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dc.contributor.advisorChatterjee, Piyali
dc.contributor.advisorSaini, Tarun Deep
dc.contributor.authorDey, Sahel
dc.date.accessioned2024-01-01T09:19:39Z
dc.date.available2024-01-01T09:19:39Z
dc.date.submitted2023
dc.identifier.urihttps://etd.iisc.ac.in/handle/2005/6334
dc.description.abstractThe solar atmosphere is embedded with an enormous no of plasma structures with a vast range of spatial and temporal scales. Solar spicules are one of such ubiquitous features of the sun, which are present in millions over the entire solar disk and often appear as a forest of jets. These are magnetized plasma jets, impinging periodically on the dynamic interface region between the visible solar surface and the hot corona. From the time first reported in 1877, solar spicules have remained a puzzle to the community mainly in the absence of a unified formation mechanism, their highly dynamic nature, and their potential effect on the solar corona. A large fraction of these jets are observed in a nearly open magnetic field region on the sun, where a stream of highly energetic charged particles escapes the solar gravity, known as the solar wind. This wind fills up the entire heliosphere as a primary component of the space weather and sets up the connection between the Sun and Earth. Hence it becomes of paramount interest to investigate solar spicules, which predominately originate from the same source location of solar winds and contribute to the mass budget and acceleration of winds. In the first part of the thesis, we will focus on the genesis of the solar spicule forest with the help of radiative Magnetohydrodynamics (rMHD) modeling and their similitudes with Laboratory experiments of fluids. Several mechanisms have been suggested to account for the formation of solar spicules, including granular squeezing, shocks and pulses, solar global acoustic waves, Alfvén waves, magnetic reconnection, magnetic tension release aided by ion-neutral coupling, and the Lorentz force. Although, models based on these drivers have not been able to quantitatively match the heights and abundance of the observed solar spicules. To start with a simple rMHD model, we do not consider the turbulent solar convection zone. We mimic the generation of global acoustic oscillations with a dominant period of 5 min instead by a sinusoidal forcing at the photospheric base of a gravitationally stratified solar-like atmospheric model. Here, the constant magnetic fields are imposed in the vertical direction. With sufficiently large amplitude forcing, these simplistic models are able to assemble a forest of synthetic spicules. In the Laboratory experiment, we find a similar jetting phenomenon at the free surface of a polymeric fluid layer when high amplitude vertical oscillations are applied by a subwoofer speaker, also known as the Faraday excitation. To understand the threshold acceleration required to drive jets in both systems, we perform a series of numerical and tabletop experiments with different driving frequencies and accelerations. The phase plots clearly depict a similar behavior of the threshold acceleration curve with frequency for two disparate systems. One can also find the analogous nature of the interaction of magnetic fields with the plasma turbulence to long-chain polymers with thermal fluctuations in a viscoelastic fluid. The role of anisotropy in the jet formation process is further demonstrated by comparing two fluid solutions with similar viscosity and surface tension,e.g., visco-elastic fluid in the presence of polymer chains and isotropic Newtonian fluids. Under harmonic driving, numerous coherent jets form in the viscoelastic solution, whereas a large number of droplets are observed for the Newtonian fluid. The droplet formation occurs due to underlying Plateau-Rayleigh Instability, which is arrested by the turbulent energy absorption of polymer chains. For the MHD counterpart, it may be concluded that the magnetic field provides this anisotropy by collimating the rising plasma to form coherent jet structures via Maxwell’s stress tensor, thereby suppressing the Kelvin-Helmholtz instability. Finally, we report the sufficient conditions (a) fluid medium, (b) gravity, (c) large-amplitude quasi-periodic driving, (d) anisotropy of the medium to excite a forest of jets irrespective of the nature of the medium by inducing non-linear development. As a next step in realistic modeling, we include the turbulent convection zone and couple it with the stratified solar atmosphere, reaching coronal height. With sub-surface convection, solar global surface oscillation is excited self-consistently by the turbulent convection process. These dominant 5-min acoustic oscillations at the photosphere impinge on the chromosphere, transition regions, and assemble a forest of spicules with a remarkable agreement to observed characteristics. The impact of photospheric undulations steepens to large amplitude acceleration fronts or shock fronts like the Domino effect in the presence of a much less dense upper atmosphere and induces jet structures. Upon careful analysis with the Lagrangian tracking technique, we find a range of heights (6-25 Mm) amongst the simulated spicular jets. The shortest spicules generally form above the convective down-flow regions or the intergranular regions where the magnetic field opens up as a funnel. The formation of a convective plume squeezes the magnetic flux tube, forcing the plasma trapped inside to shoot upwards along the magnetic field lines into the upper atmosphere. On the contrary, long spicules are located in regions above the convective granules where the magnetic field is organized in the form of low-lying loops. It is known from previous studies that the power of the solar global oscillations reaching the atmosphere is higher over a granule than above an intergranular lane. At the chromospheric height, the resultant plasma motions can push oppositely directed magnetic field lines together, causing magnetic reconnection between the open field lines and these low-lying loops. This mechanism aids the plasma to eject along the newly opened field lines leading to the generation of taller and faster spicules. The acceleration fronts in the higher atmosphere are generated by several mechanisms, including, e.g., (i) squeezing by granular buffeting (ii) solar global modes (iii) aided by magnetic reconnection. However, each process is ultimately controlled by the same underlying driver: solar convection. In addition to the forest feature of spicules, our model also captures their main properties, similar to observed counterparts, even in the absence of chromospheric microphysics of ambipolar diffusion and non-local thermodynamic equilibrium of the partially ionized plasma. Finally, we shed light on the morphology of spicular jets and several observed features of their motion by combining three-dimensional rMHD simulation data sets and high-resolution solar observations. The standard perception of the spatial structure of an astrophysical jet is a conical shape. The shape and extent of jets directly relate to the filling factor of the solar atmosphere, hence of tremendous importance in estimating mass, momentum, and energy fluxes to the solar corona. In our 3D rMHD model, spicules are detected as pleated drapery of compressible plasma, in contrast to conical or tube-like geometry. These extended spicular plasma structures mainly follow the slow MHD wavefronts resulting from global acoustic oscillations. A fluted sheet-like morphology has already been observationally conjectured for spicules, although the dense sheets are reported as tangential discontinuities like in current sheets. Our laboratory fluid experiments also direct to such morphology of jets. However, the line-of-sight (LOS) integrated profile retains the bright conical structure by taking into consideration of several warps of nonuniform plasma sheets. The dynamical complexity of solar spicules is another fascinating area, which is recently advanced with the availability of high-resolution, high-cadence observations. Transverse oscillations and bulk spinning motions of spicules constitute a significant part of their rapid evolutionary phase. The observed lateral swaying or the transverse oscillation is interpreted as a manifestation of propagating fast magnetohydrodynamic (MHD) kink mode through spicules. At the same time, the bulk spinning motion is still debated and believed to be either due to torsional Alfvén waves or because of mini-filament eruption. We detect several cases of bulk spinning motions amongst clusters of spicules in our self-sustained 3D rMHD simulation data cubes. For comparison, we analyze an observed dataset from the Broadband Filter Imager (BFI) of the Solar Optical Telescope (SOT) instrument onboard the Hinode satellite, focusing on a region near the northern polar coronal hole. Time-distance diagram of LOS integrated images shows the common inter-crossing feature of spicular strands as an outcome of the bulk spinning motion for both observed and simulated jets with similar order of magnitude time periods. Upon investigating the rudimentary mechanism behind this robust spinning motion, we identify several highly dynamic, coherent helical velocity streamlines that interact with spicules in the solar atmosphere and make them rotate. We introduce the term – coronal swirling conduits (CoSCo)– for the stream tubes since they are tall, cylindrical structures, have rotational speeds of 2–20 km/s, and average lifetime of 20–120 s. The CoSCos are parallel to the local magnetic field and, according to the dominant vorticity source, may be categorized into two classes. The CoSCo-I, or magnetic tension-driven swirls, whose origin may be traced to the lower atmosphere, where magnetic fields are continuously perturbed by the turbulent convective motions. They are characterized by co-spatial magnetic, velocity swirls, and oppositely phased velocity and magnetic field perturbations. The twisted magnetic fields at the exact location of the swirl support a strong tension force and therefore driving of CoSCo-I swirls. The second kind, CoSCo-II, is triggered next to spicules by the misaligned density and temperature gradients existing at their periphery, which is formally known as Baroclinic instability. These newly reported swirls are indirectly induced by the spicular jets in the atmosphere. For CoSCo-II, we find a prominent velocity swirl, but there is no signature of the corresponding magnetic swirl. Unlike CoSCo-I, the horizontal velocity and magnetic field vectors in CoSCo-II do not follow a 180-degree phase difference over the entire height range of the swirl. Apart from several differences, both types of CoSCos frequently overlap and rotate spicules along with them and propagate above the chromosphere regions with local Alfvén speed. Therefore detection of the rotating clusters of spicules both in observations and simulations confirms that rotating spicules are indirect tracers of CoSCos in the solar atmosphere. Further, we comprehensively discuss the energy and mass flux contribution of spicules and swirls to the solar corona, which can support against the local radiative losses and supply mass and momentum to the solar wind. The rotational energy of swirls and mass flux contribution of spicules are anti-correlated in time as the spinning motion is mostly dominant at the falling phase of spicules under solar gravity. A curious phenomenon is also noticed regarding the mass and the Poynting fluxes carried by spicules. The Poynting flux curve is anti-correlated with spicular mass flux, although it correlates with the rotational energy of swirls. It strongly indicates that CoSCos form by drawing energy from the thermal and magnetic energy reservoir of spicular plasma and transport it to much higher heights in the atmosphere due to their tall structure. One could also deduce that the Poynting fluxes are not just transported by spicules but also by the surrounding regions that include the dimmer regions of the drapery as well as the swirls. For the horizontally averaged mass flux over the domain, a periodic profile in time is prominent due to the solar global acoustic oscillation and the Domino effect on the subsequent higher atmosphere. It is found that the simulated mass flux is also adequate to support the mass reservoir of the fast solar wind. Therefore for a complete picture of momentum and energy transport to the solar wind, it is necessary to consider this interplay effect between spicules and CoSCos. Finally, we compare our rMHD model results with two publicly available rMHD setups from the Bifrost code. Surprisingly we find some remarkable similarities in the qualitative and quantitative aspects of the Pencil and Bifrost data cubes. However, several differences are also present in terms of the characteristics of spicules, swirls, thermodynamic quantities, and overall magnetic topology. Hence it helps us to understand essential physical processes responsible for such parallels and opens up the door to future studies for coherent physical outcomes across different numerical frameworks.en_US
dc.language.isoen_USen_US
dc.relation.ispartofseries;ET00346
dc.rightsI grant Indian Institute of Science the right to archive and to make available my thesis or dissertation in whole or in part in all forms of media, now hereafter known. I retain all proprietary rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertationen_US
dc.subjectchromosphereen_US
dc.subjectcoronaen_US
dc.subjectmagnetohydrodynamicsen_US
dc.subjectradiative transferen_US
dc.subjectSunen_US
dc.subjectSolar spiculesen_US
dc.subjectBroadband Filter Imageren_US
dc.subjectSolar Optical Telescopeen_US
dc.subjectCoSCosen_US
dc.subject.classificationResearch Subject Categories::NATURAL SCIENCES::Physics::Astronomy and astrophysics::Solar physicsen_US
dc.titleInsights into the genesis and dynamics of the solar spicule forest: aided by simulations and laboratory experimentsen_US
dc.typeThesisen_US
dc.degree.namePhDen_US
dc.degree.levelDoctoralen_US
dc.degree.grantorIndian Institute of Scienceen_US
dc.degree.disciplineFaculty of Scienceen_US


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