explanation blue bibcodes open ADS page with paths to full text
Author name code: hillier
ADS astronomy entries on 2022-09-14
author:Hillier, Andrew
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Title: Intermediate shocks in compressible turbulent magnetic
reconnection
Authors: Snow, Ben; Hillier, Andrew
2022cosp...44.1492S Altcode:
Compressible magnetohydrodynamic (MHD) turbulence is a common feature
of astrophysical systems such as the solar atmosphere and interstellar
medium. Such systems are rife with shock waves that can redistribute
and dissipate energy, and hence understanding the role of shocks
in compressible turbulence is critical for determining the energy
balance of these dynamic systems. However, automated detection
and classification of shocks in turbulent systems is inherently
difficult due to the highly dynamic medium. Here we present a method
for detecting and classifying the full range of MHD shocks (slow,
fast and intermediate) applied to the Orszag-Tang vortex. We find
that the system is dominated by fast and slow shocks, however we
also detect many intermediate shocks (which feature a reversal in the
magnetic field) that appear near reconnection sites. We propose that
the formation mechanism for these intermediate shocks is related to
turbulent reconnection, where slight variations of the inflow parameters
allow super-Alfvenic to sub-slow transitions to exist. This acts as
an indirect metric for identifying reconnection regions in turbulent
simulations and improves our understanding of the structures and
dissipation that occurs in such systems.
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Title: Collisional ionisation and recombination effects on coalescence
instability in chromospheric partially ionised plasmas
Authors: Murtas, Giulia; Hillier, Andrew; Snow, Ben
2022arXiv220511091M Altcode:
Plasmoid-mediated fast magnetic reconnection plays a fundamental
role in driving explosive dynamics and heating, but relatively
little is known about how it develops in partially ionised plasmas
(PIP) of the solar chromosphere. Partial ionisation might largely
alter the dynamics of the coalescence instability, which promotes
fast reconnection and forms a turbulent reconnecting current
sheet through plasmoid interaction, but it is still unclear to what
extent PIP effects influence this process. We investigate the role of
collisional ionisation and recombination in the development of plasmoid
coalescence in PIP through 2.5D simulations of a two-fluid model. The
aim is to understand whether these two-fluid coupling processes
play a role in accelerating reconnection. We find that in general
ionisation-recombination process slow down the coalescence. Unlike
the previous models in G. Murtas, A. Hillier \& B. Snow, Physics
of Plasmas 28, 032901 (2021) that included thermal collisions only,
ionisation and recombination stabilise current sheets and suppress
non-linear dynamics, with turbulent reconnection occurring in
limited cases: bursts of ionisation lead to the formation of thicker
current sheets, even when radiative losses are included to cool the
system. Therefore, the coalescence time scale is very sensitive to
ionisation-recombination processes. However, reconnection in PIP is
still faster than in a fully ionised plasma environment having the same
bulk density: the PIP reconnection rate ($M_{_{\operatorname{IRIP}}}
= 0.057$) increases by a factor of $\sim 1.2$ with respect to the MHD
reconnection rate ($M_{_{\operatorname{MHD}}} = 0.047$).
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Title: Correction to: The magnetic Rayleigh-Taylor instability in
solar prominences
Authors: Hillier, Andrew
2021RvMPP...5....5H Altcode:
No abstract at ADS
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Title: Rayleigh-Taylor and Richtmyer-Meshkov instabilities: A journey
through scales
Authors: Zhou, Ye; Williams, Robin J. R.; Ramaprabhu, Praveen; Groom,
Michael; Thornber, Ben; Hillier, Andrew; Mostert, Wouter; Rollin,
Bertrand; Balachandar, S.; Powell, Phillip D.; Mahalov, Alex; Attal, N.
2021PhyD..42332838Z Altcode:
Hydrodynamic instabilities such as Rayleigh-Taylor (RT) and
Richtmyer-Meshkov (RM) instabilities usually appear in conjunction
with the Kelvin-Helmholtz (KH) instability and are found in many
natural phenomena and engineering applications. They frequently
result in turbulent mixing, which has a major impact on the overall
flow development and other effective material properties. This can
either be a desired outcome, an unwelcome side effect, or just an
unavoidable consequence, but must in all cases be characterized in any
model. The RT instability occurs at an interface between different
fluids, when the light fluid is accelerated into the heavy. The RM
instability may be considered a special case of the RT instability,
when the acceleration provided is impulsive in nature such as that
resulting from a shock wave. In this pedagogical review, we provide
an extensive survey of the applications and examples where such
instabilities play a central role. First, fundamental aspects of the
instabilities are reviewed including the underlying flow physics at
different stages of development, followed by an overview of analytical
models describing the linear, nonlinear and fully turbulent stages. RT
and RM instabilities pose special challenges to numerical modeling,
due to the requirement that the sharp interface separating the fluids
be captured with fidelity. These challenges are discussed at length
here, followed by a summary of the significant progress in recent
years in addressing them. Examples of the pivotal roles played
by the instabilities in applications are given in the context of
solar prominences, ionospheric flows in space, supernovae, inertial
fusion and pulsed-power experiments, pulsed detonation engines
and Scramjets. Progress in our understanding of special cases of
RT/RM instabilities is reviewed, including the effects of material
strength, chemical reactions, magnetic fields, as well as the roles the
instabilities play in ejecta formation and transport, and explosively
expanding flows. The article is addressed to a broad audience, but
with particular attention to graduate students and researchers who
are interested in the state-of-the-art in our understanding of the
instabilities and the unique issues they present in the applications
in which they are prominent.
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Title: Stability of two-fluid partially ionized slow-mode shock fronts
Authors: Snow, B.; Hillier, A.
2021MNRAS.506.1334S Altcode: 2021MNRAS.tmp.1714S; 2021arXiv210604199S
A magnetohydrodynamic (MHD) shock front can be unstable to the
corrugation instability, which causes a perturbed shock front to
become increasingly corrugated with time. An ideal MHD parallel shock
(where the velocity and magnetic fields are aligned) is unconditionally
unstable to the corrugation instability, whereas the ideal hydrodynamic
(HD) counterpart is unconditionally stable. For a partially ionized
medium (for example, the solar chromosphere), both HD and MHD species
coexist and the stability of the system has not been studied. In this
paper, we perform numerical simulations of the corrugation instability
in two-fluid partially ionized shock fronts to investigate the stability
conditions, and compare the results to HD and MHD simulations. Our
simulations consist of an initially steady two-dimensional parallel
shock encountering a localized upstream density perturbation. In MHD,
this perturbation results in an unstable shock front and the corrugation
grows with time. We find that for the two-fluid simulation, the neutral
species can act to stabilize the shock front. A parameter study is
performed to analyse the conditions under which the shock front is
stable and unstable. We find that for very weakly coupled or very
strongly coupled partially ionized system the shock front is unstable,
as the system tends towards MHD. However, for a finite coupling, we
find that the neutrals can stabilize the shock front, and produce new
features including shock channels in the neutral species. We derive an
equation that relates the stable wavelength range to the ion-neutral
and neutral-ion coupling frequencies and the Mach number. Applying
this relation to umbral flashes gives an estimated range of stable
wavelengths between 0.6 and 56 km.
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Title: Observation of bi-directional jets in a prominence
Authors: Hillier, A.; Polito, V.
2021A&A...651A..60H Altcode:
Quiescent prominences host a large range of flows, many driven
by buoyancy, which lead to velocity shear. The presence of these
shear flows could bend and stretch the magnetic field resulting
in the formation of current sheets which can lead to magnetic
reconnection. Though this has been hypothesised to occur in
prominences, with some observations that are suggestive of
this process, clear evidence has been lacking. In this paper
we present observations performed on June 30, 2015 using the
Interface Region Imaging Spectrograph Si IV and Mg II slit-jaw
imagers of two bi-directional jets that occur inside the body of
the prominence. Such jets are highly consistent with what would be
expected from magnetic reconnection theory. Using this observation,
we estimate that the prominence under study has an ambient field
strength in the range of 4.5−9.2 G with `turbulent' field strengths
of 1 G. Our results highlight the ability of gravity-driven flows
to stretch and fold the magnetic field of the prominence, implying
that locally, the quiescent prominence field can be far from a
static, force-free magnetic field. <P />Movies are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201935774/olm">https://www.aanda.org</A>
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Title: Dispersion relations for waves in visco-gravitating anisotropic
magnetoplasmas
Authors: Desta, Ephrem Tesfaye; Hillier, A.; Eritro, Tigistu Haile
2021PhPl...28d2901D Altcode:
The effect of Braginskii's full viscosity tensor on an infinite
non-conducting, gravitating anisotropic plasma in which the medium
is trapped in a strong magnetic field is discussed in the context
of Braginskii's magnetohydrodynamic model with Chew-Goldberger-Low
double adiabatic approximation and finite Larmor radius (FLR)
correction. Through linearization of the perturbed equations, the
general dispersion relation is derived for the separate compression,
shear, and drift viscosity components as well as the FLR corrections. We
investigate the stability for parallel and transverse perturbations with
respect to the direction of the magnetic field, and both gravitational
and fire-hose instabilities are found. The role of each viscous term is
to suppress instability, but each component works in different ways. The
FLR acts in a way that is very similar to the drift viscosity. The
instability threshold is found to be independent of viscosity for
compression and shear viscosity, but both the drift viscosity and FLR
corrections can change the critical wavenumber for the instability. The
compression viscosity is most effective at reducing the growth rate
of the gravitational instability, whereas the shear viscosity works
to suppress the fire-hose instability. The result of the present
study may be useful for the study of large scale compression, shear,
and drift plasma flow in and around clusters of galaxies and galactic
disks and for the solar and stellar wind.
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Title: Coalescence instability in chromospheric partially ionized
plasmas
Authors: Murtas, Giulia; Hillier, Andrew; Snow, Ben
2021PhPl...28c2901M Altcode: 2021arXiv210201630M
Fast magnetic reconnection plays a fundamental role in driving explosive
dynamics and heating in the solar chromosphere. The reconnection
time scale of traditional models is shortened at the onset of
the coalescence instability, which forms a turbulent reconnecting
current sheet through plasmoid interaction. In this work, we aim
to investigate the role of partial ionization in the development of
fast reconnection through the study of the coalescence instability
of plasmoids. Unlike the processes occurring in fully ionized coronal
plasmas, relatively little is known about how fast reconnection develops
in partially ionized plasmas (PIPs) of the chromosphere. We present
2.5D numerical simulations of coalescing plasmoids in a single fluid
magnetohydrodynamic (MHD) model and a two-fluid model of a partially
ionized plasma (PIP). We find that in the PIP model, which has the
same total density as the MHD model but an initial plasma density
two orders of magnitude smaller, plasmoid coalescence is faster than
the MHD case, following the faster thinning of the current sheet
and secondary plasmoid dynamics. Secondary plasmoids form in the PIP
model where the effective Lundquist number S = 7.8 × <SUP>10 3</SUP>
, but are absent from the MHD case where S = 9.7 × <SUP>10 3</SUP>
: these are responsible for a more violent reconnection. Secondary
plasmoids also form in linearly stable conditions as a consequence of
the nonlinear dynamics of the neutrals in the inflow. In the light of
these results, we can affirm that two-fluid effects play a major role
in the processes occurring in the solar chromosphere.
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Title: Collisional ionisation, recombination, and ionisation potential
in two-fluid slow-mode shocks: Analytical and numerical results
Authors: Snow, B.; Hillier, A.
2021A&A...645A..81S Altcode: 2020arXiv201006303S
Context. Shocks are a universal feature of warm plasma environments,
such as the lower solar atmosphere and molecular clouds, which
consist of both ionised and neutral species. Including partial
ionisation leads to the existence of a finite width for shocks,
where the ionised and neutral species decouple and recouple. As
such, drift velocities exist within the shock that lead to frictional
heating between the two species, in addition to adiabatic temperature
changes across the shock. The local temperature enhancements within
the shock alter the recombination and ionisation rates and hence
change the composition of the plasma. <BR /> Aims: We study the role
of collisional ionisation and recombination in slow-mode partially
ionised shocks. In particular, we incorporate the ionisation potential
energy loss and analyse the consequences of having a non-conservative
energy equation. <BR /> Methods: A semi-analytical approach is used
to determine the possible equilibrium shock jumps for a two-fluid
model with ionisation, recombination, ionisation potential, and
arbitrary heating. Two-fluid numerical simulations are performed
using the (P<underline>I</underline>P) code. Results are
compared to the magnetohydrodynamic (MHD) model and the semi-analytic
solution. <BR /> Results: Accounting for ionisation, recombination,
and ionisation potential significantly alters the behaviour of
shocks in both substructure and post-shock regions. In particular,
for a given temperature, equilibrium can only exist for specific
densities due to the radiative losses needing to be balanced by the
heating function. A consequence of the ionisation potential is that
a compressional shock will lead to a reduction in temperature in
the post-shock region, rather than the increase seen for MHD. The
numerical simulations pair well with the derived analytic model
for shock velocities. Conclusion. Multi-fluid effects can lead to a
significant departure from MHD results. The results in this paper are
applicable to a wide range of partially ionised plasmas, including
the solar chromosphere and molecular clouds. <P />The simulation data
from this study are available from BS upon reasonable request. The
(P<underline>http://I</underline>http://P)
code is available at <A
href="https://github.com/AstroSnow/PIP">https://github.com/AstroSnow/PIP</A>.
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Title: Estimating the Energy Dissipation from Kelvin-Helmholtz
Instability Induced Turbulence in Oscillating Coronal Loops
Authors: Hillier, Andrew; Van Doorsselaere, Tom; Karampelas,
Konstantinos
2020ApJ...897L..13H Altcode: 2020arXiv200709068H
Kelvin-Helmholtz instability induced turbulence is one promising
mechanism by which loops in the solar corona can be heated by
MHD waves. In this Letter we present an analytical model of the
dissipation rate of Kelvin-Helmholtz instability induced turbulence
ɛ<SUB>D</SUB>, finding it scales as the wave amplitude (d) to the
third power (ɛ<SUB>D</SUB> ∝ d<SUP>3</SUP>). Based on the concept of
steady-state turbulence, we expect the turbulence heating throughout
the volume of the loop to match the total energy injected through its
footpoints. In situations where this holds, the wave amplitude has to
vary as the cube-root of the injected energy. Comparing the analytic
results with those of simulations shows that our analytic formulation
captures the key aspects of the turbulent dissipation from the numerical
work. Applying this model to the observed characteristics of decayless
kink waves we predict that the amplitudes of these observed waves are
insufficient to turbulently heat the solar corona.
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Title: Mode conversion of two-fluid shocks in a partially-ionised,
isothermal, stratified atmosphere
Authors: Snow, B.; Hillier, A.
2020A&A...637A..97S Altcode: 2020arXiv200402550S
Context. The plasma of the lower solar atmosphere consists of mostly
neutral particles, whereas the upper solar atmosphere is mostly made up
of ionised particles and electrons. A shock that propagates upwards in
the solar atmosphere therefore undergoes a transition where the dominant
fluid is either neutral or ionised. An upwards propagating shock also
passes a point where the sound and Alfvén speed are equal. At this
point the energy of the acoustic shock can separated into fast and
slow components. The way the energy is distributed between the two
modes depends on the angle of magnetic field. <BR /> Aims: We aim
to investigate the separation of neutral and ionised species in a
gravitationally stratified atmosphere. The role of two-fluid effects
on the structure of the shocks post-mode-conversion and the frictional
heating is quantified for different levels of collisional coupling. <BR
/> Methods: Two-fluid numerical simulations were performed using the
(P<underline>I</underline>P) code of a wave steepening into
a shock in an isothermal, partially-ionised atmosphere. The collisional
coefficient was varied to investigate the regimes where the plasma
and neutral species are weakly, strongly, and finitely coupled. <BR />
Results: The propagation speeds of the compressional waves hosted by
neutral and ionised species vary and, therefore, velocity drift between
the two species is produced as the plasma attempts to propagate faster
than the neutrals. This is most extreme for a fast-mode shock. We
find that the collisional coefficient drastically impacts the features
present in the system, specifically the mode conversion height, type of
shocks present, and the finite shock widths created by the two-fluid
effects. In the finitely-coupled regime, fast-mode shock widths can
exceed the pressure scale height, which may lead to a new potential
observable of two-fluid effects in the lower solar atmosphere.
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Title: Coronal Cooling as a Result of Mixing by the Nonlinear
Kelvin-Helmholtz Instability
Authors: Hillier, Andrew; Arregui, Iñigo
2019ApJ...885..101H Altcode: 2019arXiv190911351H
Recent observations show cool, oscillating prominence threads fading
when observed in cool spectral lines and appearing in warm spectral
lines. A proposed mechanism to explain the observed temperature
evolution is that the threads were heated by turbulence driven
by the Kelvin-Helmholtz instability that developed as a result
of wave-driven shear flows on the surface of the thread. As the
Kelvin-Helmholtz instability is an instability that works to mix
the two fluids on either side of the velocity shear layer, in the
solar corona it can be expected to work by mixing the cool prominence
material with that of the hot corona to form a warm boundary layer. In
this paper, we develop a simple phenomenological model of nonlinear
Kelvin-Helmholtz mixing, using it to determine the characteristic
density and temperature of the mixing layer. For the case under study,
with constant pressure across the two fluids, these quantities are
{ρ }<SUB>mixed</SUB>}=\sqrt{{ρ }<SUB>1</SUB>{ρ }<SUB>2</SUB>}
and {T}<SUB>mixed</SUB>}=\sqrt{{T}<SUB>1</SUB>{T}<SUB>2</SUB>}. One
result from the model is that it provides an accurate—as determined
by comparison with simulation results—determination of the kinetic
energy in the mean velocity field. A consequence of this is that
the magnitude of turbulence—and with it, the energy that can be
dissipated on fast timescales—as driven by this instability can be
determined. For the prominence-corona system, the mean temperature rise
possible from turbulent heating is estimated to be less than 1% of the
characteristic temperature (which is found to be T <SUB>mixed</SUB> =
10<SUP>5</SUP> K). These results highlight that mixing, and not heating,
is likely to be the cause of the observed transition between cool to
warm material. One consequence of this result is that the mixing creates
a region with higher radiative loss rates on average than either of
the original fluids, meaning that this instability could contribute
a net loss of thermal energy from the corona, i.e., coronal cooling.
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Title: Achievements of Hinode in the first eleven years
Authors: Hinode Review Team; Al-Janabi, Khalid; Antolin, Patrick;
Baker, Deborah; Bellot Rubio, Luis R.; Bradley, Louisa; Brooks,
David H.; Centeno, Rebecca; Culhane, J. Leonard; Del Zanna, Giulio;
Doschek, George A.; Fletcher, Lyndsay; Hara, Hirohisa; Harra,
Louise K.; Hillier, Andrew S.; Imada, Shinsuke; Klimchuk, James A.;
Mariska, John T.; Pereira, Tiago M. D.; Reeves, Katharine K.; Sakao,
Taro; Sakurai, Takashi; Shimizu, Toshifumi; Shimojo, Masumi; Shiota,
Daikou; Solanki, Sami K.; Sterling, Alphonse C.; Su, Yingna; Suematsu,
Yoshinori; Tarbell, Theodore D.; Tiwari, Sanjiv K.; Toriumi, Shin;
Ugarte-Urra, Ignacio; Warren, Harry P.; Watanabe, Tetsuya; Young,
Peter R.
2019PASJ...71R...1H Altcode:
Hinode is Japan's third solar mission following Hinotori (1981-1982)
and Yohkoh (1991-2001): it was launched on 2006 September 22 and is in
operation currently. Hinode carries three instruments: the Solar Optical
Telescope, the X-Ray Telescope, and the EUV Imaging Spectrometer. These
instruments were built under international collaboration with the
National Aeronautics and Space Administration and the UK Science and
Technology Facilities Council, and its operation has been contributed
to by the European Space Agency and the Norwegian Space Center. After
describing the satellite operations and giving a performance evaluation
of the three instruments, reviews are presented on major scientific
discoveries by Hinode in the first eleven years (one solar cycle long)
of its operation. This review article concludes with future prospects
for solar physics research based on the achievements of Hinode.
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Title: Dynamic Evolution of Current Sheets, Ideal Tearing, Plasmoid
Formation and Generalized Fractal Reconnection Scaling Relations
Authors: Singh, K. A. P.; Pucci, Fulvia; Tenerani, Anna; Shibata,
Kazunari; Hillier, Andrew; Velli, Marco
2019ApJ...881...52S Altcode: 2019arXiv190400755S
Magnetic reconnection may be the fundamental process allowing energy
stored in magnetic fields to be released abruptly, with solar flares and
coronal mass ejection being archetypal natural plasma examples. Magnetic
reconnection is much too slow of a process to be efficient on the
large scales, but accelerates once small enough scales are formed in
the system. For this reason, the fractal reconnection scenario was
introduced to explain explosive events in the solar atmosphere; it was
based on the recursive triggering and collapse via tearing instability
of a current sheet originally thinned during the rise of a filament in
the solar corona. Here we compare the different fractal reconnection
scenarios that have been proposed, and derive generalized scaling
relations for the recursive triggering of fast, “ideal” —i.e.,
Lundquist number independent—tearing in collapsing current sheet
configurations with arbitrary current profile shapes. An important
result is that the Sweet-Parker scaling with Lundquist number, if
interpreted as the aspect ratio of the singular layer in an ideally
unstable sheet, is universal and does not depend on the details of
the current profile in the sheet. Such a scaling, however, must not
be interpreted in terms of stationary reconnection, rather it defines
a step in the accelerating sequence of events of the ideal tearing
mediated fractal cascade. We calculate scalings for the expected number
of plasmoids for such generic profiles and realistic Lundquist numbers,
showing that in ideal tearing scenarios a smaller number of plasmoids,
by orders of magnitude, is generated compared to the original fractal
model.
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Title: Ion-neutral decoupling in the nonlinear Kelvin-Helmholtz
instability: Case of field-aligned flow
Authors: Hillier, A.
2019PhPl...26h2902H Altcode: 2019arXiv190712507H
Nonlinear magnetic Kelvin-Helmholtz instability (KHI), and the
turbulence it creates appear in many astrophysical systems. This
includes those systems where the local plasma conditions are such
that the plasma is not fully ionized, for example in the lower solar
atmosphere and molecular clouds. In a partially ionized system, the
fluids couple via collisions which occur at characteristic frequencies,
therefore neutral and plasma species become decoupled for sufficiently
high-frequency dynamics. Here, we present high-resolution 2D two-fluid
simulations of the nonlinear KHI for a system that traverses the dynamic
scales between decoupled fluids and coupled dynamics. We discover some
interesting phenomena, including the presence of a density coupling
that is independent of the velocity coupling. Using these simulations,
we analyze the heating rate, and two regimes appear. The first is a
regime where the neutral flow is decoupled from the magnetic field that
is characterized by a constant heating rate, then at larger scales,
the strong coupling approximation holds the heating rate with the
KHI layer width to the power of -2. There is an energy cascade in
the simulation, but the nature of the frictional heating means the
heating rate is determined by the largest scale of turbulent motions,
a fact that has consequences for understanding turbulent dissipation
in multifluid systems.
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Title: Intermediate shock sub-structures within a slow-mode shock
occurring in partially ionised plasma
Authors: Snow, B.; Hillier, A.
2019A&A...626A..46S Altcode: 2019arXiv190412518S
Context. Slow-mode shocks are important in understanding fast magnetic
reconnection, jet formation and heating in the solar atmosphere,
and other astrophysical systems. The atmospheric conditions in the
solar chromosphere allow both ionised and neutral particles to exist
and interact. Under such conditions, fine sub-structures exist
within slow-mode shocks due to the decoupling and recoupling of
the plasma and neutral species. <BR /> Aims: We study numerically
the fine sub-structure within slow-mode shocks in a partially
ionised plasma, in particular, analysing the formation of an
intermediate transition within the slow-mode shock. <BR /> Methods:
High-resolution 1D numerical simulations were performed using the
(P<underline>I</underline>P) code using a two-fluid
approach. <BR /> Results: We discover that long-lived intermediate
(Alfvén) shocks can form within the slow-mode shock, where there
is a shock transition from above to below the Alfvén speed and a
reversal of the magnetic field across the shock front. The collisional
coupling provides frictional heating to the neutral fluid, resulting in
a Sedov-Taylor-like expansion with overshoots in the neutral velocity
and neutral density. The increase in density results in a decrease of
the Alfvén speed and with this the plasma inflow is accelerated to
above the Alfvén speed within the finite width of the shock leading
to the intermediate transition. This process occurs for a wide range
of physical parameters and an intermediate shock is present for all
investigated values of plasma-β, neutral fraction, and magnetic
angle. As time advances the magnitude of the magnetic field reversal
decreases since the neutral pressure cannot balance the Lorentz
force. The intermediate shock is long-lived enough to be considered
a physical structure, independent of the initial conditions. <BR />
Conclusions: Intermediate shocks are a physical feature that can exist
as shock sub-structure for long periods of time in partially ionised
plasma due to collisional coupling between species.
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Title: On Kelvin-Helmholtz and parametric instabilities driven by
coronal waves
Authors: Hillier, Andrew; Barker, Adrian; Arregui, Iñigo; Latter,
Henrik
2019MNRAS.482.1143H Altcode: 2018MNRAS.tmp.2618H; 2018arXiv181002773H
The Kelvin-Helmholtz instability has been proposed as a mechanism to
extract energy from magnetohydrodynamic (MHD) kink waves in flux tubes,
and to drive dissipation of this wave energy through turbulence. It
is therefore a potentially important process in heating the solar
corona. However, it is unclear how the instability is influenced
by the oscillatory shear flow associated with an MHD wave. We
investigate the linear stability of a discontinuous oscillatory
shear flow in the presence of a horizontal magnetic field within
a Cartesian framework that captures the essential features of MHD
oscillations in flux tubes. We derive a Mathieu equation for the
Lagrangian displacement of the interface and analyse its properties,
identifying two different instabilities: a Kelvin-Helmholtz instability
and a parametric instability involving resonance between the oscillatory
shear flow and two surface Alfvén waves. The latter occurs when the
system is Kelvin-Helmholtz stable, thus favouring modes that vary
along the flux tube, and as a consequence provides an important and
additional mechanism to extract energy. When applied to flows with
the characteristic properties of kink waves in the solar corona, both
instabilities can grow, with the parametric instability capable of
generating smaller scale disturbances along the magnetic field than
possible via the Kelvin-Helmholtz instability. The characteristic
time-scale for these instabilities is ∼100 s, for wavelengths of
200 km. The parametric instability is more likely to occur for smaller
density contrasts and larger velocity shears, making its development
more likely on coronal loops than on prominence threads.
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Title: The magnetic Rayleigh-Taylor instability in solar prominences
Authors: Hillier, Andrew
2018RvMPP...2....1H Altcode:
The magnetic Rayleigh-Taylor instability is a fundamental instability of
many astrophysical systems, and recent observations are consistent with
this instability developing in solar prominences. Prominences are cool,
dense clouds of plasma that form in the solar corona that display a
wide range of dynamics of a multitude of spatial and temporal scales,
and two different phenomena that have been discovered to occur in
prominences can be understood as resulting from the Rayleigh-Taylor
instability. The first is that of plumes that rise through quiescent
prominences from low density bubbles that form below them. The second
is that of a prominence eruption that fragments as the material falls
back to the solar surface. To identify these events as the magnetic
Rayleigh-Taylor instability, a wide range of theoretical work, both
numerical and analytical has been performed, though alternative
explanations do exist. For both of these sets of observations,
determining that they are created by the magnetic Rayleigh-Taylor
instability has meant that the linear instability conditions and
nonlinear dynamics can be used to make estimates of the magnetic field
strength. There are strong connections between these phenomena and
those in a number of other astro, space and plasma systems, making
these observations very important for our understanding of the role
of the Rayleigh-Taylor instability in magnetised systems.
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Title: Observations of the Kelvin-Helmholtz Instability Driven by
Dynamic Motions in a Solar Prominence
Authors: Hillier, Andrew; Polito, Vanessa
2018ApJ...864L..10H Altcode: 2018arXiv180802286H
Prominences are incredibly dynamic across the whole range of their
observable spatial scales, with observations revealing gravity-driven
fluid instabilities, waves, and turbulence. With all of these complex
motions, it would be expected that instabilities driven by shear in
the internal fluid motions would develop. However, evidence of these
have been lacking. Here we present the discovery in a prominence,
using observations from the Interface Region Imaging Spectrograph,
of a shear flow instability, the Kelvin-Helmholtz sinusoidal-mode of
a fluid channel, driven by flows in the prominence body. This finding
presents a new mechanism through which we can create turbulent motions
from the flows observed in quiescent prominences. The observation of
this instability in a prominence highlights their great value as a
laboratory for understanding the complex interplay between magnetic
fields and fluid flows that play a crucial role in a vast range of
astrophysical systems.
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Title: Three-dimensional Velocity Measurements in Solar Prominence
Bubbles and Combined Kelvin-Helmholtz/Rayleigh-Taylor Instability
Authors: Berger, Thomas; Hillier, Andrew; Liu, Wei
2018cosp...42E.293B Altcode:
We present measurements of flow velocities in solar prominences that
display so-called "prominence bubble" events. Prominence bubbles
are large-scale buoyant intrusions into prominences that rise from
below and penetrate into the overlying plasma. They are believed to
be due to magnetic flux emergence below prominences and can trigger
Rayleigh-Taylor and Kelvin-Helmholtz instability flows as they interact
with the overlying prominence. Prominence bubbles frequently result
in the formation of plumes that rise into, or entirely through, the
overlying prominence. This presents a mechanism for increasing magnetic
flux and helicity in the associated coronal magnetic flux tubes,
which are key for their eventual loss of equilibrium and eruptions
as coronal mass ejections (CMEs). In this presentation, Hinode/Solar
Optical Telescope (SOT) and Interface Region Imaging Spectrograph (IRIS)
observations are analyzed to infer three-dimensional flow vectors
in the "boundary layer" above several prominence bubble events. IRIS
Doppler velocity measurements indicate flow speeds of 50-100 km/sec
perpendicular to the sky plane, consistent with flow speeds inferred
from combined Kelvin-Helmholtz/Rayleigh-Taylor instability analysis
using typical quiescent prominence density and magnetic flux density
values. With these typical values, flow speeds and magnetic flux
densities within the bubbles can be inferred to be on the order of
100 km/sec and 10 Gauss, respectively. We discuss the implications of
these novel results, and in particular, the potential for strong shear
flows at the bubble boundary to trigger Kelvin-Helmholtz instability
waves that develop into large-scale Rayleigh-Taylor instability plumes.
---------------------------------------------------------
Title: Observations of a shear-flow instability driven by dynamic
prominence motions
Authors: Hillier, Andrew; Polito, . V.
2018cosp...42E1460H Altcode:
Prominences are incredibly dynamic across the whole range of their
observable spatial scales, with observations revealing gravity-driven
fluid instabilities, waves, and turbulence. With all these complex
motions, it would be expected that instabilities driven by shear in the
fluid motions contained in the prominence body would develop. However,
evidence of these have been lacking. Here we present the discovery in
a prominence, using observations from the Interface Region Imaging
Spectrograph (IRIS), of a shear flow instability, a mode of the
Kelvin-Helmholtz instability that makes streams of fluid develop
serpentine patterns, driven by transonic motions in the prominence
body. This finding presents a new mechanism through which we can
create turbulence from the flows observed in quiescent prominences. The
observation of this instability in a prominence highlights their great
value as a laboratory for understanding the complex interplay between
magnetic fields and fluid flows that play a crucial role in a vast
range of astrophysical systems.
---------------------------------------------------------
Title: Onset of 2D magnetic reconnection in the solar photosphere,
chromosphere, and corona
Authors: Snow, B.; Botha, G. J. J.; McLaughlin, J. A.; Hillier, A.
2018A&A...609A.100S Altcode: 2017arXiv171100683S
<BR /> Aims: We aim to investigate the onset of 2D time-dependent
magnetic reconnection that is triggered using an external (non-local)
velocity driver located away from, and perpendicular to, an
equilibrium Harris current sheet. Previous studies have typically
utilised an internal trigger to initiate reconnection, for example
initial conditions centred on the current sheet. Here, an external
driver allows for a more naturalistic trigger as well as the study
of the earlier stages of the reconnection start-up process. <BR />
Methods: Numerical simulations solving the compressible, resistive
magnetohydrodynamic (MHD) equations were performed to investigate the
reconnection onset within different atmospheric layers of the Sun,
namely the corona, chromosphere and photosphere. <BR /> Results: A
reconnecting state is reached for all atmospheric heights considered,
with the dominant physics being highly dependent on atmospheric
conditions. The coronal case achieves a sharp rise in electric field
(indicative of reconnection) for a range of velocity drivers. For the
chromosphere, we find a larger velocity amplitude is required to trigger
reconnection (compared to the corona). For the photospheric environment,
the electric field is highly dependent on the inflow speed; a sharp
increase in electric field is obtained only as the velocity entering
the reconnection region approaches the Alfvén speed. Additionally,
the role of ambipolar diffusion is investigated for the chromospheric
case and we find that the ambipolar diffusion alters the structure
of the current density in the inflow region. <BR /> Conclusions: The
rate at which flux enters the reconnection region is controlled by
the inflow velocity. This determines all aspects of the reconnection
start-up process, that is, the early onset of reconnection is dominated
by the advection term in Ohm's law in all atmospheric layers. A lower
plasma-β enhances reconnection and creates a large change in the
electric field. A high plasma-β hinders the reconnection, yielding a
sharp rise in the electric field only when the velocity flowing into
the reconnection region approaches the local Alfvén speed.
---------------------------------------------------------
Title: Quiescent Prominence Dynamics Observed with the Hinode
Solar Optical Telescope. II. Prominence Bubble Boundary Layer
Characteristics and the Onset of a Coupled Kelvin-Helmholtz
Rayleigh-Taylor Instability
Authors: Berger, Thomas; Hillier, Andrew; Liu, Wei
2017ApJ...850...60B Altcode: 2017arXiv170705265B
We analyze solar quiescent prominence bubble characteristics and
instability dynamics using Hinode/Solar Optical Telescope data. We
measure the bubble expansion rate, prominence downflows, and the
profile of the boundary layer brightness and thickness as a function
of time. The largest bubble analyzed rises into the prominence with a
speed of about 1.3 {km} {{{s}}}<SUP>-1</SUP> until it is destabilized by
a localized shear flow on the boundary. Boundary layer thickness grows
gradually as prominence downflows deposit plasma onto the bubble with
characteristic speeds of 20{--}35 {km} {{{s}}}<SUP>-1</SUP>. Lateral
downflows initiate from the thickened boundary layer with characteristic
speeds of 25{--}50 {km} {{{s}}}<SUP>-1</SUP>, “draining” the
layer of plasma. Strong shear flow across one bubble boundary leads
to an apparent coupled Kelvin-Helmholtz Rayleigh-Taylor (KH-RT)
instability. We measure shear flow speeds above the bubble of 10 {km}
{{{s}}}<SUP>-1</SUP> and infer interior bubble flow speeds on the order
of 100 {km} {{{s}}}<SUP>-1</SUP>. Comparing the measured growth rate
of the instability to analytic expressions, we infer a magnetic flux
density across the bubble boundary of ∼10<SUP>-3</SUP> T (10 Gauss)
at an angle of ∼ 70^\circ to the prominence plane. The results are
consistent with the hypothesis that prominence bubbles are caused by
magnetic flux that emerges below a prominence, setting up the conditions
for RT, or combined KH-RT, instability flows that transport flux,
helicity, and hot plasma upward into the overlying coronal magnetic
flux rope.
---------------------------------------------------------
Title: The non-linear growth of the magnetic Rayleigh-Taylor
instability
Authors: Carlyle, Jack; Hillier, Andrew
2017A&A...605A.101C Altcode: 2017arXiv170707987C
This work examines the effect of the embedded magnetic field strength on
the non-linear development of the magnetic Rayleigh-Taylor instability
(RTI) (with a field-aligned interface) in an ideal gas close to the
incompressible limit in three dimensions. Numerical experiments are
conducted in a domain sufficiently large so as to allow the predicted
critical modes to develop in a physically realistic manner. The ratio
between gravity, which drives the instability in this case (as well
as in several of the corresponding observations), and magnetic field
strength is taken up to a ratio which accurately reflects that of
observed astrophysical plasma, in order to allow comparison between the
results of the simulations and the observational data which served as
inspiration for this work. This study finds reduced non-linear growth
of the rising bubbles of the RTI for stronger magnetic fields, and that
this is directly due to the change in magnetic field strength, rather
than the indirect effect of altering characteristic length scales with
respect to domain size. By examining the growth of the falling spikes,
the growth rate appears to be enhanced for the strongest magnetic field
strengths, suggesting that rather than affecting the development of the
system as a whole, increased magnetic field strengths in fact introduce
an asymmetry to the system. Further investigation of this effect
also revealed that the greater this asymmetry, the less efficiently
the gravitational energy is released. By better understanding the
under-studied regime of such a major phenomenon in astrophysics,
deeper explanations for observations may be sought, and this work
illustrates that the strength of magnetic fields in astrophysical
plasmas influences observed RTI in subtle and complex ways.
---------------------------------------------------------
Title: Prominence Bubble Shear Flows and the Coupled Kelvin-Helmholtz
— Rayleigh-Taylor Instability
Authors: Berger, Thomas; Hillier, Andrew
2017SPD....4820103B Altcode:
Prominence bubbles are large arched structures that rise from below
into quiescent prominences, often growing to heights on the order of
10 Mm before going unstable and generating plume upflows. While there
is general agreement that emerging flux below pre-existing prominences
causes the structures, there is lack of agreement on the nature of
the bubbles and the cause of the instability flows. One hypothesis is
that the bubbles contain coronal temperature plasma and rise into the
prominence above due to both magnetic and thermal buoyancy, eventually
breaking down via a magnetic Rayleigh-Taylor (RT) instability to
release hot plasma and magnetic flux and helicity into the overlying
coronal flux rope. Another posits that the bubbles are actually just
“arcades” in the prominence indicating a magnetic separator line
between the bipole and the prominence fields with the observed upflows
and downflows caused by reconnection along the separator. We analyze
Hinode/SOT, SDO/AIA, and IRIS observations of prominence bubbles,
focusing on characteristics of the bubble boundary layers that may
discriminate between the two hypotheses. We find speeds on the order
of 10 km/s in prominence plasma downflows and lateral shear flows
along the bubble boundary. Inflows to the boundary gradually increase
the thickness and brightness of the layer until plasma drains from
there, apparently around the dome-like bubble domain. In one case,
shear flow across the bubble boundary develops Kelvin-Helmholtz (KH)
vortices that we use to infer flow speeds in the low-density bubble
on the order of 100 km/sec. IRIS spectra indicate that plasma flows on
the bubble boundary at transition region temperatures achieve Doppler
speeds on the order of 50 km/s, consistent with this inference. Combined
magnetic KH-RT instability analysis leads to flux density estimates
of 10 G with a field angle of 30° to the prominence, consistent with
vector magnetic field measurements. In contrast, we find no evidence
of the impulsive brightening or bi-directional jets that are expected
from reconnection driven flows at bubble boundaries. We conclude that
observations to date are consistent with the hot bubble/Rayleigh-Taylor
instability hypothesis.
---------------------------------------------------------
Title: Differences between Doppler velocities of ions and neutral
atoms in a solar prominence
Authors: Anan, T.; Ichimoto, K.; Hillier, A.
2017A&A...601A.103A Altcode: 2017arXiv170302132A
Context. In astrophysical systems with partially ionized plasma,
the motion of ions is governed by the magnetic field while the
neutral particles can only feel the magnetic field's Lorentz force
indirectly through collisions with ions. The drift in the velocity
between ionized and neutral species plays a key role in modifying
important physical processes such as magnetic reconnection, damping
of magnetohydrodynamic waves, transport of angular momentum in plasma
through the magnetic field, and heating. <BR /> Aims: This paper
aims to investigate the differences between Doppler velocities of
calcium ions and neutral hydrogen in a solar prominence to look for
velocity differences between the neutral and ionized species. <BR />
Methods: We simultaneously observed spectra of a prominence over an
active region in H I 397 nm, H I 434 nm, Ca II 397 nm, and Ca II
854 nm using a high dispersion spectrograph of the Domeless Solar
Telescope at Hida observatory. We compared the Doppler velocities,
derived from the shift of the peak of the spectral lines presumably
emitted from optically-thin plasma. <BR /> Results: There are instances
when the difference in velocities between neutral atoms and ions is
significant, for example 1433 events ( 3% of sets of compared profiles)
with a difference in velocity between neutral hydrogen atoms and
calcium ions greater than 3σ of the measurement error. However, we
also found significant differences between the Doppler velocities of
two spectral lines emitted from the same species, and the probability
density functions of velocity difference between the same species is not
significantly different from those between neutral atoms and ions. <BR
/> Conclusions: We interpreted the difference of Doppler velocities as
being a result of the motions of different components in the prominence
along the line of sight, rather than the decoupling of neutral atoms
from plasma. <P />The movie attached to Fig. 1 is available at <A
href="http://www.aanda.org/10.1051/0004-6361/201629979/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Investigating prominence turbulence with Hinode SOT
Dopplergrams
Authors: Hillier, A.; Matsumoto, T.; Ichimoto, K.
2017A&A...597A.111H Altcode: 2016arXiv161008281H
Quiescent prominences host a diverse range of flows, including
Rayleigh-Taylor instability driven upflows and impulsive downflows,
and so it is no surprise that turbulent motions also exist. As
prominences are believed to have a mean horizontal guide field,
investigating any turbulence they host could shed light on the
nature of magnetohydrodynamic (MHD) turbulence in a wide range of
astrophysical systems. In this paper we have investigated the nature
of the turbulent prominence motions using structure function analysis
on the velocity increments estimated from Hα Dopplergrams constructed
with observational data from Hinode Solar Optical Telescope (SOT). The
probability density function of the velocity increments shows that as
we look at increasingly small spatial separations the distribution
displays greater departure from a reference Gaussian distribution,
hinting at intermittency in the velocity field. Analysis of the
even order structure functions for both the horizontal and vertical
separations showed the existence of two distinct regions displaying
different exponents of the power law with the break in the power
law at approximately 2000 km. We hypothesise this to be a result
of internal turbulence excited in the prominence by the dynamic
flows of the system found at this spatial scale. We found that the
scaling exponents of the pth order structure functions for these
two regions generally followed the p/ 2 (smaller scales) and p/ 4
(larger scales) laws that are the same as those predicted for weak MHD
turbulence and Kraichnan-Iroshnikov turbulence respectively. However,
the existence of the p/ 4 scaling at larger scales than the p/ 2
scaling is inconsistent with the increasing nonlinearity expected
in MHD turbulence. We also found that as we went to higher order
structure functions, the dependence of the scaling exponent on the
order p is nonlinear implying that intermittency may be playing an
important role in the turbulent cascade. Estimating the heating from
the turbulent energy dissipation showed that this turbulence would be
very inefficient at heating the prominence plasma, but that the mass
diffusion through turbulence driven reconnection was of the order
of 10<SUP>10</SUP> cm<SUP>2</SUP> s<SUP>-1</SUP>. This is of similar
order to that of the expected value of the ambipolar diffusion and a
few orders of magnitude greater than Ohmic diffusion for a quiescent
prominence. <P />The movie associated to Fig. 4 is available at <A
href="http://www.aanda.org/10.1051/0004-6361/201527766/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: On the nature of the magnetic Rayleigh-Taylor instability in
astrophysical plasma: the case of uniform magnetic field strength
Authors: Hillier, Andrew S.
2016MNRAS.462.2256H Altcode: 2016arXiv161008317H
The magnetic Rayleigh-Taylor instability has been shown to play a key
role in many astrophysical systems. The equation for the growth rate of
this instability in the incompressible limit, and the most-unstable
mode that can be derived from it, are often used to estimate the
strength of the magnetic field that is associated with the observed
dynamics. However, there are some issues with the interpretations
given. Here, we show that the class of most unstable modes k<SUB>u</SUB>
for a given θ, the class of modes often used to estimate the strength
of the magnetic field from observations, for the system leads to the
instability growing as σ<SUP>2</SUP> = 1/2Agk<SUB>u</SUB>, a growth
rate which is independent of the strength of the magnetic field and
which highlights that small scales are preferred by the system, but
not does not give the fastest growing mode for that given k. We also
highlight that outside of the interchange (k ṡ B = 0) and undular
(k parallel to B) modes, all the other modes have a perturbation pair
of the same wavenumber and growth rate that when excited in the linear
regime can result in an interference pattern that gives field aligned
filamentary structure often seen in 3D simulations. The analysis was
extended to a sheared magnetic field, where it was found that it was
possible to extend the results for a non-sheared field to this case. We
suggest that without magnetic shear it is too simplistic to be used
to infer magnetic field strengths in astrophysical systems.
---------------------------------------------------------
Title: The formation and evolution of reconnection-driven, slow-mode
shocks in a partially ionised plasma
Authors: Hillier, A.; Takasao, S.; Nakamura, N.
2016A&A...591A.112H Altcode: 2016arXiv160201112H
The role of slow-mode magnetohydrodynamic (MHD) shocks in magnetic
reconnection is of great importance for energy conversion and transport,
but in many astrophysical plasmas the plasma is not fully ionised. In
this paper, we use numerical simulations to investigate the role of
collisional coupling between a proton-electron, charge-neutral fluid and
a neutral hydrogen fluid for the one-dimensional (1D) Riemann problem
initiated in a constant pressure and density background state by a
discontinuity in the magnetic field. This system, in the MHD limit,
is characterised by two waves. The first is a fast-mode rarefaction
wave that drives a flow towards a slow-mode MHD shock wave. The
system evolves through four stages: initiation, weak coupling,
intermediate coupling, and a quasi-steady state. The initial stages
are characterised by an over-pressured neutral region that expands
with characteristics of a blast wave. In the later stages, the system
tends towards a self-similar solution where the main drift velocity
is concentrated in the thin region of the shock front. Because of
the nature of the system, the neutral fluid is overpressured by the
shock when compared to a purely hydrodynamic shock, which results in
the neutral fluid expanding to form the shock precursor. Once it has
formed, the thickness of the shock front is proportional to ξ<SUB>
I</SUB><SUP>-1.2</SUP> , which is a smaller exponent than would be
naively expected from simple scaling arguments. One interesting result
is that the shock front is a continuous transition of the physical
variables of subsonic velocity upstream of the shock front (a c-shock)
to a sharp jump in the physical variables followed by a relaxation to
the downstream values for supersonic upstream velocity (a j-shock). The
frictional heating that results from the velocity drift across the
shock front can amount to ~2 per cent of the reference magnetic energy.
---------------------------------------------------------
Title: Nonlinear instability and intermittent nature of magnetic
reconnection in solar chromosphere
Authors: Singh, K. A. P.; Hillier, Andrew; Isobe, Hiroaki; Shibata,
Kazunari
2015PASJ...67...96S Altcode: 2016arXiv160201999S; 2015PASJ..tmp..234S
The recent observations of Singh et al. (2012, ApJ, 759, 33) have shown
multiple plasma ejections and the intermittent nature of magnetic
reconnection in the solar chromosphere, highlighting the need for
fast reconnection to occur in highly collisional plasma. However,
the physical process through which fast magnetic reconnection occurs
in partially ionized plasma, like the solar chromosphere, is still
poorly understood. It has been shown that for sufficiently high
magnetic Reynolds numbers, Sweet-Parker current sheets can become
unstable leading to tearing mode instability and plasmoid formation,
but when dealing with a partially ionized plasma the strength of
coupling between the ions and neutrals plays a fundamental role
in determining the dynamics of the system. We propose that as the
reconnecting current sheet thins and the tearing instability develops,
plasmoid formation passes through strongly, intermediately, and weakly
coupled (or decoupled) regimes, with the time scale for the tearing
mode instability depending on the frictional coupling between ions
and neutrals. We present calculations for the relevant time scales for
fractal tearing in all three regimes. We show that as a result of the
tearing mode instability and the subsequent non-linear instability due
to the plasmoid-dominated reconnection, the Sweet-Parker current sheet
tends to have a fractal-like structure, and when the chromospheric
magnetic field is sufficiently strong the tearing instability can
reach down to kinetic scales, which are hypothesized to be necessary
for fast reconnection.
---------------------------------------------------------
Title: Superflare Occurrence and Energies on G-, K-, and M-type Dwarfs
Authors: Candelaresi, S.; Hillier, A.; Maehara, H.; Brandenburg, A.;
Shibata, K.
2014ApJ...792...67C Altcode: 2014arXiv1405.1453C
Kepler data from G-, K-, and M-type stars are used to study conditions
that lead to superflares with energies above 10<SUP>34</SUP> erg. From
the 117,661 stars included, 380 show superflares with a total of 1690
such events. We study whether parameters, like effective temperature
or rotation rate, have any effect on the superflare occurrence
rate or energy. With increasing effective temperature we observe a
decrease in the superflare rate, which is analogous to the previous
findings of a decrease in dynamo activity with increasing effective
temperature. For slowly rotating stars, we find a quadratic increase
of the mean occurrence rate with the rotation rate up to a critical
point, after which the rate decreases linearly. Motivated by standard
dynamo theory, we study the behavior of the relative starspot coverage,
approximated as the relative brightness variation. For faster rotating
stars, an increased fraction of stars shows higher spot coverage,
which leads to higher superflare rates. A turbulent dynamo is used
to study the dependence of the Ohmic dissipation as a proxy of the
flare energy on the differential rotation or shear rate. The resulting
statistics of the dissipation energy as a function of dynamo number is
similar to the observed flare statistics as a function of the inverse
Rossby number and shows similarly strong fluctuations. This supports
the idea that superflares might well be possible for solar-type G stars.
---------------------------------------------------------
Title: The Rayleigh-Taylor Instability and the role of Prominences
in the Chromosphere-Corona Mass Cycle
Authors: Berger, Thomas; Liu, Wei; Hillier, Andrew; Scullion, Eamon;
Low, Boon Chye
2014AAS...22421201B Altcode:
We review recent results in the study of so-called "prominence
bubbles", a buoyant instability discovered in quiescent solar
prominences by the Hinode/SOT instrument in 2007. Analysis of the
plasma flows along the boundary of the bubbles indicates that shear
flows leading to Kelvin-Helmholtz instability waves can develop into
the seed perturbations triggering the Rayleigh-Taylor instability. The
non-linear phase of the RT instability leads to the formation of large
turbulent plumes that transport the bubble plasma (and presumably
magnetic flux) into the overlying coronal flux rope. We propose that
the upward turbulent transport of hot bubble plasma and the downflows
of cooler chromospheric plasma in the prominence are related aspects
of a large-scale "chromosphere-corona mass cycle" in which hot plasma
and magnetic flux and helicity from the chromosphere are transported
upwards while the cooler prominence plasma downflows, which decouple
from the magnetic field they are originally frozen-into, represent
the condensation return flows of the cycle. This cycling enables a
mechanism by which magnetic flux and helicity build up in the coronal
flux rope while mass drains out of the flux rope, eventually triggering
a "loss of confinement" eruption in the form of a CME.
---------------------------------------------------------
Title: The Generation and Damping of Propagating MHD Kink Waves in
the Solar Atmosphere
Authors: Morton, R. J.; Verth, G.; Hillier, A.; Erdélyi, R.
2014ApJ...784...29M Altcode: 2013arXiv1310.4650M
The source of the non-thermal energy required for the heating of the
upper solar atmosphere to temperatures in excess of a million degrees
and the acceleration of the solar wind to hundreds of kilometers
per second is still unclear. One such mechanism for providing the
required energy flux is incompressible torsional Alfvén and kink
magnetohydrodynamic (MHD) waves, which are magnetically dominated
waves supported by the Sun's pervasive and complex magnetic field. In
particular, propagating MHD kink waves have recently been observed
to be ubiquitous throughout the solar atmosphere, but, until now,
critical details of the transport of the kink wave energy throughout
the Sun's atmosphere were lacking. Here, the ubiquity of the
waves is exploited for statistical studies in the highly dynamic
solar chromosphere. This large-scale investigation allows for the
determination of the chromospheric kink wave velocity power spectra, a
missing link necessary for determining the energy transport between the
photosphere and corona. Crucially, the power spectra contain evidence
for horizontal photospheric motions being an important mechanism for
kink wave generation in the quiescent Sun. In addition, a comparison
with measured coronal power spectra is provided for the first time,
revealing frequency-dependent transmission profiles, suggesting that
there is enhanced damping of kink waves in the lower corona.
---------------------------------------------------------
Title: Within the International Collaboration CHAIN: a Summary of
Events Observed with Flare Monitoring Telescope (FMT) in Peru
Authors: Ishitsuka, J.; Asai, A.; Morita, S.; Terrazas, R.; Cabezas,
D.; Gutierrez, V.; Martinez, L.; Buleje, Y.; Loayza, R.; Nakamura,
N.; Takasao, S.; Yoshinaga, Y.; Hillier, A.; Otsuji, K.; Shibata, K.;
Ishitsuka, M.; Ueno, S.; Kitai, R.; Ishii, T.; Ichimoto, K.; Nagata,
S.; Narukage, N.
2014SunGe...9...85I Altcode:
In 2008 we inaugurated the new Solar Observatory in collaboration with
Faculty of Sciences of San Luis Gonzaga de Ica National University,
300 km south of Lima. In March of 2010 a Flare Monitoring Telescope
of Hida Observatory of Kyoto University arrived to Ica, part of CHAIN
Project (Continuous H-alpha Imaging Network). In October of the same
year we hosted the First FMT Workshop in Ica, then in July of 2011 the
Second FMT Workshop was opened. Since that we are focused on two events
registered by FMT in Peru to publish results. FMT is a good tool to
introduce young people from universities into scientific knowledge;
it is good also for education in Solar Physics and outreach. Details
of this successful collaboration will be explained in this presentation.
---------------------------------------------------------
Title: Investigating the Dynamics and Density Evolution of Returning
Plasma Blobs from the 2011 June 7 Eruption
Authors: Carlyle, Jack; Williams, David R.; van Driel-Gesztelyi,
Lidia; Innes, Davina; Hillier, Andrew; Matthews, Sarah
2014ApJ...782...87C Altcode: 2014arXiv1401.4824C
This work examines in-falling matter following an enormous coronal mass
ejection on 2011 June 7. The material formed discrete concentrations,
or blobs, in the corona and fell back to the surface, appearing as dark
clouds against the bright corona. In this work we examined the density
and dynamic evolution of these blobs in order to formally assess the
intriguing morphology displayed throughout their descent. The blobs
were studied in five wavelengths (94, 131, 171, 193, and 211 Å)
using the Solar Dynamics Observatory Atmospheric Imaging Assembly,
comparing background emission to attenuated emission as a function
of wavelength to calculate column densities across the descent of
four separate blobs. We found the material to have a column density of
hydrogen of approximately 2 × 10<SUP>19</SUP> cm<SUP>-2</SUP>, which is
comparable with typical pre-eruption filament column densities. Repeated
splitting of the returning material is seen in a manner consistent
with the Rayleigh-Taylor instability. Furthermore, the observed
distribution of density and its evolution is also a signature of this
instability. By approximating the three-dimensional geometry (with data
from STEREO-A), volumetric densities were found to be approximately 2
× 10<SUP>-14</SUP> g cm<SUP>-3</SUP>, and this, along with observed
dominant length scales of the instability, was used to infer a magnetic
field of the order 1 G associated with the descending blobs.
---------------------------------------------------------
Title: Determination of Prominence Plasma β from the Dynamics of
Rising Plumes
Authors: Hillier, Andrew; Hillier, Richard; Tripathi, Durgesh
2014IAUS..300...94H Altcode:
Observations of quiescent prominences show rising plumes, dark in
chromospheric lines, that propagate from large bubbles. In this paper
we present a method that may be used to determine the plasma β (ratio
of gas pressure to magnetic pressure) from the rising plumes. Using
the classic fluid dynamic solution for flow around a circular cylinder,
the compression of the prominence material can be estimated. Application
to a prominence gave an estimate of the plasma β as β=0.47-1.13 for
a ratio of specific heats of γ=1.4-1.7.
---------------------------------------------------------
Title: A Statistical Study of Transverse Oscillations in a Quiescent
Prominence
Authors: Hillier, A.; Morton, R. J.; Erdélyi, R.
2013ApJ...779L..16H Altcode: 2013arXiv1310.8009H
The launch of the Hinode satellite has allowed for seeing-free
observations at high-resolution and high-cadence making it well suited
to study the dynamics of quiescent prominences. In recent years it
has become clear that quiescent prominences support small-amplitude
transverse oscillations, however, sample sizes are usually too small
for general conclusions to be drawn. We remedy this by providing a
statistical study of transverse oscillations in vertical prominence
threads. Over a 4 hr period of observations it was possible to
measure the properties of 3436 waves, finding periods from 50 to
6000 s with typical velocity amplitudes ranging between 0.2 and 23
km s<SUP>-1</SUP>. The large number of observed waves allows the
determination of the frequency dependence of the wave properties and
derivation of the velocity power spectrum for the transverse waves. For
frequencies less than 7 mHz, the frequency dependence of the velocity
power is consistent with the velocity power spectra generated from
observations of the horizontal motions of magnetic elements in the
photosphere, suggesting that the prominence transverse waves are
driven by photospheric motions. However, at higher frequencies the two
distributions significantly diverge, with relatively more power found
at higher frequencies in the prominence oscillations. These results
highlight that waves over a large frequency range are ubiquitous in
prominences, and that a significant amount of the wave energy is found
at higher frequency.
---------------------------------------------------------
Title: Can Superflares Occur on Our Sun?
Authors: Shibata, Kazunari; Isobe, Hiroaki; Hillier, Andrew; Choudhuri,
Arnab Rai; Maehara, Hiroyuki; Ishii, Takako T.; Shibayama, Takuya;
Notsu, Shota; Notsu, Yuta; Nagao, Takashi; Honda, Satoshi; Nogami,
Daisaku
2013PASJ...65...49S Altcode: 2012arXiv1212.1361S
Recent observations of Sun-like stars, similar to our Sun in their
surface temperature (5600-6000 K) and slow rotation (rotational period
> 10 d), using the Kepler satellite by Maehara et al. (2012, Nature,
485, 478) have revealed the existence of superflares (with energy
of 10<SUP>33</SUP>-10<SUP>35</SUP> erg). From statistical analyses
of these superflares, it was found that superflares with energy
of 10<SUP>34</SUP> erg occur once in 800 yr, and superflares with
10<SUP>35</SUP> erg occur once in 5000 yr. In this paper, we examine
whether superflares with energy of 10<SUP>33</SUP>-10<SUP>35</SUP>
erg could occur on the present Sun through the use of simple
order-of-magnitude estimates based on current ideas related to
the mechanisms of the solar dynamo. If magnetic flux is generated
by differential rotation at the base of the convection zone, as
assumed in typical dynamo models, it is possible that the present Sun
would generate a large sunspot with a total magnetic flux of ∼2 ×
10<SUP>23</SUP> Mx (= G cm<SUP>2</SUP>) within one solar cycle period,
and lead to superflares with an energy of 10<SUP>34</SUP> erg. To
store a total magnetic flux of ∼10<SUP>24</SUP> Mx, necessary for
generating 10<SUP>35</SUP> erg superflares, it would take ∼40 yr. Hot
Jupiters have often been argued to be a necessary ingredient for the
generation of superflares, but we found that they do not play any
essential role in the generation of magnetic flux in the star itself,
if we consider only the magnetic interaction between the star and the
hot Jupiter. This seems to be consistent with Maehara et al.'s finding
of 148 superflare-generating solar-type stars that do not have a hot
Jupiter-like companion. Altogether, our simple calculations, combined
with Maehara et al.'s analysis of superflares on Sun-like stars,
show that there is a possibility that superflares of 10<SUP>34</SUP>
erg would occur once in 800 yr on our present Sun.
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Title: On the Support of Solar Prominence Material by the Dips of
a Coronal Flux Tube
Authors: Hillier, Andrew; van Ballegooijen, Adriaan
2013ApJ...766..126H Altcode: 2013arXiv1303.4130H
The dense prominence material is believed to be supported against
gravity through the magnetic tension of dipped coronal magnetic
field. For quiescent prominences, which exhibit many gravity-driven
flows, hydrodynamic forces are likely to play an important role in
the determination of both the large- and small-scale magnetic field
distributions. In this study, we present the first steps toward creating
a three-dimensional magneto-hydrostatic prominence model where the
prominence is formed in the dips of a coronal flux tube. Here 2.5D
equilibria are created by adding mass to an initially force-free
magnetic field, then performing a secondary magnetohydrodynamic
relaxation. Two inverse polarity magnetic field configurations are
studied in detail, a simple o-point configuration with a ratio of the
horizontal field (B<SUB>x</SUB> ) to the axial field (B<SUB>y</SUB>
) of 1:2 and a more complex model that also has an x-point with a
ratio of 1:11. The models show that support against gravity is either
by total pressure or tension, with only tension support resembling
observed quiescent prominences. The o-point of the coronal flux tube
was pulled down by the prominence material, leading to compression of
the magnetic field at the base of the prominence. Therefore, tension
support comes from the small curvature of the compressed magnetic field
at the bottom and the larger curvature of the stretched magnetic field
at the top of the prominence. It was found that this method does not
guarantee convergence to a prominence-like equilibrium in the case
where an x-point exists below the prominence flux tube. The results
imply that a plasma β of ~0.1 is necessary to support prominences
through magnetic tension.
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Title: Simulations of the Dynamics of the Magnetic Rayleigh-Taylor
Instability in Solar Prominences
Authors: Hillier, A.; Berger, T.; Shibata, K.; Isobe, H.
2013ASPC..474..147H Altcode:
The magnetic Rayleigh-Taylor instability plays an important role in the
mass and magnetic flux transport in many astrophysical bodies. Solar
prominences also display this instability and recent observations using
the Solar Optical Telescope onboard the Hinode satellite have revealed
these dynamics in amazing detail. The observations show rising plumes,
approximately 1 Mm in width, that propagate through the dense prominence
material from low-density bubbles, i.e. the situation expected when the
magnetic Rayleigh-Taylor instability occurs. To study this phenomenon,
we performed 3D simulations of the magnetic Rayleigh-Taylor instability
in the Kippenhahn-Schlüter prominence model. The plumes formed in
these simulations are filamentary structures that are aligned with
the magnetic field created as 3D modes of the magnetic Rayleigh-Taylor
instability. The plumes rise, developing large structures from smaller
structures through an inverse cascade process driven by nonlinear
interaction. The results suggest that the plumes observed in the
prominence may be used to study the conditions inside the prominence.
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Title: Determination of Prominence Plasma β from the Dynamics of
Rising Plumes
Authors: Hillier, Andrew; Hillier, Richard; Tripathi, Durgesh
2012ApJ...761..106H Altcode: 2012arXiv1211.0742H
Observations by the Hinode satellite show in great detail the
dynamics of rising plumes, dark in chromospheric lines, in quiescent
prominences that propagate from large (~10 Mm) bubbles that form at
the base of the prominences. These plumes present a very interesting
opportunity to study magnetohydrodynamic (MHD) phenomena in quiescent
prominences, but obstacles still remain. One of the biggest issues is
that of the magnetic field strength, which is not easily measurable
in prominences. In this paper we present a method that may be
used to determine a prominence's plasma β when rising plumes are
observed. Using the classic fluid dynamic solution for flow around
a circular cylinder with an MHD correction, the compression of the
prominence material can be estimated. This has been successfully
confirmed through simulations; application to a prominence gave an
estimate of the plasma β as β = 0.47 ± 0.079 to 1.13 ± 0.080
for the range γ = 1.4-1.7. Using this method it may be possible to
estimate the plasma β of observed prominences, therefore helping our
understanding of a prominence's dynamics in terms of MHD phenomena.
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Title: Numerical Simulations of the Magnetic Rayleigh-Taylor
Instability in the Kippenhahn-Schlüter Prominence
Model. II. Reconnection-triggered Downflows
Authors: Hillier, Andrew; Isobe, Hiroaki; Shibata, Kazunari; Berger,
Thomas
2012ApJ...756..110H Altcode: 2011arXiv1106.2613H
The launch of the Hinode satellite has allowed high-resolution
observations of supersonic bright downflows in quiescent prominences,
known as prominence knots. We present observations in the Ca
II H spectral line using the Solar Optical Telescope on board
the Hinode satellite of a descending plasma knot of size ~900
km. The knot initially undergoes ballistic motion before undergoing
impulsive accelerations at the same time as experiencing increases
in intensity. We also present a subset of our three-dimensional
magnetohydrodynamic simulations, performed to investigate the
nonlinear stability of the Kippenhahn-Shlüter prominence model
to the magnetic Rayleigh-Taylor instability in which interchange
reconnection occurs. The interchange reconnection in the model
breaks the force balance along the field lines which initiates the
downflows. The downflows propagate with a downward fluid velocity
of ~15 km s<SUP>-1</SUP> and a characteristic size of ~700 km. We
conclude that the observed plasma blob and the simulated downflow are
driven by the breaking of the force balance along the magnetic field
as a result of a change in magnetic topology caused by reconnection
of the magnetic field.
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Title: Spicule Dynamics over Plage Region
Authors: Anan, T.; Kitai, R.; Hillier, A.; Kawate, T.; Ichimoto, K.;
Shibata, K.
2012ASPC..454...91A Altcode:
We have studied spicular jets over a plage region and derived their
dynamic characteristics using Hinode Solar Optical Telescope (SOT)
high-resolution Ca II H images. We have identified 169 spicules over
the target plage. This sample size permits us to derive statistically
reliable results regarding spicular dynamics. The properties of plage
spicules can be summarized as follows: (1) In a plage area, we clearly
identify spicular jet features. (2) They are shorter in length than
the quiet-region limb spicules, and follow ballistic motion under
constant deceleration. (3) The majority (80%) of the plage spicules
show a full rise and retreat (which we call ‘parabolic’ spicules),
while 10% of them fade out without a complete retreat phase(which we
call ‘fade out’ spicules). (4) The deceleration of the spicule is
proportional to the velocity of ejection (i.e. the initial velocity).
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Title: Simulations of the Magnetic Rayleigh-Taylor Instability in
the Kippenhahn-Schlüter Prominence Model
Authors: Hillier, A.; Berger, T.; Shibata, K.; Isobe, H.
2012ASPC..456..157H Altcode:
The launch of the Hinode satellite, with the Solar Optical Telescope,
allowed for high resolution, high time cadence observations of
prominences to be performed in the seeing free environment of
space. The most striking discovery from these observations is of
plumes, approximately 1 Mm in width, that propagate through the
prominence material. The plumes initiate from underdense bubbles that
form beneath prominences, rise at constant speeds of approximately 20
km s<SUP>-1</SUP> and are formed in the conditions required for the
magnetic Rayleigh-Taylor instability to occur. To study this phenomenon,
we performed 3D simulations of the magnetic Rayleigh-Taylor instability
in the Kippenhahn-Schlüter prominence model. The plumes formed in
these simulations are filamentary structures that are aligned with
the magnetic field created as 3D modes of the magnetic Rayleigh-Taylor
instability. The plumes rise, developing large structures from smaller
structures through an inverse cascade process driven by nonlinear
interaction. The results suggest that the plumes observed in the
prominence may be used to study the conditions inside the prominence.
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Title: Numerical Simulations of the Magnetic Rayleigh-Taylor
Instability in the Kippenhahn-Schlüter Prominence Model. I. Formation
of Upflows
Authors: Hillier, Andrew; Berger, Thomas; Isobe, Hiroaki; Shibata,
Kazunari
2012ApJ...746..120H Altcode:
The launch of the Hinode satellite led to the discovery of rising
plumes, dark in chromospheric lines, that propagate from large
(~10 Mm) bubbles that form at the base of quiescent prominences. The
plumes move through a height of approximately 10 Mm while developing
highly turbulent profiles. The magnetic Rayleigh-Taylor instability
was hypothesized to be the mechanism that drives these flows. In this
study, using three-dimensional (3D) MHD simulations, we investigate the
nonlinear stability of the Kippenhahn-Schlüter prominence model for the
interchange mode of the magnetic Rayleigh-Taylor instability. The model
simulates the rise of a buoyant tube inside the quiescent prominence
model, where the interchange of magnetic field lines becomes possible
at the boundary between the buoyant tube and the prominence. Hillier
et al. presented the initial results of this study, where upflows of
constant velocity (maximum found 6 km s<SUP>-1</SUP>) and a maximum
plume width ≈1.5 Mm which propagate through a height of approximately
6 Mm were found. Nonlinear interaction between plumes was found to be
important for determining the plume dynamics. In this paper, using
the results of ideal MHD simulations, we determine how the initial
parameters for the model and buoyant tube affect the evolution of
instability. We find that the 3D mode of the magnetic Rayleigh-Taylor
instability grows, creating upflows aligned with the magnetic field
of constant velocity (maximum found 7.3 km s<SUP>-1</SUP>). The width
of the upflows is dependent on the initial conditions, with a range
of 0.5-4 Mm which propagate through heights of 3-6 Mm. These results
are in general agreement with the observations of the rising plumes.
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Title: Simulations of the magnetic Rayleigh-Taylor instability in
a quiescent prominence model to study the dark upflows observed
in prominences
Authors: Hillier, A. S.; Berger, T. E.; Shibata, K.; Isobe, H.
2011AGUFMSH33A2033H Altcode:
Observations of quiescent prominences by the Solar Optical Telescope
(SOT) on board the Hinode satellite show plumes of hot, underdense
material rising through the prominence. These plumes form at the
boundary between the prominence and low density bubbles, approximately
10 Mm in size, that appear beneath the prominence, and then rise
through the prominence material at speeds of approximately 20 km/s and
widths of approximately 1.5 Mm. The plume profile ranges from highly
turbulent to smooth, suggesting that the prominence conditions, as well
as those of the bubble, are important in determining the dynamics. To
investigate this phenomenon, we perform simulations of the magnetic
Rayleigh-Taylor instability in a local prominence model. The instability
creates rising plumes of hot, underdense material that propagate through
the prominence material at a velocity of approximately 6-7 km/s and
widths of approximately 1.5 Mm, in rough agreement with the Hinode
observations. Nonlinear effects, in which the interaction between
plumes drives an inverse cascade process creating large plumes from
smaller plumes, are found to be important. Increasing the magnetic
field strength creates smoother plume structures. The addition of a
strong guide field, which is suggested in some prominence models, does
not hinder plume formation but does change the dynamic scaling. The
Rayleigh-Taylor instability drives an upward flow of magnetic energy
and a downward flow of mass. The results from the simulations well
match the characteristics of the observed plumes, suggesting that
the magnetic Rayleigh-Taylor instability could be important in
determining prominence structure as well as changing the magnetic
energy distribution in overlying coronal cavities which ultimately
erupt as coronal mass ejections.
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Title: Numerical Simulations of the Magnetic Rayleigh-Taylor
Instability in the Kippenhahn-Schlüter Prominence Model
Authors: Hillier, Andrew; Isobe, Hiroaki; Shibata, Kazunari; Berger,
Thomas
2011ApJ...736L...1H Altcode: 2011arXiv1107.4882A
The launch of the Hinode satellite has allowed unprecedented
high-resolution, stable images of solar quiescent prominences to
be taken over extended periods of time. These new images led to the
discovery of dark upflows that propagated from the base of prominences,
developing highly turbulent profiles. As yet, how these flows are driven
is not fully understood. To study the physics behind these phenomena,
we use three-dimensional magnetohydrodynamic simulations to investigate
the nonlinear stability of the Kippenhahn-Shlüter prominence model
to the magnetic Rayleigh-Taylor instability. The model simulates the
rise of a buoyant tube inside a quiescent prominence, where the upper
boundary between the tube and prominence model is perturbed to excite
the interchange of magnetic field lines. We found upflows of constant
velocity (maximum found 6 km s<SUP>-1</SUP>) and a maximum plume
width ≈1500 km which propagate through a height of approximately 6
Mm in the no guide field case. The case with the strong guide field
(initially B<SUB>y</SUB> = 2B<SUB>x</SUB> ) results in a large plume
that rises through the prominence model at ~5 km s<SUP>-1</SUP> with
width ~900 km (resulting in width of 2400 km when viewed along the
axis of the prominence), reaching a height of ~3.1 Mm. In both cases,
nonlinear processes were important for determining plume dynamics.
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Title: Observations of Plasma Blob Ejection from a Quiescent
Prominence by Hinode Solar Optical Telescope
Authors: Hillier, Andrew; Isobe, Hiroaki; Watanabe, Hiroko
2011PASJ...63L..19H Altcode: 2011arXiv1103.3750H
We report findings from 0”.2 resolution observations of the
2007 October 03 quiescent prominence observed with the Solar
Optical Telescope on the Hinode satellite. The observations show
clear ejections from the top of the quiescent prominence of plasma
blobs. The ejections, originating from the top of prominence threads,
are impulsively accelerated to approximately Alfvén velocities and
then undergo ballistic motion. The ejections have a characteristic
size between ∼ 1000-2000 km. These characteristics are similar
to downwardly propagating knots (typical size ∼ 700 km) that have
been observed in prominence threads, we suggest that the plasma blob
ejections could be the upward moving counterpart to the downwardly
propagating knots. We discuss the tearing instability as a possible
mechanism to explain the ejections.
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Title: Magneto-thermal convection in solar prominences
Authors: Berger, Thomas; Testa, Paola; Hillier, Andrew; Boerner, Paul;
Low, Boon Chye; Shibata, Kazunari; Schrijver, Carolus; Tarbell, Ted;
Title, Alan
2011Natur.472..197B Altcode:
Coronal cavities are large low-density regions formed by
hemispheric-scale magnetic flux ropes suspended in the Sun's outer
atmosphere. They evolve over time, eventually erupting as the dark
cores of coronal mass ejections. Although coronal mass ejections are
common and can significantly affect planetary magnetospheres, the
mechanisms by which cavities evolve to an eruptive state remain poorly
understood. Recent optical observations of high-latitude `polar crown'
prominences within coronal cavities reveal dark, low-density `bubbles'
that undergo Rayleigh-Taylor instabilities to form dark plumes rising
into overlying coronal cavities. These observations offered a possible
mechanism for coronal cavity evolution, although the nature of the
bubbles, particularly their buoyancy, was hitherto unclear. Here we
report simultaneous optical and extreme-ultraviolet observations of
polar crown prominences that show that these bubbles contain plasma at
temperatures in the range (2.5-12)×10<SUP>5</SUP> kelvin, which is
25-120 times hotter than the overlying prominence. This identifies a
source of the buoyancy, and suggests that the coronal cavity-prominence
system supports a novel form of magneto-thermal convection in the solar
atmosphere, challenging current hydromagnetic concepts of prominences
and their relation to coronal cavities.
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Title: MHD simulations of quiescent prominence upflows in the
Kippenhahn-Schlüter prominence model
Authors: Hillier, A. S.; Isobe, H.; Shibata, K.; Berger, T. E.
2011ASInC...2..331H Altcode:
Images from the Hinode satellite have led to the discovery of dark
upflows that propagate from the base of prominences, developing highly
turbulent profiles. The magnetic Rayleigh-Taylor instability has been
hypothesized as the mechanism to create these plumes. To study the
physics behind this phenomenon we use 3D magnetohydrodynamic simulations
to investigate the nonlinear stability of the Kippenhahn-Shlüter
prominence model to the magnetic Rayleigh-Taylor instability. The model
simulates the rise of a buoyant tube inside a quiescent prominence,
where the upper boundary between the tube and prominence model is
perturbed to excite the interchange of magnetic field lines. We
find upflows of constant velocity (maximum found 6 km s^{-1}) and a
maximum plume width ≈ 1500 km which propagate through a height of
approximately 6 Mm, in general agreement with the Hinode observations.
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Title: Evolution of the Kippenhahn-Schlüter Prominence Model Magnetic
Field under Cowling Resistivity
Authors: Hillier, Andrew; Shibata, Kazunari; Isobe, Hiroaki
2010PASJ...62.1231H Altcode: 2010arXiv1007.1909H
We present the results from 1.5D diffusion simulations of the
Kippenhahn-Schlüter prominence model magnetic field evolution under the
influence of the ambipolar terms of Cowling resistivity. We show that
initially the evolution is determined by the ratio of the horizontal
and vertical magnetic fields, which gives current sheet thinning
(thickening) when this ratio is small (large) and a marginal case
where a new characteristic current sheet length scale is formed. After
a timespan greater than the Cowling resistivity time, the current
sheet thickens as a power law of t independent of the ratio of the
field strengths. These results imply that when Cowling resistivity is
included in the model, the tearing instability time scale is reduced
by more than one order of magnitude when the ratio of the horizontal
field to the vertical field is 20% or less. These results imply that,
over the course of its lifetime, the structure of the prominence can
be significantly altered by Cowling resistivity, and in some cases
will allow the tearing instability to occur.
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Title: Spicule Dynamics over a Plage Region
Authors: Anan, Tetsu; Kitai, Reizaburo; Kawate, Tomoko; Matsumoto,
Takuma; Ichimoto, Kiyoshi; Shibata, Kazunari; Hillier, Andrew; Otsuji,
Kenichi; Watanabe, Hiroko; Ueno, Satoru; Nagata, Shin'ichi; Ishii,
Takako T.; Komori, Hiroyuki; Nishida, Keisuke; Nakamura, Tahei; Isobe,
Hiroaki; Hagino, Masaoki
2010PASJ...62..871A Altcode: 2010arXiv1002.2288A
We studied spicular jets over a plage area and derived their
dynamic characteristics using Hinode Solar Optical Telescope (SOT)
high-resolution images. A target plage region was near to the west limb
of the solar disk. This location permitted us to study the dynamics
of spicular jets without any overlapping effect of spicular structures
along the line of sight. In this work, to increase the ease with which
we could identify spicules on the disk, we applied the image processing
method `MadMax' developed by Koutchmy et al. (1989). It enhances fine,
slender structures (like jets), over a diffuse background. We identified
169 spicules over the target plage. This sample permited us to derive
statistically reliable results regarding spicular dynamics. The
properties of plage spicules can be summarized as follows: (1) In a
plage area, we clearly identified spicular jet features. (2) They were
shorter in length than the quiet region limb spicules, and followed a
ballistic motion under constant deceleration. (3) The majority (80%)
of the plage spicules showed a cycle of rise and retreat, while 10% of
them faded out without a complete retreat phase. (4) The deceleration
of the spicule was proportional to the velocity of ejection (i.e.,
the initial velocity).
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Title: MHD simulations of upflows in the Kippenhahn-Schlueter
prominence model
Authors: Hillier, Andrew; Shibata, Kazunari; Isobe, Hiroaki; Berger,
Thomas
2010cosp...38.2914H Altcode: 2010cosp.meet.2914H
The launch of SOT on the Hinode satellite, with it's previously
unprecedented high resolution, high cadence images of solar prominences,
led to the discovery of small scale, highly dynamic flows in quiescent
prominences. Berger et al. (2008) reported dark upflows that propagated
from the base of the prominence through a height of approximately 10
Mm before ballooning into the familiar mushroom shape often associated
with the Rayleigh-Taylor instability. Whether such phenomena can be
driven by instabilities and, if so, how the instability evolve is yet
to be fully investigated. In this study, we use the Kippenhahn-Schlueter
(K-S) prominence model as the base for 3D numerical MHD simulations. The
K-S prominence model is linearly stable for ideal MHD perturbationss,
but can be made unstable through nonlinear perturbations, which we
impose through inserting a low density (high temperature) tube through
the centre of the prominence. Our simulations follow the linear and
nonlinear evolution of upflows propagating from the hot tube through the
K-S prominence model. We excited Rayleigh-Taylor like modes inside the
K-S model with a wave along the contact discontinuity created between
the hot tube and the K-S prominence, and solved the pertur-bations
of this system. For such a complex setting, the linear evolution of
the instability has 0.7 not been studied, and we found the growth
rate to be ∼ ( ρ+ -ρ- - 0.05)k 0.22 . The most ρ+ +ρ- unstable
wavelength was ∼ 100 km which, through the inverse cascade process,
created upflows of ∼ 300 km. The rising plumes obtained a constant
rise velocity in the nonlinear stage due to the creation of adverse
magnetic and gas pressure gradients at the top of the plume.
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Title: Preservation of Lantern Slides for Use in Today's Technology
Authors: Hillier, A. S.
2007ASPC..377..308H Altcode: 2007lisa.conf..308H
Lantern slides will keep a long time, which is a good quality for
preservation. However, as I have found, they break. Unless there is
a lantern slide projector available, there is no way to show these
valuable assets to others. This poster will explain my project to
bring these pictures to life, to use them in education projects, and
to simply show a bit of history to an attentive audience. With today's
technology they can be placed on computers and stored more easily and
be a joy to all.
