Contribution of mode coupling and phase-mixing of Alfvén waves to coronal heating

Contribution of mode coupling and phase-mixing of Alfvén waves to coronal heating

P. Pagano School of Mathematics and Statistics, University of St Andrews, North Haugh, St Andrews, Fife, Scotland KY16 9SS, UK. pp25@st-andrews.ac.uk    I. De Moortel School of Mathematics and Statistics, University of St Andrews, North Haugh, St Andrews, Fife, Scotland KY16 9SS, UK. pp25@st-andrews.ac.uk
Key Words.:
Abstract

Context:Phase-mixing of Alfvén waves in the solar corona has been identified as one possible candidate to explain coronal heating. While this scenario is supported by observations of ubiquitous oscillations in the corona carrying sufficient wave energy and by theoretical models that have described the concentration of energy in small scale structures, it is still unclear whether this wave energy can actually be converted into thermal energy in order to maintain the million degree solar corona.

Aims:The aim of this work is to assess how much energy can be converted into thermal energy by a phase-mixing process triggered by the propagation of Alfvénic waves in a cylindric coronal structure, such as a coronal loop, and to estimate the impact of this conversion on the coronal heating and thermal structure of the solar corona.

Methods:To this end, we run 3D MHD simulations of a magnetised cylinder where the Alfvén speed varies through a boundary shell and a footpoint driver is set to trigger kink modes which mode couple to torsional Alfvén modes in the boundary shell. These Alfvén waves are expected to phase-mix and the system allows us to study the subsequent thermal energy deposition. We run a reference simulation to explain the main process and then we vary simulation parameters, such as the size of the boundary shell, its structure and the persistence of the driver.

Results:When we take into consideration high values of magnetic resistivity and strong footpoint drivers, we find i) that phase-mixing leads to a temperature increase of the order of K or less, depending on the structure of the boundary shell, ii) that this energy is able to balance the radiative losses only in the localised region involved in the heating, iii) and how the boundary layer and the persistence of the driver influence the thermal structure of the system.

Conclusions:Our conclusion is that due to the extreme physical parameters we adopted and the moderate impact on the heating of the system, it is unlikely that phase-mixing can contribute on a global scale to the heating of the solar corona.

1 Introduction

The coronal heating problem has been open for several decades already and it is still unclear what the mechanism is behind the million degree solar corona. Of course, all this time has not passed in vain, and different generations of instruments and models have brought further insights to the problem. We recommend De Moortel & Browning (2015) and references therein for a comprehensive review.

One of the candidates to explain coronal heating is phase-mixing of Alfvén waves (Heyvaerts & Priest, 1983), which is one of the main models where Alfvén waves are put forward to explain the thermal structure of the solar corona. (see Arregui, 2015, for a more extended review). In this specific model localised small scale gradients develop when Alfvén waves propagate at different speed and these are preferred locations where magnetic and kinetic energy can be converted into heating. Recently, this model has fallen under careful scrutiny from the coronal physics community because of theoretical results and observations that have opened the possibility for phase-mixing to explain coronal heating (e.g. Cargill et al., 2016).

Oscillations in the solar corona have been observed for more than a decade and some studies have already suggested that the damping of waves could be connected with coronal heating (Nakariakov et al., 1999). However, only more recently observations of ubiquitous Alfvén waves have concluded that waves carry enough energy to account for the coronal heating (Tomczyk et al., 2007; Jess et al., 2009; McIntosh et al., 2011), where wave disturbances are sufficiently intense to power quiet Sun and coronal holes, while active regions remain off the range by an order of magnitude (see also De Moortel & Nakariakov, 2012, for a more comprehensive review). Morton & McLaughlin (2013) focused on low amplitude oscillations where they found that wave activity is low over an extended period of time and these would not be able to match the energy requirements of active regions. Threlfall et al. (2013) identified unambiguous wave propagations simultaneously measuring velocity and displacement of observed coronal loops that López Ariste et al. (2015) have interpreted as combined propagation of kink and sausage wave modes.

At the same time, a number of studies have shown that transverse waves are damped in the solar corona, opening the way for the possibility that the energy previously observed in the form of waves could be converted into coronal heating. Morton et al. (2014) analysed the power spectra of transverse motions in the solar corona and chromosphere and have discovered a frequency-dependent transmission profiles motivated by the damping of kink waves in the lower corona. Hahn et al. (2012) came to similar conclusions observing the line width of coronal lines in coronal holes, adding that such damping could account for a major portion of the energy required to heat these structures. Pascoe et al. (2016) observed and analysed the damping of some coronal loop oscillations making a detailed seismologic analysis to retrive the loop parameters, while Goddard et al. (2016) examined a large set of kink oscillations to carry out a statistical study on how these oscillations undergo damping in the solar corona. Additionally, other studies have confronted observed heating properties with models in order to constraint the models. Van Doorsselaere et al. (2007) found that coronal loop heating profiles better match with a resistive wave heating mechanism than a viscous one. Okamoto et al. (2015) observed the oscillations of threads of a coronal prominence, idenfied as standing Alfvén waves, and their subsequent damping and Antolin et al. (2015) argued that the observed damping of the oscillations is enhanced by the development of Kelvin-Helmotz Instabilities at the boundaries of the threads.

On the theoretical side the highly structured solar corona suggests the presence of numerous interfaces between the many magnetic structures where plasma and magnetic field vary and offer the ideal environment where Alfvén waves can propagate at different speeds. At the same time, such coronal structures are anchored to the base of the corona where footpoints move horizontally due to photospheric and chromospheric motions, thus transversally to the magnetic field and likely to generate transverse waves to be phase-mixed. To this extend, the MHD numerical experiments of Pascoe et al. (2010), Pascoe et al. (2011), Pascoe et al. (2012), and Pascoe et al. (2013) have shown that phase-mixing can be triggered in the solar corona when kink oscillations of coronal loops lead to the propagation of Alfvénic waves along the inhomogeneous flux tube. It has been robustly proven that this process leads to the concentration of wave energy in the inhomogeneous boundary shell and the formation of small scale structures. This model has also described how the structures generated by the phase-mixing on the boundary shell become increasingly smaller. This result has been validated in different and increasingly realistic configurations and concluded that this energy needs to be eventually dissipated. Earlier on, ideal MHD simulations by Poedts & Boynton (1996) had estimated the heating due to phase-mixing of Alfvén waves propagating along a magnetized cylinder by assuming turbulent heating following the ideal evolution. Soler et al. (2016) have run a similar analysis of propagating Alfvén waves in prominences approaching the problem from an analytical point of view and, estimating also the heating that can derive from this process, they found that wave heating can contribute a fraction of the radiative losses and only in certain conditions.

These data and numerical experiments have opened up the possibility that the damping of kink oscillations could contribute to the coronal heating and with this regard phase-mixing is a likely candidate mechanim that could enhance the damping of waves and lead to the conversion into coronal heating. The aim of this work is therefore to test this hypothesis and to assess the possible contribution to coronal heating that can derive from the damping of kink waves via phase mixing.

To this end we continue the study of Pascoe et al. (2010) where MHD simulations of a magnetized cylinder with the presence of a driver at one of the footpoints are used to simulate the propagation of a kink mode in a coronal loop and the subsequent mode coupling with the () Alfvén mode of the loop and dissipation of these waves through phase-mixing. In our simulations we also account for non-ideal terms as magnetic resistivity and thermal condution in order to investigate how much energy is converted into heating and how the plasma temperature changes following this process. To do so we first run a reference MHD simulation where a single pulse driver is set in and we analyse the energy deposition in the boundary shell and we then run a series of simulations where we investigate the role of the width and shape of the boundary shell, and how the results change when a continuous driver acts, instead of a single pulse.

The paper is structured as follows: in Sec.2 we describe our reference model, in Sec.3 we analyse in full details our reference simulation, in Sec.4 we vary some properties of the boundary shell and the driver, and in Sec.5 we discuss our results and we draw some conclusions.

2 Model

In order to investigate the deposition of thermal energy in the solar corona during the mode coupling and phase mixing we devise a numerical experiment following the same pattern introduced by Pascoe et al. (2010), Pascoe et al. (2011), Pascoe et al. (2012).

2.1 Initial Condition

We consider a cylindrical flux tube where we define an interior region, a boundary shell and an exterior region (Fig.1). The system is set in a cartesian reference frame with being the direction along the cylinder axis and and define the plane across the cylinder section. The origin of the axes is placed at the centre of one footpoint of the cylinder. The cylinder has radius , the interior region has radius and the boundary shell covers a fraction of the cylinder radius. The boundary shell is the ring between radii and , and the exterior region is the rest of the domain beyond radius .

Figure 1: Sketch to illustrate the geometry of our system and the cartesian axes.

The interior region is denser than the exterior and the density increases over the boundary shell defined as a function of , , , and :

(1)

where is the radial distance from the centre of the cylinder, is the density in the exterior region, and is the density in the interior. The temperature of the plasma, , is assumed uniform at , and the thermal pressure, , is set by the equation of state

(2)

where is the proton mass and is the Boltzmann constant. The flux tube is initially in equilibrium and in order to allow the propagation of kink waves and provide magnetohydrostatic equilibrium we set a non uniform magnetic field, , along the -direction:

(3)

that balances the varying thermal pressure, where is the value of the field in the exterior region, is the thermal pressure in the exterior region (derived from and ), and is the local value of the thermal pressure. In Tab.1 we list the values of the free parameters used to set up our specific experiment.

Parameter value Units
G
Table 1: Parameters
Figure 2: Cuts of our initial condition for our MHD model across the cylinder. (a) density, (b) component of the magnetic field, (c) Alfvén speed.

Fig.2 shows the value of , and the corresponding Alfvén speed () across the cylinder. In particular is higher in the exterior region and decreases in the boundary shell, where the maximum gradient is at . With the present parameters the plasma is uniformly .

2.2 The driver

A driver acts on the system in order to initiate the propagation of Alfvénic waves within the system (represented in pink in our sketch in Fig.1). The driver is prescribed by the displacement along the x axis of the flux tube below the lower boundary of the cylinder and by the consequent velocity perturbation.

Figure 3: Settings for the driver. (a) time evolution of the cylinder centre footpoint displacement as a function of time (blue line) and the consequent velocity displacement (red line). (b) map of the density of the driver at with overplotted velocity arrow. The maximum velocity in the cylinder at this time is as from Tab.2

Specifically, Fig.3a (blue line) shows the position of the centre of the flux tube as a function of time, whose expression is given by:

(4)

where is the amplitude of the speed of the displacement, is the angular frequency derived by the period of the oscillations , and is the following spatial displacement of the centre of the flux tube. The drivers sets in at and stops at . Such a displacement of the flux tube leads to a uniform velocity perturbation within the interior region and the boundary shell that is simply described by the time derivative of Eq.4 (Fig.3a, red line):

(5)

As in Pascoe et al. (2011), this choice for the driver combines an oscillatory motion at the footpoint () with an envelope of to ensure a smooth (continuous) acceleration at . In the exterior region, we assume the system reacts to the flux tube motion as a two dimensional dipole velocity configuration, so that the and components of the velocity in the exterior region can be written as function of time and space as:

(6)
(7)

This choice allows a smooth transition between the boundary shell and the exterior region as the velocity amplitude is never discontinuous and mimics plasma flows around the magnetised cylinder when in motion. Fig.3b shows the driver configuration at when the velocity is maximum and the flux tube is at its rest position.

Parameter value Units
Table 2: Parameters

Tab.2 summarises the values we have used to set up the driver in our numerical experiment.

The value of we choose is one tenth of the Alfvén speed in the interior and it is a relatively large value with respect to the usually considered horizontal foot point motions (Threlfall et al., 2013). The period of the driver is designed to produce a visible effect on the present numerical experiment and it is not meant to represent a significative frequency in the power spectrum of the corona. At the same time, while the period is well below the peak of the 5 minutes, oscillations of the order of seconds could still contribute to the power spectrum of the velocity perturbation at the coronal footpoints.

2.3 MHD simulation

In order to study the evolution of the system due to the footpoint driver we use the MPI-AMRVAC software (Porth et al., 2014) to solve the MHD equations, where thermal conduction, magnetic diffusion and joule heating are treated as source terms:

(8)
(9)
(10)
(11)

where is time, velocity, the magnetic resistivity, the speed of light, the current density, the conductive flux (Spitzer, 1962). The total energy density is given by

(12)

where denotes the ratio of specific heats.

In the present numerical experiments we adopt a value of which is set uniformly as where is the classical value (Spitzer, 1962).

The computational domain is composed of cells, distributed on a uniform grid. The simulation domain extends from to , from to (where we model only half of a flux tube) and from to in the direction of the initial magnetic field. The boundary conditions are treated with a system of ghost cells and we have periodic boundary conditions at both boundaries, reflective boundary conditions at the boundary crossing the centre of the flux tube and outflow boundary conditions at the other boundary. The driver is set as a boundary condition at the lower boundary and outflow boundary conditions are set at the upper boundary.

3 Reference simulation

The evolution of the MHD simulation allows us to analyse how the process of mode coupling and phase mixing leads to the deposition of thermal energy in the system. As soon as the driver sets in, a wavetrain propagates into the domain and we illustrate the evolution that follows in Fig.4, where we show maps of density contrast , -component of the velocity and temperature of the plasma at and in a projection of the 3D simulation box where we cut two vertical planes at and and a horizontal plane at the coordinates of the trail of the wavetrain propagating at the Alfvén speed of the interior region.

Figure 4: 3D cuts of the MHD simulation where show maps of density contrast (left-hand side column), (centre column), and temperature change (right-hand side column) on the and plane and the horizontal plane at the coordinate at (upper row) and (lower row)

The evolution described by the plane is the expected evolution of an oscillation of the flux tube along the direction where we see weak compression and rarefaction according to the phase of the driver, alternate positive and negative regions and no significant temperature change. Additionally, an acoustic mode follows the propagation of the transverse wave and leads to a visible compression and rarefaction at at .

The evolution on the plane is of greater interest for the present work. The velocity patterns are in line with what was already found and thoroughly analysed by Pascoe et al. (2010) where the mode coupling and phase mixing lead to the concentration of velocity structures in the boundary shell. The amplitude of the driver (10% of the Alfvén speed) leads to weakly non-linear dynamics.

As the purpose of the work is to identify the capacity of these phenomena to convert the magnetic and kinetic energy concentrated in the boundary shell into plasma heating we address the temperature change in the boundary shell at (Fig.4 righ-hand side column). We find that a region of increased temperature along the vertical direction inside the boundary shell is formed. This region has slightly variable width and it extends over about on the plane as we see in the horizontal cut at . In this simulation the heating is of the order to , going up to in some regions.

Figure 5: Temperature difference cuts. (a) cuts at and (where the maximum temperature is found) between (thinnest line) and (thickest line). The vertical red dashed lines are the borders of the boundary shell and the red lines are placed at the location of the maximum gradient of Alfvén speed at each time. (b) staggered cuts along z direction on the at the initial location of maximum gradient of Alfvén speed. In both plots the green line follows the location of the maximum of temperature increase.

Fig.5a shows the temperature increase profile at from to with a cadence of (from thinnest to thickest line) where the red vertical line markes the position of the maximum of gradient of Alfvén speed. The temperature starts to increase in the boundary shell near its border with the exterior region, where the kinetic energy is higher. Then the temperature increase profile maximum drifts towards the centre of the boundary shell and stops at the position where the gradient of Alfvén speed is maximum and from there it develops in a temperature increase profile centred at that position. The green line in Fig.5a follows the maximum of the temperature increase that gets closer to the centre of the boundary shell in three approaching segments, each determined by the arrival of one of the velocity peaks induced by the driver. The location of the maximum gradient of the Alfvén speed does not change over this time frame. Fig.5b stacks cuts of the temperature increase along the -direction at different times every at the coordinate where the maximum of the gradient of the Alfvén speed is situated. We notice that the bump in the temperature increase is located at higher z as the evolution progresses until , when it settles at about . At the same time, its magnitude increases as well. At the maximum temperature increase is . It also should be noted that at each time the maximum temperature increase lags the perturbation as is visible from the wave that propagates from the origin towards the edge of the domain.

Figure 6: Maps on the plane of (a) , and (b) temperature difference at with overplotted Poynting flux vectors where it is more intense. In (a) the dashed blue lines define the borders of the boundary shell.

Similar conclusions can be justified and motivated from Fig.6 where we show maps of the quantity (Fig.6a) and temperature increase (Fig.6b) at on the plane. The black arrows represent the Poynting flux (only where its intensity is above a threshold) and we choose because it is some order of magnitude larger than and . As we are investigating a non-ideal heating mechanism we focus on because where this term is large, the magnetic diffusivity operates and magnetic energy is converted into heating. Where is locally large, the Poynting vector shows a transfer of energy from the exterior to the boundary shell. This creates favourable conditions for the diffusivity term to act and thus, the conversion into thermal energy starts. Therefore, the temperature increase becomes visible only after (or following) the transfer of energy to the boundary shell has occurred, together with the formation of regions with high .

Figure 7: Temperature difference evolution of two points on the plane at the location of the maximum gradient of the Alfvén speed at (red line - where the maximum temperature in the simulation is reached at ) and at (blue line)

Additionally, the heating takes place over a bounded period of time from when the phase mixing starts to occur until the process of conversion of energy into thermal energy is concluded. Fig.7 shows the temperature evolution of two points at (blue line) and (red line) at the plane at the location of maximum gradient of Alfvén speed. The vertical lines show when a wave propagating from the lower boundary at the Alfvén speed of the exterior region reaches the two points (fronts) and when a wave propagating from the lower boundary when the driver ends at the speed of the Alfvén speed of the interior region (trails). The temperature remains constant as long as no perturbation reaches the point, it then steadily increases as the energy concentrated in the boundary shell is dissipated, and it finally remains constant again. While this evolution is common for both points, the final temperature depends on the coordinate because the amount of energy that can be converted into heating depends on the intensity of the gradients of the magnetic field, which become steeper in z, the more out of phase adjacent propagating waves are. At the same time because of the progressive damping of waves, there is more kinetic energy available at lower z-coordinates than higher up. In the present analysis the time integral of the kinetic energy in the boundary shell that crosses the surface is more than the one that crosses the .

In order to further investigate how the energy is converted into heating, we run a simulation where we set to exclude the resistivity effects. In MHD simulation terms this correspond to the adoption of numerical resistivity which is the lowest resistivity value that a given MHD simulation can achieve. The simulation with shows an evolution very similar to the one in Sec.3, except that no significant temperature increase occurs due to phase mixing (less than ).

Figure 8: (a) Difference in energy in the boundary shell between time and : magnetic (blue lines - , magenta lines - ), kinetic (green lines), and thermal (red lines) energy for the simulations with and . (b) Difference in the energy in the boundary shell between the simulation with and normalized to the thermal energy difference between the two simulations at each time. Note that the horizontal axis in both panels starts at , i.e. when the boundary driving has been terminated.

Fig.8a shows the change of magnetic energy (associated with and ), kinetic, and thermal energy in the boundary shell compared to time (when the driver has stopped) as a function of time in the two simulations. The magnetic energy associated with remains negligible over the entire evolution. The main difference is that the thermal energy significantly increases in the simulation where , while it shows only a very modest change in the simulation without resistivity. This corresponds to a visible drop in magnetic () and kinetic energy in the simulation with resistivity, when the driver has fed energy into the system. In addition, the localised heating leads to an increase in the plasma pressure in the boundary shell and hence (to conserve pressure balance), a decrease in . This decrease is clearly visible from the magenta lines in Fig.8a. The oscillations in the -magnetic energy are due to the driver-induced transverse wave motions, as the equilibrium magnetic field is not constant in the domain. This analysis shows again that the non-ideal MHD terms are essential to convert the energy concentrated in the boundary shell into heating. In Fig.8b we set equal to the difference of the thermal energy in the boundary shell at any given time between the simulation with resistivity and the one without. The corresponding difference in magnetic () and kinetic energy is about the same, at a value . This result suggests that the kinetic and magnetic energy contribute to the same extend in providing the system with thermal energy as expected for Alfvén waves and predicted in Heyvaerts & Priest (1983).

Figure 9: Thermal energy difference as a function of time (a) in the same point as in Fig.7 at and (b) integrated in the entire plane. In both plots the red dashed line is an estimation of the radiative losses for the interested region.

However, the outstanding question is whether this energy input can match the radiative losses in order to answer to what extend this heating mechanism can contribute to the coronal heating. In Fig.9a we show the internal energy evolution in a plasma element located on the phase mixing shell at (same as red line in Fig.7) as a function of time compared with an estimation of the cumulated radiative losses following from the density and temperature evolution of the same plasma element. We find that the energy deposition in a single plasma element can largely overcome the radiative losses. However, the question is not if the heating mechanism can maintain coronal temperature in a single favourable location, but if it can supply sufficient energy for entire coronal structures. Fig.9b shows the result when the same analysis is carried out and integrated on the upper three quarters of the plane of the present simulation (we exclude the lower part to avoid spurious variations due to the lower boundary of the simulation and the propagation of the acoustic mode). As the heating is localised in a narrow region near the phase mixing shell, the energy contribution to this extended domain turns out to be insufficient to balance the energy lost by radiation of the plasma on the plane . This spatial domain is arbitrary and just to show the effect of the spatial limitation of the heating. Conditions would be even less favourable to maintain a coronal temperature in this regime if we considered a domain instead.

4 Parameters space investigation

In Sec.3 we have described the mechanism that leads to the deposition of thermal energy in the boundary shell due to the phase mixing of propagating Alfvén waves. Our estimation of the thermal energy contribution from this mechanism seems not to be conclusive whether it can definitely overcome the radiative losses. In light of this, it is essential to address the role of the boundary shell in this mechanism in order to assess how much the energy deposition can differ from what we have analysed so far. To do so we run simulations where the boundary shell of the cylinder varies in width and where it has a more complex structure.

4.1 Boundary shell width

Using the setup explained in Sec.2 we run two more simulations where we only change the width of the boundary shell by using (wider boundary shell) and (narrower boundary shell) with respect to Tab.1. These are analysed in combination with the reference simulation with described in Sec.3 to investigate how the mode coupling and phase mixing are affected by Alfvén speed gradients. A narrower boundary shell leads to a steeper Alfvén speed profile (as all other parameters in the setup have remained unchanged), implying that phase mixing will become more efficient (i.e. the damping length will be shorter Heyvaerts & Priest (1983)). The mode coupling process which feeds kinetic and magnetic energy into the shell region also depends on the Alfvén speed profile in the shell region, but now a wider shell (less steep Alfvén speed gradient) leads to more efficient mode coupling (Pascoe et al., 2010, 2012, 2013). Hence, the deposition of thermal energy is dependent on the combined efficiency of mode coupling and phase mixing and how the resistivity effects interact with these mechanisms, so that it is not a priori obvious which configuration will be most efficient. The purpose of this experiment is to assess which effect dominates the dynamics and how this influences the non-ideal effects in the simulation, in particular the heating.

Figure 10: 3D cuts as in Fig.4 of left-hand side column and temperature difference, , right-hand side column at for the simulation with (upper row) and with (lower row).

Fig.10 shows cuts of and on the and planes at for the simulations with and , to be compared with the simulation with (Fig.4). The velocity phase-mixing patterns are visibly narrower in the simulation with and visibly wider in the simulation with as they simply fill the boundary shell. Similar variations are found in the extension of the temperature increase region, where the temperature increase is however dependent on the size of the boundary shell. The region where the heating occurs is narrowest in the simulation with and the maximum temperature increase is , while the simulation with show the largest heated region where .

Figure 11: Evolution of the wave energy (blue lines) and thermal energy (red lines) in the loop boundary layer as a function of time for different widths of the boundary (, dashed line; , continuous line; , dotted line).

In order to assess which configuration (wide or narrow boundary shell) leads to more efficient deposition of thermal energy, we analyse the time evolution of the wave energy (kinetic magnetic energy associated to ) and the thermal energy in the boundary shell (Fig.11). Initially (), energy is injected into the domain (blue lines), including the boundary shell, by the driver. This initial increase is followed by a second rise, where the mode coupling process transfers wave energy into the shell region. However, phase mixing is already taking place at this stage, as is evident from the increase in thermal energy. Indeed, the thermal energy starts to rise before the wave energy reaches its maximum value. Following this maximum, dissipation through phase mixing is clearly the dominant process in the shell region: wave energy is converted into thermal energy at a faster rate than it is transferred into the layer by mode coupling. Comparing the thermal energy curves (red lines), it is clear that the increase in thermal energy is largest for the narrow boundary layer (dashed red line). This is in agreement with Fig.10 which showed a larger increase in temperature in the boundary layer at for the simulation with the smallest boundary width ().

Figure 12: Time evolution of the time derivative of the average kinetic energy in the boundary shell (continuous lines) and in the interior region (dashed lines) for the three simulations with (red lines), (black lines), (blue lines)

In order to further describe how the width of the boundary shell affects the energy transfer to the boundary shell we show the time derivative of the average kinetic energy in the interior region and in the boundary shell in Fig.12. We focus on the average kinetic energy because i) it is the form of energy that is initially pumped into the system and more clearly drifts to the boundary shell and ii) averaging over the domain we compensate for two effect: first the narrower boundary shell simulation has more total kinetic energy because it has more plasma in the simulation box and second the broader boundary shell simulation has more kinetic energy in the boundary shell because the shell is larger. In Fig.12 the time derivative is large and positive for while the driver is pumping kinetic energy into both the interior and boundary shell. After the average kinetic energy in the interior diminishes and it increases in the boundary shell. The time derivative of the average kinetic energy in the interior is negative for all simulations, and the simulation with is the one where it is minimum. The time derivatives of the interior region and the boundary shell are opposite in sign and the total kinetic energy remains roughly constant, with variations of the order of 1%. This confirms that in this first phase the mode coupling dominates the dynamics, that the kinetic energy is transferred from the interior to the boundary shell, and that a broader boundary shell makes the energy transfer via mode coupling more efficient. This phase continues until when the time derivative of the average kinetic energy in the boundary shell changes sign. In this second phase, dissipation dominates and hence, the time derivative of the average kinetic energy in the boundary shell is negative for all the simulations. In this regime the narrower boundary shell simulation is more efficient in dissipating energy as its time derivative is lower. At the same time, the time derivative in the interior region approaches zero because there is less and less energy to dissipate, without ever showing a regime change, as the waves keep damping in that region. After the propagating waves interact with the upper boundary of the simulation box and the kinetic energy undergoes variations because part leaves the domain.

Fig.13 shows the temperature evolution of the point on the plane that reaches the highest temperature at the end of the simulation from the time when heating starts. We find that the final temperature depends on the width of the boundary shell, while the time scale over which thermal energy energy is deposited is not dependent on the width of the boundary shell, as the time taken from the start of the temperature increase to the final temperature is the same for all three simulations. In all three simulations, the final temperature is reached in about one period. Fig.13 also shows that the maximum of temperature is reached at lower z (and earlier in time), as the boundary shell becomes narrower.

Figure 13: Temperature difference evolution of the points with highest temperature on the plane at the location of maximum gradient of Alfvén speed in each simulation: (dotted line), (continuous line), (dashed line). Note that the time axes have been shifted so that we plot the elapsed time from when the heating starts. At the end of each line we report the -coordinate of the point, and the time at when the heating of the point starts.

4.2 Structure of the boundary shell

The boundary shell structure we have investigated so far is relatively simple, where the density smoothly increases towards the interior region and the gradient of the Alfvén speed peaks near the centre of the boundary shell. In order to address how the location of the heating depends on the structure of the boundary shell, we devise a different simulation where we change the structure of the boundary into a profile with two peaks in the gradient of the Alfvén speed (Fig.14).

Figure 14: Density (blue lines) and gradient of Alfvén speed (red lines) across the cylinder for the simulation with a smooth boundary shell (dashed lines) and the simulation with a two steps boundary shell (continuous lines).

In this simulation the boundary shell extends from to , and the density increases in two steps alike Eq.1, each extending for half of the boundary shell. The density profile is given by:

(13)

This density profile prescribes a boundary shell of width where the density in the interior and exterior region are the same as in the simulation with . The two gradient of Alfvén speed peaks are smaller and larger than the equivalent profile for the simulation with a smooth boundary shell.

Figure 15: 3D cuts as in Fig.4 of left-hand side panel and tempertaure difference, , right-hand side panel at for the simulation with and a two steps boundary shell.

Fig.15 shows the evolution of the system at equivalent to Fig.10 and the mode coupling and temperature increase patterns do not show big differences with respect to the simulation with a smooth boundary profile. We only find that the phase-mixing pattern becomes slightly more structured with a variable width and that the temperature increases at two different locations on the boundary shell, where the temperature increase is more signficant at the more external peak.

Figure 16: Temperature difference, , cut across the cylinder on the plane at for the simulations with with the smooth boundary shell (dashed line) and the two steps boundary shell (continuous line).

Fig.16 shows the temperature increase on the plane at at for the two simulations with boundary shell . We find that the temperature increase follows the structure of the boundary shell, with as many temperature peaks as gradient of Alfvén speed peaks. The peaks are also located in the same positions of the peaks of gradient of Alfvén speed and the temperature increase is proportional to the intensity of the Alfvén speed gradient.

Figure 17: Thermal energy different between t and in the boundary shell as a function of time for the simulation with and a smooth boundary shell (dashed line) and a two steps boundary shell (continuous line).

However, as shown in Fig.17, this does not lead to a visible change in the thermal energy deposited in the boundary shell where in the simulation with a two step boundary shell only more thermal energy is deposited in the boundary shell.

4.3 Continuous driver

To conclude our investigation we analyse the evolution of the system when a continuous driver (instead of a single pulse) is used to perturb the system. This is a step towards a configuration where the footpoints are continuously displaced by photospheric motion. In this simulation we use all the parameters outlined in Sec.2, with the only difference that we do not switch off the driver after one pulse, but we let it continue.

Figure 18: 3D cuts as in Fig.4 of left-hand side column and temperature difference, , right-hand side column at (upper row) and (lower row) for the simulation with continuous driver.

Fig.18 shows the evolution of the system after and . The pattern shown on the plane indicates that the phase-mixing occurs in a similar way for each consecutive period of the driver as the same pattern visible in Fig.4 repeats. However, as each wave train encounters conditions increasingly departing from the initial condition, the shape of the phase-mixing pattern becomes less regular and with more internal structuring. The temperature increase pattern shows significant differences with respect to our previous simulation, as the plasma reaches higher temperatures and the heated region broadens in time.

Figure 19: Temperature difference evolution of the point on the plane at the location of the maximum gradient of the Alfvén speed at for the simulation with continuous driver (black continuous line), compared to the same temperature evolution for the simulation with single pulse (red line). The overplotted grid is horizontally spaced by and vertically spaced by the final temperature increase in the single pulse simulation. The blue straight line would be the temperature evolution is the same initial temperature increase is gained at each pulse.

Fig.19 shows the temperature increase evolution of a single plasma element located at the steepest Alfvén speed location on the plane at and the overplotted grid is spaced one period horizontally and vertically by the temperature increase after one pulse. The temperature increase starts after , and it increases mostly linearly until , when 4 pulses have crossed the plasma element. After , the temperature increase is no longer linear in time and each pulse contributes with an increasingly smaller temperature rise. At the temperature increase slightly exceeds .

Figure 20: Difference in energy in the boundary shell between time and for the simulations with continuous driver as function of time: wave energy (blue line) and thermal energy (red line)

.

Similar conclusions can be drawn from the evolution of energy in the boundary shell (Fig.20), where we find that the thermal energy steadily increase until . After , the thermal energy increases at the expense of the wave energy that enters the boundary shell. However, the wave energy remains constant after , when the entire z-extension of the domain is filled by the wave propagation. After , the thermal energy gets saturated.

Figure 21: Thermal energy difference as a function of time integrated in the entire plane for the simulation with continuous driver (red line) and for the simulation with single pulse (black line). The red dashed line is an estimation of the radiative losses in the same region.

Fig.21 compares the thermal energy increase in the boundary shell at with an estimation of the radiative losses (as in Fig.9). In this simulation the thermal energy deposition largely overcomes the radiative losses because of the continuous injection of energy in the system. It also should be noted that the increase in thermal energy is not linear and each pulse deposits an increasingly higher amount of thermal energy because the deposition of energy drifts to lower and higher z coordinates after the shells initially involved saturate.

Figure 22: Maps on the plane of temperature difference, (left-hand side column), and (right-hand side column) at (upper row) and (lower row) for the simulation with continuous driver.

Additionally, the prolonged effect of the driver on the system leads to the fragmentation of the structure of the boundary shell on the time span of a few periods. Fig.22 shows the evolution of and the temperature difference on a cross section placed at at and . The cross sections show regular patterns at , when that cross section has been reached by only one pulse. The velocity patterns are concentric around the centre and negative and positive velocity regions alternate. The temperature increase is contained within an annular arc region of the boundary shell. Once more pulses reach this location, the patterns of and become more irregular and smaller scale structures appear. The temperature increase extends to higher radial distances from the centre of the cylinder and it is no longer an annular arc. The velocity pattern also becomes irregular and involves more external shells from the centre of the cylinder. These structures develop when the system loses the initial symmetry about the plane and analysis of the involved forces suggest that the loss of symmetry originates from the oscillations of the magnetic field induced by the driver. Such an evolution is comparable with the development Kelvin-Helmholtz instabilities as in Terradas et al. (2008), Antolin et al. (2015), and Magyar & Van Doorsselaere (2016) with the difference that in our model the conditions for development of the instability are built through the passage of several propagating waves, while in those studies the instability is triggered by standing oscillations. It has been shown that the development of Kelvin-Helmholtz instabilities amplifies the effect of resonant absorption (or phase-mixing) of Alfvén waves on plasma heating by developing smaller scale structures at the boundary shell where wave energy is more favourably converted into heating (Browning & Priest, 1984). The present simulation shows comparable dynamics, as the thermal energy deposition in the boundary shell significantly accelerates after , when the first Kelvin-Helmholtz instability-like structures appear. However, the high resistivity we have adopted prohibits the development of visible small scale vortices.

Figure 23: Temperature difference, , cut across the cylinder on the plane at from to with cadence from the thinnest to the thickest line. The red vertical dashed lines mark the borders of the boundary shell.

One of the consequences of the expansion of the region involved in the temperature increase is that the heating is not bounded to the initial boundary shell, but it extends in time to regions initially outside of the boundary shell. Fig.23 shows the temperature increase on the plane at from to with cadence (from the thinnest to the thickest line). The temperature increase profile is always a curve with one peak that reaches at the end of the simulation. We also see that the position of the centre of the peak slightly moves in time, but more importantly while at the beginning the heating is only appreciable in the initial boundary shell (between ) at the end it extends from to . This results is also in line with what has been found by Antolin et al. (2015).

5 Discussion and Conclusions

In the present work we have developed a simple model of a magnetised cylinder that is perturbed at one of its footpoints by a transverse displacement. The configuration we have adopted leads to the phase mixing of propagating Alfvén waves in the loop shell region and we have focused our analysis on the heating that follows. The aim of this modelling effort is to investigate the contribution of phase-mixing of Alfvén waves to the coronal heating problem.

In many respects, our model is a simplification of a coronal loop structure and it does not address more complex features of these magnetic structures. Certainly, our modelling starts from a condition where the coronal loop is already in place and with a temperature above the chromospheric and photospheric temperatures. Therefore, the key to interpret our results is whether such a mechanism can at least maintain the loop structure against the radiative losses.

Although our study is not conclusive on the matter, it sheds light on some relevant aspects of the contribution from phase-mixing to coronal heating and it helps identify some points where this model is in disagreement with observations.

First of all, our numerical experiments show that phase-mixing is a plausible mechanism in the corona, where we have used plasma and magnetic field parameters that are in the observed or measured range. We have also shown that a time limited oscillation propagating from the footpoint leads to a time limited energy deposition that increases the plasma temperature by a certain amount. This process is in line with the impulsive manner how coronal heating is observed to work (Warren et al., 2011; Reale, 2010). Finally, we have shown that the energy deposition is comparable with the radiative losses thus keeping phase-mixing as a candidate to maintain the high temperature of the solar corona. Given the physics and geometry of the solar corona where magnetic field structures act as vertical wave guides and observed horizontal motions at the base of the corona perturb these magnetic structures out of equilibrium, inevitably leading to the upward propagation of Alfvénic waves, phase-mixing has to occur. The unanswered question is whether this is just a marginal process in the solar corona or whether it dominates the temperature evolution.

Here, major concerns are still challenging phase-mixing as a mechanism to justify coronal heating on a larger scale and here we list some. Fist of all, the model can match the radiative losses estimation only with the adoption of a very high magnetic resistivity. In our simulations we have used a resistivity times the value predicted by the classical theory (at a 2MK temperature) and such an amplification of the resistivity is not only essential to overcome the numerical resistivity (inherent to any MHD code) but, more importantly, also to achieve a significant increase in the plasma temperature in the model. Similarly, we have to adopt a relatively large amplitude driver, in order to stimulate an appreciable deposition of thermal energy in the model. Usually velocities of the order of 10 km/s are measured as motions of coronal structures, while in our case the peak velocity is above 100 km/s. Our velocity is also much higher than the one used in Pascoe et al. (2010) and at about of the Alfvén speed, our system probably evolves in a weakly non linear regime. Moreover, the measured power spectrum of the oscillations of the solar corona peaks at 5 minutes because of the forcing photospheric oscillations. The period of our driver is instead at 6 seconds which may not be a significant frequency of the coronal power spectrum. This is especially relevant because longer oscillation periods would probably lead to slower heating time scales in this model. Finally, our model has certainly led to the heating of the boundary shell, but does not address how the centre of the loop would get a significant amount of thermal energy to sustain the loop structure against radiative cooling. This is a concern intrinsic to the phase-mixing process that concentrates wave energy only on the boundary shell (Cargill et al., 2016). Even the single case we have addressed where the temperature increase eventually involves regions beyond the boundary shell, it needs several oscillations to set in. The question remains open whether this extension needs to be triggered earlier in order to balance radiative losses, particularly in the core of the loop.

The aforementioned problems seem to pose major concerns for the validation of phase-mixing as a mechanism for coronal heating on a global scale. The model we have described seems to match the observations only by pushing the physical parameters to conditions extremely unlikely to occur in the solar corona. At the same time, it is useful to outline some uncertainties about the coronal physics that could still open the way to the possibility that phase-mixing can explain coronal heating. It is widely accepted that the combination of a classical resistivity value and the spatial resolution at which we resolve coronal structure are not able to explain the magnetic energy conversion in the solar corona. One possibility is that the development of gradients on spatial scales much smaller than our current spatial resolution can affect the larger scale evolution and thus amplify the role of the classical diffusivity terms. Another possibility is that the classical theory simply fails when small spatial scales become relevant. In any case, the question on how the ohmic heating operates in the solar corona still holds and the efficiency of any magnetic energy conversion mechanism remains uncertain. The intensity of the driver has also been questioned, but it is currently not possible to disentangle the line of sight projection effects when coronal flows are measured from doppler velocities. Therefore, while we can state that average observed speeds are of the order of , it is more difficult to determine whether this average comes from a collection of comparable motions or from the residual of the line of sight cancellation of much faster motions (De Moortel & Pascoe, 2012). Similarly, it needs to be addressed whether the development of Kelvin-Helmholtz instabilities triggered by phase-mixing could make the whole mechanism more efficient. Finally, while it is true that our model takes into account only a monochromatic wave packet with a specific period that seems not to be relevant for the solar corona, it is also true that such simple drivers are unlikely to occur at the dynamic base of the solar corona. It is more plausible that actual drivers in the solar corona are composed of a more complex spectrum. This means that also frequencies of the order of seconds are a component of the spectrum and that a realistic and energy richer wave packet would provide the system with more energy than what we have modelled with a monochromatic pulse.

Our partial conclusion is that phase-mixing seems not to be a viable mechanism to explain the large scale heating of the solar corona, even though it is likely that some energy deposition takes place through this mechanism. At the same time, further analysis is needed to validate the present results and to complete the investigation. To begin with we will run a comparable analysis over longer time scales and with a different class of drivers, also including in the energy balance the effects of radiative losses and various kinds of background heating, in order to more conclusively address whether phase-mixing can sustain the thermal structure of a coronal loop.

Acknowledgements.
This research has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 647214) and from the UK Science and Technology Facilities Council. This work used the DiRAC Data Centric system at Durham University, operated by the Institute for Computational Cosmology on behalf of the STFC DiRAC HPC Facility (www.dirac.ac.uk. This equipment was funded by a BIS National E-infrastructure capital grant ST/K00042X/1, STFC capital grant ST/K00087X/1, DiRAC Operations grant ST/K003267/1 and Durham University. DiRAC is part of the National E-Infrastructure. We acknowledge the use of the open source (gitorious.org/amrvac) MPI-AMRVAC software, relying on coding efforts from C. Xia, O. Porth, R. Keppens.

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