A sharp EUI & SPICE look into the EUV variability and fine-scale structure associated with coronal rain

(Solar Orbiter nugget #14 by P. Antolin¹, A. Dolliou², F. Auchère², L.P. Chitta³, S. Parenti², D. Berghmans³ & EUI Team)

Introduction

The solar atmosphere is exceptionally hot, millions of degrees hotter than the underlying surface. However, it can also dramatically cool down due to thermal instability under very specific spatial and temporal conditions of the heating mechanism. For strongly stratified and high-frequency heating (when heating events occur at a rate faster than the radiative cooling time of the coronal structure), the structures become radiatively unstable and undergo cycles of heating (evaporation) and cooling (condensation), also known as TNE-TI (for thermal non-equilibrium - thermal instability) [1, 2]. TI leads to the generation of dense and cool clumpy plasma that falls down towards the solar surface in a process known as coronal rain [3, 4].

Aims & Methods

In this work we use the high-resolution EUI data (with HRIEUV) [5] on Solar Orbiter [6] from the early Spring 2022 perihelion to identify observable features of the TNE-TI scenario. We aim to understand the role TNE-TI plays in the observed EUV variability and morphology of the solar corona.

Figure 1. (Left) HRIEUV field-of-view (FOV) on 2022-03-30 showing an active region on the East limb (figure has been rotated). Coronal rain (red paths) was seen along a coronal loop. (Middle) A zoomed-in portion of the loop footpoint. The rain is seen in EUV absorption (black arrows). (Right) A zoomed-in portion on a rain clump (black arrows), with the bright compression region underneath (red arrows) that produces the fireball effect (see animation).

Small scale features

HRIEUV detects EUV absorption features as small as 260km produced by coronal rain, with a width average of 500 ± 200 km. The rain is observed to fall at speeds in the plane-of-the-sky (POS) of 10-150 km/s, generating in some cases a brightening immediately underneath the clumps as they fall [7]. We interpret this brightening as a result of compression and heating, similar to the fireball effect for meteors (although without ablation)

Figure 2. Time-distance diagram along path 3 (shown in Figure 1) followed by rain clumps. Zero distance corresponds to the chromosphere. The dark features correspond to rain, while the bright features immediately underneath correspond to the fireball (compression an heating). The bright wide feature with positive slope indicates a surge and shock wave propagating upwards.

We also observe the appearance of EUV strands prior/during the rain, having similar widths as the rain, which may be the result of the CCTR (Condensation Corona Transition Region) [8]. Just prior to impact, the entire region beneath the rain is observed to brighten, producing a flash-like EUV event along a few strands. This is only observed for the fastest events, and may be a result of very strong compression.

Figure 3. (Left) HRIEUV FOV on 2022-04-01 showing another active region on the East limb. Coronal rain (red paths) was seen along several coronal structures. Four regions were selected. (Right) A zoom-in on Region 3. Some rain paths are shown in dashed white curves. Note the appearance of multi-stranded structure (see animation).

Large scale features

SPICE [9] detects sequential brightening of loops, from hotter to cooler spectral bands, just prior to the appearance of coronal rain. This matches the cooling expected in the TNE-TI scenario.

Figure 4. Multi-spectral view on a portion of Region 2 shown in Figure 3. The blue-green maps correspond to SPICE rasters on several spectral lines (whose ion is indicated on top of each map, with respective maximum formation temperature). The yellow map corresponds to an HRIEUV synthetic raster, following the SPICE rastering characteristics. The maps are ordered by decreasing temperature from right to left. The red paths in each map correspond to coronal rain paths (as shown also in Figure 3). Three map groups are shown, separated by 16 min (cadence of raster). The red arrow shows a loop that reaches maximum brightness at that specific raster time. Sequential brightening from right (hot) to left (cool) is seen, indicating cooling.

HRI_EUV detects the the impact of rain showers for the first time. We observe an upward propagating EUV perturbance at speeds of 50-130 km/s, which we interpret as a combination of hot flows and slows modes that partially reheat the loop [10].

Observations with AIA 171 [11] on board Solar Dynamics Observatory [12] in quadrature only capture a small fraction of the observed rain events. On the other hand, AIA 304 shows widespread coronal rain, in agreement with SPICE observations that show abundance of cool structures (in the lower and upper transition region lines). This shows that detecting coronal rain in EUV absorption needs high spatial resolution and depends on the line-of-sight as well.

Figure 5. Multi-spectral view of the full FOV of SPICE on 2022-04-01. Each map corresponds to a specific spectral line indicated on top, together with the time of the raster. The red paths show the rain trajectories common to Figure 3. Note the presence of many coronal structures at relatively cool temperatures (lower to upper transition region). 

Conclusions

Coronal rain generates EUV structure and variability over a wide range of scales, from coronal loops to the smallest resolvable scales (strand widths, fireballs). This establishes the major role that TNE-TI plays in the observed EUV morphology and variability of the corona.

This study has been published in Antolin, P. et al. A&A in press, doi.org/10.1051/0004-6361/202346016

Acknowledgements:

P.A. acknowledges funding from STFC Ernest Rutherford Fellowship No. ST/R004285/2.

Affiliations:

¹Department of Mathematics, Physics and Electrical Engineering, Northumbria University, Newcastle upon Tyne NE1 8ST, UK

²Institut d'Astrophysique Spatiale, Université Paris-Saclay, Orsay, France

³Max Planck Institut for Solar System Research, Göttingen, Germany

⁴Royal Observatory of Belgium, Brussels, Belgium

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