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The Solar Ultraviolet Imaging Telescope (SUIT) is one of the payloads onboard the Aditya-L1 mission. It will perform full disk imaging of the Sun in the near-Ultraviolet wavelength range of 200-400 nm. This provides near-simultaneous observations of the Sun from the photosphere and chromosphere. A back-illuminated, enhanced UV Charge Coupled Device (CCD) of size 4096 (H) × 4136 (V) pixels, with a pixel size of 12 μm, is used as an imaging element in SUIT. The CCD characterization and the readout electronics development and testing were performed in-house at the Space Astronomy Group (SAG), UR Rao Satellite centre, ISRO. The test setup and procedures are explained and the measured values of various parameters including noise, dark current, gain, linearity, and cross-talk are presented in this paper. The results show a satisfactory performance from the CCD as well as the readout electronics to meet the specifications required by the SUIT payload.

We derive the non-thermal velocities (NTVs) in the transition region of an active region using the \ion{Si}{4}~1393.78~{\AA} line observed by the Interface Region Imaging Spectrograph (IRIS) and compare them with the line-of-sight photospheric magnetic fields obtained by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). The active region consists of two strong field regions with opposite polarity, separated by a weak field corridor, that widened as the active region evolved. The means of the NTV distributions in strong-field regions (weak field corridors) range between $\sim$18{--}20 (16{--}18)~km~s$^{-1}$, albeit the NTV maps show much larger range. In addition, we identify a narrow lane in the middle of the corridor with significantly reduced NTV. The NTVs do not show a strong center-to-limb variation, albeit somewhat larger values near the disk center. The NTVs are well correlated with redshifts as well as line intensities. The results obtained here and those presented in our companion paper on Doppler shifts suggest two populations of plasma in the active region emitting in \ion{Si}{4}. The first population exists in the strong field regions and extends partway into the weak field corridor between them. We attribute this plasma to spicules heated to $\sim$0.1 MK (often called type II spicules). They have a range of inclinations relative to vertical. The second population exists in the center of the corridor, is relatively faint, and has smaller velocities, likely horizontal. These results provide further insights into the heating of the transition region., Comment: 23 pages, 8 figures, accepted for publication in The Astrophysical Journal

To study the dynamical evolution of trans-equatorial loops (TELs) using imaging and spectroscopy. We have used the images recorded by the Atmospheric Imaging Assembly and the Helioseismic Magnetic Imager on-board the Solar Dynamics Observatory and spectroscopic observations taken from the Extreme-Ultraviolet Imaging Spectrometer on-board Hinode. The data from AIA 193 {\AA} channel show that TELs are formed between AR 12230 and a newly emerging AR 12234 and evolved during December 10-14, 2014. The xt-plots for December 12, 2014 obtained using AIA 193 {\AA} data reveal signatures of inflow and outflow towards an X-region. High cadence AIA images also show recurrent intensity enhancements in close proximity to the X-region (P2), which is observed to have higher intensities for spectral lines formed at log T[K] =6.20 and voids at other higher temperatures. The electron densities and temperatures in the X-region (and P2) are maintained steadily at log N_e =8.5-8.7 /cc and log T[K] =6.20, respectively. Doppler velocities in the X-region show predominant redshifts by about 5-8 km/s when closer to the disk centre but blueshifts (along with some zero-velocity pixels) when away from the centre. The Full-Width-Half-Maxima (FWHM) maps reveal non-thermal velocities of about 27-30 km/s for Fe XII, Fe XIII and Fe XV lines. However, the brightest pixels have non-thermal velocities of about 62 km/s for Fe XII and Fe XIII lines. On the contrary, the dark X-region for Fe XV line have the highest non-thermal velocity (roughly 115 km/s). We conclude that the TELs are formed due to magnetic reconnection. We further note that the TELs themselves undergo magnetic reconnection leading to reformation of loops of individual ARs. Moreover, this study, for the first time, provides measurements of plasma parameters in X-regions thereby providing essential constraints for theoretical studies., Comment: Accepted by Astronomy and Astrophysics (May 26, 2020) 17 pages, 15 figures

A comprehensive understanding of the structure of Doppler motions in transition region including the center-to-limb variation and its relationship with the magnetic field structure is vital for the understanding of mass and energy transfer in the solar atmosphere. In this paper, we have performed such a study in an active region using the Si IV 1394~{\AA} emission line recorded by the Interface Region Imaging Spectrograph (IRIS) and the line-of-sight photospheric magnetic field obtained by the Helioseismic and Magnetic Imager (HMI) on-board the Solar Dynamics Observatory (SDO). The active region has two opposite polarity strong field regions separated by a weak field corridor, which widened as the active region evolved. On average the strong field regions (corridor) show(s) redshifts of 5{--}10 (3{--}9)~km~s$^{-1}$ (depending on the date of observation). There is, however, a narrow lane in the middle of the corridor with near-zero Doppler shifts at all disk positions, suggesting that any flows there are very slow. The Doppler velocity distributions in the corridor seem to have two components---a low velocity component centered near 0 km/s and a high velocity component centered near 10~km~s$^{-1}$. The high velocity component is similar to the velocity distributions in the strong field regions, which have just one component. Both exhibit a small center-to limb variation and seem to come from the same population of flows. To explain these results, we suggest that the emission from the lower transition region comes primarily from warm type II spicules, and we introduce the idea of a `chromospheric wall'---associated with classical cold spicules---to account for a diminished center-to-limb variation., Comment: 15 pages, 16 figures

A comprehensive study of the physical parameters of active region fan loops is presented using the observations recorded with the Interface Region Imaging Spectrometer (IRIS), the EUV Imaging Spectrometer (EIS) on-board Hinode and the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager (HMI) on-board the Solar Dynamics Observatory (SDO). The fan loops emerging from non-flaring AR~11899 (near the disk-center) on 19th November, 2013 are clearly discernible in AIA 171~{\AA} images and those obtained in \ion{Fe}{8} and \ion{Si}{7} images using EIS. Our measurements of electron densities reveal that the footpoints of these loops are approximately at constant pressure with electron densities of $\log\,N_{e}=$ 10.1 cm$^{-3}$ at $\log\,[T/K]=5.15$ (\ion{O}{4}), and $\log\,N_{e}=$ 8.9 cm$^{-3}$ at $\log\,[T/K]=6.15$ (\ion{Si}{10}). The electron temperature diagnosed across the fan loops by means of EM-Loci suggest that at the footpoints, there are two temperature components at $\log\,[T/K]=4.95$ and 5.95, which are picked-up by IRIS lines and EIS lines respectively. At higher heights, the loops are nearly isothermal at $\log\,[T/K]=5.95$, that remained constant along the loop. The measurement of Doppler shift using IRIS lines suggests that the plasma at the footpoints of these loops is predominantly redshifted by 2-3~km~s$^{-1}$ in \ion{C}{2}, 10-15~km~s$^{-1}$ in \ion{Si}{4} and $~$15{--}20~km~s$^{-1}$ in \ion{O}{4}, reflecting the increase in the speed of downflows with increasing temperature from $\log\,[T/K]=4.40$ to 5.15. These observations can be explained by low frequency nanoflares or impulsive heating, and provide further important constraints on the modeling of the dynamics of fan loops., Comment: Accepted for publication in The Astrophysical Journal; 8 Figures, 11 pages

The Solar Ultraviolet Imaging Telescope (SUIT) is an instrument onboard the Aditya-L1 spacecraft, the first dedicated solar mission of the Indian Space Research Organization (ISRO), which will be put in a halo orbit at the Sun-Earth Langrage point (L1). SUIT has an off-axis Ritchey–Chretien configuration with a combination of 11 narrow and broad bandpass filters which will be used for full-disk solar imaging in the Ultravoilet (UV) wavelength range 200-400 nm. It will provide near simultaneous observations of lower and middle layers of the solar atmosphere, namely the Photosphere and Chromosphere. These observations will help to improve our understanding of coupling and dynamics of various layers of the solar atmosphere, mechanisms responsible for stability, dynamics and eruption of solar prominences and Coronal Mass ejections, and possible causes of solar irradiance variability in the Near and Middle UV regions, which is of central interest for assessing the Sun’s influence on climate.

The observations of post eruption arcades (PEAs) in X-rays and EUV reveal strong localised brightenings at the loop top regions. The origin of these brightenings and their dynamics is not well understood to date. Here, we study the dynamics of PEAs using one-dimensional hydrodynamic modelling with the focus on the the understanding of the formation of localised brightening.Our findings suggest that these brightenings are the result of collisions between the counter-streaming chromospheric evaporation from both the foot points. We further perform forward modelling of the emission observed in simulated results in various spectral lines observed by the Extreme-Ultraviolet Imaging Telescope aboard Hinode. The forward modelled intensities in various spectral lines are in close agreement with a flare observed in December 17, 2006 by EIS., Accepted by ApJ

The Solar Ultraviolet Imaging Telescope (SUIT) is an instrument onboard the Aditya-L1 mission of ISRO that will measure and monitor the solar radiation emitted in the near-ultraviolet wavelength range (200-400 nm). SUIT will simultaneously map the photosphere and the chromosphere of the Sun using 11 filters sensitive to different wavelengths and covering different heights in the solar atmosphere and help us understand the processes involved in the transfer of mass and energy from one layer to the other. SUIT will also allow us to measure and monitor spatially resolved solar spectral irradiance that governs the chemistry of oxygen and ozone in the stratosphere of Earth's atmosphere. This is central to our understanding of the Sun climate relationship., 5 Figures, Published in Current Science