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Issue
A&A
Volume 512, March-April 2010
Article Number L2
Number of page(s) 4
Section Letters
DOI https://doi.org/10.1051/0004-6361/200913752
Published online 19 March 2010
A&A 512, L2 (2010)

LETTER TO THE EDITOR

Recurrent solar jets in active regions

V. Archontis1 - K. Tsinganos2 - C. Gontikakis3

1 - School of Mathematics and Statistics. St. Andrews University, St. Andrews, KY16 9SS, UK
2 - Section of Astrophysics, Astronomy and Mechanics, Department of Physics, University of Athens, Panepistimiopolis, Zografos 157 84, Athens, Greece
3 - Research Center for Astronomy and Applied Mathematics, Academy of Athens, 4 Soranou Efessiou Str., Athens 11527, Greece

Received 27 November 2009 / Accepted 24 February 2010

Abstract
Aims. We study the emergence of a toroidal flux tube into the solar atmosphere and its interaction with a pre-existing field of an active region. We investigate the emission of jets as a result of repeated reconnection events between colliding magnetic fields.
Methods. We perform 3D simulations by solving the time-dependent, resistive MHD equations in a highly stratified atmosphere.
Results. A small active region field is constructed by the emergence of a toroidal magnetic flux tube. A current structure is build up and reconnection sets in when new emerging flux comes into contact with the ambient field of the active region. The topology of the magnetic field around the current structure is drastically modified during reconnection. The modification results in a formation of new magnetic systems that eventually collide and reconnect. We find that reconnection jets are taking place in successive recurrent phases in directions perpendicular to each other, while in each phase they release magnetic energy and hot plasma into the solar atmosphere. After a series of recurrent appearance of jets, the system approaches an equilibrium where the efficiency of the reconnection is substantially reduced. We deduce that the emergence of new magnetic flux introduces a perturbation to the active region field, which in turn causes reconnection between neighboring magnetic fields and the release of the trapped energy in the form of jet-like emissions. This is the first time that self-consistent recurrency of jets in active regions is shown in a three-dimensional experiment of magnetic flux emergence.

Key words: magnetohydrodynamics (MHD) - Sun: corona - magnetic fields

1 Introduction

Jet-like emissions of plasma in the solar atmosphere have been extensively observed over a range of wavelengths (X-ray, EUV, H$\alpha$). They usually occur in active regions and polar coronal holes. It is believed that many jets and surges are produced directly by magnetic reconnection (Shibata et al. 2007) when oppositely directed magnetic field lines come into contact. Due to reconnection, the magnetic energy of the fields is converted into heat and kinetic energy of the ejected plasma. Observations (Chae et al. 1999) have also shown that EUV jets and H$\alpha$-surges occur in regions of magnetic cancellation between emerging and pre-existing magnetic fields of opposite polarity. Thus, the idea that the jet formation is due to an interaction of magnetic fields is widely supported by various measurements and numerical experiments (e.g. Yokoyama & Shibata 1996; Moreno-Insertis et al. 2008; Archontis et al. 2005).

In many cases, the appearance of jets is recurrent. Chifor et al. (2008b,a) have shown a recurrent jet emission in an active region. They found that the emission was associated with magnetic flux cancellation and they suggested that the emission was coming from the chromosphere in the process of evaporation. Chae et al. (1999) suggested that magnetic reconnection driven by emerging flux would be a possible scenario for the recurrency of EUV recurrent jets in active regions. Wang & Sheeley (2002) found numerous jet-like ejections, originated from active regions located inside or near the boundaries of nonpolar coronal holes. The jets were apparently triggered when the magnetic loop systems of the active region reconnected with the overlying open flux.

Murray et al. (2009) studied the emergence of magnetic flux in a coronal hole via 2.5 MHD numerical simulations. They found that oscillatory reconnection occurs between the rising field and the open ambient field of the coronal hole environment. A cyclic evolution of temperature and repeated reconnection outflows were reported as a consequence of the oscillatory reconnection.

In a previous study (Gontikakis et al. 2009), hereafter Paper I, we showed the formation and emission of a reconnection jet, driven by the emergence of a toroidal loop at the edge of an active region. The physical properties of the jet were in good qualitative and quantitative agreement with observations of an active region jet. Here we study the long-term evolution of the system focusing on the characteristics of the reconnection process and the jets. We find a persistent behavior of reconnection (similar to the 2.5D oscillatory reconnection by Murray et al. 2009) between the interacting magnetic fields and also recurrent emission of jets. This is the first reported instance of recurrent jets in 3D, driven by reconnection that is initiated by flux emergence into pre-existing closed loops of an active region.

\begin{figure} \par\includegraphics[width=4.5cm,height=5.0cm]{13752f1a.eps}\incl... ...752f1c.eps}\includegraphics[width=4.5cm,height=5.0cm]{13752f1d.eps} \end{figure} Figure 1:

Recurrent emission of jets due to reconnection at t=144 1a), t=184 1b), t=192 1c) and t=228 1d) in the x=10 plane. Specifically, in panels  1a) and 1c) jets are emitted in the 1 and 7 o'clock directions, toward the arcade and envelope loops. In panels  1b) and 1c) jets are emitted in the 4 and 10 o'clock directions while there is inflowing plasma from the arcade and envelope loops. Color contours show the temperature distribution. Velocity field (arrows) and magnetic field lines (white lines) are overplotted onto the plane.

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2 Model

The results in our experiments are obtained from a 3D magnetohydrodynamic simulation using a Lagrangian remap scheme (Arber et al. 2001). The basic setup follows the simulation in Paper I. The initial state consists of an hydrostatic atmosphere and two toroidal magnetic flux loops. All variables are made dimensionless by choosing photospheric values for the density, $\rho_{{\rm ph}}=3$ $\times$ $10^{-7}~{\rm g}~{\rm cm}^{-3}$, pressure, $p_{{\rm ph}}=1.4$ $\times$ $10^{5}~{\rm erg}~{\rm cm}^{-3}$, and pressure scale height, $H_{{\rm ph}}=170~{\rm km}$, and by derived units (e.g., magnetic field strength $B_{\rm ph}=1300~{\rm G}$, velocity $V_{\rm ph}=6.8~{\rm km}~{\rm s}^{-1}$ and time $t_{{\rm ph}}=25~{\rm s}$). As in Paper I, the atmosphere includes a subsurface layer $(-25\le z<0)$, photosphere $(0\le z<10)$, transition region $(10\le z<20)$ and corona $(20\le z\le 100)$. The (dimensionless) size of the numerical domain in the longitudinal (y) and transverse (x) directions is [-80,80] $\times$ [-80,80]. The toroidal loops are imposed below the photosphere along the y-axis. The crest of the first toroidal loop must rise $1.2~{\rm Mm}$ to meet the surface (leading loop). The corresponding distance for the second loop is $2.2~{\rm Mm}$ (following loop). To initiate the emergence, the entire loops are made buoyant by setting the temperature within the tubes equal to the temperature of the background atmosphere. The density deficit and excess pressure along the loops have been introduced in Hood et al. (2009) and in Paper I. The first magnetic elements of the leading tube that reach the photosphere have a field strength of ${\approx}2~{\rm KG}$. The following loop arrives at the photosphere with a weaker field strength, around $1.7~{\rm KG}$. The values for the twist, the minor and the major radius of the toroidal flux loops are the same as those of Paper I.

3 Results

3.1 Recurrent appearance of jets

\begin{figure} \par\includegraphics[width=18cm,height=7.0cm]{13752f2.eps} \end{figure} Figure 2:

3d visualization of the jets (velocity isosurfaces, yellowish/grey) at t=144 ( left) and t=184 ( right). Side views are shown for the two snapshots. The current sheets (colored red) are visualized by calculating J/B. The horizontal slice is a magnetogram at z=2. Note that the two upward elongated jets are emitted along similar directions (oblique-left). The arrows (black color) show the direction of the full magnetic field vector.

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Figure 1 (panels 1a-1d) shows the emission of bi-directional flows (jets) at four different times during the evolution of the system. The colored slice is a 2D horizontal plane (x=10) that shows the distribution of the temperature. Shown also is the projection of the full velocity vector onto the plane (arrows) and the magnetic field lines (white lines). At t=144, the leading magnetic loop has risen well into the corona, producing an external ambient field for the following loop to come into. When the two loops meet, a current layer is formed at their interface (-8<y<-12, 24<z<28). The fieldlines on the two sides of the interface are oppositely directed and, thus, they reconnect along the current structure. The reconnected fieldlines form two new magnetic domains, above and below the edges of the interface (marked with A and B, panel 1a). The fieldlines in domain A form an arcade-like structure. The domain B consists of an envelope field that overlies the rising toroidal loops. The dynamical interaction between the four magnetic domains is important for the recurrency of jets.

It was shown in Paper I that the initial interaction between the two magnetic loops leads to the formation of a hot and high-velocity reconnection jet. This is also shown here in panel 1a. Firstly, the lateral expansion of the leading loop and the adiabatic, rising motion of the following loop drive inflows that bring magnetized plasma into contact. Then the magnetic field lines that press against each other reconnect, producing outflows (jets), which are directed towards the arcade and envelope fields. The bidirectional flows are accelerated by the tension force of the reconnected fieldlines. This is the first episode, but not the last, in the dynamical evolution of magnetic fields that results in jet formation. The velocity of the jets may reach values in the range 100-200 km s-1. The emitted hot plasma has temperatures of a few MK during the evolution.

Eventually (panel 1b) the topology of the flow around the reconnection site experiences a substantial change. The arcade field undergoes an apparent vertical expansion due to the addition of reconnected fieldlines at the top of the arcade. In this way the crest of the arcade approaches the envelope field, forming a new current layer, which is located higher in the atmosphere ( -11<y<-15,33<z<36) and its cross section undergoes a rotation by  ${\approx}90$ degrees relative to the interface at t=144. This time it is the fieldlines of the arcade and envelope fields that reconnect to produce jets. The reconnection jets move toward the two emerging fields, which were previously possessing inflows. The reversal in the direction of the velocity flow leads to new reconnection events and a recurrency of jets. During the experiment we are witnessed two more episodes of reconnection outflows, at $t\approx192$ (panel 1c) and $t\approx228$ (panel 1d). The change in the topology of the flow field occurs alternately: at t=144 and t=192 the inflow regions are the emerging toroidal loops, while at the intervening time t=184 and t=228 the inflows emanate from the arcade and envelope fields.

The physical properties of the recurrent jets change over time. Their velocity, for example, does not appear so high in all episodes. In the last event the bidirectional outflows do not have speeds more than 50 km s-1. The temperature along the jets may also drop from a few MK during the first ejection to ${\approx}500.000~{\rm K}$ in the last emission. At that stage of evolution, the apparent enhancement of temperature along the reconnection outflows is also due to the compression of neighboring magnetic fields. The initial plasma density of the jets is more than ten times the density of the background atmospheric plasma. The density in the following jets may decrease by a factor of 2. After t=240, the system approaches a stage where the occurrence of jets is drastically diminished. The recurrent jets do not have the same properties because the magnetic systems that come into contact have a specific initial reservoir of magnetic flux and energy. Each time they reconnect, energy is released and the flux is eventually exchausted. Consequently, each reconnection event between the same magnetic flux systems is less effective than the previous one. As a result, the recurrent jets appear to have different physical properties (e.g. temperature).

Figure 2 shows the three-dimensional emission of the jets. At t=144 (panel 2a) the leading and following loops (green and blue fieldlines respectively) reconnect along the current structure (transparent red isosurface), which adopts an arch-like shape. The orange field lines that join the positive polarity of the leading loop with the negative polarity of the following loop represent the magnetic domain of the envelope field. The arcade magnetic field is shown by the red fieldlines. At the beginning of the emission, the jet is directed vertically above the current, but eventually it becomes collimated along the reconnected fieldlines of the envelope field. The 3D visualization reveals that the jet adopts a double-peak structure (panel 2b). The spikes are developed at the leading edge of the jet and are moving along parallel fieldlines that belong to the same (envelope) field. At t=184 (panel 2b), the jets are emitted sideways from the rims of the current structure. They are directed on opposite sides along the ambient fieldlines.

We note that the initial emission of the recurrent jets occurs in perpendicular directions. Eventually, the upward jets move along the reconnected fieldlines that envelope the leading loop. As a result, the final direction of these jets is similar: they are pointing along the negative direction of the y-axis, in an oblique-left orientation. On the other hand, the relative orientation of the downward jets makes an angle of ${\approx}90$ degrees. No specific observations to date have indicated this feature. We believe that the geometry of the overall system plays an important role in determining the final direction of the jets. If, for example, the emerging field reconnects with a (constant and uniform) pre-existing vertical (or oblique) field the direction of all recurrent jets will be vertical (or oblique). If on the other hand the pre-existing field evolves dynamically into the 3D space the direction of the jets depends on the relative orientation of the fieldlines of the magnetic systems at the time of their contact.

3.2 Driving mechanism

Now we study the effect of the reconnection process on the recurrency of the jets. Figure 3 shows the time evolution of the maximum current density ${\vec J} = \vert{\bf\nabla} \times {\vec B}\vert$at the evolved current structure between the interacting magnetic fields. For the calculation we measured the maximum J within the current structure. We also plot the maximum value of the parallel electric field  $E_{\parallel }$(in the same region), which is a rough estimate of the reconnection rate between the magnetic fields into contact. Based on the reversal of the flow topology around the diffusion region, it is possible to distinguish four reconnection phases (RP1 to RP4). In each phase, J first (initial stage) reaches a maximum value and then (later stage) drops before the next flow reversal. The duration of each phase is between 9 and 13 min. In RP1 and RP3 inflows bring the two emerged toroidal loops into the diffusion region. In RP2 and RP4 the inflows to the current structure and the outflows from the diffusion region have a reversed direction. Figure 1 shows the emission of the reconnection outflows at a time when the value of J is maximum in each reconnection phase.

Figure 3 shows that there is a good correlation between J and  $E_{\parallel }$. During the initial stage of RP1 (130<t<144), the following loop is emerging and comes into contact with the pre-existing field for first time. Thus, the magnetic stresses throughout the region of the interface (mainly on the side of the following loop) are large, increasing the compression at the interface. As a result, the current density is enhanced, reaching a peak value at t=144. $E_{\parallel }$ follows a similar evolution. The reconnection rate is minimal during the initial stage, but thereafter it increases as the current structure builds up at the interface. As the fieldlines diffuse in through the plasma and cancel, the fluid is expelled out of the ends of the current layer, which eventually dissipates (later stage). Consequently, the reconnection becomes less effective and the reconnection rate quickly drops to a low value. This is shown by the decrease of  $E_{\parallel }$ in the later stage of RP1.

The next time that these two systems press against each other is during RP3. Compared to RP1, the maximum J is smaller. The same trend in the evolution of the current is followed during the other two phases: J is larger when fieldlines from the arcade and envelope fields reconnect for the first time (RP2) and smaller during the second time (RP4). Also, it is stronger during the reconnection between the main emerging fields (RP1 and RP3) and weaker during RP2 and RP4. In all phases, the reconnection rate undergoes a parallel evolution to the current density. They develop a similar trend and reach local maxima and minima at approximately the same time. Their behavior after RP4, might be described as convergent evolution towards an equilibrium.

\begin{figure} \par\includegraphics[height=5.1cm,width=7.8cm,clip]{13752f3a.eps}... ...e*{3mm} \includegraphics[height=5.5cm,width=7cm,clip]{13752f3b.eps} \end{figure} Figure 3:

Top: time evolution of the maximum J (black line) and  $E_{\parallel }$ (red line). Bottom: time evolution of the Lorentz force (dashed) and the total pressure gradient (solid).

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To study the mechanism that drives the recurrency of jets we investigated the forces around the diffusion region. More precisely, we calculated the maximum values of the Lorentz force and the total (magnetic and gas) pressure gradient within a 3d sub-volume of the arcade field, in a very close vicinity of the current structure (e.g.,  6<x<14, -5<y<-9, 22<z<24 at t=144). At the initial stage of RP1, the Lorentz force in the arcade increases. This is mainly due to the tension force of the bent magnetic fieldlines, which are accumulated on the arcade during reconnection. The tension force is directed towards the outflow region and, thus acts against a possible upward motion of the arcade field. The stretching of the fieldlines in the arcade is apparent in Fig. 1 (panel 1b). Reconnected fieldlines are added to both outflow regions (arcade and envelope), increasing also the compression and the total pressure there. Thus, the total pressure gradient increases, although with a slower rate compared to the Lorentz force. The pressure gradient is directed towards the inflow regions and their interface. At the later stage of RP1, the pressure gradient continues to increase and eventually becomes larger than the Lorentz force. This change in the forces signals the onset of the initial stage of RP2. During this phase, the pressure gradient overwhelms the Lorentz force and causes the envelope and arcade fields to reconnect. The inflow regions in RP1, were pulled apart and reconnection between them has been stopped. After t=184, the current structure diffuses away and the pressure gradient in the inflow (outflow) regions decreases (increases). Consequently, the arcade field retreats. It is actually shoved aside by the toroidal loops, which have regained enough stress to push against each other for the second time. The re-joining of the loops occurs: firstly, because the outward acting pressure gradient force increases during RP2 and secondly, because the magnetic field of the following loop continues to emerge and expand laterally.

A similar evolution of the forces occurs in the last two reconnection phases. This suggests that the same mechanism underlies the successive reconnection events and the recurrency of jets. It is the work of the total pressure force against the work of the Lorentz force on the four magnetic domains, which is responsible for the persistent behavior of the system. The difference between the early and late reconnection phases is that the amplitude of the forces is smaller. This can be understood as follows: it has been shown (Archontis & Török 2008) that the expansion of the emerging field cannot continue for ever. Eventually, the amount of emerging flux is exhausted and the dynamical rise slows down and reaches an equilibrium. In the present experiments, this equilibrium occured for the leading tube at $t\approx 100$. The emergence of the following loop and the contact with the pre-existing field is an event that causes an initial disturbance of this equilibrium. The disturbance leads successive reconnection events, which occur with less efficiency over time, and the overall system approaches a new equilibrium stage.

4 Conclusion

Although our results are based on a single experiment as a first approach, it is clear in identifying the important processes and effects and in establishing the connection between flux emergence, reconnection and recurrent jets in a 3D environment. We expect observations to test whether recurrent jets appear in active regions due to successive reconnection events triggered by flux emergence and whether such magnetic systems evolve towards equilibrium. In a forthcoming study, we will investigate the recurrency of jets in a broader range of interacting systems.

Acknowledgements
Financial support by the European Comission through the SOLAIRE network (MTRM-CT-2006-035484) is gratefully acknowledged. UKMHD consortium cluster funded by STFC and a SRF grant to the University of St Andrews.

References

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All Figures

  \begin{figure} \par\includegraphics[width=4.5cm,height=5.0cm]{13752f1a.eps}\incl... ...752f1c.eps}\includegraphics[width=4.5cm,height=5.0cm]{13752f1d.eps} \end{figure} Figure 1:

Recurrent emission of jets due to reconnection at t=144 1a), t=184 1b), t=192 1c) and t=228 1d) in the x=10 plane. Specifically, in panels  1a) and 1c) jets are emitted in the 1 and 7 o'clock directions, toward the arcade and envelope loops. In panels  1b) and 1c) jets are emitted in the 4 and 10 o'clock directions while there is inflowing plasma from the arcade and envelope loops. Color contours show the temperature distribution. Velocity field (arrows) and magnetic field lines (white lines) are overplotted onto the plane.

Open with DEXTER
In the text

  \begin{figure} \par\includegraphics[width=18cm,height=7.0cm]{13752f2.eps} \end{figure} Figure 2:

3d visualization of the jets (velocity isosurfaces, yellowish/grey) at t=144 ( left) and t=184 ( right). Side views are shown for the two snapshots. The current sheets (colored red) are visualized by calculating J/B. The horizontal slice is a magnetogram at z=2. Note that the two upward elongated jets are emitted along similar directions (oblique-left). The arrows (black color) show the direction of the full magnetic field vector.

Open with DEXTER
In the text

  \begin{figure} \par\includegraphics[height=5.1cm,width=7.8cm,clip]{13752f3a.eps}... ...e*{3mm} \includegraphics[height=5.5cm,width=7cm,clip]{13752f3b.eps} \end{figure} Figure 3:

Top: time evolution of the maximum J (black line) and  $E_{\parallel }$ (red line). Bottom: time evolution of the Lorentz force (dashed) and the total pressure gradient (solid).

Open with DEXTER
In the text

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