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Imaging and spectroscopic observations of extreme-ultraviolet brightenings using EUI and SPICE on board Solar Orbiter

(Solar Orbiter nugget #6 by Z. Huang¹, L. Teriaca¹ , R. Aznar Cuadrado¹ , L. P. Chitta¹ , S. Mandal¹ , H. Peter¹ , U. Schühle¹ , S.K. Solanki¹ , F. Auchère² , D. Berghmans³ , É. Buchlin² , M. Carlsson^{4, 5} , A. Fludra⁶ , T. Fredvik^{4, 5} , A. Giunta⁶ , T. Grundy⁶ , D. Hassler⁷ , S. Parenti² , and F. Plaschke⁸  and the SPICE team)

Introduction

The smallest coronal transient extreme-ultraviolet (EUV) brightening events that were observed so far, so-called campfires, were discovered by Berghmans et al.^[1] in 2021 using data from the High Resolution Extreme Ultraviolet telescope (HRI_EUV), which is part of the Extreme Ultraviolet Imager^[2] (EUI) on board the ESA and NASA Solar Orbiter mission^[3]. The high spatial and temporal resolution of HRI_EUV enables us to study the small EUV brightenings (hereafter EUI brightenings) that are barely detected in other observations.

The discovery of EUI brightenings extends the boundary of EUV brightening event sizes to a smaller scale and helps to identify their potential role in coronal heating. To understand that, temperature is an important property and spectral data are necessary to study the temperature evolution of solar events. A first successful attempt at obtaining suitable simultaneous observations of the same region on the Sun with the instruments HRI_EUV and SPectral Imaging of the Coronal Environment ^[4,5] (SPICE) occurred in 2021, during the cruise phase of Solar Orbiter. These observations provided the opportunity to investigate EUI brightenings, especially their thermal properties using both imaging and spectral data.

Combination of EUI and SPICE data

For the purpose of studying their temperature properties, We selected EUI brightening events that can be identified in HRI_EUV data and are also covered by the slit of SPICE. In this study, we focused on three cases from two different parts of the spacecraft orbit. The first dataset was obtained on 23 February 2021 and the other datasets were obtained on 12 and 13 September 2021. Information about the datasets is included in Table 1. In dataset 1, SPICE took both context and small high-cadence rasters. The two context rasters are obtained by stepping the slit in 192 steps, while for each small raster, the slit scanned over three spatial positions. In data set 2, SPICE observed in sit-and-stare mode.

Table 1. Overview of the observations.

* All times refer to the starting times of the observations

Figure 1 shows overviews of the HRI_EUV field of view that is covered in these two data sets. To accurately align the data from the two instruments, we applied a two-step approach and made use of time-series data. After alignment, we determined the positions of the EUI brightenings. Since the SPICE slit covered just a narrow region, only three EUI brightenings were identified in the HRI_EUV and SPICE data. The white boxes in Fig. 1 mark the region also covered by the SPICE slit and the blue arrows point to the positions of EUI brightenings. SPICE spectra are dominated by the instrumental profile, which can be reasonably fitted by a Gaussian function. We used this to retrieve the integrated line radiance.

Fig.1 HRI_EUV images of the solar corona taken from the analyzed data sets. (a) Image obtained on 23 February 2021 at 17:17:28 UT (data set 1). The region that is also covered by SPICE slit (3-step) is marked by the white box. (b) Image obtained on 12 September 2021 at 22:06:10 UT (data set 2). The region also covered by the SPICE slit (sit and stare) is marked by the white box. (c) Image obtained on 13 September 2021 at 00:21:42 UT (data set 2). The region also covered by the SPICE slit (sit and stare) is marked by the white box. Blue arrows in all panels point to the positions of EUI brightenings.

Temperature properties of EUI brightenings

EUI brightening E-1 (see Fig. 1a) is recognized in dataset 1. E-1 is very small, with an area of about 2.2 Mm² and only survives for about 50s. Thus, the detection of E-1 is at the limit of the SPICE capabilities. It could not have been independently identified without the aid of HRI_EUV observations. In the spectral data (Ne VIII, CIII, Ly/beta and O VI) from SPICE, only the O VI spectra show an increase around the time when the EUI brightening appears. No obvious clue of the EUI brightening is found in Ne VIII, which might indicate that this EUI brightening has a sufficient emission measure (to be detected by SPICE) only at temperatures close to the typical temperature of O VI (0.3 MK) and none, or very little, at coronal temperatures.

In dataset 2, two EUI brightenings (E-2 and E-3, see Fig. 1b and 1c) occur at the position of the SPICE slit. Compared with E-1, these brightenings are larger. They also last longer (i.e., 6 min for E-2 and 4 min for E-3, instead of less than a minute for E-1) and were observed by SPICE at higher time cadence (i.e., 10.2 s instead of 20 s) and with a longer exposure time (i.e., 10 s instead of 4.7 s). From the integrated line radiances of lines provided by SPICE (N IV, Ne VIII, S V, O IV, C III, Ly/beta and O VI), these two EUI brightenings are observed up to Ne VIII temperatures (0.6 MK). They are also detectable in O VI (0.3 MK) and in some other transition region (TR) lines.

As shown in Fig. 2a, E-3 is a loop-like event south of a larger brightening. In E-3, the EUI brightening emission in some TR lines shows two peaks around the peak time of the HRI_EUV data and of Ne VIII. This can be seen in greater detail in Fig. 2b, where the temporal change of the C III, O VI and Ne VIII intensities are shown. The uncertainties of SPICE data are estimated by square-summing the photon noise component to the readout noise and to the dark noise. The different peak times may indicate the thermal evolution of E-3, that is, the temperature of this EUI brightening increases from the typical emission temperature of C III (log T[/K] = 4.8) to that of O VI (log T[/K] = 5.5) and then continues to rise to Ne VIII (log T[/K] = 5.8), which is the temperature of the lower corona.
After the Ne VIII peak time, the EUI brightening starts to cool down to TR temperatures, which explains the appearance of the second emission peak in O VI and C III lines.

Fig.2 EUI brightening (E-3). Panel (a): HRI_EUV image taken on 13 September 2021 at 00:21:40 UT, at about the time when the intensity of the EUI brightening peaked. The smaller dashed box shows the region covered by one SPICE-binned pixel (4′′ × 4.392′′). The C III (green), O VI (blue) and Ne VIII (orange) intensities (normalized) at this pixel in panel (b) is calculated from Gaussian fits to the line in the SPICE data. Their peaks are marked by arrows in different colors. The error bars were calculated from the uncertainties of the fitting parameters. This region was also used to calculate the HRI_EUV intensity (dashed line) in panel (b). The larger solid box shows the region we used to compute the light curve (solid line) shown in panel (b). The horizontal bar in the lower left corner shows the SPICE exposure duration, which is about 10 s.

Conclusion

Spectral data from SPICE allow us to follow the thermal properties of EUI brightenings. Our results indicate that at least some EUI brightenings barely reach coronal temperatures.

This work is now accepted in Astronomy and Astrophysics, forthcoming articles.

DOI: https://doi.org/10.1051/0004-6361/202345988

Affiliations

1 Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany

2 Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405, Orsay, France

3 Solar-Terrestrial Centre of Excellence – SIDC, Royal Observatory of Belgium, Ringlaan -3- Av. Circulaire, 1180 Brussels, Belgium

4 Rosseland Centre for Solar Physics, University of Oslo, P.O. Box 1029 Blindern, NO-0315 Oslo, Norway

5 Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029 Blindern, NO-0315 Oslo, Norway

6 RAL Space, UKRI STFC, Rutherford Appleton Laboratory, Didcot, UK

7 Southwest Research Institute, Boulder, CO, USA

8 Institut für Geophysik und extraterrestrische Physik, Technische Universität Braunschweig, Mendelssohnstrasse 3, 38106 Braunschweig, Germany

Acknowledgements

Solar Orbiter is a space mission of international collaboration between ESA and NASA, operated by ESA. The development of SPICE has been funded by ESA member states and ESA. It was built and is operated by a multi-national consortium of research institutes supported by their respective funding agencies: STFC RAL (UKSA, hardware lead), IAS (CNES, operations lead), GSFC (NASA), MPS (DLR), PMOD/WRC (Swiss Space Office), SwRI (NASA), UiO (Norwegian Space Agency). The EUI instrument was built by CSL, IAS, MPS, MSSL/UCL, PMOD/WRC, ROB, LCF/IO with funding from the Belgian Federal Science Policy Office (BELSPO/PRODEX PEA C4000134088); the Centre National d’Etudes Spatiales (CNES); the UK Space Agency (UKSA); the Bundesministerium für Wirtschaft und Energie (BMWi) through the Deutsches Zentrum für Luft- und Raumfahrt (DLR); and the Swiss Space Office (SSO). L.P.C. gratefully acknowledges funding by the European Union (ERC, ORIGIN, 101039844). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them. S.P. acknowledges the funding by CNES through the MEDOC data and operations center.

References

[1] Berghmans, D., Auchère, F., Long, D. M., et al. 2021, A&A, 656, L4

[2] Rochus, P., Auchère, F., Berghmans, D., et al. 2020, A&A, 642, A8

[3] Müller, D., St. Cyr, O. C., Zouganelis, I., et al. 2020, A&A, 642, A1

[4] Spice Consortium, Anderson, M., Appourchaux, T., et al. 2020, A&A, 642, A14

[5] Fludra, A., Caldwell, M., Giunta, A., et al. 2021, A&A, 656, A38

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