arXiv subjects

Soumitra Hazra

Publications and source records attributed to Soumitra Hazra.

At least 19 recordsLinked to original sources

arXiv2026

Early-Time Reshaping of Laser-Induced Plasma Profiles

Laser-induced plasmas are primarily characterized by their temporal density decay. Using time-resolved transverse optical diffractometry, we reveal their concurrent spatial evolution, demonstrating pronounced broadening and flattening during the first 200 ps following ionization. This non-self-similar evolution arises from the local density dependence of electron-ion recombination, without particle transport. Exploiting the continuum of initial densities within a single plasma disc, we experimentally reconstruct an effective, time-dependent local recombination law over a broad density range. Applied locally to independently measured initial plasma profiles, this kinetic map quantitatively predicts their subsequent evolution. These results are relevant to transient diffractive optics, plasma-based optical elements and waveguides, and the development of laser-induced plasmas as platforms for gas-phase THz plasmonics.

physics.plasm-ph↗

arXiv2026

Direct Time-Domain Observation of l-Doubling via Centrifugal-Distortion Pre-compensation

We demonstrate direct time-domain observation of l-doubling contributions in molecular rotational dynamics using shaped femtosecond laser pulses. By imposing a tailored spectral phase on the excitation pulse, we pre-compensate centrifugal distortion, which otherwise leads to temporally broadened, multi-cycle revival structures that obscure fine rotational features. A cubic spectral phase [Phys. Rev. A 107, 053108 (2023)] compresses selected revivals into near single-cycle events, in agreement with an analytic expression derived from molecular rotational constants, enabling predictive pulse design beyond numerical optimization. The resulting distortion-free revivals reveal temporally separated l-doubling contributions that remain unresolved in conventional impulsive alignment experiments. The method proves robust against experimental imperfections, including spatial light modulator discretization. While selective control of individual l-doubling components becomes feasible, here we focus on their direct observation in the time domain.

quant-ph↗

arXiv2026

Exploring the Origin and Dynamics of Solar Magnetic Fields

The Sun is a magnetically active star and is the source of the solar wind, electromagnetic radiation and energetic particles which affect the heliosphere and the Earths atmosphere. The magnetic field of the Sun is responsible for most of the dynamic activity of the Sun. This thesis research seeks to understand solar magnetic field generation and the role that magnetic fields play in the dynamics of the solar atmosphere. Specifically, this thesis focuses on two themes: in the first part, we study the origin and behaviour of solar magnetic fields using magnetohydrodynamic dynamo theory and modelling, and in the second part, utilizing observations and data analysis we study two major problems in solar physics, namely, the coronal heating problem and initiation mechanisms of solar flares.

astro-ph.SR↗

arXiv2025

Can meridional flow variations explain the observed rising/declining phase asymmetry in the solar cycle?

Accurate prediction of the 11-year solar cycle remains a major challenge in solar physics and is important for space weather forecasting. A persistent property of the cycle is its asymmetry: the rise phase is usually much shorter than the decay phase. This asymmetry is often linked to variations in the Sun's meridional circulation, but it is unclear whether these variations are mainly deterministic, produced by Lorentz-force feedback, or stochastic in nature. We investigate this question using kinematic flux-transport dynamo simulations that include three types of time-dependent meridional flow: deterministic variations, stochastic fluctuations, and hybrid combinations of both. We evaluate cycle asymmetry using the ratio of rise to decay times and correlations of cycle amplitude with rise time, rise rate, and decay rate. Our results show that the temporal evolution of the meridional flow strongly controls cycle asymmetry. When both the meridional circulation and the Babcock-Leighton mechanism vary stochastically, the model does not produce cycles in which the decay phase is consistently longer than the rise phase. In contrast, deterministic variations motivated by Lorentz-force feedback and linked to the latitude of maximum toroidal field reproduce the observed asymmetry. In these cases, the meridional flow weakens near cycle maximum, stays reduced for some time, and then recovers, producing a longer decay phase. Hybrid models that mix deterministic and stochastic variability also match the observed rise-decay asymmetry. Across all simulations, cycle amplitude correlates strongly with rise rate, while correlations with rise time and decay rate are weaker but remain significant. These results highlight the key role of meridional flow variability in shaping solar cycle asymmetry and show that incorporating such variability can improve forecasting tools such as Solar Predict.

astro-ph.SR↗

arXiv2025

An MHD Simulation of the Possible Modulations of Stellar CMEs Radio Observations by an Exoplanetary Magnetosphere

Type II radio bursts are the indicator of adverse space weather in a stellar system. These radio bursts are the consequence of shock wave acceleration due to the coronal mass ejection (CME). In this study, we conduct a series of magnetohydrodynamic (MHD) simulations of CME-driven star-planet systems to investigate how close-in exoplanets modulate radio burst characteristics. We use a model for the stellar wind with a close-in exoplanet, and a CME model based on the eruption of a flux rope. We are able to generate synthetic radio burst images from our MHD simulations. We find that radio burst like phenomena is most likely to be observed for moderately active solar like stars and close-in exoplanetary systems have significant influence on the nature of radio burst spectrum. We find that when the exoplanet's magnetic field is relatively weak, its magnetosphere compresses the CME plasma, increasing local density and shifting the radio emission to higher frequencies. Conversely, a strong planetary magnetic field results in a large magnetosphere that prevents effective CME-shock development, producing weaker radio emission concentrated at lower frequencies, particularly at the flanks of the CME. For highly active solar-like stars, strong overlying stellar magnetic fields suppress the CME shock, greatly diminishing radio burst visibility. For HD 189733 (moderate stellar field), only intensity difference is visible when the CME arrives the planet. We also do not find significant modulation in the radio emission by a close-in exoplanet system when the stellar magnetic field is complex. In summary, our findings highlight that the nature of the radio burst spectrum is strongly dependent on both the topology of the stellar magnetic field and the magnetic strength of close-in exoplanets.

astro-ph.EP↗

arXiv2024

Tomographic diffractometry of laser-induced plasma formations

A sensitive optical diffractometry method is developed and utilized for advanced tomography of laser-induced air plasma formations. Using transverse diffractometry and Supergaussian plasma distribution modelling we extract the main parameters of the plasma being the plasma density, width and shape with 20 micrometer spatial resolution throughout the plasma formation. The experimentally recorded diffraction patterns fitted by the Supergaussian plasma model are found to capture unprecedentedly delicate traits in the evolution of the plasma from its effective birth and on. Key features in the spatial evolution of the plasma such as the 'escape position', the 'turning point' and the refocusing dynamics of the beam are identified and explored in details. Our work provides experimental and theoretical access into the highly nonlinear dynamics of laser-induced air plasma.

physics.plasm-ph↗

arXiv2023

Enhanced transmission at the zeroth-order mode of a terahertz Fabry-Perot cavity

A planar Fabry-Perot cavity with inter-mirror spacing significantly shorter than the probing wavelength is explored for its "zero-order mode" terahertz transmission. The enhanced transmission observed as d approaches 0 indicates that such cavities satisfy the resonance condition across a broad terahertz bandwidth. The experimental signatures from this elusive, "technically challenging" regime are evidenced using time-domain terahertz spectroscopy and are complemented by numerical calculations. The results raise intriguing possibilities for terahertz field modulation and pave new paths for strong coupling of multiple transition frequencies simultaneously.

physics.optics↗

arXiv2022

Exoplanet Radio Transits as a Probe for Exoplanetary Magnetic Fields -- Time-dependent MHD Simulations

We perform a series of time dependent Magnetohydrodynamic simulations of the HD 189733 star-planet system in order to predict radio transit modulations due to the interaction between the stellar wind and planetary magnetic field. The simulation combines a model for the stellar corona and wind with an exoplanet that is orbiting the star in a fully dynamic, time-dependent manner. Our simulations generate synthetic radio images that enable us to obtain synthetic radio lightcurves in different frequencies. We find a clear evidence for the planetary motion in the radio light curves. Moreover, we find specific repeated features in the light curves that are attributed to the passage of the planetary magnetosphere in front of the star during transit. More importantly, we find a clear dependence in the magnitude and phase of these lightcurve features on the strength of the planetary magnetic field. Our work demonstrates that if radio transits could be observed, they could indeed provide information about the magnetic field strength of the transiting exoplanet. Future work to parameterize these lightcurve features and their dependence on the planetary field strength would provide tools to search for these features in radio observations datasets. As we only consider the thermal radio emission from the host star for our study, very sensitive radio interferometers are necessary to detect these kinds of planetary transit in radio.

astro-ph.EP↗

arXiv2021

Modeling Solar Wind Variations over an 11-yr Cycle with Alfvén Wave Dissipation: a Parameter Study

We study the behaviour and properties of the solar wind using a 2.5D Alfvén wave driven wind model. We first systematically compare the results of an Alfvén wave (AW) driven wind model with a polytropic approach. Polytropic magnetohydrodynamic wind models are thermally driven, while Alfvén waves act as additional acceleration and heating mechanisms in the Alfvén wave driven model. We confirm that an AW-driven model is required to reproduce the observed bimodality of slow and fast solar winds. We are also able to reproduce the observed anti-correlation between the terminal wind velocity and the coronal source temperature with the AW-driven wind model. We also show that the wind properties along an eleven-year cycle differ significantly from one model to the other. The AW-driven model again shows the best agreement with observational data. Indeed, solar surface magnetic field topology plays an important role in the Alfvén wave driven wind model, as it enters directly into the input energy sources via the Poynting flux. On the other hand, the polytropic wind model is driven by an assumed pressure gradient; thus it is relatively less sensitive to the surface magnetic field topology. Finally, we note that the net torque spinning down the Sun exhibits the same trends in the two models, showing that the polytropic approach still captures correctly the essence of stellar winds.

astro-ph.SR↗

arXiv2020

Does the mean-field alpha effect have any impact on the memory of the solar cycle?

Predictions of solar cycle 24 obtained from advection-dominated and diffusion-dominated kinematic dynamo models are different if the Babcock-Leighton mechanism is the only source of the poloidal field. Yeates et al. (2008) argue that the discrepancy arises due to different memories of the solar dynamo for advection- and diffusion-dominated solar convection zones. We aim to investigate the differences in solar cycle memory obtained from advection-dominated and diffusion-dominated kinematic solar dynamo models. Specifically, we explore whether the inclusion of Parker's mean-field $α$ effect, in addition to the Babcock-Leighton mechanism, has any impact on the memory of the solar cycle. We used a kinematic flux transport solar dynamo model where poloidal field generation takes place due to both the Babcock-Leighton mechanism and the mean-field $α$ effect. We additionally considered stochastic fluctuations in this model and explored cycle-to-cycle correlations between the polar field at the minima and toroidal field at cycle maxima. Solar dynamo memory is always limited to only one cycle in diffusion-dominated dynamo regimes while in advection-dominated regimes the memory is distributed over a few solar cycles. However, the addition of a mean-field alpha effect reduces the memory of the solar dynamo to within one cycle in the advection-dominated dynamo regime when there are no fluctuations in the mean-field $α$ effect. When fluctuations are introduced in the mean-field poloidal source a more complex scenario is evident, with very weak but significant correlations emerging across a few cycles. Our results imply that inclusion of a mean-field alpha effect in the framework of a flux transport Babcock-Leighton dynamo model leads to additional complexities that may impact memory and predictability of predictive dynamo models of the solar cycle.

astro-ph.SR↗

arXiv2020

Distinguishing between Flaring and Non-Flaring Active Regions: A Machine Learning Perspective

Large scale solar eruptions significantly impact space weather and damages space-based human infrastructures. It is necessary to predict large scale solar eruptions, which will enable us to protect our vulnerable infrastructures of modern society. We aim to investigate the difference between flaring and non-flaring active regions. We use photospheric vector magnetogram data from Solar Dynamic Observatory's Helioseismic Magnetic Imager to study the time evolution of photospheric magnetic parameters on the solar surface. We build a database of flaring and non-flaring active region observed on the solar surface from the years 2010 to 2017. We train the machine learning algorithm by the time evolution of these active region parameters. Finally, we estimate the performance obtained from this machine learning algorithm. We find the strength of some magnetic parameters namely total unsigned magnetic flux, total unsigned magnetic helicity, total unsigned vertical current and total photospheric magnetic energy density in flaring active regions are much higher compared to the non-flaring ones. These magnetic parameters in the flaring active region are highly evolving and complex. We are able to obtain good forecasting capability with a relatively high value of true skill statistic (TSS). We also find that time evolution of total unsigned magnetic helicity and total unsigned magnetic flux have very high ability to distinguish flaring and non-flaring active regions. It is possible to distinguish flaring active region from the non-flaring one with good accuracy. We confirm that there is no single common parameter which can distinguish all flaring active regions from the non flaring one. However, time evolution of top few magnetic parameters namely total unsigned magnetic flux and total unsigned magnetic helicity have very high distinguishing capability.

astro-ph.SR↗

arXiv2019

The Origin of Parity Changes in the Solar Cycle

Although sunspots have been systematically observed on the Sun's surface over the last four centuries, their magnetic properties have been revealed and documented only since the early 1900s. Sunspots typically appear in pairs of opposite magnetic polarities which have a systematic orientation. This polarity orientation is opposite across the equator -- a trend that has persisted over the last century over which magnetic field observations exist. Taken together with the configuration of the global poloidal field of the Sun -- that governs the heliospheric open flux and cosmic ray flux at Earth -- this phenomenon is consistent with the dipolar parity state of an underlying magnetohydrodynamic dynamo. Although hemispheric asymmetry in the emergence of sunspots is observed in the Sun, a parity shift has never been observed. We simulate hemispheric asymmetry through the introduction of random fluctuations in a computational dynamo model of the solar cycle and demonstrate that changes in parity are indeed possible in long-term simulations covering thousands of years. Quadrupolar modes are found to exist over a significant fraction of the simulated time. In particular, we find that a parity shift in the underlying nature of the sunspot cycle is more likely to occur when sunspot activity dominates in any one hemisphere for a time which is significantly longer than the cycle period. We establish causal pathways connecting hemispheric asymmetry and cross-equatorial phase-shifts to parity flips in the underlying dynamo mechanism. Our findings indicate that the solar cycle may have resided in quadrupolar parity states in the distant past, and provides a possible pathway for predicting parity flips in the future.

astro-ph.SR↗

arXiv2018

Hemispheric Preference and Cyclic Variation of Solar Filament Chirality from 2000 to 2016

It is well known that solar filaments are features in the solar atmosphere which show a hemispheric preference in their chirality. The hemispheric preference is such that the dextral chirality dominates in the northern hemisphere while the sinistral chirality dominates in the southern. Determining the strength and cyclic variation of the degree of this hemispheric preference, however, is challenging and tedious and thus needs to be automated. In this paper, we follow Dr. Pietro Bernasconi's algorithm (Bernasconi et al. 2005) to detect filament chirality with two parallel channels of application. The algorithm is applied to H-alpha images by "Advanced Automated Filament Detection and Characterization Code" (AAFDCC) (Bernasconi et al. 2005) or the algorithm is explained to a human and the human determines the chirality of the solar filament. We have conducted this exercise on data during the month of August from years 2000 to 2016 and we found that 83 % of our visually determined filaments follow the hemispheric chirality preference, while 58 % of automatically determined filaments follow it. Our visually compiled results have over 90 % agreement with those of Pevtsov et al. (2003), yet the visually determined chiralities of filaments disagree with automated determinations significantly. We find that the hemispheric preference remained the same between solar cycles 23 and 24 but the preference is very difficult to determine during the solar minimum of 2008-2010 primarily due to the absence of filaments.

astro-ph.SR↗

arXiv2018

Excess speicific heat of the gapped sliding phonons in the incommensurate composite crystal Sr$_{14}$Cu$_{24}$O$_{41}$

We show that low temperature specific heat (C$_p$) of the incommensurate chain-ladder system Sr$_{14}$Cu$_{24}$O$_{41}$ is enriched by the presence of a rather large excess contribution of non-magnetic origin. Diluted Al doping at the Cu site or annealing the crystal in an O$_2$ atmosphere suppresses this feature considerably. Using the THz time-domain spectroscopy, we show that the occurrence of excess specific heat is associated with the presence of very low-energy ($\sim$ 1 meV) gapped phonon modes that originate due to the sliding motion of oppositely charged mutually incommensurate chain and ladder layers.

cond-mat.str-el↗

arXiv2017

Simulating coronal loop implosion and compressible wave modes in a flare hit active region

There is considerable observational evidence of implosion of magnetic loop systems inside solar coronal active regions following high energy events like solar flares. In this work, we propose that such collapse can be modeled in three dimensions quite accurately within the framework of ideal magnetohydrodynamics. We furthermore argue that the dynamics of loop implosion is only sensitive to the transmitted disturbance of one or more of the system variables, e.g. velocity generated at the event site. This indicates that to understand loop implosion, it is sensible to leave the event site out of the simulated active region. Towards our goal, a velocity pulse is introduced to model the transmitted disturbance generated at the event site. Magnetic field lines inside our simulated active region are traced in real time, and it is demonstrated that the subsequent dynamics of the simulated loops closely resemble observed imploding loops. Our work highlights the role of plasma $β$ in regards to the rigidity of the loop systems and how that might affect the imploding loops' dynamics. Compressible magnetohydrodynamic modes such as kink and sausage are also shown to be generated during such processes, in accordance with observations.

astro-ph.SR↗

arXiv2016

A proposed paradigm for solar cycle dynamics mediated via turbulent pumping of magnetic flux in Babcock-Leighton type solar dynamos

At present, Babcock-Leighton flux transport solar dynamo models appear as the most promising model for explaining diverse observational aspects of the sunspot cycle. The success of these flux transport dynamo models is largely dependent upon a single-cell meridional circulation with a deep equatorward component at the base of the Sun's convection zone. However, recent observations suggest that the meridional flow may in fact be very shallow (confined to the top 10 % of the Sun) and more complex than previously thought. Taken together these observations raise serious concerns on the validity of the flux transport paradigm. By accounting for the turbulent pumping of magnetic flux as evidenced in magnetohydrodynamic simulations of solar convection, we demonstrate that flux transport dynamo models can generate solar-like magnetic cycles even if the meridional flow is shallow. Solar-like periodic reversals is recovered even when meridional circulation is altogether absent, however, in this case the solar surface magnetic field dynamics does not extend all the way to the polar regions. Very importantly, our results demonstrate that the Parker-Yoshimura sign rule for dynamo wave propagation can be circumvented in Babcock-Leighton dynamo models by the latitudinal component of turbulent pumping -- which can generate equatorward propagating sunspot belts in the absence of a deep, equatorward meridional flow. We also show that variations in turbulent pumping coefficients can modulate the solar cycle amplitude and periodicity. Our results suggest the viability of an alternate magnetic flux transport paradigm -- mediated via turbulent pumping -- for sustaining solar-stellar dynamo action.

astro-ph.SR↗

arXiv2015

The Relationship between Solar Coronal X-Ray Brightness and Active Region Magnetic Fields: A Study Using High Resolution Hinode Observations

By using high-resolution observations of nearly co-temporal and co-spatial Solar Optical Telescope spectropolarimeter and X-Ray Telescope coronal X-ray data onboard Hinode, we revisit the problematic relationship between global magnetic quantities and coronal X-ray brightness. Co-aligned vector magnetogram and X-ray data were used for this study. The total X-ray brightness over active regions is well correlated with integrated magnetic quantities such as the total unsigned magnetic flux, the total unsigned vertical current, and the area-integrated square of the vertical and horizontal magnetic fields. On accounting for the inter-dependence of the magnetic quantities, we inferred that the total magnetic flux is the primary determinant of the observed integrated X-ray brightness. Our observations indicate that a stronger coronal X-ray flux is not related to a higher non-potentiality of active-region magnetic fields. The data even suggest a slightly negative correlation between X-ray brightness and a proxy of active-region non-potentiality. Although there are small numerical differences in the established correlations, the main conclusions are qualitatively consistent over two different X-ray filters, the Al-poly and Ti-poly filters, which confirms the strength of our conclusions and validate and extend earlier studies that used low-resolution data. We discuss the implications of our results and the constraints they set on theories of solar coronal heating.

astro-ph.SR↗

arXiv2014

A Stochastically Forced Time Delay Solar Dynamo Model: Self-Consistent Recovery from a Maunder-like Grand Minimum Necessitates a Mean-Field Alpha Effect

Fluctuations in the Sun's magnetic activity, including episodes of grand minima such as the Maunder minimum have important consequences for space and planetary environments. However, the underlying dynamics of such extreme fluctuations remain ill-understood. Here we use a novel mathematical model based on stochastically forced, non-linear delay differential equations to study solar cycle fluctuations, in which, time delays capture the physics of magnetic flux transport between spatially segregated dynamo source regions in the solar interior. Using this model we explicitly demonstrate that the Babcock-Leighton poloidal field source based on dispersal of tilted bipolar sunspot flux, alone, can not recover the sunspot cycle from a grand minimum. We find that an additional poloidal field source effective on weak fields--the mean-field alpha-effect driven by helical turbulence--is necessary for self-consistent recovery of the sunspot cycle from grand minima episodes.

astro-ph.SR↗