# Andrew Hillier's research while affiliated with University of Exeter and other places

## Publications (57)

Article
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 ionized plasmas (PIP) of the solar chromosphere. Partial ionization might largely alter the dynamics of the coalescence instability, which promotes fast reconnection and f...
Preprint
Full-text available
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 f...
Preprint
Full-text available
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. For an MHD system, three broad categories of shocks exist (slow, fast or intermediate) however the occurrence rates of...
Article
Full-text available
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) cou...
Preprint
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) cou...
Article
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...
Preprint
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 r...
Article
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 phenomenon and engineering applications. They frequently result in turbulent mixing, which has a major impact on the overall flow development and othe...
Article
Full-text available
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. For an MHD system, three broad categories of shocks exist (slow, fast, and intermediate); however, the occurrence rate...
Article
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 exis...
Preprint
Shocks are a universal feature of the lower solar atmosphere which consists of both ionised and neutral species. Including partial ionisation leads to a finite-width existing 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 speci...
Preprint
Kelvin-Helmholtz {instability induced} turbulence is one promising mechanism by which loops in the solar corona can be heated by MHD waves. In this paper we present an analytical model of the dissipation rate of {Kelvin-Helmholtz instability induced} turbulence $\varepsilon_{\rm D}$, finding it scales as the wave amplitude ($d$) to the third power...
Article
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...
Preprint
The plasma of the lower solar atmosphere consists of mostly neutral particles, whereas the upper solar atmosphere is mostly 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...
Article
The late nonlinear phase of the Rayleigh-Taylor instability is characterized by the self-similar expansion of the instability mixing layer given at late times by h ≈ αAgt². In this paper, we present a new model of this mixing layer, based on a piecewise step function approximation where the main constraint imposed is conservation of mass. This mode...
Chapter
In this chapter we focus on the magnetohydrodynamic (MHD) versions of the Rayleigh–Taylor and Kelvin–Helmholtz instabilities, taking the reader beyond the commonly presented situations to include how extra physics influences the stability of the models. After a discussion of the physical processes behind each instability we look at the general fram...
Article
Full-text available
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...
Preprint
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 this 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...
Article
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 c...
Preprint
The nonlinear magnetic Kelvin-Helmholtz instability (KHi), and the turbulence it creates, appears in many astrophysical systems. This includes those systems where the local plasma conditions are such that the plasma is not fully ionised, for example in the lower solar atmosphere and molecular clouds. In a partially ionised system, the fluids couple...
Article
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-mo...
Preprint
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 substructures exist within slow-mode shocks d...
Preprint
Magnetic reconnection may be the fundamental process allowing energy stored in magnetic fields to be released abruptly, solar flares and coronal mass ejection (CME) being archetypal natural plasma examples. Magnetic reconnection is much too slow a process to be efficient on the large scales, but accelerates once small enough scales are formed in th...
Article
Full-text available
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 sca...
Article
Full-text available
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 osc...
Preprint
Full-text available
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 osc...
Article
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 b...
Preprint
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 internal fluid motions would develop. However, evidence of these have been...
Article
Full-text available
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 o...
Article
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...
Article
We analyze solar quiescent prominence bubble characteristics and instability dynamics using Hinode Solar Optical Telescope (SOT) data. We measure 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 abo...
Article
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 lik...
Article
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...
Article
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 MHD turbulence in a...
Article
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 dynamic...
Article
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 partiall...
Article
Full-text available
Kepler data from G-, K-, and M-type stars are used to study conditions that lead to superflares with energies above 1034 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. W...
Article
Full-text available
Kepler data from G, K and M type stars are used to study conditions that lead to superflares of energies above $10^{34} {\rm erg}$. From the 117661 included stars, 795 show superflares with a total of 6830 such events. We study if parameters, like the surface temperature or the rotation rate, have any effect on the superflare occurrence rate or ene...
Article
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...
Article
Full-text available
This work examines infalling matter following an enormous Coronal Mass Ejection (CME) 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 as...
Article
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...
Article
Full-text available
Recent observations of solar type stars with the Kepler satellite by Maehara et al. have revealed the existence of superflares (with energy of 10^33 - 10^35 erg) on Sun-like stars, which are similar to our Sun in their surface temperature (5600 K - 6000 K) and slow rotation (rotational period > 10 days). From the statistical analysis of these super...
Article
The launch of Hinode satellite led to the discovery 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 stil...
Article
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 {approx}900 km. The knot initially un...
Article
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 spi...
Article
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 hypothesize...
Article
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 ac...
Article
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 full...
Article
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 cavi...
Article
Full-text available
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 rising prominence threads, are impulsively acce...
Article
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)...
Article
Full-text available
We studied spicular jets over a plage area and derived their dynamic characteristics using Hinode Solar Optical Telescope (SOT) high-resolution images. The target plage region was near the west limb of the solar disk. This location permitted us to study the dynamics of spicular jets without the overlapping effect of spicular structures along the li...
Article
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 M...

## Citations

... Traditionally, the classification of MHD waves is based on the Rankine-Hugoniot relations, which lead to a set of nonlinear algebraic relations and compatibility relations for a complex wave structure. Snow et al. 16 proposed detection method of MHD shocks based on their upstream and downstream velocity relative to the characteristic speeds of the system. This method has been specifically applied to shock identification and classification in 2D MHD compressible turbulence-Orszag-Tang vortex evolution. ...
... The Orszag-Tang vortex has been well studied in the literature (Dahlburg and Picone, 1989;Picone and Dahlburg, 1991;Jiang and Wu, 1999;Parashar et al., 2009;Uritsky et al., 2010). The initial conditions are evolved in 2D for ideal MHD equations using the 4th order central-difference solver in the (PIP) code (Hillier et al., 2016), which has been used previously to study shocks (e.g., Snow and Hillier, 2021). The simulations are performed using 1024 × 1024 cells with periodic boundary conditions. ...
... Magnetic island-coalescence (MIC) problem [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] that uses a Fadeev equilibrium [27]) i.e. ...
... At late time, the nonlinear effect causes the interfacial instability to transition into turbulent mixing. The evolution of mixing width (W), defined as the distance from the bubble front to the spike front, has been investigated extensively, which is reviewed systematically in a series of papers (Brouillette 2002;Zhou 2017a,b;Zhai et al. 2018;Zhou et al. 2021). ...
... The ionisation-recombination effects are further enhanced by the action of the ionisation potential. When collisional ionisation takes place, the energy expended by a free electron to release a bound electron results in a net loss of energy from the plasma 49 . As the recombination process is associated with changes in energy levels and photons being released, this overall effect can be modeled as a radiative loss. ...
... Assuming steady-state turbulence -all the energy injected cascades and dissipated at the same rate it is injected. Therefore, for the specific value of the energy injected into the system, the value of oscillation amplitude required to dissipate all the energy by the turbulence, was calculated by Hillier et al. (2020). In this setting, the injected energy flux scales as a third power of the velocity amplitude. ...
... However, it is known that for switch-off shocks, the finite width has a dependence on the neutral fraction and plasma-β (Hillier et al. 2016). Also, the physical width of the shocks studied in this paper is much smaller than the >300 km shocks in Snow & Hillier (2020). As such, one would expect that the stability range changes for different shock types. ...
... The different observed scenarios attributed to the quiet Sun are associated with the data used or not in the models [35]. The quiet Sun observations require instruments sensitive enough for magnetic field characterization from weak polarization signals [36]. New high-resolution instruments have been impacted in physical models and theoretical quiet Sun understanding. ...
... The numerical simulations and experimental results also indicated that the profiles of certain physical quantities, including density, mole fraction, and root mean square (rms) values of velocity components, are self-similar over time. 16,18,[31][32][33][34][35] The mixed mass m, 36,37 normalized mixed mass W, 13,36 molecular mixing degree H, 19,20,38 and mixing parameter N 12,21,32 are important statistical quantities associated with the turbulent fluctuations of the RTI. Zhou, Cabot, and Thornber 36 analyzed the asymptotic behavior of these mixing parameters at varying Atwood numbers through 3D numerical simulations. ...
... Numerical models of such a case typically provide an initial perturbation which contains all of the wave energy. Not only is this initial energy unable to provide substantial heating, ref. [95] showed that if the instability forms at the boundary of a prominence, the mixing of cold, dense plasma with much hotter and more tenuous coronal plasma can cause an increase in the radiative losses and, thus, lead to enhanced cooling. This increase in the energy loss rate is much larger than the energy dissipation that could be obtained due to the relatively low wave energy content. ...