New Insights About EUV Brightenings In The  Quiet Sun Corona From The Extreme Ultraviolet Imager

(Solar Orbiter nugget #1 by Chris Nelson¹ & the EUI team)

Ever since their detection by Ellerman^[1] in 1917 more than a century ago, localised (sub-Mm in diameter) transient (lifetimes of less than 10 minutes) brightenings have been actively studied by the solar physics community. Although such features were originally identified in active regions, where the strong magnetic fields and plasma velocities produce relatively large events, higher-resolution modern observatories have also been able to detect such events in the quiet Sun. Not only this, but localised brightenings are also observed across much of the electromagnetic spectrum, from visible light sampling the lower solar atmosphere to EUV observations of the corona. See [2] for an introductory review to some events within this extensive family of features.

One of the key early results of Solar Orbiter's^[3] Extreme Ultraviolet Imager (EUI)^[4] was identifying just how prevalent such localised transient brightenings are in the quiet Sun at EUV wavelengths (sampling temperatures of close to 1 MK). Berghmans et al.^[5] used an automated algorithm to detect more than one thousand such events, colloquially referred to as 'campfires', in only a 245 s time-series collected using the 17.4 nm High-Resolution Imager (EUI/HRI) telescope. These EUV brightenings were shown to occur right down to the instrumental spatial resolution of around several hundred km, below what could have been routinely detected by the Solar Dynamics Observatory's Atmospheric Imaging Assembly (SDO/AIA)^[6]. Using the distinct viewing angles between Solar Orbiter and SDO/AIA, Zhukov et al.^[7] were able to show that these coronal events occurred at heights of between 1 Mm and 5 Mm above the photosphere, which is barely above the chromosphere. In the left-hand panel of Figure 1, we plot an image from a different EUI/HRI time-series, of around 34 minutes in length, sampled on 8 March 2022. The blue contours outline pixels which were identified to contain an EUV brightening in this dataset in at least one frame by the algorithm employed in [5], clearly displaying the spatial ubiquity of these features. The four panels on the right plot each cover around 5 Mm by 5 Mm on the horizontal scale, with the blue contours outlining clear examples of extremely small-scale EUV brightenings returned by the algorithm.

Figure 1: (Left panel) The EUI/HRI field-of-view from 8 March 2022. The blue contours outline pixels which contained EUV brightenings in at least one frame during this time-series. See also the movie at the end of this page. (Right panels) Four examples of zoomed in (approximately 5 Mm by 5Mm) regions containing different types of EUV brightenings. Specifically, EUV brightenings that are found within larger structures are plotted in the top row, whilst more isolated examples are plotted in the bottom row.

As with many transient events in the solar atmosphere, magnetic reconnection is a leading hypothesis as to the driver of EUV brightenings. To investigate this, Panesar et al.^[8] examined line-of-sight photospheric magnetic field maps sampled by SDO's Helioseismic and Magnetic Imager (SDO/HMI)^[9] finding that EUV brightenings often occurred co-spatial to bipoles, where high gradients in the local magnetic field can facilitate energetic energy release through reconnection. Kahil et al.^[10] conducted a similar study using higher-resolution data from Solar Orbiter's Polarimetric and Helioseismic Imager (PHI)^[11] finding that, potentially, around 70 % of localised EUV brightenings occurred co-spatial to bipoles. On top of this, both the numerical simulations of Chen et al.^[12] and the magnetic field extrapolations of Barczynski et al.^[13] have provided some evidence that magnetic reconnection high in the solar atmosphere could account for these events. In Figure 2, the evolution of the photospheric line-of-sight magnetic field (positive polarity in white and negative polarity in black) sampled by SDO/HMI co-spatial to two regions of EUV brightenings (identified by the red contours) detected in the dataset from 8 March 2022 are plotted. One of groupings of EUV brightenings is located above a unipolar region (top panels) whilst the other is found co-spatial with an isolated bipole (bottom panels).

Figure 2: Time-series displaying the evolution of the line-of-sight magnetic field (positive polarity in white and negative polarity in black) as inferred by SDO/HMI co-spatial to two regions of EUV brightening identified from the dataset sampled on 8 March 2022. The top row displays EUV brightenings which occur above a seemingly unipolar patch of magnetic field while the bottom row plots an EUV brightening which occurs above a localised bipole.

If EUV brightenings are indeed magnetic reconnection driven, then it is possible (or even likely) that they could occur at the same spatial locations as other localised transient events. One could envisage overlaps in the quiet Sun with, for example, Quiet-Sun Ellerman-like Brightenings^[14] in the photosphere or Explosive Events^[15] in the transition region. Studying SDO/AIA imaging data co-spatial to EUI intensity maps, both Panesar et al.^[8] and Dolliou et al.^[16] identified cooler plasma co-spatial to some EUV brightenings. Preliminary results obtained through analysis of transition region spectra sampled by both the Interface Region Imaging Spectrograph (IRIS)^[17] and Spectral Imaging of the Coronal Environment (SPICE)^[18] have also supported the assertion that lower temperature components may also be present. More detailed statistical analysis of spectra sampled at the location of EUV brightenings will be required in the future to better understand any potential connections.

Despite this progress, it is still unclear what proportion of automatically detected EUV brightenings, if any, are driven by magnetic reconnection. Some EUV brightenings identified automatically could instead, for example, be evidence of plasma cooling through the EUI/HRI temperature response window during instances of catastrophic cooling in coronal loops. EUI data (see [19] for an overview of some data) sampled in coordination with other instruments (such as IRIS and SPICE) will play a vital role in assessing this in the near future. What role, if any, EUV brightenings play in accounting for the big problems in solar physics, namely coronal heating and the driving of the solar wind, should then become clearer.

Affiliations:

1 European Space Agency, ESTEC, Noordwijk, The Netherlands

Acknowledgments:

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).

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Movie for Figure 1