Connecting the Low to High Corona: A Method to Isolate Transients in STEREO/COR1 Images

TL;DR

This paper introduces a novel method using a temporal bandpass filter to isolate and track transient features from the low to high corona in coronagraph images, revealing their dynamics and connection to the solar wind.

Contribution

The method effectively isolates time-varying coronal features across multiple instruments, enabling detailed tracking from the low to high corona and improving understanding of solar wind sources.

Findings

Small density enhancements propagate from ~1.2 Rs to beyond 5 Rs.

Features recur during solar minimum and remain coherent into interplanetary space.

Measured feature speeds classify them as slow and fast solar wind tracers.

Abstract

We present a method that isolates time-varying components from coronagraph and EUV images, allowing sub-streamer transients propagating within streamers to be tracked from the low to high corona. The method uses a temporal bandpass filter with a transmission bandwidth of ~2.5-10 hours that suppresses both high and low frequency variations in observations made by the STEREO/SECCHI suite. We demonstrate that this method proves crucial in linking the low corona where the magnetic field is highly non-radial, to their counterparts in the high corona where the magnetic field follows a radial path through the COR1 instrument. We also applied our method to observations by the COR2 and EUVI instruments onboard SECCHI and produced height-time profiles that revealed small density enhancements, associated with helmet streamers, propagating from ~1.2 Rs out to beyond 5 Rs. Our processing method reveals that these features are common during the period of solar minimum in this study. The features recur on timescales of hours, originate very close to the Sun, and remain coherent out into interplanetary space. We measure the speed of the features and classify them as: slow (a few to tens of km/s) and fast (~100 km/s). Both types of features serve as an observable tracer of a variable component of the slow solar wind to its source regions. Our methodology helps overcome the difficulties in tracking small-scale features through COR1. As a result, it proved successful in measuring the connectivity between the low and high corona and in measuring velocities of small-scale features.