HOP 0234

abstract of observational proposal

Research in the last 10 years has shown that the corona is permeated with waves. The pervasive presence of waves provides an indirect way of measuring the local physical state of the plasma in the corona. However, most EUV and X-ray instruments lack the instrumental resolution necessary to detect low amplitude waves and probe the elementary fine-scale structure in the corona. The scarcity of good diagnostic tools is a major obstacle for understanding this part of the solar atmosphere.

Coronal loops are mostly out of hydrostatic equilibrium and exhibit heating and cooling processes con-stantly, especially in active regions. One such process is coronal rain, which corresponds to partially ionized matter catastrophically cooling down following a thermal instability in the corona. Observed in cool chro￢mospheric lines such as Hα or Ca ii H and K, the rain appears in a timescale of minutes and is observed to fall down as elongated blobs at high speeds along loop-like paths (De Groof et al. 2004; Schrijver 2001).

The advent of high-resolution telescopes such as those on board Hinode and the Swedish 1-m Solar Telescope (SST) is revealing a scenario in which coronal rain seems to be more frequent and ubiquitous than previously thought (Antolin et al. 2010; Antolin & Rouppe van der Voort 2012). Due to the chromospheric nature of the phenomenon, observations of coronal rain from the ground are possible. Such observations therefore constitute very high-resolution windows into the substructure of coronal loops. The blobs that constitute coronal rain act as tracers of the magnetic field, and have provided insight into the elementary length-scales in the solar corona. Both observations and numerical simulations indicate a relatively high optical thickness and plasma-β parameter for the coronal rain blobs, implying an interaction with the magnetic field through MHD wave generation/damping (M¨uller et al. 2003). This scenario offers a wealth of application for coronal seismology, especially in the case of transverse MHD waves in thermally unstable coronal loops (Antolin & Verwichte 2011). The existence of transverse standing MHD waves in loops can affect the dynamics of coronal rain blobs in ways specific to the dominant harmonics (Verwichte et al. 2013, in preparation). The seismological potential of coronal rain can only be fully exploited through multi-wavelength spectroscopic observations of the phenomenon with high temporal, spatial and spectral resolution instruments allowing the detection of transverse MHD waves in coronal loops and the measurement of the thermo(and)dynamical properties of the rain blobs (a few hundred km in widths).

Such requirements may be obtained by combining imaging and spectroscopic instruments covering chromospheric, transition region and coronal temperatures. This can be achieved by combining the instruments on board of Hinode with those available at the SST and those of the upcoming IRIS mission. IRIS, the Interface Region Imaging Spectrograph, whose launch is scheduled for April 2013, is specially designed for spectrometric observations of the upper chromosphere -transition region lines with high spatial and temporal resolutions, and can therefore provide a complementary picture of coronal rain to that obtained from Hinode and SST. The seismological study with coronal rain proposed here is directly relevant to two out of the three main IRIS science goals:

request to EIS

Fast cadence raster at 2 positions with the 1". slit for dynamics study of coronal rain. A coronal rain

EIS study is being submitted at the same time of this proposal, matching the information described below. Mode:

1 Context slot: 40" slot before / after the main raster for accurate co-registration with AIA.

2 Slit: Scanning raster with the 1" slit at 2 positions with a step size of 20" as described below. At both positions the slit should be roughly perpendicular to the loops axes. A slit extension of 60" is required (same as SST dimension). The exposure time should be 10-12 sec in order to keep a < 25 s cadence.

Guideline for EIS target:

1. If IRIS is observing an AR at limb: First position centered on AR but ~10" above the limb (above the spicular region). Second position ~10" below the limb (close to where the footpoints of the loops would be). In this case, the slit orientation will most likely be parallel to solar limb.

2. If IRIS if observing an AR on-disc AND IRIS is running a wave program: First position at coronal loop footpoints and the second position 20" away from the ?rst, crossing the loops (roughly perpendicular to their axes).

3. If IRIS is doing none of the above then look for AR on limb and follow number 1 above.

Lines: Transition region lines mainly and density-sensitive lines: He II 256.32 Mg VI 268.99 Mg VII 280.75 Mg VII 278.39 Si VII 275.35 Si X 258.37 Si X 261.04 Fe IX 197.86 Fe XII 195.12 Fe XII 186.88 Fe XIV 264.79 Fe XIV 274.20

remarks

Observing request

Due to their low temperature and high density, coronal rain blobs have been observed in Hα and Ca ii H and K. Optically thick blobs can also be observed in absorption in EUV lines such as those of the ions Mg vi, Mg vii, Si vii or Fe ix. Observing time at the SST has been acquired for observations in the red beam with the CRISP spectropolarimeter and in the blue beam. We request co-observations in the same time period with Hinode/SOT, Hinode/EIS and IRIS. Hinode/XRT is also requested for context active region coronal loop observations.

Time period: Ideally, the acquired observations should elapse at least a 1 hour non-interrupted observation sequence in the period August 4, 2013 -August 13, 2013, provided excellent seeing conditions at the SST. It is not necessary for observations to be on consecutive days. Observations should be conducted between 10 -15 UT (SAA-free period), in order to overlap with the best seeing at La Palma (8 12 UT). Short interruptions should be avoided within this time interval.
Target of interest: The target should be an active region towards the solar limb (preferably east limb) within 30αα.from the limb (in order to secure both good AO locking at the SST and off-limb coronal rain observations). Particularly, effort will be made to co-observe with IRIS. The target of observation at the SST will be chosen according to the following guidelines, which will be executed depending on the IRIS observing programs at the time. We would like the SOT and EIS instruments to follow the same guidelines:

Main requisite: Active Region. If IRIS is observing a QS region then no co-observation with IRIS and look for an AR on limb.

1. The main goal is to co-observe with IRIS on off-limb coronal loops with coronal rain (active regions)

2. If IRIS is running on-disk wave programs, we will follow IRIS with the observing program specified in section 2.1 (although with lower probability of detection, coronal rain can also be observed on-disk, Antolin et al. 2012)

3. Only if IRIS is not running a wave program or the main requisite is not satisfied, and there is a suitable target close to the limb, we will run our own program (see section 2.1) without IRIS co-pointing.