arXiv subjects

Durgesh Tripathi

Publications and source records attributed to Durgesh Tripathi.

At least 55 records · Page 3Linked to original sources

arXiv2020

Formation and Dynamics of Transequatorial Loops

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 Å 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 Å 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.

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arXiv2020

Wave amplitude modulation in fan loops as observed by AIA/SDO

We perform multiwavelength time-distance analysis of a fan loop system anchored in an isolated sunspot region AR 12553. The active region was observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. We measure the phase speeds of the propagating intensity disturbances by employing cross-correlation analysis, as well as by obtaining the slopes in xt-plots. We obtain original as well as de-trended light curves at different heights of the time-distance maps and characterize them by performing Fouriér and Wavelet analysis, respectively. The time-distance maps reveal clear propagation of intensity oscillations in all the coronal EUV channels except AIA 94 and 335~Å. We determine the nature of the intensity disturbances as slow magnetoacoustic waves by measuring their phase speeds. The time-distance maps, as well as the de-trended light curves, show an increase and decrease in the amplitude of propagating 3-min oscillations over time. The amplitude variations appear most prominent in AIA 171~Å, though other EUV channels also show such signatures. Fouriér power spectrum yield the presence of significant powers with several nearby frequencies between 2--3 minutes (5--8 mHz), along with many other smaller peaks between 2--4 minutes. Wavelet analysis shows an increase and decrease of oscillating power around 3-min simultaneous to the amplitude variations. We obtain the modulation period to be in the range of 20--30 minutes. Our results provide the viability of occurrence of phenomenon like `Beat' among the nearby frequencies giving rise to the observed amplitude modulation. However, we cannot, at this stage, rule out the possibility that the modulation may be driven by variability of an underlying unknown source.

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arXiv2020

Signatures of Untwisting Magnetic Field in a Small Emerging Bipole in the Solar Photosphere

We perform a study of fluid motions and its temporal evolution in and around a small bipolar emerging flux region using observations made by the Helioseismic and Magnetic Imager (HMI) on-board the Solar Dynamics Observatory (SDO). We employ local correlation tracking of the Doppler observations to follow horizontal fluid motions and line-of-sight magnetograms to follow the flux emergence. Changes in vertical vorticity and horizontal divergence are used to derive signatures of evolving twists in the magnetic field. Our analysis reveals that the two polarities of the magnetic flux swirl in opposite directions in early stages of flux emergence indicating an unwinding of the pre-emergence twists in the magnetic field. We further find that during the emergence, there is an increase in swirly motions in the neighbouring non-magnetic regions. We estimate the magnetic and kinetic energies and find that magnetic energy is about a factor of ten larger than the kinetic energy. During the evolution, when the magnetic energy decreases, an increase in the kinetic energy is observed indicating transfer of energy from the unwinding of magnetic flux tube to the surrounding fluid motions. Our results thus demonstrate the presence of pre-emergence twists in emerging magnetic field that is important in the context of the hemispheric helicity rule warranting a detailed statistical study in this context. Further, our observations point to a possible widespread generation of torsional waves in emerging flux regions due to the untwisting magnetic field with implications for upward energy transport to the corona.

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arXiv2020

On the ratios of \ion{Si}{4} lines ($λ$1394/$λ$1403) in an emerging flux region

The resonance lines of \ion{Si}{4} 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 (1394Å/1403Å), which under optically thin condition 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 \ion{Si}{4} 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 modelling of the solar atmosphere, including the initial stage of the emergence of an active region as well as quiet Sun.

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arXiv2019

On Doppler shift and its Center-to-Limb Variation in Active Regions in the Transition Region

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~Å 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.

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arXiv2018

Magnetic fields and low-frequency acoustic wave-energy supply to the solar chromosphere

The problem of solar chromospheric heating remains a challenging one with wider implications for stellar physics. Several studies in the recent past have shown that small-scale inclined magnetic field elements channel copious amount of energetic low-frequency acoustic waves, that are normally trapped below the photosphere. These magneto-acoustic waves are expected to shock at chromospheric heights contributing to chromospheric heating. In this work, exploiting simultaneous photospheric vector magnetic field, Doppler, continuum and line-core intensity (of FeI 6173 Å) observations from the Helioseismic and Magnetic Imager (HMI) and lower-atmospheric UV emission maps in the 1700 Å and 1600 Å channels of the Atmospheric Imaging Assembly (AIA), both onboard the Solar Dynamics Observatory (SDO) of NASA, we revisit the relationships between magnetic field properties (inclination and strength) and the acoustic wave propagation (phase travel time). We find that the flux of acoustic energy, in the 2 - 5 mHz frequency range, between the upper photosphere and lower chromosphere is in the range of 2.25 - 2.6 kW m$^{-2}$, which is about twice the previous estimates. We identify that the relatively less-inclined magnetic field elements in the quiet-Sun channel a significant amount of waves of frequency lower than the theoretical minimum for acoustic cut-off frequency due to magnetic inclination. We also derive indications that these waves steepen and start to dissipate within the heights ranges probed, while those let out due to inclined magnetic fields pass through. We explore connections with existing theoretical and numerical results that could explain the origin of these waves.

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arXiv2018

Quiet-Sun and Coronal Hole in \ion{Mg}{2}~k line as observed by IRIS

Coronal holes (CHs) regions are dark in comparison to the quiet-Sun (QS) at the coronal temperatures. However, at chromospheric and transition region (TR) temperatures, QS and CHs are hardly distinguishable. In this study we have used the \ion{Mg}{2}~2796.35~Å spectral line recorded by the Interface Region Imaging Spectrometer (IRIS) to understand the similarities and differences in the QS and CH at chromospheric levels. Our analysis reveals that the emission from \ion{Mg}{2}~k3 \& k2v that originates in the chromosphere is significantly lower in CH than in QS for the regions with similar magnetic field strength. The wing emissions of \ion{Mg}{2}~k that originates from the photospheric layer, however, do not show any difference between QS and CH. The difference in \ion{Mg}{2}~k3 intensities between QS and CH increases with increasing magnetic field strength. We further studied the effects of spectral resolution on these differences and found that the difference in the intensities decreases with decreasing spectral resolution. For a resolution of 11~Å, the difference completely disappears. These findings are not only important for mass and energy supply from the chromosphere to the corona but also provides essential ingredients for the modelling of the solar spectral irradiance for the understanding of the Sun-climate relationships.

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arXiv2018

Observation and Modeling of Chromospheric Evaporation in a Coronal Loop Related to Active Region Transient Brightening

Using the observations recorded by Atmospheric Imaging Assembly (AIA) on-board the Solar Dynamics Observatory (SDO), the Interface Region Imaging Spectrograph (IRIS) and the Extreme-ultraviolet Imaging Spectrometer (EIS) and X-Ray Telescope (XRT) both on-board Hinode, we present the evidence of chromospheric evaporation in a coronal loop after the occurrence of two active region transient brightenings (ARTBs) at the two footpoints. The chromospheric evaporation started nearly simultaneously in all the three hot channels of AIA such as 131~Å, 94~Å and 335~Å, which was observed to be temperature dependent, being fastest in the highest temperature channel. The whole loop became fully brightened following the ARTBs after $\approx25$~s in 131~Å, $\approx 40$~s in 94~Å, and $\approx 6.5$~min in 335~Å. The DEM measurements at the two footpoints (i.e., of two ARTBs) and the loop-top suggest that the plasma attained a maximum temperature of $\sim$10~MK at all these locations. The spectroscopic observations from IRIS revealed the presence of redshifted emission of $\sim$20~km~s$^{-1}$ in cooler lines like \ion{C}{2} and \ion{Si}{4} during the ARTBs that was co-temporal with the evaporation flow at the footpoint of the loop. During the ARTBs, the line width of \ion{C}{2} and \ion{Si}{4} increased nearly by a factor of two during the peak emission. Moreover, enhancement in the line width preceded that in the Doppler shift which again preceded enhancement in the intensity. The observed results were qualitatively reproduced by 1-D hydrodynamic simulations where energy was deposited at both the footpoints of a monolithic coronal loop that mimicked the ARTBs identified in the observations.

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arXiv2018

Energetics of Hi-C EUV Brightenings

We study the thermal structure and energetics of the point-like EUV brightenings within a system of fan loops observed in the active region \textsl{AR~11520}. These brightenings were simultaneously observed on 2012 July 11 by the HIgh-resolution Coronal (Hi-C) imager and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). We identified 27 brightenings by automatically determining intensity enhancements in both Hi-C and AIA ~193~Å light curves. The energetics of these brightenings were studied by using the Differential Emission Measure (DEM) diagnostics. The DEM weighted temperatures of these transients are in the range $\log{T} (K) = 6.2 - 6.6$ with radiative energies ${\approx}10^{24-25}$~ergs and densities ${\approx}$ a few times $10^{9}$~cm$^{-3}$. To the best of our knowledge, these are the smallest brightenings in EUV ever detected. We used these results to determine the mechanism of energy loss in these brightenings. Our analysis reveals that the dominant mechanism of energy loss for all the identified brightenings is conduction rather than radiation.

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arXiv2017

Direct observations of different sunspot waves influenced by umbral flashes

We report the simultaneous presence of chromospheric umbral flashes and associated umbral waves, and propagating coronal disturbances, in a sunspot and related active region. We have analyzed time-distance maps obtained using the observations from Atmospheric Imaging Assembly (AIA) on-board Solar Dynamics Observatory (SDO). These maps show the simultaneous occurrence of different sunspot oscillation and waves such as umbral flashes, umbral waves, and coronal waves. Analysis of the original light curves, i.e., without implementing any Fourier filtering on them, show that the amplitudes of different sunspot waves observed at different atmospheric layers change in synchronization with the light curves obtained from the umbral flash region, thus demonstrating that these oscillations are modulated by umbral flashes. This study provides the first observational evidence of the influence of sunspot oscillations within the umbra on other sunspot waves extending up to the corona. The properties of these waves and oscillations can be utilized to study the inherent magnetic coupling among different layers of the solar atmosphere above sunspots.

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arXiv2017

Fan Loops Observed by IRIS, EIS and AIA

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~Å 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.

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arXiv2016

On the Bright Loop Top Emission in Post Eruption Arcades

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.

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arXiv2016

Multiwavelength study of twenty jets emanating from the periphery of active regions

We present a multiwavelength analysis of 20 EUV jets which occurred at the periphery of active regions close to sunspots. We discuss the physical parameters of the jets and their relation with other phenomena such as H alpha surges, nonthermal type III radio bursts and hard X-ray emission. Using AIA wavelength channels sensitive to coronal temperatures, we studied the temperature distribution in the jets using the line-of-sight Differential Emission Measure technique. We also investigated the role of the photospheric magnetic field using the LOS magnetogram data from the HMI. The lifetime of jets range from 5 to 39 minutes with an average of 18 minutes and their velocities range from 87 to 532 km/s with an average of 271 km/s. Most of the jets are co-temporal with nonthermal type III radio bursts observed by the Wind/WAVES spacecraft. We confirm the source region of these bursts using the Potential Field Source Surface technique. 10 out of 20 events showed that the jets originated in a region of flux cancellation and 6 jets in a region of flux emergence. 4 events showed flux emergence and then cancellation during the jet evolution. DEM analyses showed that for most of the spires of the jets, the DEM peaked at around log T [K] = 6.2/6.3. In addition, we derived an emission measure and a lower limit of electron density at the location of the spire and the footpoint. These results are in agreement with those obtained earlier by studying individual active region jets. The observation of flux cancellation, the association with HXR emission and emission of nonthermal type III radio bursts, suggest that the initiation and therefore, heating is taking place at the base of the jet. This is also supported by the high temperature plasma revealed by the DEM analysis in the jet footpoint. Our results provide substantial constraints for theoretical modeling of the jets and their thermodynamic nature.

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arXiv2016

A 7~MK hot Flux Rope Observed by SDO/AIA

A filament eruption was observed on October 31, 2010 in the images recorded by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory (SDO) in its Extreme Ultra-Violet (EUV) channels. The filament showed a slow rise phase followed by a fast rise and was classified to be an asymmetric eruption. In addition, multiple localized brightening which was spatially and temporally associated with the slow rise phase were identified leading us to believe that the tether-cutting mechanism to be the cause of the initiation of the eruption. An associated flux rope was detected in high temperature channels of AIA namely 94Å and 131Å corresponding to 7 MK and 11 MK plasma respectively. In addition, these channels are also sensitive to cooler plasma corresponding to ~1-2 MK. In this study we have applied the algorithm devised by Warren et al. (2012) to remove cooler emission from the 94Å channel to deduce only the high temperature structure of the flux rope and to study its temporal evolution. We found that the flux rope was very clearly seen in clean 94Å channel image corresponding to Fe XVIII emission which corresponds to a plasma at a temperature of 7 MK. This temperature matched well with that obtained using DEM analysis. This study provides important constrains in the modelling of the thermodynamic structure of the flux ropes in CMEs.

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arXiv2015

Intensity Conserving Spectral Fitting

The detailed shapes of spectral line profiles provide valuable information about the emitting plasma, especially when the plasma contains an unresolved mixture of velocities, temperatures, and densities. As a result of finite spectral resolution, the intensity measured by a spectrometer is the average intensity across a wavelength bin of non-zero size. It is assigned to the wavelength position at the center of the bin. However, the actual intensity at that discrete position will be different if the profile is curved, as it invariably is. Standard fitting routines (spline, Gaussian, etc.) do not account for this difference, and this can result in significant errors when making sensitive measurements. Detection of asymmetries in solar coronal emission lines is one example. Removal of line blends is another. We have developed an iterative procedure that corrects for this effect. It can be used with any fitting function, but we employ a cubic spline in a new analysis routine called Intensity Conserving Spline Interpolation (ICSI). As the name implies, it conserves the observed intensity within each wavelength bin, which ordinary fits do not. Given the rapid convergence, speed of computation, and ease of use, we suggest that ICSI be made a standard component of the processing pipeline for spectroscopic data.

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arXiv2015

IRIS and SDO observation of recurrent explosive events

Observations of recurrent explosive events (EEs) with time scale of 3-5 minutes are reported. These EEs have been observed with the Interface Region Imaging Spectrograph (IRIS) and have a spatial dimension of $\sim1.5"$ along the slit. The spectral line profiles of \ion{C}{2}~$1335/1336$ Å and \ion{Si}{4}~$1394/1403$ Å become highly broadened both in red as well as blue wings. Several absorption lines on top of the broadened profiles were identified. In addition, emission lines corresponding to neutral lines such as \ion{Cl}{1}~1351.66~Å, \ion{C}{1}~1354.29~Å, and \ion{C}{1}~1355.84~Å were identified. The \ion{C}{1}~1354.29~Å, and \ion{C}{1}~1355.84 Å lines were found only during the EEs whereas \ion{Cl}{1}~1351.66~Å broadens during the EEs. The estimated lower limit on electron number density obtained using the line ratios of \ion{Si}{4} and \ion{O}{4} is about $10^{13.5}$ cm$^{-3}$, suggesting that the observed events are most likely occurring at heights corresponding to lower chromosphere. To the best of our knowledge, for the first time we have detected short-period variability (30 s and 60--90 s) within the EE bursts. Observations of photospheric magnetic field underneath EEs indicate that negative polarity field emerges in the neighbourhood of oppositely directed positive fields which undergo repetitive reconnection (magnetic flux cancellation) events. The dynamic changes observed in AIA 1700 Å, 1600 Å, \ion{C}{2} 1330 Å and \ion{Si}{4} 1400 Å intensity images corresponded very well with the emergence and cancellation of photospheric magnetic field (negative polarity) on the time scale of 3--5 min. The observations reported here suggests that these EEs are formed due to magnetic reconnection and are occurring in the lower chromosphere.

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arXiv2014

Spectroscopic Observations of a Coronal Loop: Basic Physical Plasma Parameters Along the Full Loop Length

Coronal loops are the basic structures of the solar transition region and corona. The understanding of physical mechanism behind the loop heating, plasma flows, and filling are still considered a major challenge in the solar physics. The mechanism(s) should be able to supply mass to the corona from the chromosphere and able to heat the plasma over 1 MK within the small distance of few hundred km from the chromosphere to the corona. This problem makes coronal loops an interesting target for detailed study. In this study, we focus on spectroscopic observations of a coronal loop, observed in its full length, in various spectral lines as recorded by the Extreme-ultraviolet Imaging Spectrometer (EIS) on-board Hinode. We derive physical plasma parameters such as electron density, temperature, pressure, column depth, and filling factors along the loop length from one foot-point to the another. The obtained parameters are used to infer whether the observed coronal loop is over-dense or under-dense with respect to gravitational stratification of the solar atmosphere. These new measurements of physical plasma parameters, from one foot-point to another, provide important constraints on the modeling of the mass and energy balance in the coronal loops.

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arXiv2014

Observations and modelling of North-South asymmetries using a Flux Transport Dynamo

The peculiar behaviour of the solar cycle 23 and its prolonged minima has been one of the most studied problems over the last few years. In the present paper, we study the asymmetries in active region magnetic flux in the northern and southern hemispheres during complete solar cycle 23 and rising phase of solar cycle 24. During the declining phase of solar cycle 23, we find that the magnetic flux in the southern hemisphere is about 10 times stronger than that in the northern hemisphere during the declining phase of the solar cycle 23 and during the rising phase of cycle 24, however, this trend reversed. The magnetic flux becomes about a factor of 4 stronger in the northern hemisphere to that of southern hemisphere. Additionally, we find that there was significant delay (about 5 months) in change of the polarity in the southern hemisphere in comparison with the northern hemisphere. These results provide us with hints of how the toroidal fluxes have contributed to the solar dynamo during the prolonged minima in the solar cycle 23 and in the rising phase of the solar cycle 24. Using a solar flux-transport dynamo model, we demonstrate that persistently stronger sunspot cycles in one hemisphere could be caused by the effect of greater inflows into active region belts in that hemisphere. Observations indicate that greater inflows are associated with stronger activity. Some other change or difference in meridional circulation between hemispheres could cause the weaker hemisphere to become the stronger one.

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