1. Introduction
The Sun’s atmosphere is a dynamic environment governed almost entirely by the evolution of its magnetic field. The release of stored magnetic energy powers a vast spectrum of phenomena, from massive solar flares and coronal mass ejections down to localized, transient brightenings, also observed in apparently calm quiet-Sun regions (Berghmans et al. Reference Berghmans, Auchère and Long2021).
A fundamental process underlying this activity is magnetic reconnection, where magnetic field lines abruptly reconfigure, converting magnetic energy into heat, kinetic energy, and particle acceleration (Priest & Forbes Reference Priest and Forbes2000). A key driver for reconnection is the continuous emergence of new magnetic flux from the solar interior. Emerging flux regions (EFRs) rise through the photosphere and expand into the upper atmosphere, interacting with the pre-existing magnetic field (van Driel-Gesztelyi & Green Reference van Driel-Gesztelyi and Green2015). This interaction often forces opposite magnetic polarities into close contact, creating conditions that are ripe for reconnection and triggering energetic events across multiple atmospheric layers, usually visible in the ultraviolet (UV) and X-ray spectral ranges.
While large-scale eruptions have significant impacts on space weather, it is the small-scale reconnection events that provide an unparalleled opportunity to study the fundamental physics of this process in detail. Prototypical examples include Ellerman Bombs (EBs) (Ellerman Reference Ellerman1917) and UV bursts (UBs) (Peter et al. Reference Peter, Tian and Curdt2014; Young et al. Reference Young, Tian and Peter2018), which are intense, compact brightenings observed in the lower solar atmosphere at a spatial scale of a few arcseconds on the solar disk. These events are usually accompanied by plasma ejecta taking the form of collimated jets or more diffuse surges. Such phenomena serve as natural laboratories for investigating the fine-structure of reconnection; yet, key questions remain about the specific magnetic topologies and physical mechanisms, such as plasmoid formation or turbulence, that govern them (Hansteen et al. Reference Hansteen, Archontis and Pereira2017, Reference Hansteen, Ortiz and Archontis2019; Rouppe van der Voort et al. Reference Rouppe van der Voort, De Pontieu and Scharmer2017; Nóbrega-Siverio & Moreno-Insertis Reference Nóbrega-Siverio and Moreno-Insertis2022). Understanding the physics of these small-scale energy release events is crucial, as they likely share the same underlying processes that operate in the Sun’s most powerful explosions.
The study of these small-scale solar phenomena greatly benefits from a multi-wavelength approach that combines imaging with spectro-polarimetry (e.g. Murabito et al. Reference Murabito, Ermolli and Guglielmino2025). High-resolution spectro-polarimetric measurements, which are often obtained using ground-based facilities, are essential for diagnosing the underlying magnetic field structure and evolution at the solar surface. They provide crucial information about the magnetic configurations where magnetic reconnection processes that drive energetic events occur. Complementing these data, the Interface Region Imaging Spectrograph (IRIS) satellite (De Pontieu et al. Reference De Pontieu, Title and Lemen2014) offered, during the last decade, an invaluable window into the consequences in the upper atmospheric layers. IRIS grants us high-resolution UV images and spectra that allow for the detailed detection and characterization of surges, jets, and UV bursts in the chromosphere and transition region (De Pontieu et al. Reference De Pontieu, Polito and Hansteen2021). The synergy between these observations, as exemplified by several works (e.g. Guglielmino et al. Reference Guglielmino, Zuccarello and Young2018, Reference Guglielmino, Young and Zuccarello2019; Joshi et al. Reference Joshi, Chandra and Schmieder2020, 2024a, b; Nóbrega-Siverio et al. Reference Nóbrega-Siverio, Guglielmino and Sainz Dalda2021, Reference Nóbrega-Siverio, Cabello and Bose2024), enables us to connect the magnetic topology, including features such as null points, to the subsequent plasma heating and energetic outflows, thereby creating a complete physical picture of the energy release mechanisms.
In this work, we briefly present preliminary results from recent observational campaigns. During these coordinated observations, several episodes of energy release, likely triggered by magnetic reconnection, were observed at small spatial scales.
2. Observations and Data Analysis
The coordinated data presented in this contribution pertain to active regions (ARs) from two different epochs of the solar activity cycle: AR NOAA 12585, which appeared in 2016 during the decay phase of Cycle 24, and ARs NOAA 13392 and NOAA 13394, which emerged on the solar disk in 2023 near the peak of Cycle 25.
Observational campaigns involved ground-based telescopes in the Canary Islands, as well as the Hinode (Kosugi et al. Reference Kosugi, Matsuzaki and Sakao2007) and IRIS satellites, complemented by full-disk observations from the SDO (Pesnell et al. Reference Pesnell, Thompson and Chamberlin2012) satellite.
2.1. SST campaign (2016)
High-resolution spectropolarimetric data for AR NOAA 12585 were obtained in September 2016 at the Swedish 1-m Solar Telescope (SST) (Scharmer et al. Reference Scharmer, Bjelksjo and Korhonen2003) in La Palma. Here, we focus on a dataset acquired during the decay phase of the active region on September 4, taken with the CRISP instrument (Scharmer et al. Reference Scharmer, Narayan and Hillberg2008). This observation covers multiple spectral lines to probe different atmospheric layers (Fig. 1), such as the Fe I 630.2 nm line pair for photospheric magnetic field diagnostics and the Ca II 854.2 nm line for information on chromospheric dynamics and magnetic fields. Data in the H
$\alpha$
656.29 nm line, acquired in spectroscopy-only mode, were also used to study chromospheric structures and activity. Note that, due to poor atmospheric seeing, this dataset comprises only a single temporal frame.
Clockwise from top-left: CRISP observations in the continumm-like, in the blue wing of the H
$\alpha$
line, in the Ca II core, and magnetogram in the Fe I 630.2 nm line. The FoV covers a portion of the AR NOAA 12585, west of the main leading sunspot. The square indicates the common FoV between CRISP and IRIS observations. In the images, solar north is on the top, west to the right.
Figure 1 Long description
Panel A: A grayscale image showing solar observations in the continuum-like wavelength. The axes are labeled Helioprojective Latitude (Solar-Y) and Helioprojective Longitude (Solar-X) with units in arcseconds. Panel B: A grayscale image showing solar observations in the blue wing of the H-alpha line. The axes are labeled Helioprojective Latitude (Solar-Y) and Helioprojective Longitude (Solar-X) with units in arcseconds. Panel C: A grayscale image showing solar observations in the Ca II core wavelength. The axes are labeled Helioprojective Latitude (Solar-Y) and Helioprojective Longitude (Solar-X) with units in arcseconds. Panel D: A grayscale image showing a magnetogram in the Fe I 630.2 nm line. The axes are labeled Helioprojective Latitude (Solar-Y) and Helioprojective Longitude (Solar-X) with units in arcseconds. Each image includes a square indicating the common field of view between CRISP and IRIS observations.
A Milne-Eddington inversion of the Fe I data yielded the magnetic field strength, inclination, and other photospheric parameters. The Ca II data were analyzed using the Weak-Field Approximation, allowing for the first quantitative characterization of the chromospheric magnetic field. We derived the line-of-sight (LOS) velocity fields within the EFR at the chromospheric level from Doppler shifts measured along the H
$\alpha$
spectral line.
IRIS observed the EFR through a sequence of four very large,
$42^{\prime\prime}$
-wide dense 128-step rasters, with a cadence of approximately 35 minutes per raster. Simultaneous slit-jaw images in the 1400 Å and 2796 Å passbands were acquired, encompassing a field-of-view (FoV) of about
$165^{\prime\prime} \times 130^{\prime\prime}$
.
2.2. GREGOR campaign (2023)
Our group was awarded a 10-day observational campaign in August 2023 at the 1.5-meter GREGOR solar telescope (Schmidt et al. Reference Schmidt2012; Kleint et al. Reference Kleint, Berkefeld and Esteves2020), located on Tenerife Island. During the campaign, in coordination with the Hinode and IRIS satellites, two ground-based instruments were operated: the High-resolution Fast Imager (HiFI+) (Denker et al. Reference Denker, Verma and Wiśniewska2023) and the GREGOR Infrared Spectrograph (GRIS) (Collados et al. Reference Collados, López and Páez2012).
During this campaign, a variety of solar targets were observed, including ARs NOAA 13392 and NOAA 13394, as well as several quiet Sun regions.
In particular, on August 5 and 6, HiFI+ took images of AR NOAA 13392 in the G band (430.5 nm) and H
$\alpha$
channels, with a cadence of about 10 s. GRIS scanned an area of
$13.5^{\prime\prime} \times 59.5^{\prime\prime}$
in about 40 minutes, acquiring spectropolarimetric measurements in the 1083 nm spectral range, which encompasses the chromospheric He I 1083 nm triplet and the photospheric Si I 1082.7 nm line. Almost simultaneously, IRIS performed a sequence of dense 16-step rasters, each
$5^{\prime\prime}$
wide, with a raster cadence of approximately 4.5 minutes, capturing slit-jaw images in all the available passbands.
3. Results
The single-frame observation acquired by CRISP captured several small-scale magnetic patches with mixed polarity surrounding the main sunspot of AR NOAA 12585 (see Fig. 1, bottom-left panel). A comparison with SDO/HMI magnetograms indicates that flux emergence occurred from 8:00 UT until 16:00 UT in the area indicated with a solid rectangle in Fig. 1, which corresponds to the common FoV between CRISP and IRIS. At the chromospheric level, observed in the H
$\alpha$
line wing and Ca II 854.2 nm line core (Fig. 1, top-right and bottom-right panels, respectively), an arch filament system (AFS) overarches this EFR. A compact brightening is observed at one of the feet of the AFS, where the positive emerging field meets pre-existing flux patches with negative polarity.
From the IRIS data, we reconstructed spectroheliograms for several lines in the UV range, as shown in Fig. 2. Interestingly, the brightening seen in CRISP observations of the chromospheric H
$\alpha$
and Ca II 854.2 nm lines has a clear counterpart in the transition region, visible both in the Si IV 140.28 nm line and the adjacent, forbidden O IV 140.12 nm line. In these transition region lines, we clearly observe elongated features with enhanced emission that follow the individual bundles of the AFS. In the chromosphere, the Mg II 279.64 nm line reveals a blueshifted, cospatial surge-like structure.
Reconstructed IRIS spectroheliograms in the Si IV 140.28 nm, O IV 140.12 nm, and Mg II k 279.64 nm lines from the third raster scan (15:20-15:55 UT) on September 4. The images show the response of the transition region to the peak brightening at the foot of the AFS and the cospatial surge observed in the chromosphere. The FoV is almost the same as indicated by the solid rectangle in Fig. 1.
The spectroscopic analysis of these IRIS data suggests the presence of bidirectional plasma flows, up to
$150 \,\textrm{km s}^{-1}$
in the brightening region, while downflows of about
$20-30 \,\textrm{km s}^{-1}$
are consistently found along the AFS bundles in the transition region.
Additionally, SDO/AIA filtergrams in all the extreme UV channels show the same compact brightening at the peak. This supports the idea that the observed event – triggered by the reconnection between the emerging flux and the pre-existing magnetic environment – involves all the upper layers of the solar atmosphere, from the chromosphere up to the coronal levels.
The GREGOR and IRIS data for AR NOAA 13392 reveal very similar events. In the moat around the leading sunspot, compact brightening events are seen by HIFI+ in the H
$\alpha$
broadband, while surges elongating to the south-west, cospatial to the brightening, are clearly visible in the H
$\alpha$
narrowband, i.e., line core. Some events are also observed by IRIS, whose spectral features indicate that they involve plasma heated to transition region temperatures and show bidirectional flows consistent with reconnection.
During the coordinated GREGOR campaign, a B-class flare occurred in AR NOAA 13392 on August 5. This event was captured by IRIS, which observed its evolution starting from the peak in slit-jaw images, while the slit captured only a ribbon footpoint. GREGOR experienced poor seeing during the flare peak; however, during the gradual phase, the seeing improved, and some high-resolution images by HIFI+ and spectropolarimetric scans by GRIS were acquired. These data reveal that the area involved by the flaring event is characterized by the presence of an AFS, as shown in Fig. 3.
Image of a portion of AR NOAA 13392 observed by HiFI+ and GRIS at GREGOR telescope at 08:03 UT on August 5, during the gradual phase of the B-class flare. The remnants of the ribbons, with enhanced emission, are clearly visible. Contours indicate the isophotes.
Another small-scale, flare-like brightening event was observed in AR NOAA 13394 on August 8. This event had a response in all SDO/AIA filters. In this case, we are investigating the chromospheric response to the localized brightening, taking advantage of the spectropolarimetric data obtained by GRIS in the He I triplet. A two-component HAZEL inversion (Asensio Ramos et al. Reference Asensio Ramos, Trujillo Bueno and Landi Degl’Innocenti2008), required to reproduce the observed He I line profiles, suggests the presence of a patch with strong redshifts (
$\approx 30 \,\mathrm{km \,s}^{-1}$
) in the lower chromosphere, reminiscent of flare-driven chromospheric condensation.
4. Conclusions
In this work, we present a preliminary analysis of high-resolution, multi-wavelength observations of dynamic events driven by flux emergence, identifying signatures of magnetic reconnection, such as bidirectional plasma flows and significant atmospheric heating. These results illustrate that a synergistic approach, combining ground-based spectropolarimetry with space-based UV/EUV data, is essential for investigating the upper atmospheric response to the evolution of emerging flux systems.
In this perspective, an upgraded version of the IBIS spectropolarimetric imager (Ermolli et al. Reference Ermolli, Cirami and Calderone2020; Viavattene et al. Reference Viavattene, Ermolli and Cirami2022; Ermolli et al. Reference Ermolli, Cirami and Sant2024), developed as part of the preparatory studies for the European Solar Telescope (EST) (Quintero Noda et al. Reference Quintero Noda, Schlichenmaier and Bellot Rubio2022), will soon be installed at the THEMIS telescope at the Teide Observatory in the Canary Islands. This instrument will enable us to obtain spectropolarimetric data in synergy with the SOLAR-C (Shimizu et al. Reference Shimizu, Imada and Kawate2019) and MUSE (De Pontieu et al. Reference De Pontieu, Martínez-Sykora and Testa2020) satellites in the UV and EUV spectral ranges. Taking advantage of their complementary capabilities, these high-resolution coordinated observations are expected to advance our understanding of the interplay between magnetic flux systems and their consequences in the solar atmosphere.
Acknowledgements
S.L. Guglielmino acknowledges support from the National Institute of Astrophysics (INAF) through the grant “Fundamental Research 2023”, no. C63C23001260005. This research was supported by the European Union’s Horizon 2020 research and innovation program under grant agreement no. 824135 (SOLARNET) and by the Italian Space Agency (ASI) under contract with the INAF no. 2021-12-HH.0 and Addendum 2021-12-HH.1-2024 “Missione Solar-C EUVST-Supporto scientifico di Fase B/C/D,” and under contract with the INAF no. 2022-29-HH.0 “Missione MUSE”.
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