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Predictions of DKIST/DL-NIRSP Observations for an Off-limb Kink-unstable Coronal Loop  

James McLaughlin   Submitted: 2018-09-04 05:14

Synthetic intensity maps are generated from a 3D kink-unstable flux rope simulation using several DKIST/DL-NIRSP spectral lines to make a prediction of the observational signatures of energy transport and release. The reconstructed large field-of-view intensity mosaics and single tile sit-and-stare high-cadence image sequences show detailed, fine-scale structure and exhibit signatures of wave propagation, redistribution of heat, flows, and fine-scale bursts. These fine-scale bursts are present in the synthetic Doppler velocity maps and can be interpreted as evidence for small-scale magnetic reconnection at the loop boundary. The spectral lines reveal the different thermodynamic structures of the loop, with the hotter lines showing the loop interior and braiding and the cooler lines showing the radial edges of the loop. The synthetic observations of DL-NIRSP are found to preserve the radial expansion, and hence the loop radius can be measured accurately. The electron number density can be estimated using the intensity ratio of the Fe XIII lines at 10747 and 10798 ?. The estimated density from this ratio is correct to within ?10% during the later phases of the evolution; however, it is less accurate initially when line-of-sight density inhomogeneities contribute to the Fe XIII intensity, resulting in an overprediction of the density by ≈30%. The identified signatures are all above a conservative estimate for instrument noise and therefore will be detectable. In summary, we have used forward modeling to demonstrate that the coronal off-limb mode of DKIST/DL-NIRSP will be able to detect multiple independent signatures of a kink-unstable loop and observe small-scale transient features including loop braiding/twisting and small-scale reconnection events occurring at the radial edge of the loop.

Authors: Snow, B., Botha, G.J. J., Scullion, E., McLaughlin, J.A., Young, P.R. and Jaeggli, S.A.
Projects: DKIST/DL-NIRSP

Publication Status: [published, open access] Astrophysical Journal, 863, 172 (2018)
Last Modified: 2018-09-04 09:19
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Modelling Quasi-Periodic Pulsations in Solar and Stellar Flares  

James McLaughlin   Submitted: 2018-02-13 04:15

Solar flare emission is detected in all EM bands and variations in flux density of solar energetic particles. Often the EM radiation generated in solar and stellar flares shows a pronounced oscillatory pattern, with characteristic periods ranging from a fraction of a second to several minutes. These oscillations are referred to as quasi-periodic pulsations (QPPs), to emphasise that they often contain apparent amplitude and period modulation. We review the current understanding of quasi-periodic pulsations in solar and stellar flares. In particular, we focus on the possible physical mechanisms, with an emphasis on the underlying physics that generates the resultant range of periodicities. These physical mechanisms include MHD oscillations, self-oscillatory mechanisms, oscillatory reconnection/reconnection reversal, wave-driven reconnection, two loop coalescence, MHD flow over-stability, the equivalent LCR-contour mechanism, and thermal-dynamical cycles. We also provide a histogram of all QPP events published in the literature at this time. The occurrence of QPPs puts additional constraints on the interpretation and understanding of the fundamental processes operating in flares, e.g. magnetic energy liberation and particle acceleration. Therefore, a full understanding of QPPs is essential in order to work towards an integrated model of solar and stellar flares.

Authors: McLaughlin, J.A., Nakariakov, V.M., Dominique, M., Jelínek, P., Takasao, S.
Projects: Fermi/GBM,GOES X-rays,Nobeyama Radioheliograph,Other,PROBA2/LYRA,RHESSI

Publication Status: [published] 2018, Space Science Reviews, 214, 45
Last Modified: 2018-02-14 11:41
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On the Periodicity of Oscillatory Reconnection  

James McLaughlin   Submitted: 2012-12-05 08:07

Oscillatory reconnection is a time-dependent magnetic reconnection mechanism that naturally produces periodic outputs from aperiodic drivers. This paper aims to quantify and measure the periodic nature of oscillatory reconnection for the first time. We solve the compressible, resistive, nonlinear MHD equations using 2.5D numerical simulations. We identify two distinct periodic regimes: the impulsive and stationary phases. In the impulsive phase, we find the greater the amplitude of the initial velocity driver, the longer the resultant current sheet and the earlier its formation. In the stationary phase, we find that the oscillations are exponentially decaying and for driving amplitudes 6.3 - 126.2 km s-1, we measure stationary-phase periods in the range 56.3 - 78.9 s, i.e. these are high frequency (0.01 - 0.02 Hz) oscillations. In both phases, we find that the greater the amplitude of the initial velocity driver, the shorter the resultant period, but note that different physical processes and periods are associated with both phases. We conclude that the oscillatory reconnection mechanism behaves akin to a damped harmonic oscillator.

Authors: McLaughlin, J. A., Thurgood, J. O., MacTaggart, D.
Projects: None

Publication Status: (2012) A&A, 548, A98
Last Modified: 2012-12-05 14:04
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Generation of Quasi-periodic Waves and Flows in the Solar Atmosphere by Oscillatory Reconnection  

James McLaughlin   Submitted: 2012-04-03 03:57

We investigate the long-term evolution of an initially buoyant magnetic flux tube emerging into a gravitationally stratified coronal hole environment and report on the resulting oscillations and outflows. We perform 2.5-dimensional nonlinear numerical simulations, generalizing the models of McLaughlin et al. (2009) and Murray et al. (2009). We find that the physical mechanism of oscillatory reconnection naturally generates quasi-periodic vertical outflows, with a transverse/swaying aspect. The vertical outflows consist of both a periodic aspect and evidence of a positively directed flow. The speed of the vertical outflow (20-60 km s-1) is comparable to those reported in the observational literature. We also perform a parametric study varying the magnetic strength of the buoyant flux tube and find a range of associated periodicities: 1.75-3.5 minutes. Thus, the mechanism of oscillatory reconnection may provide a physical explanation to some of the high-speed, quasi-periodic, transverse outflows/jets recently reported by a multitude of authors and instruments.

Authors: McLaughlin, J. A., Verth, G., Fedun, V., Erdélyi, R.
Projects: None

Publication Status: ApJ (2012) 749, 30
Last Modified: 2012-04-04 08:13
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Review Article: MHD Wave Propagation Near Coronal Null Points of Magnetic Fields  

James McLaughlin   Submitted: 2011-10-13 09:21

We present a comprehensive review of MHD wave behaviour in the neighbourhood of coronal null points: locations where the magnetic field, and hence the local Alfvén speed, is zero. The behaviour of all three MHD wave modes, i.e. the Alfvén wave and the fast and slow magnetoacoustic waves, has been investigated in the neighbourhood of 2D, 2.5D and (to a certain extent) 3D magnetic null points, for a variety of assumptions, configurations and geometries. In general, it is found that the fast magnetoacoustic wave behaviour is dictated by the Alfvén-speed profile. In a beta=0 plasma, the fast wave is focused towards the null point by a refraction effect and all the wave energy, and thus current density, accumulates close to the null point. Thus, null points will be locations for preferential heating by fast waves. Independently, the Alfvén wave is found to propagate along magnetic fieldlines and is confined to the fieldlines it is generated on. As the wave approaches the null point, it spreads out due to the diverging fieldlines. Eventually, the Alfvén wave accumulates along the separatrices (in 2D) or along the spine or fan-plane (in 3D). Hence, Alfvén wave energy will be preferentially dissipated at these locations. It is clear that the magnetic field plays a fundamental role in the propagation and properties of MHD waves in the neighbourhood of coronal null points. This topic is a fundamental plasma process and results so far have also lead to critical insights into reconnection, mode-coupling, quasi-periodic pulsations and phase-mixing.

Authors: McLaughlin, J. A., Hood, A. W. and De Moortel, I.
Projects: None

Publication Status: Space Science Reviews, 158, 205
Last Modified: 2011-10-13 09:33
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Phase mixing of nonlinear visco-resistive Alfvén waves  

James McLaughlin   Submitted: 2011-03-07 09:43

We investigate the behaviour of nonlinear, nonideal Alfvén wave propagation within an inhomogeneous magnetic environment. The governing MHD equations are solved in 1D and 2D using both analytical techniques and numerical simulations. We find clear evidence for the ponderomotive effect and visco-resistive heating. The ponderomotive effect generates a longitudinal component to the transverse Alfvén wave, with a frequency twice that of the driving frequency. Analytical work shows the addition of resistive heating. This leads to a substantial increase in the local temperature and thus gas pressure of the plasma, resulting in material being pushed along the magnetic field. In 2D, our system exhibits phase mixing and we observe an evolution in the location of the maximum heating, i.e. we find a drifting of the heating layer. Considering Alfvén wave propagation in 2D with an inhomogeneous density gradient, we find that the equilibrium density profile is significantly modified by both the flow of density due to visco-resistive heating and the nonlinear response to the localised heating through phase mixing.

Authors: McLaughlin, J. A., De Moortel, I. and Hood, A. W.
Projects: None

Publication Status: A&A, 527, A149
Last Modified: 2011-03-08 09:57
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Nonlinear fast magnetoacoustic wave propagation in the neighbourhood of a 2D magnetic X-point: oscillatory reconnection  

James McLaughlin   Submitted: 2009-02-02 10:18

This paper extends the models of Craig & McClymont (1991) and McLaughlin & Hood (2004) to include finite beta and nonlinear effects. We investigate the nature of nonlinear fast magnetoacoustic waves about a 2D magnetic X-point. We solve the compressible and resistive MHD equations using a Lagrangian remap, shock capturing code (Arber et al. 2001) and consider an initial condition in (v x B).z , a natural variable of the system. We observe the formation of both fast and slow oblique magnetic shocks. The nonlinear wave deforms the X-point into a 'cusp-like' point which in turn collapses to a current sheet. The system then evolves through a series of horizontal and vertical current sheets, with associated changes in connectivity, i.e. the system exhibits oscillatory reconnection. Our final state is non-potential (but in force balance) due to asymmetric heating from the shocks. Larger amplitudes in our initial condition correspond to larger values of the final current density left in the system. The inclusion of nonlinear terms introduces several new features to the system that were absent from the linear regime.

Authors: McLaughlin, J. A., De Moortel, I., Hood, A. W., Brady, C. S.
Projects:

Publication Status: A&A (2009) 493, 227-240
Last Modified: 2009-02-05 05:44
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3D MHD Wave Behavior in Active Regions: Individual Loop Density Structure  

James McLaughlin   Submitted: 2008-03-28 04:23

We present the numerical results from a 3D nonlinear MHD simulation of wave activity in an idealized active region in which individual, realistic loop density structure is included. The active region is modelled by an initially force-free, dipole magnetic configuration with gravitationally stratified density and contains a loop with a higher density than its surroundings. This study represents an extension to the model of Ofman & Thompson (2002). As found in their work, we see that fast wave propagation is distorted by the Alfvén speed profile and that the wave propagation generates fieldline oscillations and these oscillations are rapidly damped. We find that the addition of a high density loop significantly changes the behavior inside that loop, specifically in that the loop can support trapped waves. We also find that the impact of the fast wave impulsively excites both horizontal and vertical loop oscillations. From a parametric study of the oscillations, we find that the amplitude of oscillations decreases with increasing density contrast, whereas the period and damping time increase. This is one of the key results presented here; that individual loop density structure can influence the damping rate, specifically that the damping time increases with increasing density contrast. All these results were compared with an additional study performed on a straight coronal loop with similar parameters. Through comparison with the straight loop, we find that the damping mechanism in our curved loop is wave leakage due to curvature. The work performed here highlights the importance of including individual loop density structure in the modelling of active regions, and illustrates the need for obtaining accurate density measurements for coronal seismology.

Authors: McLaughlin, J.A. and L. Ofman
Projects: None

Publication Status: ApJ (in press)
Last Modified: 2008-09-23 21:12
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Magnetohydrodynamics wave propagation in the neighbourhood of a two-dimensional null point  

James McLaughlin   Submitted: 2007-12-11 09:04

The nature of fast magnetoacoustic and Alfvén waves is investigated in a zero β plasma. This gives an indication of wave propagation in the low β solar corona. It is found that for a two-dimensional null point, the fast wave is attracted to that point and the front of the wave slows down as it approaches the null point, causing the current density to accumulate there and rise rapidly. Ohmic dissipation will extract the energy in the wave at this point. This illustrates that null points play an important role in the rapid dissipation of fast magnetoacoustic waves and suggests the location where wave heating will occur in the corona. The Alfvén wave behaves in a different manner in that the wave energy is dissipated along the separatrices. For Alfvén waves that are decoupled from fast waves, the value of the plasma β is unimportant. However, the phenomenon of dissipating the majority of the wave energy at a specific place is a feature of both wave types.

Authors: McLaughlin, J. A. and Hood, A. W.
Projects: None

Publication Status: Astronomy and Astrophysics, 420, 1129-1140 (2004)
Last Modified: 2007-12-11 10:42
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3D MHD Coronal Oscillations About a Magnetic Null Point: Application of WKB Theory  

James McLaughlin   Submitted: 2007-12-11 09:01

This paper is a demonstration of how the WKB approximation can be used to help solve the linearised 3D MHD equations. Using Charpit's Method and a Runge-Kutta numerical scheme, we have demonstrated this technique for a potential 3D magnetic null point, B = (x,εy - (ε +1)z). Under our cold plasma assumption, we have considered two types of wave propagation: fast magnetoacoustic and Alfvén waves. We find that the fast magnetoacoustic wave experiences refraction towards the magnetic null point, and that the effect of this refraction depends upon the Alfvén speed profile. The wave, and thus the wave energy, accumulates at the null point. We have found that current build up is exponential and the exponent is dependent upon ε. Thus, for the fast wave there is preferential heating at the null point. For the Alfvén wave, we find that the wave propagates along the fieldlines. For an Alfvén wave generated along the fan-plane, the wave accumulates along the spine. For an Alfvén wave generated across the spine, the value of ε determines where the wave accumulation will occur: fan-plane (ε=1), along the x-axis (0 < ε <1) or along the y-axis (ε > 1). We have shown analytically that currents build up exponentially, leading to preferential heating in these areas. The work described here highlights the importance of understanding the magnetic topology of the coronal magnetic field for the location of wave heating.

Authors: McLaughlin, J. A., Ferguson, J.S.L. and Hood, A. W.
Projects: None

Publication Status: Solar Physics (in press)
Last Modified: 2007-12-11 09:01
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MHD mode coupling in the neighbourhood of a 2D null point  

James McLaughlin   Submitted: 2007-08-30 04:13

At this time there does not exist a robust set of rules connecting low and high beta waves across the beta ≈ 1 layer. The work here contributes specifically to what happens when a low beta fast wave crosses the beta ≈ 1 layer and transforms into high beta fast and slow waves. The nature of fast and slow magnetoacoustic waves is investigated in a finite beta plasma in the neighbourhood of a two-dimensional null point. The linearised equations are solved in both polar and cartesian forms with a two-step Lax-Wendroff numerical scheme. Analytical work (e.g. small beta expansion and WKB approximation) also complement the work. It is found that when a finite gas pressure is included in magnetic equilibrium containing an X-type null point, a fast wave is attracted towards the null by a refraction effect and that a slow wave is generated as the wave crosses the beta ≈ 1 layer. Current accumulation occurs close to the null and along nearby separatrices. The fast wave can now pass through the origin due to the non-zero sound speed, an effect not previously seen in related papers but clear seen for larger values of beta. Some of the energy can now leave the region of the null point and there is again generation of a slow wave component (we find that the fraction of the incident wave converted to a slow wave is proportional to beta). We conclude that there are two competing phenomena; the refraction effect (due to the variable Alfvén speed) and the contribution from the non-zero sound speed. These experiments illustrate the importance of the magnetic topology and of the location of the beta ≈ 1 layer in the system.

Authors: McLaughlin, J. A. and Hood, A. W.
Projects: None

Publication Status: Astronomy and Astrophysics, 459, 641-649 (2006)
Last Modified: 2007-08-30 21:35
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Magnetohydrodynamics wave propagation in the neighbourhood of two dipoles  

James McLaughlin   Submitted: 2007-08-30 04:05

Context. This paper is the third in a series of investigations by the authors.
Aims. The nature of fast magnetoacoustic and Alfvén waves is investigated in a 2D β=0 plasma in the neighbourhood of two dipoles.
Methods. We use both numerical simulations (two-step Lax-Wendroff scheme) and analytical techniques (WKB approximation). Results. It is found that the propagation of the linear fast wave is dictated by the Alfvén speed profile and that close to the null, the wave is attracted to the neutral point. However, it is also found that in this magnetic configuration some of the wave can escape the refraction effect; this had not been seen in previous investigations by the authors. The wave split occurs near the regions of very high Alfvén speed (found near the loci of the two dipoles). Also, for the set-up investigated it was found that 40% of the wave energy accumulates at the null. Ohmic dissipation will then extract the wave energy at this point. The Alfvén wave behaves in a different manner in that part of the wave accumulates along the separatrices and part escapes. Hence, the current density will accumulate at this part of the topology and this is where wave heating will occur. Conclusions. The phenomenon of wave accumulation at a specific place is a feature of both wave types, as is the result that a fraction of the wave can now escape the numerical box when propagating in this magnetic configuration.

Authors: McLaughlin, J. A. and Hood, A. W.
Projects: None

Publication Status: Astronomy and Astrophysics, 452, 603-613 (2006)
Last Modified: 2007-08-30 04:05
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Abstracts by Author
Predictions of DKIST/DL-NIRSP Observations for an Off-limb Kink-unstable Coronal Loop
Modelling Quasi-Periodic Pulsations in Solar and Stellar Flares
On the Periodicity of Oscillatory Reconnection
Generation of Quasi-periodic Waves and Flows in the Solar Atmosphere by Oscillatory Reconnection
Review Article: MHD Wave Propagation Near Coronal Null Points of Magnetic Fields
Phase mixing of nonlinear visco-resistive Alfvén waves
Nonlinear fast magnetoacoustic wave propagation in the neighbourhood of a 2D magnetic X-point: oscillatory reconnection
3D MHD Wave Behavior in Active Regions: Individual Loop Density Structure
Magnetohydrodynamics wave propagation in the neighbourhood of a two-dimensional null point
3D MHD Coronal Oscillations About a Magnetic Null Point: Application of WKB Theory
MHD mode coupling in the neighbourhood of a 2D null point
Magnetohydrodynamics wave propagation in the neighbourhood of two dipoles

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