The role of small scale EUV brightenings in the quiet Sun coronal heating

(Solar Orbiter Nugget #43 by A. Dolliou¹, S. Parenti¹, K. Bocchialini¹)

1. Introduction

How the outermost layer of the solar atmosphere, namely the solar corona, is maintained to a temperature of more than 1 MK, about 1000 times the photosphere, is an unsolved problem of paramount importance in solar physics. Observations of impulsive emissions showed that the ones associated with the largest energy scales (from 10²⁶ to 10³² erg) cannot maintain the corona by themselves [1]. Instead, one of the main coronal heating theories [2] suggests that most of the energy is dissipated through a large number of impulsive processes operating at small scales (∼10²⁴ erg). Solar Orbiter carries remote sensing instruments and regularly gets close to the Sun up to 0.29 AU, so it is suitable for investigating the coronal heating at the smaller scales over a long period of time.

On 2020 May 30, during one of its first high cadence observation at 5s, the Extreme Ultraviolet Imager/High Resolution Imager in EUV (174 A) (EUI/HRIEUV), onboard Solar Orbiter, was used to detect small (400 – 4000 km) and short lived (10-100 s) EUV brightenings, which we call “events” hereafter [3]. We define them as small-scale impulsive emissions seen in HRIEUV, that are automatically identified with the algorithm used in [3]. At that time, the HRIEUV field of view (FOV) covered a quiet Sun (QS) region, defined by the lack of large-scale magnetic structures. We studied how these events contributed to coronal heating. In particular, we investigated if coronal temperatures (T > 1 MK) were reached within these events, which is a necessary condition for them to contribute to coronal heating.
In Section 2, we used the same events dataset as in [3]. As these events seem to be present in the QS at all time, they were also detected in later HRIEUV sequences at high cadence. In Section 3, we used three more events datasets in the QS on 2022 March 8, 17 and on 2023 April 4. We found that most of the events’ emission originates from cooler plasma than the corona, referred as plasma at “transition region” (TR) temperatures, from ∼10⁴ K to 1 MK. As such, they are not the direct signature of coronal heating. 

2. Statistical analysis of the thermal behavior of the events [4]

As spectroscopic data were not available during the 2020 May 30 observation, we relied on the EUV multi-channels imager AIA, on board SDO. We measured the time lags between the intensity peaks of different pairs of AIA EUV channels [5]. We used this method to distinguish whether the events' emission is due to plasma at TR (T < 1 MK) or at coronal temperature (T > 1 MK) [4]. Indeed, the response function of most of the AIA channels have their maximum at different coronal temperatures. Taking as an example the AIA 193 A (1.5 MK) – AIA 171 A (0.9 MK) pair, a heating (or cooling) of coronal plasma can produce a positive (negative) time lag between the intensity peaks of the two AIA channels. On the other hand, the emission from TR plasma produces a zero time lag, as the response functions of the AIA channels are similar at these temperatures.

Figure 1. (a) HRIEUV sub-FOV, centered around the event pixels delimited by a white contour. (b) HRIEUV and AIA light curves (dots) extracted over the event pixel 1 (shown in (a)). The solid lines are the estimation of the background/foreground emission along the line of sight.  (c) 2D histograms of the time lags and the associated maximum correlation values for the AIA pair 193 A –171 A. The statistics are performed over all the pixels of the HRIEUV FOV.  The red histogram shows the distribution of the event pixels. The blue contours are the 20, 40, 60 and 80 percentiles of the QS pixels distribution. The green dotted lines are the confidence limits. Adapted from [4]

To obtain statistically significant results, this method was applied to all HRIEUV pixels. The pixels are categorized as either event pixels or Quiet Sun (QS) pixels. Figures 1a and 1b show the extraction of the light curves from the AIA channels and HRIEUV for one of the event pixels. Light curves are then cross-correlated between pairs of AIA channels, to obtain the maximum correlation and its associated time lag. We applied this algorithm for nine AIA pairs, to cover a large temperature range, spanning from 0.5 to 8 MK.

Figure 1c displays the maximum correlation and the time lags distribution, for the AIA pair 193 – 171. The results are compared between the event pixels and the rest of the QS. Events are characterized by highly correlated and short time lags, with a distribution centered around values below the AIA cadence (12s). After rigorous tests of statistical significance and potential bias, we concluded that the results were from the events and not from another source (i.e. QS emission, instrumental noise). The results showed that the events are characterized by short and highly correlated time lags between the light curves of multiple AIA channels. We suggested two possible interpretations: either (1) the events peak below 1 MK, where the AIA response functions behave similarly, or (2) the events do reach temperatures above 1 MK, but the short cooling time scales associated with the 12 s AIA cadence imply that the time lags are not resolved. Thus, spectroscopic observations are required to confirm the temperature of these events, as described in the following Section.

3. Temperature and density spectroscopic diagnostics [6]

We performed spectroscopic temperature and density diagnostics on events detected with HRIEUV [6]. We selected three sequences on 2022 March 8, 17 and on 2023 April 4, which included coordination between EUV imaging (Solar Orbiter/EUI/HRIEUV, SDO/AIA) and EUV spectroscopy (Solar Orbiter/SPICE, Hinode/EIS). During the 2022 March sequences, AIA was running in a special 6s high cadence mode. The objective was to follow as close as possible the cadence of HRIEUV (up to 3s). We started by detecting events in HRIEUV, then identified nine of them in both HRIEUV and SPICE, as well as two of them in HRIEUV and EIS. 

Figure 2. Example of an event detected on 2022 March 17 in HRIEUV (left) and five emission lines measured with SPICE (right). The lines are ordered in increasing temperature of maximal emission, indicated within parenthesis (log T). The center of the SPICE slit FOV at the time of the HRIEUV image is shown as the vertical white dotted line. The red rectangle is the event region. Adapted from [6].

Figure 2 shows an example of an event detected with HRIEUV and identified in SPICE. The event clearly emits in the C III, O IV and the O VI lines (logT <= 5.5). However, we only detect a weak emission in the Ne VIII (logT = 5.8) and no emission at all in the Mg IX (logT = 6.0) lines. Temperature diagnostics (EM LOCI) confirmed that the emission of this event mainly comes from plasma at TR temperatures (T < 1 MK). Similar results were found for all other events identified in HRIEUV and SPICE. At this point, a limitation stems from the fact that SPICE cannot measure strong lines in the QS emitted by plasma at temperatures above 1 MK. Thus, using SPICE alone, coronal temperatures cannot be well constrained. EIS, on the other hand, can measure a large number of coronal lines, effectively accessing a broader range of temperatures.
 

Figure 3. (a) HRIEUV and EIS Fe VIII (logT = 5.7), Fe IX (logT = 5.85) and Fe X (logT= 6.05) images of a 4000 km length EUV brightening on 2023 April 4. The position of the EIS slit at the time of the HRI-EUV image is shown as the vertical white dotted line. The cyan rectangle delimits the selected event region, while the orange and magenta regions are the background regions. (b) Differential Emission Measure (DEM) obtained with the intensity of the EIS lines averaged over the event region (cyan) and the two background regions (orange and magenta). The lines used to estimate the DEM are also indicated. Adapted from [6].

Motivated by these considerations, we performed, for the first time, plasma diagnostics on such small-scale EUV brightenings observed by EIS (Fig. 3a). We estimated the Differential Emission Measure (DEM) of the events and the nearby QS (Fig. 2b). The event DEM is superior to the background one at logT <= 6.0 and similar at logT > 6.0. Thus, the event barely reaches 1 MK, and most of its emission originates from plasma at TR temperatures. The density of this event was also estimated to be around (1.8±1.3)⋅10^10 cm^-3 using the Fe IX 188.49/197.04 lines ratio. This density value is typical of TR plasma. 

4. Conclusions

We constrained the temperature of the events detected with HRIEUV, using multi-channels imaging with AIA and spectroscopy with SPICE and EIS. The results showed that the emission of the events mainly comes from plasma at TR temperatures. As such, the events studied in this work are not the direct signatures of the coronal heating in the QS. They might however be good candidates to explain the heating in the lower and cooler part of the atmosphere.  As for the corona in the QS, the question on how it is maintained at temperatures above 1 MK remains open. 
In this work, we chose to apply a statistical analysis of the thermal behavior on a large number of events (Section 2), followed by more precise spectroscopic diagnostics of temperature and density on a limited number of events (Section 3). A possible follow-up would be to combine both approaches by applying spectroscopic diagnostics to a larger sample of events. This would be useful to determine if some of them do reach coronal temperatures, in what proportion, and to compare the results between different regions of the atmosphere (quiet Sun, active regions, coronal holes). In the long term, the future EUV spectrometer Solar-C/EUVST, expected for 2028, will allow to constrain the plasma from chromospheric to coronal temperatures with high spatial (0.4'') and temporal (up to 1s) resolutions. Using a single instrument (Solar-C/EUVST) instead of two (Solar Orbiter/SPICE and Hinode/EIS) will remove the constraints related to the coordination between different satellites. These constraints include the possible spatial shifts in the FOV pointing or the separation angle between the line of sights.
 

Affiliations

[1] Université Paris–Saclay, CNRS, Institut d’astrophysique spatiale, 91405 Orsay, France

References

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