arXiv subjects

Ramesh Chandra

Publications and source records attributed to Ramesh Chandra.

At least 37 records · Page 2Linked to original sources

arXiv2022

Extreme-Ultraviolet Wave and Accompanying Loop Oscillations

We present the observations of an extreme-ultraviolet (EUV) wave, which originated from the active region (AR) NOAA 12887 on 28 October 2021 and its impact on neighbouring loops. The event was observed by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) satellite at various wavebands and by the Solar TErrestrial RElations Observatory-Ahead (STEREO-A) with its Extreme-Ultraviolet Imager (EUVI) and COR1 instruments with a different view angle than SDO. We show that the EUV wave event consists of several waves as well as non-wave phenomena. The wave components include: the fast-mode part of the EUV wave event, creation of oscillations in nearby loops, and the appearance of wave trains. The non-wave component consists of stationary fronts. We analyze selected oscillating loops and find that the periods of these oscillations range from 230 - 549 s. Further, we compute the density ratio inside and outside the loops and the magnetic field strength. The computed density ratio and magnetic field are found in the range of 1.08 - 2.92 and 5.75 - 8.79 G, respectively. Finally, by combining SDO and STEREO-A observations, we find that the observed EUV wave component propagates ahead of the CME leading edge.

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arXiv2022

Analysis of the Evolution of a Multi-Ribbon Flare and Failed Filament Eruption

How filaments form and erupt are topics about which solar researchers have wondered since more than a century and that are still open to debate. We present observations of a filament formation, its failed eruption, and the associated flare (SOL2019-05-09T05:51) that occurred in active region (AR) 12740 using data from SDO, STEREO-A, IRIS, and NSO/GONG. AR 12740 was a decaying region formed by a very disperse following polarity and a strong leading spot, surrounded by a highly dynamic zone where moving magnetic features (MMFs) were seen constantly diverging from the spot. Our analysis indicates that the filament was formed by the convergence of fibrils at a location where magnetic flux cancellation was observed. Furthermore, we conclude that its destabilization was also related to flux cancellation associated to the constant shuffling of the MMFs. A two-ribbon flare occurred associated to the filament eruption; however, because the large-scale magnetic configuration of the AR was quadrupolar, two additional flare ribbons developed far from the two main ones. We model the magnetic configuration of the AR using a force-free field approach at the AR scale size. This local model is complemented by a global potential-field source-surface one. Based on the local model, we propose a scenario in which the filament failed eruption and flare are due to two reconnection processes, one occurring below the erupting filament, leading to the two-ribbon flare, and another one above it between the filament flux-rope configuration and the large-scale closed loops. Our computation of the reconnected magnetic flux added to the erupting flux rope, compared to that of the large-scale field overlying it, lets us conclude that the latter was large enough to prevent the filament eruption. A similar conjecture can be drawn from the computation of the magnetic tension derived from the global field model.

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arXiv2022

Dynamics and Kinematics of the EUV Wave Event on 6 May 2019

We present here the kinematics of the EUV wave associated with a GOES M1.0-class solar flare, which originates in NOAA AR 12740. The event is thoroughly observed with Atmospheric Imaging Assembly (AIA) onboard Solar Dynamics Observatory (SDO) with high spatio-temporal resolutions. This event displays many features of EUV waves, which are very decisive for the understanding of the nature of EUV waves. These features include: a fast-mode wave, a pseudo wave, a slow-mode wave and stationary fronts, probably due to mode conversion. One fast-mode wave also propagates towards the coronal hole situated close to the north pole and the wave speed does not change when it encounters the coronal hole. We intend to provide self-consistent interpretations for all these different features.

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arXiv2022

Prominence Oscillations activated by an EUV wave

Prominence oscillations are one of interesting phenomena in the solar atmosphere, which can be utilized to infer the embedded magnetic field magnitude. We present here the transverse oscillations of two different prominences located at the East solar limb on 2011 February 11 using the multi-wavebands data of the Atmospheric Imaging Assembly (AIA) on-board the Solar Dynamics Observatory (SDO) satellite. A prominence eruption was observed towards the east direction with an average speed of ~275 km/s. The eruption is fitted with the combination of a linear and an exponential functions of time. An extreme ultraviolet (EUV) wave event was associated with the prominence eruption. This EUV wave triggered the oscillations of both prominences on the East limb. We computed the period of each prominence using the wavelet analysis method. The oscillation period varies from 14 to 22 min. The magnetic field of the prominences was derived, which ranges from 14 to 20 G.

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arXiv2022

Sympathetic Quiet and Active Region Filament Eruptions

We present the observations of three sympathetic filament eruptions occurring on 19 July 2015 namely F1, F2, and F3. The events were observed in UV/EUV wavelengths by Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory and by Global Oscillation Network Group telescope in Hα line. As filament F1 starts to erupt, a part of it falls close to the location of the F2 and F3 filaments. This causes the eruption of F2 and F3 during which the two filaments merge together and trigger a medium-class CME and a long-duration GOES C2.1 class flare. We discuss the dynamics and kinematics of these three filament eruptions and related phenomena.

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arXiv2021

Empirical atmosphere model in a mini flare during magnetic reconnection

A spatio-temporal analysis of IRIS spectra of MgII, CII, and SiIV ions allows us to study the dynamics and the stratification of the flare atmosphere along the line of sight during the magnetic reconnection phase at the jet base. Strong asymmetric MgII and CII line profiles with extended blue wings observed at the reconnection site are interpreted by the presence of two chromospheric temperature clouds: one explosive cloud with blueshifts at 290 km/s and one cloud with smaller Doppler shift (around 36 km/s). Simultaneously at the same location a mini flare was observed with strong emission in multi temperatures (AIA), in several spectral IRIS lines (e.g. Oiv and Siiv, Mgii), absorption of identified chromospheric lines in Siiv line profile, enhancement of the Balmer continuum and X-ray emission by FERMI/GBM. With the standard thick-target flare model we calculate the energy of non thermal electrons observed by FERMI and compare it to the energy radiated by the Balmer continuum emission. We show that the low energy input by non thermal electrons above 20 keV was still sufficient to produce the excess of Balmer continuum.

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arXiv2021

Solar jets observed with the Interface Region Imaging Spectrograph (IRIS)

Solar jets are impulsive, collimated plasma ejections that are triggered by magnetic reconnection. They are observed for many decades in various temperatures and wavelengths, therefore their kinematic characteristics, such as velocity and recurrence, have been extensively studied.Nevertheless, the high spatial resolution of the Interface Region Imaging Spectrograph (IRIS) launched in 2013 allowed us to make a step forward in the understanding of the relationship between surges and hot jets. In this paper we report on several results of recent studies of jets observed by IRIS. Cool and hot plasma have been detected with ejections of cool blobs having a speed reaching 300 km/s during the impulsive phase of jet formation and slow velocity surges surrounding hot jets after the reconnection phase. Plasma characteristics of solar jets, such as the emission measure, temperature, and density have been quantified. A multi-layer atmosphere at the reconnection site based on observed IRIS spectra has been proposed. IRIS evidenced bidirectional flows at reconnection sites, and tilt along the spectra which were interpreted as the signature of twist in jets. The search of possible sites for reconnection could be achieved by the analysis of magnetic topology. Combining Solar Dynamics Observatory/Helioseismic Magnetic Imager (SDO/HMI) vector magnetograms and IRIS observations, it was found that reconnection site could be located at null points in the corona as well as in bald patch regions low in the photosphere. In one case study a magnetic sketch could explain the initiation of a jet starting in a bald patch transformed to a current sheet in a dynamical way, and the transfer of twist from a flux rope to the jet during the magnetic reconnection process.

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arXiv2021

Investigation of two coronal mass ejections from circular ribbon source region: Origin, Sun-Earth propagation and Geo-effectiveness

In this article, we compare the properties of two coronal mass ejections (CMEs) that show similar source region characteristics but different evolutionary behavior in the later phases. We discuss the two events in terms of their near-Sun characteristics, interplanetary evolution, and geo-effectiveness. We carefully analyzed the initiation and propagation parameters of these events to establish the precise CME-ICME connection and their near-Earth consequences. The First event was associated with poor geo-magentic storm disturbance index (Dst $\approx$-20 nT) while the second event is associated with intense geomagnetic storm of DST $\approx$-119 nT. The configuration of the sunspots in the active regions and their evolution are observed by Helioseismic and Magnetic Imager (HMI). For source region imaging, we rely on data obtained from Atmospheric Imaging Assembly (AIA) on board Solar Dynamics Observatory (SDO) and H$α$ filtergrams from Solar Tower Telescope at Aryabhatta Research Institute of Observational Sciences (ARIES). For both the CMEs, flux rope eruptions from the source region triggered flares of similar intensities ($\approx$M1). At the solar source region of the eruptions, we observed circular ribbon flare (CRF) for both the cases, suggesting fan-spine magnetic configuration in the active region corona.

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arXiv2021

Filament Eruption Driving EUV Loop Contraction then Expansion above a Stable Filament

We analyze the observations of EUV loop evolution associated with the filament eruption located at the border of an active region. The event SOL2013-03-16T14:00 was observed with a large difference of view point by the Solar Dynamics Observatory and Solar Terrestrial Relations Observatory --A spacecraft. The filament height is fitted with the sum of a linear and exponential function. These two phases point to different physical mechanisms such as: tether-cutting reconnection and a magnetic instability. While no X-ray emission is reported, this event presents the classical eruption features like: separation of double ribbons and the growth of flare loops. We report the migration of the southern foot of the erupting filament flux rope due to the interchange reconnection with encountered magnetic loops of a neighbouring AR. Parallel to the erupting filament, a stable filament remains in the core of active region. The specificity of this eruption is that coronal loops, located above the nearly joining ends of the two filaments, first contract in phase, then expand and reach a new stable configuration close to the one present at the eruption onset. Both contraction and expansion phases last around 20 min. The main difference with previous cases is that the PIL bent about 180 deg around the end of the erupting filament because the magnetic configuration is at least tri-polar. These observations are challenging for models which interpreted previous cases of loop contraction within a bipolar configuration. New simulations are required to broaden the complexity of the configurations studied.

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arXiv2021

Balmer continuum enhancement detected in a mini flare observed with IRIS

Optical and near-UV continuum emissions in flares contribute substantially to flare energy budget. Two mechanisms play an important role for continuum emission in flares: hydrogen recombination after sudden ionization at chromospheric layers and transportation of the energy radiatively from the chromosphere to lower layers in the atmosphere, the so called back-warming. The aim of the paper is to disentangle between these two mechanisms for the excess of Balmer continuum observed in a flare. Methods. We combine the observations of Balmer continuum obtained with IRIS (spectra and SJIs 2832 A) and hard X-ray (HXR) emission detected by FERMI Gamma Burst Monitor (GBM) during a mini flare. Calibrated Balmer continuum is compared to non-LTE radiative transfer flare models and radiated energy is estimated. Assuming thick target HXR emission, we calculate the energy of non-thermal electrons detected by FERMI GBM and compare it to the radiated energy. The favorable argument of a relationship between the Balmer continuum excess and the HXR emission is that there is a good time coincidence between both of them. In addition, the shape of the maximum brightness in the 2832 SJIs, which is mainly due to this Balmer continuum excess, is similar to the FERMI/GBM light curve. The electron-beam flux estimated from FERMI/GBM is consistent with the beam flux required in non-LTE radiative transfer models to get the excess of Balmer continuum emission observed in the IRIS spectra. The low energy input by non thermal electrons above 20 keV is sufficient to produce the enhancement of Balmer continuum emission. This could be explained by the topology of the reconnection site. The reconnection starts in a tiny bald patch region which is transformed dynamically in a X-point current sheet. The size of the interacting region would be under the spatial resolution of the instrument.

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arXiv2021

Fine Structures of an EUV Wave Event from Multi-Viewpoint Observations

In this study, we investigate an extreme ultraviolet (EUV) wave event on 2010 February 11, which occurred as a limb event from the Earth viewpoint and a disk event from the STEREO--B viewpoint. We use the data obtained by the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO) in various EUV channels. The EUV wave event was launched by a partial prominence eruption. Similar to some EUV wave events in previous works, this EUV wave event contains a faster wave with a speed of $\sim$445$\pm$6 km s$^{-1}$, which we call coronal Moreton wave, and a slower wave with a speed of $\sim$298$\pm$5 km s$^{-1}$, which we call "EIT wave". The coronal Moreton wave is identified as a fast-mode wave and the "EIT wave" is identified as an apparent propagation due to successive field-line stretching. We also observe a stationary front associated with the fast mode EUV wave. This stationary front is explained as mode conversion from the coronal Moreton wave to a slow-mode wave near a streamer.

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arXiv2021

Variation of Chromospheric Features as a Function of Latitude and Time using Ca-K Spectroheliograms for Solar Cycles 15-23: Implications for Meridional Flow

We have analysed the Ca-K images obtained at Kodaikanal Observatory as a function of latitude and time for the period of 1913 - 2004 covering the Solar Cycle 15 to 23. We have classified the chromospheric activity into plage, Enhanced Network (EN), Active Network (AN), and Quiet Network (QN) areas to differentiate between large strong active and small weak active regions. The strong active regions represent toroidal and weak active regions poloidal component of the magnetic field. We find that plages areas mostly up to 50 deg latitude belt vary with about 11-year Solar Cycle. We also find that weak activity represented by EN, AN and QN varies with about 11-year with significant amplitude up to about 50 deg latitude in both the hemispheres. The amplitude of variation is minimum around 50 deg latitude and again increases by small amount in the polar region. In addition, the plots of plages, EN, AN and QN as a function of time indicate the maximum of activity at different latitude occur at different epoch. To determine the phase difference for the different latitude belts, we have computed the cross-correlation coefficients of other latitude belts with 35 deg latitude belt. We find that activity shifts from mid-latitude belts towards equatorial belts at fast speed at the beginning of Solar Cycle and at slower speed as the cycle progresses. The speed of shift varies between approximately 19 and 3 m/s considering all the data for the observed period. This speed can be linked with speed of meridional flows those believed to occur between convection zone and the surface of the Sun.

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arXiv2021

Observations of a prominence eruption and loop contraction

Context. Prominence eruptions provide key observations to understand the launch of coronal mass ejections as their cold plasma traces a part of the unstable magnetic configuration. Aims. We select a well observed case to derive observational constraints for eruption models. Methods. We analyze the prominence eruption and loop expansion and contraction observed on 02 March 2015 associated with a GOES M3.7 class flare (SOL2015-03-02T15:27) using the data from Atmospheric Imaging Assembly (AIA) and the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). We study the prominence eruption and the evolution of loops using the time-distance techniques. Results. The source region is a decaying bipolar active region where magnetic flux cancellation is present for several days before the eruption. AIA observations locate the erupting prominence within a flux rope viewed along its local axis direction. We identify and quantify the motion of loops in contraction and expansion located on the side of the erupting flux rope. Finally, RHESSI hard X-ray observations identify the loop top and two foot-point sources. Conclusions. Both AIA and RHESSI observations support the standard model of eruptive flares. The contraction occurs 19 minutes after the start of the prominence eruption indicating that this contraction is not associated with the eruption driver. Rather, this prominence eruption is compatible with an unstable flux rope where the contraction and expansion of the lateral loop is the consequence of a side vortex developing after the flux rope is launched.

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arXiv2020

Multi thermal atmosphere of a mini solar flare during magnetic reconnection observed with IRIS

The Interface Region Imaging Spectrograph(IRIS) with its high spatial and temporal resolution brings exceptional plasma diagnostics of solar chromospheric and coronal activity during magnetic reconnection. The aim of this work is to study the fine structure and dynamics of the plasma at a jet base forming a mini flare between two emerging magnetic fluxes (EMFs) observed with IRIS and the Solar Dynamics Observatory (SDO) instruments. We proceed to a spatio-temporal analysis of IRIS spectra observed in the spectral ranges of Mg II, C II, and Si IV ions. Doppler velocities from Mg II lines are computed by using a cloud model technique. Strong asymmetric Mg II and C II line profiles with extended blue wings observed at the reconnection site (jet base) are interpreted by the presence of two chromospheric temperature clouds, one explosive cloud with blueshifts at 290 km/s and one cloud with smaller Dopplershift (around 36 km/s). Simultaneously at the same location (jet base), strong emission of several transition region lines (e.g. O IV and Si IV), emission of the Mg II triplet lines of the Balmer-continuum and absorption of identified chromospheric lines in Si IV broad profiles have been observed and analysed. Such observations of IRIS line and continuum emissions allow us to propose a stratification model for the white-light mini flare atmosphere with multiple layers of different temperatures along the line of sight, in a reconnection current sheet. It is the first time that we could quantify the fast speed (possibly Alfvénic flows) of cool clouds ejected perpendicularly to the jet direction by using the cloud model technique. We conjecture that the ejected clouds come from plasma which was trapped between the two EMFs before reconnection or be caused by chromospheric-temperature (cool) upflow material like in a surge, during reconnection

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arXiv2020

On the partial eruption of a bifurcated solar filament structure

The partial eruption of a filament channel with bifurcated substructures is investigated using datasets obtained from both ground-based and space-borne facilities. Small-scale flux reconnection/cancellation events in the region triggered the pile-up of ambient magnetic field, observed as bright EUV loops in close proximity of the filament channel. This led to the formation of a V-shaped cusp structure at the site of interaction between the coalesced EUV loops and the filament channel, with the presence of distinct plasmoid structures and associated bidirectional flows. Analysis of imaging data from SDO/AIA further suggests the vertical split of the filament structure into two substructures. The perturbed upper branch of the filament structure rose up and erupted with the onset of an energetic GOES M1.4 flare at 04:30 UT on January 28, 2015. The estimated twist number and squashing factor obtained from nonlinear force free-field extrapolation of the magnetic field data support the vertical split in filament structure with high twist in upper substructure. The loss in equilibrium of the upper branch due to torus instability, implying this as a potential triggering mechanism of the observed partial eruption.

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arXiv2020

The role of small-scale surface motions in the transfer of twist to a solar jet from a remote stable flux rope

Jets often have a helical structure containing ejected plasma that is both hot and also cooler and denser than the corona. Various mechanisms have been proposed to explain how jets are primarily attributed to a magnetic reconnection between the emergence of magnetic flux and environment or that of twisted photospheric motions that bring the system into a state of instability. Multi-wavelength observations of a twisted jet observed with AIA and IRIS were used to understand how the twist was injected into the jet. We followed the magnetic history of the active region based on the analysis of HMI vector magnetic field computed with the UNNOFIT code. This region is the result of the collapse of two emerging magnetic fluxes (EMFs) overlaid by arch filament systems that have been well-observed with AIA, IRIS, and NVST in H-alpha. In the magnetic field maps, we found evidence of the pattern of a long sigmoidal flux rope (FR) along the polarity inversion line between the two EMFs, which is the site of the reconnection. Before the jet, an extension of the FR was present and a part of it was detached and formed a small bipole with a bald patch (BP) region, which dynamically became an X-current sheet over the dome of one EMF where the reconnection took place. At the time of the reconnection, the Mg II spectra exhibited a strong extension of the blue wing that is decreasing over a distance of 10 Mm (from -300 km/s to a few km/s). This is the signature of the transfer of the twist to the jet. A comparison with numerical magnetohydrodynamics (MHD) simulations confirms the existence of the long FR. We conjecture that there is a transfer of twist to the jet during the extension of the FR to the reconnection site without FR eruption. There connection would start in the low atmosphere in the BP reconnection region and extend at an X-point along the current sheet formed above.

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arXiv2020

Cause and Kinematics of a Jet-Like CME

In this article, we present the multi-viewpoint and multi-wavelength analysis of an atypical solar jet based on the data from Solar Dynamics Observatory, SOlar, and Heliospheric Observatory, and Solar TErrestrial RElations Observatory. It is usually believed that the coronal mass ejections (CMEs) are developed from the large scale solar eruptions in the lower atmosphere. However, the kinematical and spatial evolution of the jet on 2013 April 28 guide us that the jet was clearly associated with a narrow CME having a width of approx 25 degrees with a speed of 450 km/s. To better understand the link between the jet and the CME, we did the coronal potential field extrapolation from the line of sight magnetogram of the AR. The extrapolations present that the jet eruption follows exactly the same path of the open magnetic field lines from the source region which provides the route for the jet material to escape from the solar surface towards the outer corona.

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arXiv2020

Eruptive-Impulsive Homologous M-class Flares Associated with Double-Decker Flux Rope Configuration in Mini-Sigmoid of NOAA 12673

We present a multiwavelength analysis of two homologous, short lived, impulsive flares of GOES class M1.4 and M7.3, that occurred from a very localized mini-sigmoid region within the active region NOAA 12673 on 2017 September 7. Both flares were associated with initial jet-like plasma ejection which for a brief amount of time moved toward east in a collimated manner before drastically changing direction toward southwest. Non-linear force-free field extrapolation reveals the presence of a compact double-decker flux rope configuration in the mini-sigmoid region prior to the flares. A set of open field lines originating near the active region which were most likely responsible for the anomalous dynamics of the erupted plasma, gave the earliest indication of an emerging coronal hole near the active region. The horizontal field distribution suggests a rapid decay of the field above the active region, implying high proneness of the flux rope system toward eruption. In view of the low coronal double-decker flux ropes and compact extreme ultra-violet (EUV) brightening beneath the filament along with associated photospheric magnetic field changes, our analysis supports the combination of initial tether-cutting reconnection and subsequent torus instability for driving the eruption.

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