pdf/small-scale-dynamos-from-idealized-models-to-solar-and-nj4ljkuo.pdf

Small-Scale Dynamos: From Idealized Models to Solar and Stellar Applications

Context. Accurate detections of frequent small-scale extreme ultraviolet (EUV) brightenings are essential to the investigation of the physical processes heating the corona. Aims. We detected small-scale brightenings, termed campﬁres, using their morphological and intensity structures as observed in coronal EUV imaging observations for statistical analysis. Methods. We applied a method based on Zernike moments and a support vector machine (SVM) classiﬁer to automatically identify and track campﬁres observed by Solar Orbiter / Extreme Ultraviolet Imager (EUI) and Solar Dynamics Observatory (SDO) / Atmospheric Imaging Assembly (AIA). Results. This method detected 8678 campﬁres (with length scales between 400 km and 4000 km) from a sequence of 50 High Resolution EUV telescope (HRI EUV ) 174 Å images. From 21 near co-temporal AIA images covering the same ﬁeld of view as EUI, we found 1131 campﬁres, 58% of which were also detected in HRI EUV images. In contrast, about 16% of campﬁres recognized in HRI EUV were detected by AIA. We obtain a campﬁre birthrate of 2 × 10 − 16 m − 2 s − 1 . About 40% of campﬁres show a duration longer than 5 s, having been observed in at least two HRI EUV images. We ﬁnd that 27% of campﬁres were found in coronal bright points and the remaining 73% have occurred out of coronal bright points. We detected 23 EUI campﬁres with a duration greater than 245 s. We found that about 80% of campﬁres are formed at supergranular boundaries, and the features with the highest total intensities are generated at network junctions and intense H I Lyman- α emission regions observed by EUI / HRI Lya . The probability distribution functions for the total intensity, peak intensity, and projected area of campﬁres follow a power law behavior with absolute indices between 2 and 3. This self-similar behavior is a possible signature of self-organization, or even self-organized criticality, in the campﬁre formation process.

pdf/automatic-detection-of-small-scale-euv-brightenings-observed-38mlvc10.pdf

Automatic detection of small-scale EUV brightenings observed by the Solar Orbiter/EUI

Context: On 2020 May 30, small and short-lived EUV brightenings were observed in the Quiet Sun (QS) during a four minutes sequence by EUI/HRIEUV on board Solar Orbiter. Their physical origin and possible impact on coronal or Transition Region (TR) heating are still to be determined. Aims: Our aim is to derive the statistical thermal evolution of these events in order to establish their coronal or TR origin. Methods. Our thermal analysis takes advantage of the multithermal sensitivity of the Atmospheric Imaging Assembly (AIA) imager on board the Solar Dynamics Observatory (SDO). We first identified these HRIEUV events in the six coronal bands of AIA. We then performed a statistical time lag analysis, which quantifies the delays between the light curves from different bands. These time lags can give significant insights into the temperature evolution of these events. The analysis is performed taking into account the possible contribution to the results from the background and foreground emissions. Results: The events are characterized by time lags inferior to the AIA cadence of 12 s, for all nine couples of AIA bands analyzed. Our interpretation is the possible co-presence of events which reach or do not reach coronal temperatures ($\approx$ 1MK). We believe that the cool population dominates the events analyzed in this work.

/pdf/temperature-of-solar-orbiter-eui-quiet-sun-small-scale-n8w2n8gc.pdf

Temperature of Solar Orbiter/EUI quiet-Sun small-scale brightenings: Evidence for a cooler component

In this paper, we propose that flux cancellation on small granular scales (≲1000 km) ubiquitously drives reconnection at a multitude of sites in the low solar atmosphere, contributing to chromospheric/coronal heating and the generation of the solar wind. We analyze the energy conversion in these small-scale flux cancellation events using both analytical models and three-dimensional, resistive magnetohydrodynamic (MHD) simulations. The analytical models—in combination with the latest estimates of flux cancellation rates—allow us to estimate the energy release rates due to cancellation events, which are found to be on the order 106–107 erg cm−2 s−1, sufficient to heat the chromosphere and corona of the quiet Sun and active regions, and to power the solar wind. The MHD simulations confirm the conversion of energy in reconnecting current sheets, in a geometry representing a small-scale bipole being advected toward an intergranular lane. A ribbon-like jet of heated plasma that is accelerated upward could also escape the Sun as the solar wind in an open-field configuration. We conclude that through two phases of atmospheric energy release—precancellation and cancellation—the cancellation of photospheric magnetic flux fragments and the associated magnetic reconnection may provide a substantial energy and mass flux contribution to coronal heating and solar wind generation.

Coronal Heating and Solar Wind Generation by Flux Cancellation Reconnection

Context. The smallest extreme-ultraviolet (EUV) brightening events that were detected so far, called campfires, have recently been uncovered by the High Resolution EUV telescope (HRI EUV ), which is part of the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter. HRI EUV has a broad bandpass centered at 17.4 nm that is dominated by Fe IX and Fe X emission at about 1 MK. Aims. We study the thermal properties of EUI brightening events by simultaneously observing their responses at different wavelengths using spectral data from the Spectral Imaging of the Coronal Environment (SPICE) also on board Solar Orbiter and imaging data from EUI. Methods. We studied three EUI brightenings that were identified in HRI EUV data that lie within the small areas covered by the slit of the SPICE EUV spectrometer. We obtained the line intensities of the spectral profiles by Gaussian fitting. These diagnostics were used to study the evolution of the EUI brightenings over time at the different line-formation temperatures. Results. We find that (i) the detection of these EUI brightenings is at the limit of the SPICE capabilities. They could not have been independently identified in the data without the aid of HRI EUV observations. (ii) Two of these EUI brightenings with longer lifetimes are observed up to Ne VIII temperatures (0.6 MK). (iii) All of the events are detectable in O VI (0.3 MK), and the two longer-lived events are also detected in other transition region (TR) lines. (iv) In one case, we observe two peaks in the intensity light curve of the TR lines that are separated by 2.7 min for C III and 1.2 min for O VI . The Ne VIII intensity shows a single peak between the two peak times of the TR line intensity. Conclusions. 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. 

/pdf/imaging-and-spectroscopic-observations-of-extreme-19rmoysb.pdf

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

Context. The Extreme Ultraviolet Imager (EUI) on board the Solar Orbiter (SO) spacecraft observed small extreme ultraviolet (EUV) bursts, termed campﬁres, that have been proposed to be brightenings near the apexes of low-lying loops in the quiet-Sun atmosphere. The underlying magnetic processes driving these campﬁres are not understood. Aims. During the cruise phase of SO and at a distance of 0.523AU from the Sun, the Polarimetric and Helioseismic Imager on Solar Orbiter (SO / PHI) observed a quiet-Sun region jointly with SO / EUI, o ﬀ ering the possibility to investigate the surface magnetic ﬁeld dynamics underlying campﬁres at a spatial resolution of about 380km. Methods. We used co-spatial and co-temporal data of the quiet-Sun network at disc centre acquired with the High Resolution Imager of SO / EUI at 17.4nm (HRI EUV , cadence 2s) and the High Resolution Telescope of SO / PHI at 617.3nm (HRT, cadence 2.5min). Campﬁres that are within the SO / PHI − SO / EUI common ﬁeld of view were isolated and categorised according to the underlying magnetic activity. Results. In 71% of the 38 isolated events, campﬁres are conﬁned between bipolar magnetic features, which seem to exhibit signatures of magnetic ﬂux cancellation. The ﬂux cancellation occurs either between the two main footpoints, or between one of the footpoints of the loop housing the campﬁre and a nearby opposite polarity patch. In one particularly clear-cut case, we detected the emergence of a small-scale magnetic loop in the internetwork followed soon afterwards by a campﬁre brightening adjacent to the location of the linear polarisation signal in the photosphere, that is to say near where the apex of the emerging loop lays. The rest of the events were observed over small scattered magnetic features, which could not be identiﬁed as magnetic footpoints of the campﬁre hosting loops. Conclusions. The majority of campﬁres could be driven by magnetic reconnection triggered at the footpoints, similar to the physical processes occurring in the burst-like EUV events discussed in the literature. About a quarter of all analysed campﬁres, however, are not associated to such magnetic activity in the photosphere, which implies that other heating mechanisms are energising these small-scale EUV brightenings.

/pdf/the-magnetic-drivers-of-campfires-seen-by-the-polarimetric-1nkjw8yb.pdf

The magnetic drivers of campfires seen by the Polarimetric and Helioseismic Imager (PHI) on Solar Orbiter

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The magnetic drivers of campfires seen by the Polarimetric and Helioseismic Imager (PHI) on Solar Orbiter