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 (Auchere et al., 2016; Froment et al., 2017). 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 (Froment et al., 2018). Recent observations have confirmed that pulsations with complete condensations are associated with coronal rain (Auchere et al., 2018).
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. The simulations also predict periodic plasma flows along the loops footpoints, with velocities up to 40 km/s. Detecting these velocities would therefore confirm that the observed intensity pulsations are indeed due to TNE, even when no complete condensation occurs. In addition, spectral measurements would help refine the temperature and density diagnostics for such events. Because loops only contribute to about 10 % of the line-of-sight, the expected flows would result in a fain asymmetry of the observed line profile, thus requiring a high signal-to-noise ratio to be detected.
In order to detect these pulsations, continuous observations of the same active 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 each of the 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 search for sets of EIS rasters such that:
- the FOV intersects with the AIA event;
- the dataset duration is higher than 3 pulsation periods;
- the gap between the rasters are not too long nor too frequent.
After filtering out events with too faint pulsations, we obtained 11 datasets. These datasets either contain rasters with long exposure times (> 15 s) thus high SNR, or have a “high” observation cadence (several rasters per pulsation period). Therefore, no datasets provide at the same time enough SNR to measure
the velocities, and enough cadence to see the pulsations. We thus need better datasets, mainly with fewer data gaps and a higher SNR.
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. |
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