The Solar Ultraviolet Imaging Telescope (SUIT) is an instrument on the Aditya-L1 mission of the Indian Space Research Organization (ISRO) launched on 2 September 2023. SUIT continuously provides near-simultaneous full-disk and region-of-interest images of the Sun, slicing through the photosphere and chromosphere and covering a field of view up to 1.5 solar radii. For this purpose, SUIT uses 11 filters tuned at different wavelengths in the 200 – 400 nm range, including the Mg ii h and k and Ca ii H spectral lines. The observations made by SUIT help us understand the magnetic coupling of the lower and middle solar atmosphere. In addition, for the first time, this allows for the measurements of spatially resolved solar broad-band radiation in the near- and mid-ultraviolet, which will help constrain the variability of the solar ultraviolet irradiance in a wavelength range that is central for the chemistry of ozone and oxygen the Earth’s stratosphere. This paper discusses the details of the instrument and data products.

We compare the size distributions of self-organized criticality (SOC) systems in the solar photosphere and the transition region, using magnetogram data from Helioseismic and Magnetic Imager (HMI) and Interface Region Imaging Spectrograph (IRIS) data. For each dataset we fit a combination of a Gaussian and a power law size distribution function, which yields information on four different physical processes: (i) Gaussian random noise in IRIS data; (ii) spicular events in the plages of the transition region (described by power law size distribution in IRIS data); (iii) salt-and-pepper small-scale magnetic structures (described by the random noise in HMI magnetograms); and (iv) magnetic reconnection processes in flares and nanoflares (described by power law size distributions in HMI data). We find a high correlation (CCC = 0.90) between IRIS and HMI data. Datasets with magnetic flux balance are generally found to match the SOC-predicted power law slope α F = 1.80 (for mean fluxes F ), but exceptions occur due to arbitrary choices of the HMI field-of-view. The matching cases confirm the universality of SOC-inferred flux size distributions, and agree with the results of Parnell et al. (ApJ, 2009, 698, 75–82), α F = 1.85 ± 0.14.

ABSTRACT New instruments and telescopes covering the optical and ultraviolet spectral regions have revealed a range of small-scale dynamic features, many which may be related. For example, the range of spicule-like features hints towards a spectrum of features and not just two types; however, direct observational evidence in terms of tracking spicules across multiple wavelengths is needed in order to provide further insight into the dynamics of the Sun’s outer atmosphere. This paper uses H α data obtained with the CRisp Imaging SpectroPolarimeter instrument on the Swedish 1-m Solar Telescope, and in the transition region using the Interface Region Imaging Spectrograph with the SJI 1400 Å channel plus spectral data via the Si iv 1394 Å line to track spicules termed rapid blueshifted excursions (RBEs). The RBEs as seen in the H α blue wing images presented here can be subdivided into two categories: a single or multithreaded feature. Based on the H α spectra, the features can be divided into events showing broadening and line core absorption, events showing broadening and line core emission, events with a pure blueshifted H α profile without any absorption in the red wing, and broadened line profile with the absorption in the blue stronger compared to the red wing. From the RBE-like events that have a Si iv 1394 Å line profile, 78 per cent of them show a Si iv line flux increase. Most of these features show a second broadened Si iv component that is slightly blueshifted.

We focus here on impulsive phenomena and Quiet-Sun features in the solar transition region, observed with the Interface Region Imaging Spectrograph (IRIS) at 1,400 Å (at formation temperatures of T e ≈ 10 4 –10 6 K). Summarizing additional literature values we find the following fractal dimensions (in increasing order): D A = 1.23 ± 0.09 for photospheric granulation, D A =1.40 ± 0.09 for chromospheric (network) patterns, D A = 1.54 ± 0.04 for plages in the transition region, D A = 1.56 ± 0.08 for extreme ultra-violet (EUV) nanoflares, D A = 1.59 ± 0.20 for active regions in photospheric magnetograms, and D A = 1.76 ± 0.14 for large solar flares. We interpret low values of the fractal dimension (1.0 ≲ D A ≲ 1.5) in terms of sparse curvi-linear flow patterns, while high values of the fractal dimension (1.5 ≲ D A ≲ 2.0) indicate quasi-space-filling transport processes, such as chromospheric evaporation in flares. Phenomena in the solar transition region appear to be consistent with self-organized criticality (SOC) models, based on their fractality and their size distributions of fractal areas A and (radiative) energies E , which show power law slopes of αAobs=2.51±0.21 (with αAtheo=2.33 predicted), and αEobs=2.03±0.18 (with αEtheo=1.80 predicted). This agreement suggests that brightenings detected with IRIS at 1,400 Å reveal the same nonlinear SOC statistics as their coronal counterparts in EUV.

Using the ULTRASPEC instrument mounted on the 2.4-m Thai National Telescope, we observed two large flares, each with a total energy close to 10^34 erg with sub-second cadence. A combination of a wavelet analysis, a Fourier transform plus an empirical mode decomposition, reveals quasi-period pulsations (QPP) which exhibit an apparent doubling of the oscillation period. Both events showed oscillations of a few minutes over a interval of several minutes, and despite the availability of sub-second cadence, there was no evidence of sub-minute oscillations. The doubling of the QPP periods and shorter lifetime of shorter-period QPP modes strongly favour resonant dynamics of magnetohydrodynamic waves in a coronal loop. We estimate loop lengths to be 0.2-0.7 R*, in agreement with a typical length of solar coronal loops. These observations presents rare and compelling evidence for the presence of compact plasma loops in a stellar corona.

ABSTRACT We report on the properties of coronal loop foot-point heating with observations at the highest resolution, from the CRisp Imaging Spectro-Polarimeter located at the Swedish 1-m Solar Telescope and co-aligned NASA Solar Dynamics Observatory observations, of Type II spicules in the chromosphere and their signatures in the extreme ultraviolet (EUV) corona. Here, we address one important issue, as to why there is not always a one-to-one correspondence, between Type II spicules and hot coronal plasma signatures, i.e. beyond TR temperatures. We do not detect any difference in their spectral properties in a quiet Sun region compared to a region dominated by coronal loops. On the other hand, the number density close to the foot-points in the active region is found to be an order of magnitude higher than in the quiet Sun case. A differential emission measure analysis reveals a peak at ∼5 × 105 K of the order of 1022 cm−5 K−1. Using this result as a constraint, we conduct numerical simulations and show that with an energy input of 1.25 × 1024 erg (corresponding to ∼10 RBEs contributing to the burst) we manage to reproduce the observation very closely. However, simulation runs with lower thermal energy input do not reproduce the synthetic AIA 171 Å signatures, indicating that there is a critical number of spicules required in order to account for the AIA 171 Å signatures in the simulation. Furthermore, the higher energy (1.25 × 1024 erg) simulations reproduce catastrophic cooling with a cycle duration of ∼5 h, matching a periodicity we observe in the EUV observations.

ABSTRACT We show signatures of spicules termed rapid blueshifted excursions (RBEs) in the Si iv 1394 Å emission line using a semi-automated detection approach. We use the H α filtergrams obtained by the CRISP imaging spectropolarimeter on the Swedish 1-m Solar Telescope and co-aligned Interface Region Imaging Spectrograph data using the SJI 1400 Å channel to study the spatiotemporal signature of the RBEs in the transition region. The detection of RBEs is carried out using an oriented coronal loop tracing algorithm on H α Dopplergrams at ±35 km s−1. We find that the number of detected features is significantly impacted by the time-varying contrast values of the detection images, which are caused by the changes in the atmospheric seeing conditions. We detect 407 events with lifetime greater than 32 s. This number is further reduced to 168 RBEs based on the H α profile and the proximity of RBEs to the large-scale flow. Of these 168 RBEs, 89 of them display a clear spatiotemporal signature in the SJI 1400 Å channel, indicating that a total of $\sim 53{{\ \rm per\ cent}}$ are observed to have co-spatial signatures between the chromosphere and the transition region.

Coronal holes (CHs) are darker than the quiet Sun (QS) when observed in coronal channels. This study aims to understand the similarities and differences between CHs and QS in the transition region using the Si iv 1394 Å line, recorded by the Interface Region Imaging Spectrograph, by considering the distribution of magnetic field measured by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. We find that Si iv intensities obtained in CHs are lower than those obtained in QS for regions with identical magnetic flux densities. Moreover, the difference in intensities between CHs and QS increases with increasing magnetic flux. For the regions with equal magnetic flux density, QS line profiles are more redshifted than those measured in CHs. Moreover, the blueshifts measured in CHs show an increase with increasing magnetic flux density unlike in the QS. The non-thermal velocities in QS, as well as in CHs, show an increase with increasing magnetic flux. However, no significant difference was observed in QS and CHs, albeit a small deviation at small flux densities. Using these results, we propose a unified model for the heating of the corona in the QS and in CHs and the formation of solar wind.

The resonance lines of Si iv formed at λ 1394 and 1403 Å are the most critical for the diagnostics of the solar transition region in the observations of the Interface Region Imaging Spectrograph (IRIS). Studying the intensity ratios of these lines (1394 Å/1403 Å), which under optically thin conditions is predicted to be two, helps us to diagnose the optical thickness of the plasma being observed. Here we study the evolution of the distribution of intensity ratios in 31 IRIS rasters recorded for four days during the emergence of an active region. We found that during the early phase of the development, the majority of the pixels show intensity ratios smaller than two. However, as the active region evolves, more and more pixels show the ratios closer to two. Besides, there are a substantial number of pixels with ratio values larger than 2. At the evolved stage of the active region, the pixels with ratios smaller than two were located on the periphery, whereas those with values larger than 2 were in the core. However, for quiet Sun regions, the obtained intensity ratios were close to two irrespective of the location on the disk. Our findings suggest that the Si iv lines observed in active regions are affected by the opacity during the early phase of the flux emergence. The results obtained here could have important implications for the modeling of the solar atmosphere, including the initial stage of the emergence of an active region as well as the quiet Sun.

The resonance lines of Si iv formed at lambda 1394 and 1403 A are the most critical for the diagnostics of the solar transition region in the observations of the Interface Region Imaging Spectrograph (IRIS). Studying the intensity ratios of these lines (1394 A/1403 A), which under optically thin conditions is predicted to be two, helps us to diagnose the optical thickness of the plasma being observed. Here we study the evolution of the distribution of intensity ratios in 31 IRIS rasters recorded for four days during the emergence of an active region. We found that during the early phase of the development, the majority of the pixels show intensity ratios smaller than two. However, as the active region evolves, more and more pixels show the ratios closer to two. Besides, there are a substantial number of pixels with ratio values larger than 2. At the evolved stage of the active region, the pixels with ratios smaller than two were located on the periphery, whereas those with values larger than 2 were in the core. However, for quiet Sun regions, the obtained intensity ratios were close to two irrespective of the location on the disk. Our findings suggest that the Si iv lines observed in active regions are affected by the opacity during the early phase of the flux emergence. The results obtained here could have important implications for the modeling of the solar atmosphere, including the initial stage of the emergence of an active region as well as the quiet Sun.

ABSTRACT Numerous studies have analysed inferred power-law distributions between frequency and energy of impulsive events in the outer solar atmosphere in an attempt to understand the predominant energy supply mechanism in the corona. Here, we apply a burst detection algorithm to high-resolution imaging data obtained by the Interface Region Imaging Spectrograph to further investigate the derived power-law index, γ, of bright impulsive events in the transition region. Applying the algorithm with a constant minimum event lifetime (of either 60 s or 110 s) indicated that the target under investigation, such as Plage and Sunspot, has an influence on the observed power-law index. For regions dominated by sunspots, we always find γ < 2; however, for data sets where the target is a plage region, we often find that γ > 2 in the energy range (∼1023, ∼1026) erg. Applying the algorithm with a minimum event lifetime of three time-steps indicated that cadence was another important factor, with the highest cadence data sets returning γ > 2 values. The estimated total radiative power obtained for the observed energy distributions is typically 10–25 per cent of what would be required to sustain the corona indicating that impulsive events in this energy range are not sufficient to solve coronal heating. If we were to extend the power-law distribution down to an energy of 1021 erg, and assume parity between radiative energy release and the deposition of thermal energy, then such bursts could provide 25–50 per cent of the required energy to account for the coronal heating problem.