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.
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.
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
I
P) code. Results are compared to the magnetohydrodynamic (MHD) model and the semi-analytic solution.
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.
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FMFMET, NUK, UL, UM, UPUK
ABSTRACT
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.
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.
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.
Methods.
Two-fluid numerical simulations were performed using the (P
I
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.
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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ABSTRACT
Explosive phenomena are known to trigger a wealth of shocks in warm plasma environments, including the solar chromosphere and molecular clouds where the medium consists of both ionized and ...neutral species. Partial ionization is critical in determining the behaviour of shocks, since the ions and neutrals locally decouple, allowing for substructure to exist within the shock. Accurately modelling partially ionized shocks requires careful treatment of the ionized and neutral species, and their interactions. Here we study a partially ionized switch-off slow-mode shock using a multilevel hydrogen model with both collisional and radiative ionization and recombination rates that are implemented into the two-fluid (PIP) code, and study physical parameters that are typical of the solar chromosphere. The multilevel hydrogen model differs significantly from magnetohydrodynamic (MHD) solutions due to the macroscopic thermal energy loss during collisional ionization. In particular, the plasma temperature both post-shock and within the finite-width is significantly cooler that the post-shock MHD temperature. Furthermore, in the mid to lower chromosphere, shocks feature far greater compression than their single-fluid MHD analogues. The decreased temperature and increased compression reveal the importance of non-equilibrium ionized in the thermal evolution of shocks in partially ionized media. Since partially ionized shocks are not accurately described by the Rankine-Hugoniot shock jump conditions, it may be incorrect to use these to infer properties of lower atmospheric shocks.
Vortex motions are frequently observed on the solar photosphere. These motions may play a key role in the transport of energy and momentum from the lower atmosphere into the upper solar atmosphere, ...contributing to coronal heating. The lower solar atmosphere also consists of complex networks of flux tubes that expand and merge throughout the chromosphere and upper atmosphere. We perform numerical simulations to investigate the behavior of vortex-driven waves propagating in a pair of such flux tubes in a non-force-free equilibrium with a realistically modeled solar atmosphere. The two flux tubes are independently perturbed at their footpoints by counter-rotating vortex motions. When the flux tubes merge, the vortex motions interact both linearly and nonlinearly. The linear interactions generate many small-scale transient magnetic substructures due to the magnetic stress imposed by the vortex motions. Thus, an initially monolithic tube is separated into a complex multithreaded tube due to the photospheric vortex motions. The wave interactions also drive a superposition that increases in amplitude until it exceeds the local Mach number and produces shocks that propagate upward with speeds of approximately 50 km s−1. The shocks act as conduits transporting momentum and energy upward, and heating the local plasma by more than an order of magnitude, with a peak temperature of approximately 60,000 K. Therefore, we present a new mechanism for the generation of magnetic waveguides from the lower solar atmosphere to the solar corona. This wave guide appears as the result of interacting perturbations in neighboring flux tubes. Thus, the interactions of photospheric vortex motions is a potentially significant mechanism for energy transfer from the lower to upper solar atmosphere.
Background:
The treatment of displaced, intra-articular calcaneus fractures is controversial. The extensile lateral approach has been historically preferred because it provides excellent exposure and ...visualization for fracture reduction. However, soft tissue complications with this approach can lead to poor outcomes for patients. Recently, there has been an interest in the minimally invasive treatment of calcaneus fractures. The purpose of the present study was to determine the radiographic reduction of displaced, intra-articular calcaneus fractures and the rate of complications using a 2-incision, minimally invasive approach.
Methods:
A dual-incision, minimally invasive approach with plate and screw fixation was utilized for the treatment of 32 patients with displaced, intra-articular calcaneus fractures. Preoperative and postoperative calcaneal measurements were taken to assess fracture reduction. Additionally, a retrospective chart review was performed to assess for complications.
Results:
The mean preoperative Bohler’s angle measurement was 12.9 (range, –5 to 36) degrees and the final postoperative Bohler’s angle was 31.7 (range, 16-40) degrees. One patient (3.1%) had postoperative numbness related to the medial incision in the calcaneal branch sensory nerve distribution. Two patients (6.2%) had a wound infection treated with local wound care and oral antibiotics, while 1 patient (3.1%) had a deep infection that required a secondary surgery for irrigation and debridement. Two patients (6.2%) returned to the operating room for removal of symptomatic hardware.
Conclusion:
Operative fixation of displaced, intra-articular calcaneus fractures treated with a 2-incision, minimally invasive approach resulted in acceptable fracture reduction with a minimal rate of complications.
Level of Evidence:
Level IV, retrospective case series.
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NUK, OILJ, SAZU, UKNU, UL, UM, UPUK
The COVID-19 pandemic has highlighted the pre-existing vulnerability of the small-scale fisheries sector in South Africa and exposed the structural inequalities and ongoing injustices facing this ...sector. The failures within the fisheries governance and management system linked to the slow pace of implementing the Small-scale Fisheries Policy of 2012, have further exacerbated their vulnerability. This paper explores the immediate impacts of the COVID-19 pandemic on the small-scale fisheries sector and exposes how governance failures within the fisheries sector have increased their vulnerability. Restrictions on fishing activities and mobility, closure of conservation areas, unfair fines and arrests, loss of markets and barriers to sale of fish products as well as lack of access to water, have had significant impacts on small-scale fishers and coastal communities. The lack of social protection and the limited emergency relief provided by government further exacerbated their precarious position. Despite their vulnerability, fishers have demonstrated a measure of resilience, supporting those in need with food, lobbying government to amend restrictions and recognise their rights, and challenging efforts to fast-track development and exclude their voices. The crisis has highlighted an urgent need for broad, national level transformation to deal with the poverty and injustices facing poor coastal communities, as well as fisheries-specific policy reform.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
We examine some leading-order flow and stability properties of smectic A (SmA) liquid crystals (LCs) in two spatial dimensions by analysing a fully nonlinear continuum theory of these materials. We ...derive a system of equations for the dynamic variables describing the flow velocity and orientation of the material under suitable assumptions upon these quantities. This system can provide insight into the leading-order behaviour under quite general circumstances, and we provide an example of utilising this system to determine the flow induced by a constant pressure gradient applied normally to the smectic layers. We then consider the effect of oscillatory perturbations on a relaxed, stationary sample of SmA, and provide criteria under which one would expect to see the onset of instability in the form of inequalities between the material parameters and perturbative wave number. We find that instability occurs for physically realisable values of these quantities, and, in particular, that certain viscosities characterising the SmA phase can act as 'destabilising agents' such that one could, for a given sample with known parameter values, manipulate the behaviour of that sample.
Rapid eye movement (REM) sleep is generated and maintained by the interaction of a variety of neurotransmitter systems in the brainstem, forebrain, and hypothalamus. Within these circuits lies a core ...region that is active during REM sleep, known as the subcoeruleus nucleus (SubC) or sublaterodorsal nucleus. It is hypothesized that glutamatergic SubC neurons regulate REM sleep and its defining features such as muscle paralysis and cortical activation. REM sleep paralysis is initiated when glutamatergic SubC cells activate neurons in the ventral medial medulla, which causes release of GABA and glycine onto skeletal motoneurons. REM sleep timing is controlled by activity of GABAergic neurons in the ventrolateral periaqueductal gray and dorsal paragigantocellular reticular nucleus as well as melanin-concentrating hormone neurons in the hypothalamus and cholinergic cells in the laterodorsal and pedunculo-pontine tegmentum in the brainstem. Determining how these circuits interact with the SubC is important because breakdown in their communication is hypothesized to underlie narcolepsy/cataplexy and REM sleep behavior disorder (RBD). This review synthesizes our current understanding of mechanisms generating healthy REM sleep and how dysfunction of these circuits contributes to common REM sleep disorders such as cataplexy/narcolepsy and RBD.
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