The Interface Region Imaging Spectrograph (IRIS) provides an unparalleled opportunity to explore the (thermal) interface between the chromosphere, transition region, and the coronal plasma observed by the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory (SDO).

The top three rows show (from top to bottom) a running time difference of SDO/AIA 171Å and “λ - time” plots of Si IV (1403Å) and Mg IIh (2803Å). The location of the IRIS slit in the running difference is shown as a red dashed line while the horizontal (black) dashed lines in the other three panels show the zero wavelength positions. The lowest panel shows the h3 wavelength (black line) and the negative of the AIA 171 Å intensity (red line). The AIA intensity has been normalized to the h3 wavelength for ease of comparison.

The SDO/AIA observations of coronal loop footpoints show strong recurring upward propagating signals - “propagating coronal disturbances” (PCDs) with apparent speeds of order 100-120 km/s. That signal has a clear signature in the slit-jaw images of IRIS in addition to identifiable spectral signatures and diagnostics in the Mg IIh (2803 ̊A) line. In analyzing the Mg IIh line, we are able to observe the presence of magnetoacoustic shock waves that are also present in the vicinity of the coronal loop footpoints. We see there is enough of a correspondence between the shock propagation in Mg IIh, the evolution of the Si IV line profiles, and the PCD evolution to indicate that these waves are an important ingredient for PCDs. In addition, the strong flows in the jet-like features in the IRIS Si IV slit-jaw images are also associated with PCDs, such that waves and flows both appear to be contributing to the signals observed at the footpoints of PCDs.