| dc.description.abstract | The 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 |
