arXiv subjects

Ramesh Chandra

Publications and source records attributed to Ramesh Chandra.

53 records · Page 3Linked to original sources

arXiv2020

North-South Distribution and Asymmetry of GOES SXR Flares during Solar Cycle 24

Here we present the results of the study of the north-south (N-S) distribution and asymmetry of GOES soft X-ray (SXR) flares during solar cycle 24. The period of study includes ascending, maximum and descending phases of the cycle. During the cycle double-peaked (2011, 2014) solar maximum has occurred. The cycle peak in the year 2011 is due to B-class flares excess activity in the northern hemisphere (NH) whereas C and M class flares excess activity in the southern hemisphere (SH) supported the second peak of the cycle in 2014. The data analysis shows that the SXR flares are more pronounced in 11 to 20 degree latitudes for each hemisphere. Cumulative values of SXR flare count show northern excess during the ascending phase of the cycle. However, in the descending phase of the cycle, southern excess occurred. In the cycle a significant SH dominated asymmetry exists. Near the maximum of the cycle, the asymmetry enhances pronouncedly and reverses in sign.

astro-ph.SR↗

arXiv2020

Development of a Confined Circular-cum-parallel Ribbon Flare and Associated Pre-flare Activity

We study a complex GOES M1.1 circular ribbon flare and related pre-flare activity on 26 January 2015 [SOL26-01-2015] in solar active region NOAA 12268. This flare activity was observed by the AIA on board SDO and the RHESSI. The examination of photospheric magnetograms during the extended period, prior to the event, suggests the successive development of a so-called 'anemone' type magnetic configuration. NLFFF extrapolation reveals a fan-spine magnetic configuration with the presence of a coronal null-point. We found that the pre-flare activity in the active region starts ~15 min prior to the main flare in the form of localized bright patches at two locations. A comparison of locations and spatial structures of the pre-flare activity with magnetic configuration of the corresponding region suggests onset of magnetic reconnection at the null-point along with the low-atmosphere magnetic reconnection caused by the emergence and the cancellation of the magnetic flux. The main flare of M1.1 class is characterized by the formation of a well-developed circular ribbon along with a region of remote brightening. Remarkably, a set of relatively compact parallel ribbons formed inside the periphery of the circular ribbon which developed lateral to the brightest part of the circular ribbon. During the peak phase of the flare, a coronal jet is observed at the north-east edge of the circular ribbon which suggests interchange reconnection between large-scale field lines and low-lying closed field lines. Our investigation suggests a combination of two distinct processes in which ongoing pre-flare null-point reconnection gets further intensified as the confined eruption along with jet activity proceeded from within the circular ribbon region which results to the formation of inner parallel ribbons and corresponding post-reconnection arcade.

astro-ph.SR↗

arXiv2020

A case-study of multi-temperature coronal jets for emerging flux MHD models

Context: Hot coronal jets are a basic observed feature of the solar atmosphere whose physical origin is still being actively debated. Aims: We study six recurrent jets occurring in the active region NOAA 12644 on April 04, 2017. They are observed in all the hot filters of AIA as well as cool surges in IRIS slit-jaw high spatial and temporal resolution images. Methods: The AIA filters allow us to study the temperature and the emission measure of the jets using the filter ratio method. We study the pre-jet phases by analyzing the intensity oscillations at the base of the jets with the wavelet technique. Results: A fine co-alignment of the AIA and IRIS data shows that the jets are initiated at the top of a canopy-like, double-chambered structure with cool emission on one side and hot emission in the other. The hot jets are collimated in the hot temperature filters, have high velocities (around 250 km/s) and accompanied by the cool surges and ejected kernels both moving at about 45 km/s. In the pre-phase of the jets, at their base we find quasi-periodic intensity oscillations in phase with small ejections; they have a period between 2 and 6 minutes and are reminiscent of acoustic or MHD waves. Conclusions: This series of jets and surges provides a good case-study to test the 2D and 3D magnetohydrodynamic (MHD) models that result from magnetic flux emergence. The double-chambered structure found in the observations corresponds to the cold and hot loop regions found in the models beneath the current sheet that contains the reconnection site. The cool surge with kernels is comparable with the cool ejection and plasmoids that naturally appear in the models.

astro-ph.SR↗

arXiv2020

Characteristics of SEPs during Solar Cycle 21-24

The study of the solar energetic particle events (SEPs) and their association with solar flares and other activities are very crucial to understand the space weather. Keeping this in view, in this paper, we present the study of the SEPs (intensity equal to or greater than 10 pfu) during the solar cycle 21 to 24 (1976-2017) in > 10 MeV energy channels associated with solar flares. For our analysis, we have used the data from different instruments onboard SOHO satellite. We have examined the flare size, source location, CMEs characteristics of associated SEPs. About 31% and 69% of the SEPs were originated from the eastern and western solar hemisphere respectively. The average CME speed and width were 1238 km/s and 253 deg respectively. About 58 % SEPs were associated with halo CMEs and 42% of SEPs associated with CMEs width varying from 10 deg to 250 deg respectively.

astro-ph.SR↗

arXiv2019

A Hot Cusp-Shaped Confined Solar Flare

We analyze a confined flare that developed a hot cusp-like structure high in the corona (H ~ 66 Mm). A growing cusp-shaped flare arcade is a typical feature in the standard model of eruptive flares, caused by magnetic reconnection at progressively larger coronal heights. In contrast, we observe a static hot cusp during a confined flare. Despite an initial vertical temperature distribution similar to that in eruptive flares, we observe a distinctly different evolution during the late (decay) phase, in the form of prolonged hot emission. The distinct cusp shape, rooted at locations of non-thermal precursor activity, was likely caused by a magnetic field arcade that kinked near the top. Our observations indicate that the prolonged heating was a result of slow local reconnection and an increased thermal pressure near the kinked apexes due to continuous plasma upflows.

astro-ph.SR↗

arXiv2019

How Rotating Solar Atmospheric Jets Become Kelvin--Helmholtz Unstable

Recent observations support the propagation of a number of magnetohydrodynamic (MHD) modes which, under some conditions, can become unstable and the developing instability is the Kelvin--Helmholtz instability (KHI). In its nonlinear stage the KHI can trigger the occurrence of wave turbulence which is considered as a candidate mechanism for coronal heating. We review the modeling of tornado-like phenomena in the solar chromosphere and corona as moving weakly twisted and spinning cylindrical flux tubes, showing that the KHI rises at the excitation of high-mode MHD waves. The instability occurs within a wavenumber range whose width depends on the MHD mode number \emph{m}, the plasma density contrast between the rotating jet and its environment, and also on the twists of the internal magnetic field and the jet velocity. We have studied KHI in two twisted spinning solar polar coronal hole jets, in a twisted rotating jet emerging from a filament eruption, and in a rotating macrospicule. The theoretically calculated KHI development times of a few minutes for wavelengths comparable to the half-widths of the jets are in good agreement with the observationally determined growth times only for high order (10 $\mathrm{\leqslant}$ \emph{m} $\mathrm{\leqslant}$ 65) MHD modes. Therefore, we expect that the observed KHI in these cases is due to unstable high-order MHD modes.

astro-ph.SR↗

arXiv2019

Kinematics and Energetics of the EUV Waves on 11 April 2013

In this study, we present the observations of extreme-ultraviolet (EUV) waves associated with an M6.5 flare on 2013 April 11. The event was observed by Solar Dynamics Observatory (SDO) in different EUV channels. The flare was also associated with a halo CME and type II radio bursts. We observed both fast and slow components of the EUV wave. The speed of the fast component, which is identified as a fast-mode MHD wave, varies in the range from 600 to 640 km s^-1 , whereas the speed of the slow-component is ~140 km s^-1 . We observed an unusual phenomenon that, as the fast-component EUV wave passes through two successive magnetic quasi-separatrix layers (QSLs), two stationary wave fronts are formed locally. We propose that part of the outward-propagating fast-mode EUV wave is converted into slow-mode magnetohydrodynamic waves, which are trapped in local magnetic field structures, forming successive stationary fronts. Along the other direction, the fast-component EUV wave also creates oscillations in a coronal loop lying ~225 Mm away from the flare site. We have computed the energy of the EUV wave to be of the order of 10^20 J.

astro-ph.SR↗

arXiv2018

ADARES: Adaptive Resource Management for Virtual Machines

Virtual execution environments allow for consolidation of multiple applications onto the same physical server, thereby enabling more efficient use of server resources. However, users often statically configure the resources of virtual machines through guesswork, resulting in either insufficient resource allocations that hinder VM performance, or excessive allocations that waste precious data center resources. In this paper, we first characterize real-world resource allocation and utilization of VMs through the analysis of an extensive dataset, consisting of more than 250k VMs from over 3.6k private enterprise clusters. Our large-scale analysis confirms that VMs are often misconfigured, either overprovisioned or underprovisioned, and that this problem is pervasive across a wide range of private clusters. We then propose ADARES, an adaptive system that dynamically adjusts VM resources using machine learning techniques. In particular, ADARES leverages the contextual bandits framework to effectively manage the adaptations. Our system exploits easily collectible data, at the cluster, node, and VM levels, to make more sensible allocation decisions, and uses transfer learning to safely explore the configurations space and speed up training. Our empirical evaluation shows that ADARES can significantly improve system utilization without sacrificing performance. For instance, when compared to threshold and prediction-based baselines, it achieves more predictable VM-level performance and also reduces the amount of virtual CPUs and memory provisioned by up to 35% and 60% respectively for synthetic workloads on real clusters.

cs.DC↗

arXiv2018

Generalisation of the Magnetic Field Configuration of typical and atypical Confined Flares

Atypical flares cannot be naturally explained with standard models. To predict such flares, we need to define their physical characteristics, in particular, their magnetic environment, and identify pairs of reconnected loops. Here, we present in detail a case-study of a confined flare preceded by flux cancellation that leads to the formation of a filament. The slow rise of the non-eruptive filament favours the growth and reconnection of overlying loops. The flare is only of C5.0 class but it is a long duration event. The reason is that it is comprised of three successive stages of reconnection. A non-linear force-free field extrapolation and a magnetic topology analysis allow us to identify the loops involved in the reconnection process and build a reliable scenario for this atypical confined flare. The main result is that a curved magnetic polarity inversion line in active regions is a key ingredient for producing such atypical flares. A comparison with previous extrapolations for typical and atypical confined flares leads us to propose a cartoon for generalizing the concept

astro-ph.SR↗

arXiv2018

Multiple Solar Jets from NOAA AR 12644

We present here the observations of solar jets observed on April 04, 2017 from NOAA active region (AR) 12644 using high temporal and spatial resolution AIA instrument. We have observed around twelve recurring jets during the whole day. Magnetic flux emergence and cancellation have been observed at the jet location. The multi-band observations evidenced that these jets were triggered due to the magnetic reconnection at low coronal null-point.

astro-ph.SR↗

arXiv2018

High mode magnetohydrodynamic waves propagation in a twisted rotating jet emerging from a filament eruption

We study the conditions under which high mode magnetohydrodynamic (MHD) waves propagating on a rotating jet emerging from the filament eruption on 2013 April 10--11 can became unstable against the Kelvin--Helmholtz instability (KHI). The evolution of jet indicates the blob like structure at its boundary which could be one of the observable features of the KHI development. We model the jet as a twisted rotating axially moving magnetic flux tube and explore the propagation characteristics of the running MHD modes on the basis of dispersion relations derived in the framework of the ideal magnetohydrodynamics. It is established that unstable MHD waves with wavelengths in the range of $12$--$15$~Mm and instability developing times from $1.5$ to $2.6$~min can be detected at the excitation of high mode MHD waves. The magnitude of the azimuthal mode number $m$ crucially depends upon the twist of the internal magnetic field. It is found that at slightly twisted magnetic flux tube the appropriate azimuthal mode number is $m = 16$ while in the case of a moderately twisted flux tube it is equal to $18$.

astro-ph.SR↗

arXiv2017

Observational and model analysis of a two-ribbon flare possibly induced by a neighbouring blowout jet

In this paper, we present unique observations of a blowout coronal jet that possibly triggered a two-ribbon confined C1.2 flare in a bipolar solar active region NOAA 12615 on 2016 December 5. The jet activity initiates at chromospheric/transition-region heights with a small brightening that eventually grows in a larger volume with well developed standard morphological jet features, viz., base and spire. The spire widens up with a collimated eruption of cool and hot plasma components, observed in the 304 and 94 A channels of AIA, respectively. The speed of the plasma ejection, which forms the jet's spire, was higher for the hot component (~200 km/s) than the cooler one (~130 km/s). The NLFF model of coronal fields at pre- and post-jet phases successfully reveal opening of previously closed magnetic field lines with a rather inclined/low-lying jet structure. The peak phase of the jet emission is followed by the development of a two-ribbon flare that shows coronal loop emission in HXRs up to ~25 keV energy. The coronal magnetic fields rooted at the location of EUV flare ribbons, derived from the NLFF model, demonstrate the pre-flare phase to exhibit an "X-type" configuration while the magnetic fields at the post-flare phase are more or less parallel oriented. The comparisons of multi-wavelength measurements with the magnetic field extrapolations suggest that the jet activity likely triggered the two-ribbon flare by perturbing the field in the interior of the active region.

astro-ph.SR↗

arXiv2014

Weak-antilocalization and Surface Dominated Transport in Topological Insulator Bi2Se2Te

We explore the phase coherence of thin films of the topological insulator material Bi2Se2Te grown through pulsed laser deposition (PLD) technique. The films were characterised using various techniques for phase and composition. The films were found to be of good quality. We carried out extensive magneto-transport studies of these films and found that they exhibit two dimensional weak antilocalization behaviour. A careful analysis revealed a relatively high phase coherence length (58nm at 1.78K) for a PLD grown film. Since PLD is an inexpensive technique, with the possibility to integrate with other materials, one can make devices which can be extremely useful for low power spintronics and topological quantum computation.

cond-mat.mes-hall↗

arXiv2014

Sunspot Waves and Triggering of Homologous Active Region Jets

We present and discuss multi-wavelength observations of five homologous recurrent solar jets that occurred in active region NOAA 11133 on 11 December, 2010. These jets were well observed by the Solar Dynamic observatory (SDO) with high spatial and temporal resolution. The speed of the jets ranged between 86 and 267 km/s. A type III radio burst was observed in association with all the five jets. The investigation of the over all evolution of magnetic field in the source regions suggested that the flux was continuously emerging on longer term. However, all the jets but J5 were triggered during a local dip in the magnetic flux, suggesting the launch of the jets during localised submergence of magnetic flux. Additionally, using the PFSS modelling of the photospheric magnetic field, we found that all the jets were ejected in the direction of open field lines. We also traced sunspot oscillations from the sunspot interior to foot-point of jets and found presence of ~ 3 minute oscillations in all the SDO/AIA passbands. The wavelet analysis revealed an increase in amplitude of the oscillations just before the trigger of the jets, that decreased after the jets were triggered. The observations of increased amplitude of the oscillation and its subsequent decrease provides evidence of wave-induced reconnection triggering the jets.

astro-ph.SR↗

arXiv2010

Ionospheric response to the space weather event of 18 November 2003 -- An investigation

The present study explores the ionospheric effects of the well cited solar flare events (M3.2, M3.9/2N) of 18 November 2003 associated with CMEs. The Hα observations of these flares (taken with 15 cm Solar Tower Telescope at ARIES, Nainital) have been analysed to see an association of these flare events with the geomagnetic storm occurred on 20 November 2003. The ionospheric data from Puerto Rico (18.5°N, 67.2°W), Dyess (32.4°N, 99.7°W) and Millstone Hill (42.6°N, 71.5°W) together with the disturbance storm time indices (Dst index) variability exhibited a corresponding associations having a delay in the solar wind parameters, triggered by these flare events.

astro-ph.SR↗

arXiv2006

Energetics and dynamics of an impulsive flare on March 10, 2001

We present the H$α$ observations from ARIES, Nainital of a compact and impulsive solar flare occurred on March 10, 2001 and associated with a CME. We have also analysed HXT, SXT/Yohkoh observations as well as radio observations from Nobeyama Radio Observatory to derive the energetics and dynamics of this impulsive flare. We co-align the H$α$, SXR, HXR, MW and magnetogram images within the instrumental spatial resolution limit. We detect a single HXR source in this flare, which is found spatially associated with one of the H$α$ bright kernel. The unusual feature of HXR and H$α$ sources, observed for the first time, is the rotation during the impulsive phase in clockwise direction. We propose that the rotation may be due to asymmetric progress of the magnetic reconnection site or may be due to the change of peak point of the electric field. In MW emission we found two sources, one is main source which is at the main flare site and another is remote source located in South-West direction. It appears that the remote source is formed by the impact of accelerated energetic electrons from the main flare site. From the spatial co-relation of multi-wavelength images of th

astro-ph↗

arXiv2004

Multi-wavelength observations of an unusual impulsive flare associated with CME

We present the results of a detailed analysis of multi-wavelength observations of a very impulsive solar flare 1B/M6.7, which occurred on 10 March, 2001 in NOAA AR 9368 (N27 W42). The observations show that the flare is very impulsive with very hard spectrum in HXR that reveal non-thermal emission was most dominant. On the other hand this flare also produced type II radio burst and coronal mass ejections (CME), which are not general characteristics for impulsive flares. In H$α$ we observed the bright mass ejecta (BME) followed by drak mass ejecta (DME). Based on the consistence of the onset times and direction of BME and CME, we conclude that these two phenomena are closely associated. It is inferred that the energy build-up took place due to photospheric reconnection between emerging positive parasitic polarity and predominant negative polarity, which resulted as a consequence of flux cancellation. The shear increased to $>80^o$ due to further emergence of positive parasitic polarity causing strongly enhanced cancellation of flux. It appears that such enhanced magnetic flux cancellation in a strongly sheared region triggered the impulsive flare.

astro-ph↗