Magnetoacoustic Waves in a Stratified Atmosphere with a Magnetic Null Point

Tarr, Lucas A.; Linton, M. G.; Leake, J. E.

doi:10.3847/1538-4357/aa5e4e

Cited by 25 publications

(32 citation statements)

References 90 publications

(188 reference statements)

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“…For example, the periodicity of current sheet reversals observed in simulations more representative of the solar atmosphere, with model transition regions, dipole fields and external wave driving (e.g. OR occurring in setups similar to Tarr et al 2017) may be meaningful despite the requirement to use relatively high resistivity in such models (e.g. S = 10 would be typical).…”

Section: Discussionmentioning

confidence: 99%

On the periodicity of linear and nonlinear oscillatory reconnection

Thurgood

Pontin

McLaughlin

2019

A&A

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Context. An injection of energy towards a magnetic null point can drive reversals of current-sheet polarity leading to time-dependent, oscillatory reconnection (OR), which may explain periodic phenomena generated when reconnection occurs in the solar atmosphere. However, the details of what controls the period of these current-sheet oscillations in realistic systems is poorly understood, despite being of crucial importance in assessing whether a specific model of OR can account for observed periodic behaviour. Aims. This paper aims to highlight that different types of reconnection reversal are supported about null points, and that these can be distinct from the oscillation in the closed-boundary, linear systems considered by a number of authors in the 1990s. In particular, we explore the features of a nonlinear oscillation local to the null point, and examine the effect of resistivity and perturbation energy on the period, contrasting it to the linear, closed-boundary case. Methods. Numerical simulations of the single-fluid, resistive MHD equations are used to investigate the effects of plasma resistivity and perturbation energy upon the resulting OR. Results. It is found that for small perturbations that behave linearly, the inverse Lundquist number dictates the period, provided the perturbation energy (i.e. the free energy) is small relative to the inverse Lundquist number defined on the boundary, regardless of the broadband structure of the initial perturbation. However, when the perturbation energy exceeds the threshold required for 'nonlinear' null collapse to occur, a complex oscillation of the magnetic field is produced which is, at most, only weakly-dependent on the resistivity. The resultant periodicity is instead strongly influenced by the amount of free energy, with more energetic perturbations producing higher-frequency oscillations. Conclusions. Crucially, with regards to typical solar-based and astrophysical-based input energies, we demonstrate that the majority far exceed the threshold for nonlinearity to develop. This substantially alters the properties and periodicity of both null collapse and subsequent OR. Therefore, nonlinear regimes of OR should be considered in solar and astrophysical contexts.

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Section: Discussionmentioning

confidence: 99%

On the periodicity of linear and nonlinear oscillatory reconnection

Thurgood

Pontin

McLaughlin

2019

A&A

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“…It has been shown that the period of OR is dependent upon the initial disturbance to the null point field, behaving akin to a damped harmonic oscillator , and therefore a deeper understanding of null collapse can enhance our understanding of OR. This may be achieved by either simple assumptions regarding the proportion of available wave energy reaching close to the null, which would be equated to the null collapse system considered here as the initial perturbation energy, or with complementary numerical modelling efforts to estimate how much wave energy makes it to the immediate vicinity of the null whilst accounting for effects such as mode-conversion and atmospheric stratification [e.g., Tarr et al, 2017]. Indeed, our results could be used in tandem with globalscale simulations of these processes, which by necessity cannot resolve the details of the current sheet and reconnection dynamics.…”

Section: Conclusion and Discussionmentioning

confidence: 99%

Implosive Collapse about Magnetic Null Points: A Quantitative Comparison between 2D and 3D Nulls

Thurgood¹,

Pontin²,

McLaughlin³

2018

ApJ

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Null collapse is an implosive process whereby MHD waves focus their energy in the vicinity of a null point, forming a current sheet and initiating magnetic reconnection. We consider, for the first time, the case of collapsing 3D magnetic null points in nonlinear, resistive MHD using numerical simulation, exploring key physical aspects of the system as well as performing a detailed parameter study. We find that within a particular plane containing the 3D null, the plasma and current density enhancements resulting from the collapse are quantitatively and qualitatively as per the 2D case in both the linear and nonlinear collapse regimes. However, the scaling with resistivity of the 3D reconnection rate -which is a global quantity -is found to be less favourable when the magnetic null point is more rotationally symmetric, due to the action of increased magnetic backpressure. Furthermore, we find that with increasing ambient plasma pressure the collapse can be throttled, as is the case for 2D nulls. We discuss this pressurelimiting in the context of fast reconnection in the solar atmosphere and suggest mechanisms by which it may be overcome. We also discuss the implications of the results in the context of null collapse as a trigger mechanism of Oscillatory Reconnection, a time-dependent reconnection mechanism, and also within the wider subject of wave-null point interactions. We conclude that, in general, increasingly rotationally-asymmetric nulls will be more favourable in terms of magnetic energy release via null collapse than their more symmetric counterparts.

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“…Therefore, we would like to caution the reader that the definition of linear wave energy density and flux does not represent the total energy density and flux associated with a wave. To get a better estimate of the energy density and flux, we would have to include the second order terms (Leroy 1985;Tarr et al 2017). Having a time dependent background and spatial inhomogeneity, it is evident that the total wave energy is not conserved.…”

Section: Energy Transportmentioning

confidence: 99%

Internal Gravity Waves in the Magnetized Solar Atmosphere. II. Energy Transport

Vigeesh

Roth

Steiner

et al. 2019

ApJ

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In this second paper of the series on internal gravity waves (IGWs), we present a study of the generation and propagation of IGWs in a model solar atmosphere with diverse magnetic conditions. A magnetic field free, and three magnetic models that start with an initial, vertical, homogeneous field of 10 G, 50 G, and 100 G magnetic flux density, are simulated using the CO 5 BOLD code. We find that the IGWs are generated in similar manner in all four models in spite of the differences in the magnetic environment. The mechanical energy carried by IGWs is significantly larger than that of the acoustic waves in the lower part of the atmosphere, making them an important component of the total wave energy budget. The mechanical energy flux (10 6 -10 3 W m −2 ) is few orders of magnitude larger than the Poynting flux (10 3 -10 1 W m −2 ). The Poynting fluxes show a downward component in the frequency range corresponding to the IGWs, which confirm that these waves do not propagate upwards in the atmosphere when the fields are predominantly vertical and strong. We conclude that, in the upper photosphere, the propagation properties of IGWs depend on the average magnetic field strength and therefore these waves can be potential candidate for magnetic field diagnostics of these layers. However, their subsequent coupling to Alfvénic waves are unlikely in a magnetic environment permeated with predominantly vertical fields and therefore they may not directly or indirectly contribute to the heating of layers above plasma-β less than 1.

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Magnetoacoustic Waves in a Stratified Atmosphere with a Magnetic Null Point

Cited by 25 publications

References 90 publications

On the periodicity of linear and nonlinear oscillatory reconnection

On the periodicity of linear and nonlinear oscillatory reconnection

Implosive Collapse about Magnetic Null Points: A Quantitative Comparison between 2D and 3D Nulls

Internal Gravity Waves in the Magnetized Solar Atmosphere. II. Energy Transport

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