arXiv subjects

Andrew Hillier

Publications and source records attributed to Andrew Hillier.

At least 19 recordsLinked to original sources

arXiv2026

Signatures of Damping Nonlinear Oscillations by KHI-induced Turbulence in Synthetic Observations

Large-amplitude decaying kink oscillations of coronal loops are strongly influenced by nonlinear processes, such as Kelvin-Helmholtz instability (KHI) and turbulence, though comprehensive theory and observational confirmation remain limited. Building on the recently developed theory on nonlinear damping by KHI-induced turbulence in impulsively driven transverse loop oscillations, we investigate its observational signatures using 3D magnetohydrodynamic simulations and forward-modelled EUV images. The simulated oscillations exhibit time-varying frequency shifts and damping rates, which are broadly consistent with nonlinear turbulence-damping theory. Additionally, they exhibit excitation of higher-order modes, slightly increased periods relative to the linear kink period, and reduced displacement amplitudes. These features are generally preserved in synthetic observations, though resolving higher-order modes requires higher spatial resolution than currently available. For loops embedded in a hotter background, hotter channels (e.g., 193 Angstroms) are more sensitive to boundary dynamics, thus their oscillations decay faster with smaller displacements and larger phase shifts than those in cooler channels (e.g., 171 Angstroms). Comparisons of simulated and synthetic oscillations show close agreement at the early stage. At later times, synthetic oscillations exhibit smaller displacements and larger phase shifts, due to turbulence-induced asymmetry in the loop cross-section. Bayesian fitting shows that the initial oscillation amplitude and kink period are robustly constrained, whereas parameters controlling the damping profile are degenerate, indicating that additional observables would aid reliable seismological inference. These results provide a quantitative basis for identifying nonlinear damping and detecting KHI-driven turbulence in transverse loop oscillations.

astro-ph.SR↗

arXiv2025

UK White Paper on Magnetic Reconnection

Magnetic reconnection powers explosive releases of magnetic energy, heating and particle acceleration throughout the plasma universe. Knowledge of this universal process is vital to understanding the Heliosphere, as it plays a key role in solar flares, coronal mass ejections, coronal heating, solar wind acceleration, geomagnetic storms, and interactions between the solar wind and planetary magnetospheres. As such, reconnection underpins multiple science objectives of multiple future space missions. The UK plays a leading role in this international field, through a combination of in situ measurements from Earth's magnetosphere and the solar wind, observations of the solar corona and chromosphere, and world-class numerical simulations and theory. This white paper identifies: Nine priority science objectives for reconnection research in the next decade; Recommendations to guide investment in theory, simulations and infrastructure; Mission priorities and required measurements to ensure the UK maintains and improves its world-class credentials in reconnection science.

astro-ph.IM↗

arXiv2025

Comparing the magnetic Rayleigh-Taylor instability dynamics in two- and three-dimensions

The magnetic Rayleigh-Taylor instability (MRTI) governs plasma mixing and transport in a wide range of astrophysical and laboratory systems. Owing to computational constraints, MRTI is often studied using two-dimensional (2D) simulations, but the extent to which 2D captures the true three-dimensional (3D) dynamics remains unclear. In this work, we perform direct numerical simulations of non-ideal, incompressible MRTI in both 2D and 3D, systematically varying the magnetic field strength from weakly to strongly magnetized regimes. We find that the 3D system exhibits richer mode interactions due to the coexistence of interchange, undular, and mixed modes structures that are inherently absent in 2D. The mixing layer in 3D has enhanced small-scale mixing and reduced fluid dispersion compared to 2D, which is characterized by large-scale plumes. Energy diagnostics reveal that the gravitational potential energy released is higher in 2D, primarily because of inefficient mixing and significant fluid dispersion. In contrast, 3D systems display greater energy dissipation and anisotropy, driven by small-scale vortical motions. The non-linear growth of the instability increases monotonically with magnetic field strength in 3D but shows a non-monotonic trend in 2D. Despite these broad differences, the rate of magnetic-to-kinetic energy conversion remains remarkably similar across dimensions, indicating that 2D simulations can meaningfully capture reconnection-driven processes but not the full turbulent evolution. Overall, our results demonstrate that 2D MRTI simulations cannot reliably represent 3D mixing, energy dynamics, or nonlinear growth, highlighting the fundamental importance of three-dimensionality in magnetized plasma instabilities.

physics.flu-dyn↗

arXiv2025

Quantifying Reconnection and it's Dynamical Role in 2D Magnetic Rayleigh-Taylor Turbulence

Magnetic Rayleigh-Taylor instability (MRTI) governs material transport and mixing in astrophysical and laboratory plasmas under the influence of gravity and magnetic fields. While magnetic reconnection is known to occur during MRTI evolution, its role in the evolution and energy dynamics remains poorly understood. Here, we present a comprehensive analysis of the role of reconnection in the two-dimensional MRTI dynamics, using high-resolution simulations. We establish that reconnection, through facilitating plume merger, relieving magnetic tension, and enabling continued instability growth, forms an essential component for the long-term instability evolution. To quantify the role of reconnection in energy dynamics, we develop a robust automated reconnection detection algorithm and perform a statistical analysis across a range of magnetic field strengths. We find that reconnection accounts for up to $80\%$ of the magnetic-to-kinetic energy transfer in the weak magnetic field regime, while contributing minimally ($\approx 3\%$) to magnetic energy dissipation. Our results establish magnetic reconnection as a critical mechanism that regulates large-scale MRTI dynamics, with implications for astrophysical plasmas and turbulent mixing in magnetized flows.

physics.plasm-ph↗

arXiv2025

Identification of Nonlinear Damping of Transverse Loop Oscillations by KHI-induced Turbulence

Kink oscillations in coronal loops have been extensively studied for their potential contributions to coronal heating and their role in plasma diagnostics through coronal seismology. A key focus is the strong damping of large-amplitude kink oscillations, which observational evidence suggests is nonlinear. However, directly identifying the nonlinearity is a challenge. This work presents an analytic formula describing nonlinear standing kink oscillations dissipated by turbulence, characterised by a time-varying damping rate and period drift. We investigate how the damping behaviour depends on the driving amplitude and loop properties, showing that the initial damping time $\tau$ is inversely proportional to the velocity disturbance over the loop radius, $V_i/R$. Using MCMC fitting with Bayesian inference, the nonlinear function better fits an observed decaying kink oscillation than traditional linear models, including exponential damping, suggesting its nonlinear nature. By applying a Bayesian model comparison, we establish regimes in which nonlinear and linear resonant absorption mechanisms dominate based on the relationship between the damping rate $\tau/P$ and $V_i/R$. Additionally, analysis of two specific events reveals that while one favours the nonlinear model, the other is better explained by the linear model. Our results suggest that this analytical approximation of nonlinear damping due to turbulence provides a valid and reliable description of large-amplitude decaying kink oscillations in coronal loops.

astro-ph.SR↗

arXiv2025

Understanding coronal rain dynamics through a point-mass model

Observations and simulations of coronal rain show that as cold and dense plasma falls through the corona it initially undergoes acceleration by gravity before the downward velocity saturates. Simulations have shown the emergence of an unexpected relation between terminal velocity of the rain and density ratio that has not been explained. Our aim is to explain this relation. In this paper we develop a simple point-mass model to understand how the evolution of the ambient corona moving with the coronal rain drop can influence the falling motion. We find that this simple effect results in the downward speed reaching a maximal value before decreasing, which is consistent with simulations with realistic coronal rain mass. These results provide an explanation for the scaling of the maximum downward speed to density ratio of the rain to the corona and as such provide a new tool that may be used to interpret observations.

astro-ph.SR↗

arXiv2025

Observational signatures of mixing-induced cooling in the Kelvin-Helmholtz instability

Cool ($\approx 10^4$K), dense material permeates the hot ($\approx 10^6$K), tenuous solar corona in form of coronal condensations, for example prominences and coronal rain. As the solar atmosphere evolves, turbulence can drive mixing between the condensations and the surrounding corona, with the mixing layer exhibiting an enhancement in emission from intermediate temperature ($\approx10^5$K) spectral lines, which is often attributed to turbulent heating within the mixing layer. However, radiative cooling is highly efficient at intermediate temperatures and numerical simulations have shown that radiative cooling can far exceed turbulent heating in prominence-corona mixing scenarios. As such the mixing layer can have a net loss of thermal energy, i.e., the mixing layer is cooling rather than heating. Here, we investigate the observational signatures of cooling processes in Kelvin-Helmholtz mixing between a prominence thread and the surrounding solar corona through 2D numerical simulations. Optically thin emission is synthesised for Si IV, along with optically thick emission for H$\alpha$, Ca II K and Mg II h using Lightweaver The Mg II h probes the turbulent mixing layer, whereas H$\alpha$ and Ca II K form within the thread and along its boundary respectively. As the mixing evolves, intermediate temperatures form leading to an increase in Si IV emission, which coincides with increased radiative losses. The simulation is dominated by cooling in the mixing layer, rather than turbulent heating, and yet enhanced emission in warm lines is produced. As such, an observational signature of decreased emission in cooler lines and increased emission in hotter lines may be a signature of mixing, rather than an implication of heating.

astro-ph.SR↗

arXiv2024

Nonlinear evolution of fluting oscillations in coronal flux tubes

Magnetic flux tubes in the solar corona support a rich variety of transverse oscillations, which are theoretically interpreted as magnetohydrodynamic (MHD) modes with a fast and/or Alfv\'enic character. In the standard flux tube model made of a straight cylindrical tube, these modes can be classified according to their azimuthal wavenumber, $m$. Sausage $m=0$ modes produce periodic expansion and contraction of the tube cross section and are observed during solar flares. Kink $m=1$ modes laterally displace the tube axis and are related to, for example, post-flare global transverse oscillations of coronal loops. Fluting $m \geq 2$ modes produce disturbances that are mainly confined to the tube boundary, but their observation remains elusive to date. We use 3D ideal MHD numerical simulations to investigate the nonlinear evolution of fluting modes in coronal flux tubes with transversely nonuniform boundaries. The simulations show that fluting modes are short-lived as coherent, collective motions of the flux tube. Owing to the process of resonant absorption, fluting oscillations become overdamped modes in tubes with wide enough nonuniform boundaries. During the nonlinear evolution, shear flows drive the Kelvin-Helmholtz instability at the tube boundary, which further disrupts the coherent fluting oscillation. For large-enough oscillation amplitudes, baroclinic instabilities of Rayleigh-Taylor type are also present at locations in the boundary where the plasma acceleration is normal to the boundary. The evolution of the instabilities drives turbulence in the flux tube, which may inhibit the resonant damping. However, the oscillations remain strongly damped even in this case. As a result of the combination of the strong damping and the induced instabilities, it is unlikely that coronal flux tubes can support fluting modes as sufficiently enduring coherent oscillations.

astro-ph.SR↗

arXiv2024

Self-similarity and growth of non-linear magnetic Rayleigh-Taylor instability -- Role of the magnetic field strength

The non-linear regime of the magnetic Rayleigh-Taylor instability (MRTI) has been studied in the context of several laboratory and astrophysical systems. Yet, several fundamental aspects remain unclear. One of them is the self-similar evolution of the instability. Studies have assumed that non-linear MRTI has a self-similar, quadratic growth similar to hydrodynamic (HD) RTI. However, neither self-similarity nor quadratic growth has been proved analytically. Furthermore, an explicit understanding of the factors that control the growth of non-linear instability remains unclear. Magnetic fields are known to play a crucial role in the evolution of the instability. Yet, a systematic study discussing how the magnetic field influences the instability growth is missing. These issues were addressed by performing an analytical and numerical study of the MRTI with a uniform magnetic field. Our study reveals that the imposed magnetic field does not conform to the HD self-similar evolution. However, the influence of the imposed magnetic field decays with time (t) as 1/t relative to the other non-linear terms, making the MRTI conform to the HD self-similarity. Thus, the HD RTI self-similar scaling becomes relevant to MRTI at late times, when nonlinear dynamics dominate. Based on energy conservation, an equation for the mixing layer height (h) is derived, which demonstrates the quadratic growth of h in time. This gave insight into various factors that could influence the non-linear growth of the instability. By studying MRTI at different magnetic field strengths, we demonstrate the role of magnetic field strength on the nonlinear growth of MRTI. Thus, the current study analytically and numerically proves the role of magnetic fields on the evolution of MRTI.

physics.flu-dyn↗

arXiv2024

Kink instability of flux ropes in partially-ionised plasmas

In the solar atmosphere, flux ropes are subject to current driven instabilities that are crucial in driving plasma eruptions, ejections and heating. A typical ideal magnetohydrodynamics (MHD) instability developing in flux ropes is the helical kink, which twists the flux rope axis. The growth of this instability can trigger magnetic reconnection, which can explain the formation of chromospheric jets and spicules, but its development has never been investigated in a partially-ionised plasma (PIP). Here we study the kink instability in PIP to understand how it develops in the solar chromosphere, where it is affected by charge-neutral interactions. Partial ionisation speeds up the onset of the non-linear phase of the instability, as the plasma $β$ of the isolated plasma is smaller than the total plasma $β$ of the bulk. The distribution of the released magnetic energy changes in fully and partially-ionised plasmas, with a larger increase of internal energy associated to the PIP cases. The temperature in PIP increases faster also due to heating terms from the two-fluid dynamics. PIP effects trigger the kink instability on shorter time scales, which is reflected in a more explosive chromospheric flux rope dynamics. These results are crucial to understand the dynamics of small-scale chromospheric structures - mini-filament eruptions - that this far have been largely neglected but could significantly contribute to chromospheric heating and jet formation.

astro-ph.SR↗

arXiv2024

TeV Solar Gamma Rays as a probe for the Solar Internetwork Magnetic Fields

The magnetic fields that emerge from beneath the solar surface and permeate the solar atmosphere are the key drivers of space weather and, thus, understanding them is important to human society. Direct observations, used to measure magnetic fields, can only probe the magnetic fields in the photosphere and above, far from the regions the magnetic fields are being enhanced by the solar dynamo. Solar gamma rays produced by cosmic rays interacting with the solar atmosphere have been detected from GeV to TeV energy range, and revealed that they are significantly affected by solar magnetic fields. However, much of the observations are yet to be explained by a physical model. Using a semi-analytic model, we show that magnetic fields at and below the photosphere with a large horizontal component could explain the $\sim$1 TeV solar gamma rays observed by HAWC. This could allow high-energy solar gamma rays to be a novel probe for magnetic fields below the photosphere.

astro-ph.SR↗

arXiv2024

Nonlinear wave damping by Kelvin-Helmholtz instability induced turbulence

Magnetohydrodynamic kink waves naturally form as a consequence of perturbations to a structured medium, for example transverse oscillations of coronal loops. Linear theory has provided many insights in the evolution of linear oscillations, and results from these models are often applied to infer information about the solar corona from observed wave periods and damping times. However, simulations show that nonlinear kink waves can host the Kelvin-Helmholtz instability (KHi) which subsequently creates turbulence in the loop, dynamics which are beyond linear models. In this paper we investigate the evolution of KHi-induced turbulence on the surface of a flux tube where a non-linear fundamental kink-mode has been excited. We control our numerical experiment so that we induce the KHi without exciting resonant absorption. We find two stages in the KHi turbulence dynamics. In the first stage, we show that the classic model of a KHi turbulent layer growing $\propto t$is applicable. We adapt this model to make accurate predictions for damping of the oscillation and turbulent heating as a consequence of the KHi dynamics. In the second stage, the now dominant turbulent motions are undergoing decay. We find that the classic model of energy decay proportional to $t^{-2}$ approximately holds and provides an accurate prediction of the heating in this phase. Our results show that we can develop simple models for the turbulent evolution of a non-linear kink wave, but the damping profiles produced are distinct from those of linear theory that are commonly used to confront theory and observations.

astro-ph.SR↗

arXiv2024

On the Ambipolar Diffusion Formulation for Ion-neutral drifts in the non-negligible drift velocity limit

The ambipolar diffusion approximation is used to model partially ionised plasma dynamics in a single fluid setting. To correctly apply the commonly used version of ambipolar diffusion, a set of criteria should be satisfied including the requirement that the difference in velocity between charges and neutral species (known as drift velocity) is much smaller than the thermal velocity, otherwise the drift velocity will drive a non-negligible level of further collisions between the two species. In this paper we explore the consequence of relaxing this assumption. We show that a new induction equation can be formulated without this assumption. This formulation reduces to the ambipolar induction equation in the case the drift velocity is small. In the large drift velocity limit, the feedback of the drift velocity on the collision frequency results in decreased diffusion of the magnetic field compared with the standard ambipolar diffusion approximation for the same parameters. This has a natural consequence of reducing the frictional heating that can occur. Applying this to results from flux emergence simulations where the expansion of the magnetic field leads to strong adiabatic cooling of the partially ionised chromosphere resulted in a noticeable reduction in the magnitude of the predicted drift velocities.

astro-ph.SR↗

arXiv2023

Shocks and instabilities in the partially ionised solar atmosphere

The low solar atmosphere is composed of mostly neutral particles, but the importance of the magnetic field for understanding observed dynamics means that interactions between charged and neutral particles play a very important role in controlling the macroscopic fluid motions. As the exchange of momentum between fluids, essential for the neutral fluid to effectively feel the Lorentz force, is through collisional interactions, the relative timescale of these interactions to the dynamic timescale determines whether a single-fluid model or, when the dynamic frequency is higher, the more detailed two-fluid model is the more appropriate. However, as many MHD phenomena fundamentally contain multi-time-scale processes, even large-scale, long-timescale motions can have an important physical contribution from two-fluid processes. In this review we will focus on two-fluid models, looking in detail at two areas where the multi-time-scale nature of the solar atmosphere means that two-fluid physics can easily develop: shock-waves and instabilities. We then connect these ideas to observations attempting to diagnose two-fluid behaviour in the solar atmosphere, suggesting some ways forward to bring observations and simulations closer together.

astro-ph.SR↗

arXiv2023

The role of cooling induced by mixing in the mass and energy cycles of the solar atmosphere

In many astrophysical systems, mixing between cool and hot temperature gas/plasma through Kelvin-Helmholtz-instability-driven turbulence leads to the formation of an intermediate temperature phase with increased radiative losses that drive efficient cooling. The solar atmosphere is a potential site for this process to occur with interaction between either prominence or spicule material and the solar corona allowing the development of transition region material with enhanced radiative losses. In this paper, we derive a set of equations to model the evolution of such a mixing layer and make predictions for the mixing-driven cooling rate and the rate at which mixing can lead to the condensation of the coronal material. These theoretical predictions are benchmarked against 2.5D MHD simulations. Applying the theoretical scalings to prominence threads or fading spicules, we found that as a mixing layer grows on their boundaries this would lead to the creation of transition region material with a cooling time of ~100 s, explaining the warm emission observed as prominence threads or spicules fade in cool spectral lines without the requirement for any heating. For quiescent prominences, dynamic condensation driven by the mixing process could restore ~18 per cent of the mass lost from a prominence through downflows. Overall, this mechanism of thermal energy loss through radiative losses induced by mixing highlights the importance for considering dynamical interaction between material at different temperatures when trying to understand the thermodynamic evolution of the cool material in the solar corona.

astro-ph.SR↗

arXiv2023

Connecting theory of plasmoid-modulated reconnection to observations of solar flares

The short timescale of the solar flare reconnection process has long proved to be a puzzle. Recent studies suggest the importance of the formation of plasmoids in the reconnecting current sheet, with quantifying the aspect ratio of the width to length of the current sheet in terms of a negative power $α$ of the Lundquist number, i.e. $S^{-α}$, being key to understanding the onset of plasmoids formation. In this paper we make the first application of theoretical scalings for this aspect ratio to observed flares to evaluate how plasmoid formation may connect with observations. We find that for three different flares showing plasmoids a range of $α$ values of $α= 0.27$ to $0.31$. The values in this small range implies that plasmoids may be forming before the theoretically predicted critical aspect ratio ($α=1/3$) has been reached, potentially presenting a challenge for the theoretical models.

astro-ph.SR↗

arXiv2022

Collisional ionisation and recombination effects on coalescence instability in chromospheric partially ionised plasmas

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 ($M_{_{\operatorname{MHD}}} = 0.047$).

physics.plasm-ph↗

arXiv2021

Stability of two-fluid partially-ionised slow-mode shock fronts

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 ionised medium (for example the solar chromosphere), both hydrodynamic and magnetohydrodynamic 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-ionised shock fronts to investigate the stability conditions, and compare the results to HD and MHD simulations. Our simulations consist of an initially steady 2D parallel shock encountering a localised 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 stabilise 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-ionised system the shock front is unstable, as the system tends towards MHD. However, for a finite coupling, we find that the neutrals can stabilise 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 give an estimated range of stable wavelengths between 0.6 and 56 km.

astro-ph.SR↗