Main Objective:
Constraining the properties of coronal heating mechanisms based on the observed plasma cooling characteristics
Scientific Justification:
Numerical simulations and observations have shown that coronal loops, the building blocks of coronae, undergo heating and cooling cycles (Goldsmith 1971, Antiochos 1999) over large portions of the active corona (Froment et al. 2015). Footpoint heating produces chromospheric evaporation leading to dense, strongly radiating loops, which end up cooling catastrophically, a thermal non-equilibrium process that sets the corona in a dynamic and strong thermally inhomogeneous state. The cooling phase is driven by thermal instability, and produces a partially ionised, dense, multi-thermal (ranging 2,000 K to 150,000 K) and clumpy plasma that accretes towards the solar surface (Antolin & Rouppe van der Voort 2012). This phenomenon is known as coronal rain and is deeply linked to coronal heating and thermal instability (Leroy 1972, Antolin et al. 2012). In this project we aim at investigating the cool side of the corona, the degree of spatial and thermal inhomogeneity, and use the advantages of studying cold material to gain insight into the nature of coronal heating.
Through the thermometer and high resolution capability of ALMA in Cycle 5, the atmosphere above an active region can be probed for thermal bremsstrahlung from the rain (Wedemeyer et al. 2016). Through coordination with the instruments from the SDO and IRIS missions, and other observatories from the ground (SST, BBSO), the full temperature range of the rain, from 2000 K to 200,000 K, can be covered, which allows to follow and characterise the evolution of thermal instability in active region loops. In turn this allows to firmly assess the rain’s role in the chromosphere-corona mass and energy cycle of the solar atmosphere.
Furthermore, at the very high cadence offered by ALMA, we can observe and directly measure the triggering of thermal instability, the formation process of coronal rain, and follow its impact into the lower solar atmosphere. Following catastrophic cooling, as the plasma recombines from coronal down to chromospheric temperatures (in a timescale of minutes), the MHD thermal mode (also known as the entropy mode, a solution to the MHD equations) is expected to be triggered and act as seed for neighbouring rain clumps. The progressive cooling in time (as the clumps fall) and further (cascade-like) triggering of catastrophic cooling along neighbouring field lines can be captured at the very high cadence of ALMA. The formation process and tracing of the rain is therefore a perfect test ground into the limits of MHD validity in the solar corona.
For ALMA:
4 scheduling blocks, 2 hour each, 2 bands (band 3 & 6): 2 modes:
-LF mode: Large FOV: 60 x 120 arcsec^2 (bands 3 & 6)
duration: 2 hours x 2, cadence: <15 min
-HC mode: High cadence mode: 60x60 (band 3), 25x25 (band 6)
duration: 2 hours x 2, cadence: 2 s
1 mode / day preferable (bands 3 & 6 for same target).
For all instruments: Pointing is based on ALMA target. Please point Hinode so SOT (after correcting for the SOT offset) is at the limb adjacent to the ALMA target; this is an exception to the usual 15 arcsec inside the limb rule. XRT and EIS may use internal offsets to match the center of the ALMA FOV. |
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