Main Objective: To detect and characterise plasma downflows in loop footpoints that are associated with long-period intensity pulsations.
Scientific Justification: Long-period intensity pulsations have been recently detected in coronal loops with EUV images of both SoHO/EIT (Auchere et al., 2014) and SDO/AIA (Froment et al., 2015). They are observed with periods ranging from 3 to 16 h, and in loops at coronal temperatures, thus visible at EUV wavelengths. These pulsations have been interpreted as resulting from thermal non-equilibrium (TNE), thus providing a signature of a highly-stratified and quasi-constant heating at the loops footpoints (Froment et al., 2017; Auchere et al., 2016). Recent observations have permitted the unification of this phenomenon with coronal rain (Auchere et al., 2018, in prep.). Depending on the adequacy between the geometry of the loop and the characteristics of the heating, this can result in either complete (at chromospheric temperatures, i.e. those of coronal rain) or incomplete (> 1 MK) condensation and evaporation cycles, that are responsible for the observed intensity pulsations.
Various simulation studies indicate that TNE may occur in coronal loops where the heating is highly-stratified and quasi-constant. Using 1D hydrodynamic simulations, Froment et al. (2017) were able to reproduce the observed intensity, temperature, and density pulsations, with incomplete condensation for the active region studied in their previous paper. However, the simulations also predict periodic plasma upflows and downflows along the loops, with velocities up to 40 km/s. Detecting these velocities is thus necessary in order to confirm that the observed intensity pulsations are indeed due to TNE. In addition, spectral measurements would help refine the temperature and density diagnostics for such events.
In order to detect these pulsations, continuous observations of the same region during several periods are required. This translates to several days for periods around 10 h. First, we tried to find such datasets in the EIS archive. For 3181 pulsation events detected with SDO/AIA between 2010 and 2016 (see Auchere et al., 2014, for the detection method and Froment, 2016, for the events), we systematically searched for sets of EIS rasters such that:
- the FOV of all rasters intersected with the AIA event;
- the set covered more than 3 pulsation periods;
- the gaps between the rasters werenüft too long nor too frequent;
- the FOV of the rasters was greater than 55 arcsec;
- all rasters had the same study numbers.
After filtering out events with too faint pulsations, we obtained 8 datasets. However, none was perfect, because they still contained lots of gaps, had short exposure times, or lacked lines essential for density diagnostics. From these datasets, we could identify variations in density, but the expected pulsations in velocity could not be detected. We thus need better datasets, mainly with less data gaps and better signal to noise ratio.
Hence the present HOP proposal. This study aims at tracking an active region during most of its lifetime, from the east limb to the west limb. Such observations will allow us to characterise the long term variations of the plasma velocity, temperature and density in the loops. Given that these events appear to be common in active regions (Auchere et al., 2014), it is highly probable to observe pulsations in any active region.