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

Abstract

Abstract We present a method that isolates time-varying components from coronagraph and extreme ultraviolet images, allowing substreamer transients propagating within streamers to be tracked from the low to the high corona. The method uses a temporal bandpass filter with a transmission bandwidth of ∼2.5–10 hr 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 features in the low corona, where the magnetic field is highly nonradial, to their counterparts in the high corona, where the magnetic field follows a radial path, through the COR1 instrument. We also apply our method to observations by the COR2 and EUVI instruments on board SECCHI and produce height–time profiles that reveal small density enhancements, associated with helmet streamers propagating from ∼1.2 R ⊙ out to beyond 5 R ⊙. 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 kilometers per second) or fast (∼100 km s−1). 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 proves successful in measuring the connectivity between the low and high corona and in measuring the velocities of small-scale features.