James McLaughlin

MHD wave propagation in the neighbourhood of a two-dimensional null point

The nature of fast magnetoacoustic and Alfven 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 Alfven wave behaves in a different manner in that the wave energy is dissipated along the separatrices. For Alfven 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.

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

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 β=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.

Three-dimensional Magnetohydrodynamic Wave Behavior in Active Regions: Individual Loop Density Structure

We present the numerical results from a three-dimensional (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 modeled 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. As found in their work, we see that fast wave propagation is distorted by the Alfven speed profile and that the wave propagation generates field line oscillations, which 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 the 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, and 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 modeling of active regions and illustrates the need for obtaining accurate density measurements for coronal seismology.

Hi-C and AIA observations of transverse magnetohydrodynamic waves in active regions

The recent launch of the High resolution Coronal imager (Hi-C) provided a unique opportunity of studying the EUV corona with unprecedented spatial resolution. We utilize these observations to investigate the properties of low-frequency (50−200 s) active region transverse waves, whose omnipresence had been suggested previously. The five-fold improvement in spatial resolution over SDO/AIA reveals coronal loops with widths 150−310 km and that these loops support transverse waves with displacement amplitudes <50 km. However, the results suggest that wave activity in the coronal loops is of low energy, with typical velocity amplitudes <3 km s-1. An extended time-series of SDO data suggests that low-energy wave behaviour is typical of the coronal structures both before and after the Hi-C observations.

MHD mode coupling in the neighbourhood of a 2D null point

Context: At this time there does not exist a robust set of rules connecting low and high β waves across the β ≈ 1 layer. The work here contributes specifically to what happens when a low β fast wave crosses the β ≈ 1 layer and transforms into high β fast and slow waves. Aims: The nature of fast and slow magnetoacoustic waves is investigated in a finite β plasma in the neighbourhood of a two-dimensional null point. Methods: .The linearised equations are solved in both polar and cartesian forms with a two-step Lax-Wendroff numerical scheme. Analytical work (e.g. small β expansion and WKB approximation) also complement the work. Results: 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 β ≈ 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 β. 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 β). We conclude that there are two competing phenomena; the refraction effect (due to the variable Alfven speed) and the contribution from the non-zero sound speed. Conclusions: These experiments illustrate the importance of the magnetic topology and of the location of the β ≈ 1 layer in the system.

GENERATION OF QUASI-PERIODIC WAVES AND FLOWS IN THE SOLAR ATMOSPHERE BY OSCILLATORY RECONNECTION

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. and Murray et al. 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.

Magnetohydrodynamics wave propagation in the neighbourhood of two dipoles

Context: This paper is the third in a series of investigations by the authors. Aims: The nature of fast magnetoacoustic and Alfven 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 Alfven 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 Alfven 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 Alfven 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.

MHD wave propagation in the neighbourhood of two null points

The nature of fast magnetoacoustic and Alfven waves is investigated in a zero β plasma in the neighbourhood of a pair of two-dimensional null points. This gives an indication of wave propagation in the low β solar corona, for a more complicated magnetic configuration than that looked at by McLaughlin & Hood (2004, A&A, 420, 1129). It is found that the fast wave is attracted to the null points and that the front of the wave slows down as it approaches the null point pair. Here, the wave splits and part of the wave accumulates at one null and the rest at the other. Current density will then accumulate at these points and ohmic dissipation will then extract the energy in the wave at these points. This suggests locations where wave heating will occur in the corona. The Alfven wave behaves in a different manner in that the wave accumulates along the separatrices. Hence, the current density will accumulate at this part of the topology and this is where wave heating will occur. However, the phenomenon of wave accumulation at a specific place is a feature of both wave types, and illustrates the importance of studying the topology of the corona when considering MHD wave propagation.

On the Periodicity of Oscillatory Reconnection

Context. Oscillatory reconnection is a time-dependent magnetic reconnection mechanism that naturally produces periodic outputs from aperiodic drivers. Aims. This paper aims to quantify and measure the periodic nature of oscillatory reconnection for the first time. Methods. We solve the compressible, resistive, nonlinear magnetohydrodynamics (MHD) equations using 2.5D numerical simulations. Results. 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. 2k ms −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. Conclusions. We conclude that the oscillatory reconnection mechanism behaves akin to a damped harmonic oscillator.

First direct measurements of transverse waves in solar polar plumes using SDO/AIA

There is intense interest in determining the precise contribution of Alfvenic waves propagating along solar structures to the problems of coronal heating and solar wind acceleration. Since the launch of SDO/AIA, it has been possible to re- solve transverse oscillations in off-limb solar polar plumes and recently McIntosh et al. (2011, Nature, 475, 477) concluded that such waves are energetic enough to play a role in heating the corona and accelerating the fast solar wind. However, this result is based on comparisons to Monte Carlo simulations and confirmation via direct measurements is still outstanding. Thus, this letter reports on the first direct measurements of transverse wave motions in solar polar plumes. Over a 4 hour period, we measure the transverse displacements, periods and velocity am- plitudes of 596 distinct oscillations observed in the 171 u channel of SDO/AIA. We find a broad range of non-uniformly distributed parameter values which are well described by log-normal distributions with peaks at 234 km, 121 s and 8 km s −1 , and mean and standard deviations of 407±297 km, 173±118 s and 14±10 km s −1 . Within standard deviations, our direct measurements are broadly con- sistent with previous results. However, accounting for the whole of our observed non-uniform parameter distribution we calculate an energy flux of 9−24 W m −2 , which is 4 − 10 times below the energy requirement for solar wind acceleration. Hence, our results indicate that transverse MHD waves as resolved by SDO/AIA cannot be the dominant energy source for fast solar wind acceleration in the open-field corona.