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 0.523 AU from the Sun, the Polarimetric and Helioseismic Imager on Solar Orbiter (SO/PHI) observed a quiet-Sun region jointly with SO/EUI, offering the possibility to investigate the surface magnetic field dynamics underlying campfires at a spatial resolution of about 380 km. 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 17.4 nm (HRI_EUV, cadence 2 s) and the High Resolution Telescope of SO/PHI at 617.3 nm (HRT, cadence 2.5 min). 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.

In July 2024, Sunrise completed its third successful science flight. The Sunrise iii observatory had been upgraded significantly after the two previous successful flights in 2009 and 2013, to tackle the most recent science challenges concerning the solar atmosphere. Three completely new instruments focus on the small-scale physical processes and their complex interaction from the deepest observable layers in the photosphere up to chromospheric heights. Previously poorly explored spectral regions and lines are exploited to paint a three-dimensional picture of the solar atmosphere with unprecedented completeness and level of detail. The full polarimetric information is captured by all three instruments to reveal the interaction between the magnetic fields and the hydrodynamic processes. Two slit-based spectropolarimeters, the Sunrise UV Spectropolarimeter and Imager (SUSI) and the Sunrise Chromospheric Infrared spectro-Polarimeter (SCIP), focus on the near-ultraviolet (309 – 417 nm) and the near-infrared (765 – 855 nm) regions respectively, and the imaging spectropolarimeter Tunable Magnetograph (TuMag) simultaneously obtains maps of the full field-of-view of $46\times 46$ 46 × 46 Mm² in the photosphere and the chromosphere in the visible (525 and 517 nm). The instruments are operated in an orchestrated mode, benefiting from a new Image Stabilization and Light Distribution unit (ISLiD), with the Correlating Wavefront Sensor (CWS) providing the autofocus control and an image stability with a root-mean-square value smaller than 0.005”. A new gondola was constructed to significantly improve the telescope pointing stability, required to achieve uninterrupted observations over many hours. Sunrise iii was launched successfully on 10 July 2024, from the Esrange Space Center of the Swedish Space Corporation near Kiruna (Sweden). It reached the landing site between the Mackenzie River and the Great Bear Lake in Canada after a flight duration of 6.5 days. In this paper, we give an overview of the Sunrise iii observatory and its instruments.

S unrise iii is a stratospheric balloon-borne solar observatory with a 1 m diameter telescope and three postfocus instruments, along with an image stabilisation system, all within a protective gondola. It samples the lower solar atmosphere, from the solar surface to the middle chromosphere, at a resolution approaching 50 km on the Sun. S unrise iii flew successfully for 6.5 days suspended from a zero-pressure stratospheric balloon from northern Sweden to northwestern Canada in 2024 July, gathering around 200 TB of data. The present issue of the Astrophysical Journal Letters focuses on the first scientific results from the data collected during that flight. This Letter introduces this Focus Issue, providing a very brief overview of the capabilities of the instrumentation, the flight, and of the gathered data. Challenges for the measurements, data reduction, and interpretation are also briefly touched upon. The Letter ends with an overview of the first set of science results obtained from these data, as presented in the current Focus Issue.

The Sunrise balloon-borne solar observatory is equipped with a one-meter aperture optical telescope, offering a unique platform for uninterrupted seeing-free observations across ultraviolet, visible, and infrared wavelengths from altitudes higher than 33 km. For the third flight of the upgraded Sunrise observatory conducted in 2024, now called Sunrise iii , a new spectro-polarimeter called the Sunrise Chromospheric Infrared spectroPolarimeter (SCIP) was developed for observing near-infrared wavelength ranges around 770 nm and 850 nm. These wavelength ranges contain many spectral lines, including two of the Ca ii infrared triplet, K i D1 and D2 lines, and multiple Fe i lines, that are sensitive to solar magnetic fields and velocities in the photosphere and chromosphere. SCIP consists of a grating spectrograph in which polarimetric measurements are conducted using a rotating waveplate as a modulator and polarizing beam splitters placed in front of the cameras. The spatial and spectral resolutions are $0.21''$ 0.21 ″ and $1\times10^{5}$ 1 × 10 5 , respectively, and a polarimetric sensitivity of 0.03% (1 $\sigma$ σ ) of the continuum intensity is achieved with a 10 s integration time per a resolution element. To achieve high-precision detection of small polarization signals, we carefully designed the optical and mechanical systems, polarization components, control electronics, and onboard data processing. Together with the other post-focus instrumentation developed for Sunrise iii , the Sunrise Ultraviolet Spectropolarimeter and Imager (SUSI) and the Tunable Magnetograph (TuMag) visible imaging spectro-polarimeter, SCIP provides novel observations that help elucidate energy transfer and time-dependent phenomena across the solar photosphere and chromosphere.

Sunrise iii is a balloon-borne solar observatory dedicated to the investigation of the processes governing the physics of the magnetic field and the plasma flows in the lower solar atmosphere. The gondola hosts a 1-m aperture telescope that feeds three post-focus instruments. One of these instruments, the Tunable Magnetograph (TuMag), is a tunable imaging spectropolarimeter in visible wavelengths. It is designed to probe the vector magnetic field, $\bf{B}$ , and the line-of-sight (LoS) velocity, $v_{\mathrm{LoS}}$ v LoS , of the photosphere and the lower chromosphere. It provides polarized images with a $63^{\prime \prime }\times \, 63^{\prime \prime }$ 63 ″ × 63 ″ field of view (FoV) of the Sun in four polarization states. These images can later be processed on ground to retrieve maps of the aforementioned solar physical quantities. The quasi-simultaneous observation of two spectral lines sensitive to $\bf{B}$ and $v_{\mathrm{LoS}}$ v LoS in the photosphere and lower chromosphere provides excellent diagnostic measurements of the magnetic and dynamic coupling in these layers. When combined with the other two instruments on board, observing in the infrared and ultraviolet regions of the spectrum, TuMag’s diagnostic potential is expected to be greatly enhanced. Building upon heritage of instruments like IMaX and SO/PHI, the key technologies employed for TuMag are a liquid-crystal-variable-retarder-based polarimeter and a solid, LiNbO ₃ Fabry–Pérot etalon as a spectrometer. However, it also incorporates several innovative features, such as in-house-made, high-sensitivity scientific cameras and a double filter wheel. The latter makes TuMag the first balloon-borne instrument of its type capable of tuning between spectral lines. Specifically, it can sequentially observe any two out of the three spectral lines of Fe i at 525.02 and 525.06 nm and of Mg i at 517.3 nm. Time cadences range from 30 to 100 seconds, depending on the observing mode and the specific pair of spectral lines targeted. Laboratory measurements have demonstrated good image quality, spectral resolution, and polarimetric efficiency. Here we report on the concept, design, calibration, and integration phases of the instrument, as well as on the data reduction pipeline.

Context. Earth-side observations of solar p modes can be used to image and monitor magnetic activity on the Sun’s far side. In this work, we use magnetograms of the far side obtained by the Polarimetric and Helioseismic Imager (PHI) on board Solar Orbiter (SO) to directly assess the validity of far-side helioseismic holography for the first time. Aims. We wish to co-locate the positions of active regions in helioseismic images and magnetograms and to calibrate the helioseismic measurements in terms of the magnetic field strength. Methods. We identified three magnetograms displaying a total of six active regions on the far side from 18 November 2020, 3 October 2021, and 3 February 2022. The first two dates are from the SO cruise phase and the third is from the beginning of the nominal operation phase. We computed contemporaneous seismic phase maps for these three dates using helioseismic holography applied to the time series of Dopplergrams from the Helioseismic and Magnetic Imager (HMI) at the Solar Dynamics Observatory (SDO). Results. Among the six active regions seen in SO/PHI magnetograms, five of them are identified on the seismic maps at almost the same positions as on the magnetograms. One region is too weak to be detected above the seismic noise. To calibrate the seismic maps, we fit a linear relationship between the seismic phase shifts and the unsigned line-of-sight magnetic field averaged over the active region areas extracted from the SO/PHI magnetograms. Conclusions. SO/PHI provides the strongest evidence so far that helioseismic imaging is able to provide reliable information on active regions on the far side, including their positions, areas, and the mean unsigned magnetic field.