-A, STEREO-B, and SDO), which allows us to study the evolution of large portions of the corona at high cadences for long periods of complete visibility.

A key step in our methodology is the proper scaling and preprocessing of the data. Intensity histogram averaging of quiet sun regions over extended series of images is used to scale the instrument's data to each other, and along with a running average of coronal hole intensity, is utilized to fashion limb brightening correction curves, which when applied, remove natural disk-to-limb intensity variations. An image deconvolution using the instrument's known point-spread-functions is also used to reduce scattered light which can artificially brighten coronal holes. Our coronal hole detection method is discussed, which relies on an iterative double-threshold “flood-fill” algorithm. Time-dependent coronal hole maps produced by our method will be presented, and challenges of data flow, processing, and storage will be discussed.