The Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO) returns high-resolution images of the solar atmosphere in seven extreme ultraviolet wavelength channels. The images are processed on the ground to remove intensity spikes arising from energetic particles hitting the instrument, and the despiked images are provided to the community. In this work a three-hour series of images from the 171 A channel obtained on 2017 February 28 was studied to investigate how often the despiking algorithm gave false positives caused by compact brightenings in the solar atmosphere. The latter were identified through spikes appearing in the same detector pixel for three consecutive frames, and 1096 examples were found from the 900 image frames. These three-spikes were assigned to 126 dynamic solar features, and it is estimated that the three-spike method identifies 25% of the total number of features affected by despiking. For any 10 minute sequence of AIA 171 A images there are therefore around 28 solar features that have their intensity modified by despiking. The features are found in active regions, quiet Sun and coronal holes and, in relation to solar surface area, there is a greater proportion within coronal holes. In 96% of the cases, the despiked structure is a compact brightening of size 2 arcsec or less and the remaining 4% have narrow, elongated structures. In all cases, the events are not rendered invisible by the AIA processing pipeline, but the total intensity over the events lifetimes can be reduced by up to 67%. Scientists are recommended to always restore the original intensities to AIA data when studying short-lived or rapidly-evolving features that exhibit fine-scale structure.
تحميل البحث