Modelling QuasiPeriodic Pulsations in Solar and Stellar Flares 

James McLaughlin Submitted: 20180213 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 quasiperiodic pulsations (QPPs), to emphasise that they often contain apparent amplitude and period modulation. We review the current understanding of quasiperiodic 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, selfoscillatory mechanisms, oscillatory reconnection/reconnection reversal, wavedriven reconnection, two loop coalescence, MHD flow overstability, the equivalent LCRcontour mechanism, and thermaldynamical 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 Xrays,Nobeyama Radioheliograph,Other,PROBA2/LYRA,RHESSI

Publication Status: [published] 2018, Space Science Reviews, 214, 45
Last Modified: 20180214 11:41



On the Periodicity of Oscillatory Reconnection 

James McLaughlin Submitted: 20121205 08:07
Oscillatory reconnection is a timedependent 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 stationaryphase 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: 20121205 14:04



Generation of Quasiperiodic Waves and Flows in the Solar Atmosphere by Oscillatory Reconnection 

James McLaughlin Submitted: 20120403 03:57
We investigate the longterm 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 2.5dimensional 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 quasiperiodic 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 (2060 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: 1.753.5 minutes. Thus, the mechanism of oscillatory reconnection may provide a physical explanation to some of the highspeed, quasiperiodic, 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: 20120404 08:13



Review Article: MHD Wave Propagation Near Coronal Null Points of Magnetic Fields 

James McLaughlin Submitted: 20111013 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 Alfvénspeed 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 fanplane (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, modecoupling, quasiperiodic pulsations and phasemixing.
Authors: McLaughlin, J. A., Hood, A. W. and De Moortel, I.
Projects: None

Publication Status: Space Science Reviews, 158, 205
Last Modified: 20111013 09:33



Phase mixing of nonlinear viscoresistive Alfvén waves 

James McLaughlin Submitted: 20110307 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 viscoresistive 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 viscoresistive 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: 20110308 09:57



Nonlinear fast magnetoacoustic wave propagation in the neighbourhood of a 2D magnetic Xpoint: oscillatory reconnection 

James McLaughlin Submitted: 20090202 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 Xpoint.
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 Xpoint into a 'cusplike' 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 nonpotential (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, 227240
Last Modified: 20090205 05:44



3D MHD Wave Behavior in Active Regions: Individual Loop Density Structure 

James McLaughlin Submitted: 20080328 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 forcefree, 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: 20080923 21:12



3D MHD Coronal Oscillations About a Magnetic Null Point: Application of WKB Theory 

James McLaughlin Submitted: 20071211 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 RungeKutta 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 fanplane, 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: fanplane (ε=1), along the xaxis (0 < ε <1) or along the yaxis (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: 20071211 09:01




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