While the amount of solar coronal energy flux leaving the "quiet" Sun, coronal holes, and active regions has been estimated by many, similar calculations for transients like coronal mass ejections (CMEs) based on observations are few. Using spectroscopic observations in the 5303 & Aring; coronal emission line with the Visible Emission Line Coronagraph on board Aditya-L1, we report estimates of the Alfv & eacute;n wave energy flux before and during a transient coronal dimming caused by a CME. Our analysis indicates that the coronal energy flux associated with the dimming is approximate to 7.3 & times; 105 erg cm-2 s-1, which is approximate to 7% of the typical energy loss of 107 erg cm-2 s-1 from an active region due to conduction, radiative, and solar wind fluxes.

The Visible Emission Line Coronagraph on board Aditya-L1 provides continuous spectroscopic observations of the solar corona in the Fe XIV 5303 Å emission line, which are a valuable tool to understand the plasma properties and dynamics in the near-Sun solar corona. One of the widely debated topics in the spectral line observations are the observed line widths, which are always in excess of the thermal width due to the ion temperature. In this context, we report estimates of the intensity, width, and Doppler velocity of the 5303 Å emission line along a preflare coronal loop in the heliocentric distance ( r ) range 1.10–1.24 R _⊙ . The Doppler velocity of the line shows a significant blueward shift (≈11 km s ^–1 ) along one of the legs of the loop, indicating upflow of plasma. There is a simultaneous increase in the line width (≈0.2 Å) also. The changes in the above two parameters are strongly correlated. The correlation coefficient is ≈−0.9. The results indicate preflare activities are likely the cause for the excess line widths in the present case. The large change in the Doppler velocity, toward the blue side, confirms the related plasma upflows. The respective line parameters along either leg are nearly equal at the apex of the loop. The changes in the width and Doppler velocity in the other leg are comparatively minimal. The estimated upper limit for the Alfvén wave energy flux density at r ≈ 1.1 R _⊙ in the loop is ≈39%.

We present Aditya-L1/VELC spectroscopic observations of 5303Å coronal emission line widths before and after coronal mass ejections (CMEs) which showed coronal dimming. The sit & stare mode of observations enabled us to study the changes in the line widths for the `limb' CMEs noticed on 16 July 2024 and 05 August 2024. The emission line widths during the pre-CME phase are higher than the thermal values in both the cases. After the onset of the CMEs, the widths increased further by ≈15% on 16 July 2024 and ≈7% on 05 August 2024. We find that the power spectral density (PSD) distributions of the line widths for the two events exhibits a power-law behavior. The PSD slopes, measured before and after the CMEs are nearly the same, and close to the Kolmogorov slope of -5/3. The results suggest that the observed larger than thermal width of the 5303Å emission line, before and after CMEs, is mostly due to turbulence. The additional increase in the line widths after the onset of the CMEs in both the cases are likely because of enhanced turbulence caused by the CME associated coronal dimmings and subsequent coronal magnetic field reconfiguration.

We carried out radiometric calibration of the Visible Emission Line Coronagraph (VELC) onboard Aditya-L1 at 5303Å using observations of the Sun itself, a novel approach compared to the calibration using bright stars in the other currently operational space solar coronagraphs. The measured VELC detector count corresponding to the center of Sun's disk is ≈(1.70±0.12)×10^8 sec^-1Å^-1, which relates to Sun's flux at 1 AU in 5303Å, ≈3.0456×10^6 erg sec^-1 cm^-2 Å^-1. We verified the above calibration using observations of the bright star Sirius-A whose estimated flux at 1 AU in 5303Å is ≈1.44×10^-8 erg sec^-1 cm^-2 Å^-1. The expected detector count with VELC in this case is ≈19±1.4 sec^-1Å^-1, and the measured count is ≈23±0.4 sec^-1Å^-1. The reasonable agreement between the expected and observed values for Sirius-A in this initial consistency check after two years of observations with VELC, indicates that measurements of coronal brightness at 5303Å with the VELC and its conversion into absolute physical units using observations of the Sun disk light are in order. Using the Sirius-A observations we measured the point spread function (PSF) of the VELC in its 5303Å channel also. Its full width at half maximum (FWHM) is ≈3.8^''.

Constraining the plasma properties and energetics of coronal mass ejections (CMEs) in the inner corona is essential for understanding their early evolution, yet remains challenging because of limited observations. We investigate the mass and density evolution, and energy partitioning of two CMEs observed on 2025 September 14 and 16, and assess the potential of coordinated Proba-3/ASPIICS and Aditya-L1/VELC observations to constrain CME energetics in the low corona. We present the first coordinated observations of CMEs obtained simultaneously by Proba-3/ASPIICS and Aditya-L1/VELC. Using ASPIICS white-light observations, we estimate the CME mass, volume, number density, and the evolution of kinetic, thermal, and magnetic energies. Magnetic energies are estimated from observed CME properties using observationally constrained, physically motivated assumptions. Simultaneous VELC Fe XIV 5303 Angstorm observations provide independent estimates of the emission measure, electron number density, thermal energy, and CME lateral extent. The two events exhibit markedly different energy partitioning in the low corona. For the 14 September CME, the kinetic and magnetic energies are comparable, while the thermal energy remains nearly two orders of magnitude smaller, indicating limited plasma heating. In contrast, the 16 September CME exhibits a substantial thermal-energy enhancement, with thermal energy eventually becoming comparable to kinetic energy. For both CMEs, the estimated magnetic energy remains comparable to or exceeds the kinetic energy over the observed height. Our results demonstrate the scientific potential of synergetic ASPIICS and VELC observations for constraining CME mass, density, and energetics in the inner corona, providing new observational constraints on the early evolution of CMEs.

We report long duration observations of changes in the 5303 Å (Fe XIV) solar coronal emission line parameters at heliocentric distance r≈1.07R_⊙ , using data obtained with the Visible Line Emission Coronagraph (VELC) onboard Aditya-L1 in the sit and stare mode. The observed changes are due to a flare near the east limb of the Sun. The intensity and width of the line are enhanced during the event. There is no change in the Doppler velocity. Our analysis indicates that the increase in line width is most likely due to an increase in temperature due to flare heating.

Reports using total solar eclipse data indicate that the 5303 Å emission line is a promising tool for observing the thermodynamic changes due to coronal mass ejections (CMEs) close to the Sun. The Visible Emission Line Coronagraph (VELC) on board ADITYA-L1, the recently launched first Indian space solar mission, has now provided an opportunity to regularly observe the solar corona in the 5303 Å emission line. We present the first long-duration temporal observations of activity associated with a flareless CME from an active region at the west limb of the Sun, with the VELC. The observations were in the sit-and-stare mode for ≈7 hr. Our analysis shows a steady increase in the intensity of the line by ≈57%, followed by a gradual decrease to the initial level. The width of the line also showed changes, but opposite to that in the intensity. The width slowly decreased from 0.97 ± 0.01 Å to 0.87 ± 0.01 Å and then increased back. The change is ≈10%. The effective temperatures corresponding to each of the above two widths are 3.39 ± 0.1 × 10 ^6 K and 2.74 ± 0.1 × 10 ^6 K, respectively. The Doppler velocity changed gradually from 0 ± 0.5 km s ^−1 to −3 ± 0.5 km s ^−1 during the abovementioned intensity increase phase. It slowly reverted to 0 ± 0.5 km s ^−1 when the intensity decreased.

We carried out radiometric calibration of the Visible Emission LineCoronagraph (VELC) onboard Aditya-L1 at 5303Å using observations of thesolar disk through a neutral density filter, and the bright star Sirius-A in the deepspace. The measured detector counts due to the Sun, after correcting for filter−1transmission and aperture size, is ≈(1.70±0.06)×108 sec−1 Å . Using this, andthe reported flux values for Sun and Sirius-A, we calculated the expected detector−1counts for Sirius-A at 1 AU (i.e. at the location of Sun) to be ≈19 sec−1 Å . The−1measured Sirius-A count with VELC is ≈11.6±0.4 sec−1 Å . The close agree-ment between the expected and observed values indicates that measurementsof coronal brightness at 5303Å with the VELC can be reliably converted into absolute physical units.

The Ca-K line profiles as functions of solar latitude and time were obtained through our observations from the Kodaikanal Solar Observatory using the solar tunnel telescope and spectrograph with a CCD detector. Observations were conducted on all days with favourable sky conditions. We analysed the data collected over a period of about 10 yr to study the variations in the CaII K line profiles recorded between 2015 and 2024, of which 709 d of data were found useful. The temporal and time-averaged latitudinal variations of the K-1 width, K-2 width, K-3 intensity and the intensity ratios of K-2v/K-2r and K-2v/K-3 were computed using a semi-automated program. The parameters showed asymmetric increases towards the higher latitudes, with the rates of increase being higher in the Southern hemisphere. The temporal plots for K-1 width and K-3 intensity showed positive correlations with the plage and spot filling factors, whereas the temporal plots for K-2 width, K-2v/K-2r, and K-2v/K-3 intensity ratios showed negative correlations. The time-averaged latitudinal plot for K 1 width has small peaks near 25 degrees N and 20 degrees S. The K-2 width has a small peak at 0 degrees. The K-3 intensity has peaks at 20 degrees N and 15 degrees S. The K-2v/K-2r intensity ratio shows peaks at 50 degrees N, 0 degrees, and 40 degrees S. The K-2v /K-3 intensity ratio shows peaks at 60 degrees N, 0 degrees, and 60 degrees S. Slope profiles show spectral response to magnetic activity peaks near K-3 with north-south asymmetries. Such variations in the line profiles are important in the studies of solar irradiance, surface flux transport, and solar dynamo.

Using observations of the solar corona in the 5303 Å (Fe XIV ) emission line with the Visible Emission Line Coronagraph (VELC) on board Aditya-L1, we estimated the electron density ( N _e ), thermal energy ( E _CME ), and mass ( M _CME ) of a coronal mass ejection (CME) very close to the Sun at heliocentric distance r ≈ 1.06 R _⊙ . The corresponding values are N _e ≈ 3.7 × 10 ^8 cm ^−3 , E _CME ≈ 9.4 × 10 ^28 erg, and M _CME ≈ 2.7 × 10 ^14 g, respectively. The procedure adopted suggests a possibility to understand the properties of the CMEs in the visible wavelength range, particularly during the temporal phase close to their onset. The widths and Doppler velocities of the line are nearly constant in the VELC field of view (FoV ≈ 1.06–1.50 R _⊙ ). The values are ≈0.85 Å and ≈+2 km s ^−1 , respectively.

We analyze historical Ca ii K images from the Kodaikanal Observatory (KO) spanning 1907 to 1996, encompassing Solar Cycles 14 through 22. These digitized images were processed using the Equal Contrast Technique (ECT) to ensure uniform data quality for studying long and short-term variations. From these standardized images, we identify and compute the areas of both plages and network regions in both solar hemispheres in every image. We then utilizy this revised, uniform Ca ii K plage area time series for Solar Cycles 14 to 22. Our primary objective is to investigate the presence of short, Rieger-type periods and quasi-biennial oscillations (QBOs), specifically those near ≈ 1.3 years. To achieve this, we employ both Lomb-Scargle periodograms and Morlet wavelet maps. Our power spectrum analysis consistently shows that Rieger-type periods are significant across all solar cycles, in both the northern and southern hemispheres and in the whole disk data. However, the wavelet analysis reveals that both Rieger-type and QBO periodicities are intermittent, exhibiting varying periods in different cycles and hemispheres. This indicates that plages and network areas demonstrate asymmetric behavior between the two hemispheres. We have also discussed the potential reasons behind these observed periodicities.

Aditya-L1, India’s first dedicated mission to study the Sun and its atmosphere from the Sun-Earth Lagrangian L1 location was successfully launched on September 2, 2023. It carries seven payloads. The Visible Emission Line Coronagraph (VELC) is a major payload on Aditya-L1. VELC is designed to carry out imaging and spectroscopic observations (the latter in three emission lines of the corona), simultaneously. Images of the solar corona in the continuum at 5000 Å, with a field of view (FoV) from 1.05 R_⊙ to 3 R_⊙ can be obtained at variable intervals depending on the data volume that can be downloaded. Spectroscopic observations of the solar corona in three emission lines, namely 5303 Å Fexiv, 7892 Å Fexi, and 10,747 Å Fexiii are possible simultaneously, with different exposure times and cadence. Four slits, each of width 50 μ m, separated by 3.75 mm help to simultaneously obtain spectra at four positions in the solar corona in all the aforementioned lines. A Linear Scan Mechanism (LSM) makes it possible to scan the solar corona up to ± 1.5 R_⊙ . The instrument has the facility to carry out spectropolarimetric observations at 10,747 Å also in the FoV range 1.05 – 1.5 R_⊙ . Various components of the instrument were tested interferometrically on the optical bench before installation. The individual components were aligned and performance of the payload was checked in the laboratory using a laser source and tungsten lamp. Wavelength calibration of the instrument was verified using the Sun as a light source. All the detectors were calibrated for different parameters such as dark current and its variation with exposure time. Here, we discuss the various features of the VELC, alignment, calibration, performance, possible observations, initial data analysis, and results of initial tests conducted in-orbit.

Differential rotation is one of the basic characteristics of the Sun, and it plays an important role in generating the magnetic fields and its activities. We investigated rotation rate using chromospheric features such as plages, enhanced network (EN), active network (AN), and quiet network (QN) separately (for the first time). The digitized Ca-K images from Kodaikanal Observatory for 1907–1996 are used to study rotation over 0°–80° latitudes at an interval of 10°. We find that plages and all types of networks exhibit the differential rotation of the chromosphere. Furthermore, the rotation rate shows a decreasing pattern as one move from the equator to the higher polar latitudes for all the features used in the study. At the equator the rotation rate (rotation period) is obtained to be ∼13.98° day ^−1 (25.74 days), ∼13.91° day ^−1 (25.88 days), ∼13.99° day ^−1 (25.74 days), and ∼14.11° day ^−1 (25.51 days) for plage, EN, AN, and QN areas, respectively. By analyzing how the area of chromospheric features varies over time, we can effectively map the Sun’s rotation rate at all latitudes, including the polar regions. Interestingly, both plages and small-scale networks exhibit a similar differential rotation rate. This suggests these features likely rooted at the same layer below the visible surface of the Sun. Therefore, the long-term Ca-K data is very useful for studying the solar rotation rate at all latitudes including the polar regions.

The magnetic field strength and its topology play an important role in understanding the formation, evolution, and dynamics of the solar corona. Also, it plays a significant role in addressing long-standing mysteries such as coronal heating problem, origin and propagation of coronal mass ejections, drivers of space weather, origin and acceleration of solar wind, and so on. Despite having photospheric magnetograms for decades, we do not have reliable observations of coronal magnetic field strengths today. To measure the coronal magnetic field precisely, the spectropolarimetry channel of the Visible Emission Line Coronagraph (VELC) on board the Aditya-L1 mission is designed. Using the observations of coronal emission line Fe XIII [10747Å ], it is possible to generate full Stokes maps (I, Q, U, and V) that help in estimating the Line-of-Sight (LOS) magnetic field strength and to derive the magnetic field topology maps of solar corona in the Field of View (FOV) (1.05 – 1.5 R _⊙ ). In this article, we summarize the instrumental details of the spectropolarimetry channel and detailed calibration procedures adopted to derive the modulation and demodulation matrices. Furthermore, we have applied the derived demodulation matrices to the observed data in the laboratory and studied their performance.

We report on the onset of a coronal mass ejection (CME) using spectroscopic observations in the 5303 & Aring; coronal emission line with the Visible Emission Line Coronagraph (VELC) onboard ADITYA-L1, the recently launched first Indian space solar mission. The CME was observed on 2024 July 16 in association with an X1.9 class soft X-ray flare from heliographic location S05W85. The VELC observations were near the west limb of the Sun during the CME. The results obtained helped to constrain the onset time of the CME. In addition, they indicate a approximate to 50% decrease in the coronal intensity near the source region of the CME due to mass depletion, a approximate to 15% enhancement in the emission line width, and a redshifted Doppler velocity of about approximate to 10 km s-1. The nonthermal velocity associated with the line broadening is approximate to 24.87 km s-1.

Aditya-L1 is the first Indian space mission to explore the Sun and solar atmosphere with seven multi-wavelength payloads, with Visible Emission Line Coronagraph (VELC) being the prime payload. It is an internally occulted coronagraph with four channels to image the Sun at 5000 Å in the field of view 1.05 - 3 R_⊙ , and to pursue spectroscopy at 5303 Å, 7892 Å and 10747 Å channels in the FOV (1.05 - 1.5 R_⊙ ). In addition, spectropolarimetry is planned at 10747 Å channel. Therefore, VELC has three sCMOS detectors and one InGaAs detector. In this article, we aim to describe the technical details and specifications of the detectors achieved by way of thermo-vacuum calibration at the CREST campus of the Indian Institute of Astrophysics, Bangalore, India. Furthermore, we report the estimated conversion gain, full-well capacity, and readout noise at different temperatures. Based on the numbers, it is thus concluded that it is essential to operate the sCMOS detectors and InGaAs detector at -5^∘ and -17^∘ C, respectively, at the spacecraft level.

ADITYA-L1 is India′s first dedicated mission to observe the Sun and its atmosphere from a halo orbit around L1 point. Visible emission line coronagraph (VELC) is the primary payload on board Aditya-L1 to observe the Sun’s corona. VELC is designed as an internally occulted reflective coronagraph to meet the observational requirements of wide wavelength band and the field of view close to the solar limb (1.05 RSun). Images of the solar corona in continuum and spectra in three emission lines 5303Å [Fe xiv], 7892Å [Fe xi] and 10747Å [Fe xiii] obtained with high cadence are to be analyzed using software algorithms automatically. A reasonable part of these observations will be made in a synoptic mode in which the data are processed through a standard pipeline and the resulting products will be made available for public use. The procedure involves the calibration of instrument and detectors, converting the images into the FITS format, correcting the images and spectra for the instrumental effects, and aligning the images (in terms of position angles and image scales) etc. Then, develop image processing algorithms to detect the occurrence of energetic events using continuum images. Also derive physical parameters, such as temperature and velocity structure of solar corona using emission line observations. Here, we describe the calibration of detectors and the development of software algorithms to detect the occurrence of CMEs and analyze the spectroscopic data.

The network structure seen in the solar images is the outline of supergranulation, which is the large-scale convection in the Sun with a size of about 30 Mm and a lifetime of 24 hr. We have obtained the supergranulation lane widths from the autocorrelation function of image windows from the Ca ii K spectroheliograms. The images are obtained from the 100 yr Kodaikanal data, which contains information on more than nine solar cycles. The lane widths are known to show a positive correlation with the sunspot number. It is now found that the lane widths, obtained near the mid-latitudes during the sunspot cycle minima, are strongly correlated to the following sunspot number maxima. A straight-line fit adequately describes the variation. It is also found that the correlation is weak or insignificant at other times. The strong correlation of the two parameters thus provides a simple way to predict the maximum sunspot number about 4–5 yr in advance. The results are important in space weather predictions and solar irradiance variations.

The Ca-K spectroheliograms obtained at the Kodaikanal observatory are used to generate a uniform time series using the equal-contrast technique for studying the long- and short-term variations in the solar chromosphere. The percentages of plage, enhanced network, and active and quiet network areas at various latitudes is compared with the activity at 35° latitude and also with the sunspot number for the period of 1907–1984. The values of the phase differences indicate that the activity begins at ∼45° latitude and shifts progressively to a lower latitude at a speed of ∼9.4 m s ^−1 . The shift speed slows down gradually and reaches ∼3 m s ^−1 at ∼5° latitude. No phase difference between the variations of Ca-K activity at 55°, 65°, and 75° latitude belts implies that changes in the activity are happening simultaneously. The analysis shows that the activity at polar latitude belts is anticorrelated with the sunspot number. This study indicates that a multicell meridional flow pattern could exist in the latitude direction. One type of cell could transport the magnetic elements from mid- to low-latitude belts through meridional flows, and the other cell type could be operating in the polar region.