arXiv subjects

B. Ravindra

Publications and source records attributed to B. Ravindra.

At least 19 recordsLinked to original sources

arXiv2024

Equator to Pole Solar Chromospheric Differential Rotation using Ca-K Features Derived from Kodaikanal Data

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, active network, and quiet network separately (for the first time). The digitized Ca-K images from Kodaikanal Observatory for 1907-1996 are used to study rotation over 0-80 degrees latitudes at an interval of 10$^{\circ}$ . 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. 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 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 to study the solar rotation rate at all latitudes including the polar regions.

astro-ph.SR↗

arXiv2023

Data processing of Visible Emission Line Coronagraph Onboard ADITYA L1

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 prime payload on board at 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 close to the solar limb (1.05 Ro). Images of the solar corona in continuum and spectra in three emission lines 5303Å [Fe xiv], 7892Å [Fe xi] and 10747 [Fe xiii] obtained with high cadence to be analyzed using software algorithms automatically. A reasonable part of observations will be made in synoptic mode, those, need to be analyzed and results made available for public use. The procedure involves the calibration of instrument and detectors, converting the images into fits format, correcting the images and spectra for the instrumental effects, align the images 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.

astro-ph.SR↗

arXiv2023

Variation of small scale magnetic fields over a century using Ca-K images as proxy

A combined uniform and long-time series of Ca-K images from the Kodaikanal Observatory (KO), Mount Wilson Observatory (MWO), and Mauna Loa Solar Observatory (MLSO) were used to identify and study the Ca-K small-scale features and their solar cycle variations over a century. The small scale features are classified into three distinct categories: enhanced network (EN), active network (AN), and quiet network (QN). All these features show that their areas vary according to the 11-year solar cycle. The relative amplitude of the Ca-K network variations agree with that of the sunspot cycle. The total area of these small-scale features varies from about 5% during the minimum phase of the solar cycle to about 20% during its maximum phase. Considering the average intensity and the amplitude of their area variations, we find that the total contribution of EN, AN and QN to the irradiance variation of the Sun is about 3%.

astro-ph.SR↗

arXiv2022

Magnetic imprints of eruptive and non-eruptive Solar flares as observed by Solar Dynamics Observatory

The abrupt and permanent changes of photospheric magnetic field in the localized regions of active regions during solar flares called magnetic imprints (MIs), have been observed for the past nearly three decades. The well known "coronal implosion" model is assumed to explain such flare associated changes but the complete physical understanding is still missing and debatable. In this study, we made a systematic analysis of flare-related changes of photospheric magnetic field during 21 flares (14 eruptive and 7 non-eruptive) using the high-cadence (\texttt{135s}) vector-magnetogram data obtained from Helioseismic and Magnetic Imager. The MI regions for eruptive flares are found to be strongly localised, whereas the majority of non-eruptive events ($>70~\%$) have scattered imprint regions. To quantify the strength of the MIs, we derived the integrated change of horizontal field and total change of Lorentz force over an area. These quantities correlate well with the flare strength, irrespective of whether flares being eruptive or not, short or long duration. Further, the free-energy (FE), determined from virial-theorem estimates, exhibits statistically significant downward trend which starts around the flare time is observed in majority of flares. The change of FE during flares do not depend on eruptivity but have a strong positive correlation ($\approx 0.8$) with the Lorentz force change, indicating that the part of FE released would penetrate into the photosphere. While these results strongly favor the idea of significant feedback from corona on the photospheric magnetic field, the characteristics of MIs are quite indistinguishable for flares being eruptive or not.

astro-ph.SR↗

arXiv2020

Solar-Cycle Characteristics in Kodaikanal Sunspot Area: North--South Asymmetry, Phase Distribution and Gnevyshev Gap

The solar activity is asymmetric in both hemispheres in almost all cycles. This asymmetry is observed both in cycle amplitude and period. We have used about 90~years of sunspot-area data from the Kodaikanal Solar Observatory to study the North--South asymmetry in sunspot activity. The monthly mean sunspot-area showed the northern hemisphere dominated in Solar Cycles 16, 19, and 20, and the southern hemisphere dominated in Cycles 18, 22, and 23. The 13-months smoothed data indicated that in Cycle 17 and 21, the northern and southern hemisphere showed equal amplitude. Cumulative sunspot area showed that the northern hemisphere dominated in Cycles 18, 19, 20, and 21, with a large difference between the two hemispheres in Cycles 19 and 20. The northern hemisphere activity led by 12, 15, and 2 months in Cycles 20, 21, and 22, respectively. No significant phase difference is found between the two hemispheres in Cycles 16, 17, 18, 19, and 23. The wavelet technique is used to find Rieger-type periodicities in the sunspot cycles. The cross-wavelet analysis of these data sets showed several statistically significant common periodicities like the Rieger-type periodicities and Quasi-biennial oscillations. The Gnevyshev gap was found in both the hemispheric data in Cycles 16, 18, 21, 22, and 23. These results are consistent with the earlier reported characteristics of North--South asymmetry in sunspot-area data. These results suggest that the Kodaikanal Observatory data complement the existing sunspot data from other observatories to study solar activity over long and short periods.

astro-ph.SR↗

arXiv2020

Determining the Variations of Ca-K index and Features using a Century Long Equal Contrast Images from Kodaikanal Observatory

In the earlier analysis of Ca-K spectroheliograms obtained at Kodaikanal Observatory, the "Good" images were used to investigate variations in the chromosphere. Still, the contrast of the images varied on a day-to-day basis. We developed a new methodology to generate images to form a uniform time series. We adjusted each image's contrast until the FWHM of the normalized intensity distribution attained a value between 0.10 and 0.11. This methodology of the "Equal Contrast technique" is expected to compensate for the change of emulsion, development, contrast of the images due to centering of Ca-K line on the exit slit and sky transparency. Besides, this procedure will correct variations in density-to-intensity conversion for different images. We find that the correlation between sunspot and Ca-K line data improves by a large amount. For example, correlation coefficient (CC) between monthly averaged sunspots and Ca-K plage areas for the equal contrast data improves to 0.9 compared to 0.75 for the "Good" data with unequal contrast. The CC for equal contrast images improves to $\sim$0.78 from $\sim$0.46 for the "Okay" data with unequal contrast. Even the CC between the plage area and the daily sunspot number is 0.85 for 100 years of data. This methodology also permits us to study the variations in Enhanced, Active, and Quiet networks with time along with good accuracy for about a century, for the first time. Further, this procedure can be used to combine data from different observatories to make a long time series.

astro-ph.SR↗

arXiv2020

Time-Latitude Distribution of Prominences for 10 Solar Cycles: A study using Kodaikanal, Meudon and Kanzelhohe Data

Solar prominences are structures of importance because of their role in polar field reversal. We study the long-term variation of the time latitude distribution of solar prominences in this article. To accomplish this, we primarily used the digitised disc-blocked Ca II K spectroheliograms as recorded from Kodaikanal Solar Observatory for the period of 1906 -- 2002. For improving the data statistics we included full disc H_alpha images from Meudon and Kanzelhohe Observatory which are available after 1980. We developed an automated technique to identify the latitudinal locations of prominences in daily images from all three datasets. Derived time-latitude distribution clearly depicted pole-ward migration of prominence structures for 10 cycles (15-24). Unlike previous studies, we separated the rate of pole-ward migration during on-set and near pole, using piece-wise linear fits. In most cases, we found acceleration in pole-ward migration with the change occurring near +/- 70 degree latitudes. The derived migration rates for such large number of solar cycles can provide important inputs towards understanding polar field build-up process.

astro-ph.SR↗

arXiv2019

Finding the critical decay index in solar prominence eruptions

The background field is assumed to play prime role in the erupting structures like prominences. In the flux rope models, the critical decay index ($n_c$) is a measure of the rate at which background field intensity decreases with height over the flux rope or erupting structure. In the real observations, the critical height of the background field is unknown, so a typical value of $n_{c}=1.5$ is adopted from the numerical studies. In this study, we determined the $n_c$ of 10 prominence eruptions (PEs). The prominence height in 3D is derived from two-perspective observations of \textit{Solar Dynamics Observatory} and \textit{Solar TErrestrial RElations Observatory}. Synoptic maps of photospheric radial magnetic field are used to construct the background field in the corona. During the eruption, the height-time curve of the sample events exhibits the slow and fast-rise phases and is fitted with the linear-cum-exponential model. From this model, the onset height of fast-rise motion is determined and is considered as the critical height for the onset of the torus-instability because the erupting structure is allowed to expand exponentially provided there is no strapping background field. Corresponding to the critical height, the $n_c$ values of our sample events are varied to be in the range of 0.8-1.3. Additionally, the kinematic analysis suggests that the acceleration of PEs associated with flares are significantly enhanced compared to flare-less PEs. We found that the flare magnetic reconnection is the dominant contributor than the torus-instability to the acceleration process during the fast-rise phase of flare-associated PEs in low corona ($<1.3R_{\odot}$).

astro-ph.SR↗

arXiv2019

Formation and eruption of sigmoidal structure from a weak field region of NOAA 11942

Using observations from Solar Dynamics Observatory, we studied an interesting example of a sigmoid formation and eruption from small-scale flux canceling regions of active region (AR) 11942. Analysis of HMI and AIA observations infer that initially the AR is compact and bipolar in nature, evolved to sheared configuration consisting of inverse J-shaped loops hosting a filament channel over a couple of days. By tracking the photospheric magnetic features, shearing and converging motions are observed to play a prime role in the development of S-shaped loops and further flux cancellation leads to tether-cutting reconnection of J-loops. This phase is co-temporal with the filament rise motion followed by sigmoid eruption at 21:32 UT on January 6. The flux rope rises in phases of slow (v$_{avg}$ = 26 km~s$^{-1}$) and fast (a$_{avg}$= 55 ms$^{-2}$) rise motion categorizing the CME as slow with an associated weak C1.0 class X-ray flare. The flare ribbon separation velocity peaks at around peak time of the flare at which maximum reconnection rate (2.14 Vcm$^{-1}$) occurs. Further, the EUV light-curves of 131, 171Å~have delayed peaks of 130 minutes compared to 94Å~and is explained by differential emission measure. Our analysis suggests that the energy release is proceeded in a much long time duration, manifesting the onset of filament rise and eventual eruption driven by converging and canceling flux in the photosphere. Unlike strong eruption events, the observed slow CME and weak flare are indications of slow runway tether-cutting reconnection where most of the sheared arcade is relaxed during the extended post phase of the eruption.

astro-ph.SR↗

arXiv2018

Forecasting solar radiation during dust storms using deep learning

Dust storms are common in arid zones on the earth and others planets such as Mars. The impact of dust storms on solar radiation has significant implications for solar power plants and autonomous vehicles powered by solar panels. This paper deals with the analysis of solar radiation and power output of a rooftop photovoltaic plant during a dust storm and proposes a forecasting methodology using deep learning network. The increased aerosol content due to dust storms increases the diffuse component of the solar radiation. This effect persists for a long duration and can impact the quality of forecasting of solar radiation. Deep learning networks that capture long range structure can improve the quality of solar radiation forecasting during dust storms. These results can help explain the sudden drop in power output of solar plants due to dust storms originating in another continent. They can shed light on mysterious cleaning events in autonomous vehicles powered by solar panels to be used in space missions.

physics.ao-ph↗

arXiv2018

Statistical study of magnetic non-potential measures in confined and eruptive flares

Using the HMI/SDO vector magnetic field observations, we studied the relation of degree of magnetic non-potentiality with the observed flare/CME in active regions. From a sample of 77 flare/CME cases, we found a general relation that degree of non-potentiality is positively correlated with the flare strength and the associated CME speeds. Since the magnetic flux in the flare-ribbon area is more related to the reconnection, we trace the strong gradient polarity inversion line (SGPIL), Schrijver's R value manually along the flare-ribbon extent. Manually detected SGPIL length and R values show higher correlation with the flare strength and CME speed than the automatically traced values without flare-ribbon information. It highlights the difficulty of predicting the flare strength and CME speed a priori from the pre-flare magnetograms used in flare prediction models. Although the total, potential magnetic energy proxies show weak positive correlation, the decrease in free energy exhibits higher correlation (0.56) with the flare strength and CME speed. Moreover, the eruptive flares have threshold of SGPIL length (31Mm), R value ($1.6\times10^{19}$Mx), free-energy decrease ($2\times10^{31}$erg) compared to confined ones. In 90\% eruptive flares, the decay-index curve is steeper reaching $n_{crit}=1.5$ within 42Mm, whereas it is beyond 42Mm in $>70$% confined flares. While indicating the improved statistics in the predictive capability of the AR eruptive behavior with the flare-ribbon information, our study provides threshold magnetic properties for a flare to be eruptive.

astro-ph.SR↗

arXiv2017

Development of algorithm to model dispersed gas-liquid flow using lattice Boltzmann method

In this paper, we present the algorithm for the simulation of a single bubble rising in a stagnant liquid using Euler-Lagrangian (EL) approach. The continuous liquid phase is modeled using BGK approximation of lattice Boltzmann method (LBM), and a Lagrangian particle tracking (LPT) approach has been used to model the dispersed gas (bubble) phase. A two-way coupling scheme is implemented for the interface interaction between two phases. The simulation results are compared with the theoretical and experimental data reported in the literature and it was found that the presented modeling technique is in good agreement with the theoretical and experimental data for the relative and terminal velocity of a bubble. We also performed the grid independence test for the current model and the results show that the grid size does not affect the rationality of the results. The stability test has been done by finding the relative velocity of a bubble as a function of time for the different value of dimensionless relaxation frequency. The present study is relevant for understanding the bubble-fluid interaction module and helps to develop the accurate numerical model for bioreactor simulation.

physics.flu-dyn↗

arXiv2017

Performance of a crystalline silicon photovoltaic power plant during sandstorms

Solar photovoltaic power generation has achieved grid parity in a number of countries during the last decade. This revolution has begun in countries where the solar radiation intensity is not very high. Solar resource maps created by a number of agencies worldwide indicate that areas of high intensity of solar radiation suitable solar for power generation are in deserts and semi-arid zones. These regions are dusty and are subjected to both anthropogenic pollution and natural sand storms. The impact of these events on the solar photovoltaic (PV) plant yield is significant. An increase in diffuse component of solar radiation is accompanied by a naturally occurring sand storm. Often the solar panels are tilted at an optimal angle and the global tilted radiation value falling on the panels changes significantly during and after the sand storms. It is noted that the decrease in the power output of a solar photovoltaic plant during such events is high enough to warrant further consideration while forecasting the plant output. Inverter performance during such sand storms is found to be satisfactory. Integrating dust storm monitoring with PV power plant yield prediction algorithms is necessary for an improved operation of PV plants.

physics.soc-ph↗

arXiv2017

Prominence eruption initiated by helical kink-instability of an embedded flux rope

We study the triggering mechanism of a limb-prominence eruption and the associated coronal mass ejection near AR 12342 using SDO and LASCO/SOHO observations. The prominence is seen with an embedded flux thread (FT) at one end and bifurcates from the middle to a different footpoint location. The morphological evolution of the FT is similar to an unstable flux rope (FR), which we regard as prominence embedded FR. The FR twist exceeds the critical value. In addition, the morphology of the prominence plasma in 304Å~images marks the helical nature of the magnetic skeleton with a total of 2.96 turns along arc length. The potential field extrapolation model indicates that the critical height of the background magnetic field gradient falls within the inner corona (105Mm) consistent with the extent of coronal plasma loops. These results suggest that the helical kink instability in the embedded FR caused the slow rise of the prominence to a height of the torus instability domain. Moreover, the differential emission measure analysis unveils heating of the prominence plasma to coronal temperatures during eruption, suggesting a reconnection-related heating underneath the upward rising embedded FR. The prominence starts with a slow rise motion of 10km/s, followed by fast and slow acceleration phases having an average acceleration of $28.9m/s^2$, $2.4m/s^2$ in C2, C3 field of view respectively. As predicted by previous numerical simulations, the observed synchronous kinematic profiles of the CME leading edge and the core supports the involved FR instability in the prominence initiation.

astro-ph.SR↗

arXiv2017

Long term study of the solar filaments from the Synoptic Maps as derived from $H_α$ Spectroheliograms of Kodaikanal Observatory

The century long (1914-2007) $H_α$ (656.28 nm) spectroheliograms from Kodaikanal Solar Observatory (KSO) have been recently digitised. Using these newly calibrated, processed images we study the evolution of dark elongated on disk structures called filaments, potential representatives of magnetic activities on the Sun. To our knowledge this is the oldest uniform digitised dataset with daily images available today in $H_α$. We generate Carrington maps for entire time duration and try to find the correspondences with maps of same rotation from Ca II K KSO data. Filaments are segmented from Carrington maps using a semi-automated technique and are studied individually to extract their centroids and tilts. We plot the time-latitude distribution of filament centroids producing Butterfly diagram, which clearly shows presence of poleward migration. We separate polar filaments for each cycle and try to estimate the delay between the polar filament number cycle and sunspot number cycle peaks. We correlate this delay with the same between polar reversal and sunspot number maxima. This provides new insight on the role of polar filaments on polar reversal.

astro-ph.SR↗

arXiv2016

Full-disk Synoptic Observations of the Chromosphere Using H$_α$ Telescope at the Kodaikanal Observatory

This paper reports on the installation and observations of a new solar telescope installed on 7th October, 2014 at the Kodaikanal Observatory. The telescope is a refractive type equipped with a tunable Lyot H$_α$ filter. A CCD camera of 2k$\times$2k size makes the image of the Sun with a pixel size of 1.21$^{\prime\prime}$ pixel$^{-1}$ with a full field-of-view of 41$^{\prime}$. The telescope is equipped with a guiding system which keeps the image of the Sun within a few pixels throughout the observations. The FWHM of the Lyot filter is 0.4Å~and the filter is motorized, capable of scanning the H$_α$ line profile at a smaller step size of 0.01Å. Partial-disk imaging covering about 10$^{\prime}$, is also possible with the help of a relay lens kept in front of the CCD camera. In this paper, we report the detailed specifications of the telescope, filter unit, its installation, observations and the procedures we have followed to calibrate and align the data. We also present preliminary results with this new full-disk telescope.

astro-ph.SR↗

arXiv2016

Sunspot Rotation as a Driver of Major Solar Eruptions in NOAA Active Region 12158

We studied the developing conditions of sigmoid structure under the influence of magnetic non-potential characteristics of a rotating sunspot in the active region (AR) 12158. Vector magnetic field measurements from Helioseismic Magnetic Imager and coronal EUV observations from Atmospheric Imaging Assembly reveal that the erupting inverse-S sigmoid had roots in the location of the rotating sunspot. Sunspot rotates at a rate of 0-5deg/h with increasing trend in the first half followed by a decrease. Time evolution of many non-potential parameters had a well correspondence with the sunspot rotation. The evolution of the AR magnetic structure is approximated by a time series of force free equilibria. The NLFFF magnetic structure around the sunspot manifests the observed sigmoid structure. Field lines from the sunspot periphery constitute the body of the sigmoid and those from interior overly the sigmoid similar to a fluxrope structure. While the sunspot is being rotating, two major CME eruptions occurred in the AR. During the first (second) event, the coronal current concentrations enhanced (degraded) consistent with the photospheric net vertical current, however the magnetic energy is released during both the cases. The analysis results suggest that the magnetic connections of the sigmoid are driven by slow motion of sunspot rotation, which transforms to a highly twisted flux rope structure in a dynamical scenario. An exceeding critical twist in the flux rope probably leads to the loss of equilibrium and thus triggering the onset of two eruptions.

astro-ph.SR↗

arXiv2016

Butterfly Diagram and Carrington Maps for Century-Long Ca II K Spectroheliograms from Kodaikanal Observatory

The century-long (1907-2007) Ca II K spectroheliograms from Kodaikanal Solar Observatory (KSO) are calibrated, processed and analysed in the present study to follow the evolution of bright on disc structures called plages, the possible representatives of magnetic activity on the Sun. This has been the longest dataset studied in Ca II K till date covering about 9.5 cycles of 11 year periods. Plages were segmented with area $\geq 1\:\textrm {arcmin}^2$ using global thresholds for individual full disc images and subsequent application of morphological closing operation. Plage index was calculated and seen to have close positive correlation with fractional disc area covered by plages. The newly generated plage area cycle (from KSO) was compared with the same from Mount Wilson observatory (Correlation~$95.6\%$) for the overlapping years i.e. 1915-2000. Study illustrated time-latitude distribution of plage centroids rendering butterfly diagram (as observed for sunspots). The 3D visualisation of the diagram showed one to one mapping between plage location, time and area. This work further delineated positional correlation between magnetic patches and plage regions through comparison of synoptic maps derived from both Kodaikanal Ca II K images and space based full disc LOS (line of sight) magnetograms. Regular synoptic magnetograms from ground based observatories are available only after 1970s. Thus the long term Ca II K data from KSO can be used as a proxy for estimating magnetic activity locations and their strengths at earlier times.

astro-ph.SR↗