Temperature of quiet Sun small scale brightenings observed by EUI on board Solar Orbiter: Evidence for a cooler component

Context. On May 30, 2020, small and short-lived extreme-UV (EUV) brightenings in the quiet Sun were observed over a four-minute sequence by the EUV channel of the Extreme Ultraviolet Imager - High Resolution Imager (EUIHRI_EUV) on board the Solar Orbiter. The brightenings' physical origin and possible impact on coronal or transition region (TR) heating are still to be determined.
Aims: Our aim is to derive the statistical thermal evolution of these events in order to establish their coronal or TR origin.
Methods: Our thermal analysis took advantage of the multithermal sensitivity of the Atmospheric Imaging Assembly (AIA) imager on board the Solar Dynamics Observatory. We first identified the HRI_EUV events in the six coronal bands of AIA. We then performed a statistical time lag analysis that quantified the delays between the light curves from different bands, as these time lags can give significant insight into the temperature evolution of the events. The analysis was performed taking into account the possible contribution of the background and foreground emissions to the results.
Results: For all nine couples of AIA bands analyzed, the brightening events are characterized by time lags inferior to the AIA cadence of 12 s. Our interpretation for these short time lags is the possible copresence of events that reach or do not reach coronal temperatures (≈1 MK). We believe that the cool population dominates the events analyzed in this work.