Recurrent solar jets in active regions
aa r X i v : . [ a s t r o - ph . S R ] M a r Astronomy&Astrophysicsmanuscript no. 13752 c (cid:13)
ESO 2018October 30, 2018 L etter to the E ditor Recurrent solar jets in active regions.
V. Archontis , K. Tsinganos , and C. Gontikakis School of Mathematics and Statistics. St. Andrews University, St. Andrews, KY16 9SS Section of Astrophysics, Astronomy and Mechanics, Department of Physics, University of Athens, Panepistimiopolis, Zografos157 84, Athens, Greece Research Center for Astronomy and Applied Mathematics, Academy of Athens, 4 Soranou Efessiou Str., Athens 11527, GreeceReceived 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 activeregion. 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 upand reconnection sets in when new emerging flux comes into contact with the ambient field of the active region. The topology of themagnetic field around the current structure is drastically modified during reconnection. The modification results in a formation of newmagnetic systems that eventually collide and reconnect. We find that reconnection jets are taking place in successive recurrent phasesin 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 e ffi ciency of the reconnection issubstantially reduced. We deduce that the emergence of new magnetic flux introduces a perturbation to the active region field, whichin 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 magneticflux emergence. Key words.
Magnetohydrodynamics (MHD) – Methods: numerical – Sun: activity – Sun: corona – Sun: magnetic fields
1. Introduction
Jet-like emissions of plasma in the solar atmosphere have beenextensively observed over a range of wavelengths (X-Ray, EUV,H α ). They usually occur in active regions and polar coronalholes. It is believed that many jets and surges are produceddirectly by magnetic reconnection (Shibata et al. 2007) whenoppositely directed magnetic field lines come into contact. Dueto reconnection, the magnetic energy of the fields is convertedinto heat and kinetic energy of the ejected plasma. Observations(Chae et al. 1999) have also shown that EUV jets and H α -surgesoccur in regions of magnetic cancellation between emerging andpre-existing magnetic fields of opposite polarity. Thus, the ideathat the jet formation is due to an interaction of magnetic fieldsis widely supported by various measurements and numericalexperiments (e.g. Yokoyama & Shibata 1996; Archontis et al.2005; Moreno-Insertis et al. 2008)In many cases, the appearance of jets is recurrent. Chifor et al.(2008b,a) have shown a recurrent jet emission in an activeregion. They found that the emission was associated with mag-netic flux cancellation and they suggested that the emission wascoming from the chromosphere in the process of evaporation.Chae et al. (1999) suggested that magnetic reconnection drivenby emerging flux would be a possible scenario for the recurrencyof EUV recurrent jets in active regions. Wang & Sheeley (2002)found numerous jet-like ejections, originated from active re-gions located inside or near the boundaries of nonpolar coronalholes. The jets were apparently triggered when the magneticloop systems of the active region reconnected with the overlyingopen flux. Murray et al. (2009) studied the emergence of magnetic flux ina coronal hole via 2.5 MHD numerical simulations. They foundthat oscillatory reconnection occurs between the rising field andthe open ambient field of the coronal hole environment. A cyclicevolution of temperature and repeated reconnection outflowswere 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 anactive region. The physical properties of the jet were in goodqualitative and quantitative agreement with observations of anactive region jet. Here we study the long-term evolution of thesystem focusing on the characteristics of the reconnection pro-cess 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 re-current emission of jets. This is the first reported instance ofrecurrent jets in 3D, driven by reconnection that is initiated byflux emergence into pre-existing closed loops of an active region.
2. Model
The results in our experiments are obtained from a 3D magne-tohydrodynamic simulation using a Lagrangian remap scheme(Arber et al. 2001). The basic setup follows the simulation inpaper I. The initial state consists of an hydrostatic atmosphereand two toroidal magnetic flux loops. All variables are madedimensionless by choosing photospheric values for the density, ρ ph = × − g cm − , pressure, p ph = . × ergs cm − , and V. Archontis et al.: Recurrent solar jets in active regions. Fig. 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 = arcade and envelope loops. Inpanels 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 overplottedonto the plane.pressure scale height, H ph =
170 km, and by derived units (e.g.,magnetic field strength B ph = V ph = . − and time t ph =
25 s). As in paper I, the atmosphere includes asubsurface layer ( − ≤ z < ≤ z < ≤ z <
20) and corona (20 ≤ z ≤ y ) and transverse ( x ) directions is [ − , × [ − , y -axis. The crest of the first toroidal loop must rise 1 . Mm tomeet the surface ( leading loop). The corresponding distance forthe second loop is 2 . Mm ( following loop). To initiate the emer-gence, the entire loops are made buoyant by setting the tempera-ture within the tubes equal to the temperature of the backgroundatmosphere. The density deficit and excess pressure along theloops have been introduced in Hood et al. (2009) and in paperI. The first magnetic elements of the leading tube that reach thephotosphere have a field strength of ≈ KG . The following looparrives at the photosphere with a weaker field strength, around1 . KG . The values for the twist, the minor and the major radiusof the toroidal flux loops are the same as those of paper I.
3. Results
Figure 1 (panels 1a-1d) shows the emission of bi-directionalflows (jets) at four di ff erent times during the evolution of thesystem. The colored slice is a 2D horizontal plane ( x =
10) thatshows the distribution of the temperature. Shown also is the pro-jection of the full velocity vector onto the plane (arrows) andthe magnetic field lines (white lines). At t = leading magnetic loop has risen well into the corona, producing an ex-ternal ambient field for the following loop to come into. Whenthe two loops meet, a current layer is formed at their interface( − < y < −
12, 24 < z < 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 dynam-ical interaction between the four magnetic domains is importantfor 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 adi-abatic, rising motion of the following loop drive inflows thatbring magnetized plasma into contact. Then the magnetic fieldlines 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 ofthe reconnected fieldlines. This is the first episode, but not thelast, in the dynamical evolution of magnetic fields that results injet formation. The velocity of the jets may reach values in therange 100 − Km / sec . The emitted hot plasma has tempera-tures of a few MK during the evolution.Eventually (panel 1b) the topology of the flow around the recon-nection site experiences a substantial change. The arcade fieldundergoes an apparent vertical expansion due to the additionof reconnected fieldlines at the top of the arcade . In this waythe crest of the arcade approaches the envelope field, forminga new current layer, which is located higher in the atmosphere( − < y < − , < z <
36) and its cross section undergoesa rotation by ≈
90 degrees relative to the interface at t = arcade and envelope fieldsthat reconnect to produce jets. The reconnection jets move to-ward the two emerging fields, which were previously possessinginflows. The reversal in the direction of the velocity flow leadsto new reconnection events and a recurrency of jets. During theexperiment we are witnessed two more episodes of reconnectionoutflows, at t ≈
192 (panel 1c) and t ≈
228 (panel 1d). Thechange in the topology of the flow field occurs alternately: at t =
144 and t =
192 the inflow regions are the emerging toroidalloops, while at the intervening time t =
184 and t =
228 theinflows 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 allepisodes. In the last event the bidirectional outflows do not havespeeds more than 50 Km / sec . The temperature along the jets mayalso drop from a few MK during the first ejection to ≈ . K in the last emission. At that stage of evolution, the apparent en-hancement of temperature along the reconnection outflows isalso due to the compression of neighboring magnetic fields. Theinitial plasma density of the jets is more than ten times the den-sity of the background atmospheric plasma. The density in thefollowing jets may decrease by a factor of 2. After t = . Archontis et al.: Recurrent solar jets in active regions. 3 Fig. 2.
3d visualization of the jets (velocity isosurfaces, yellowish / grey) at t =
144 (left) and t =
184 (right). Side views are shownfor 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 thedirection of the full magnetic field vector.the system approaches a stage where the occurrence of jets isdrastically diminished. The recurrent jets do not have the sameproperties because the magnetic systems that come into contacthave a specific initial reservoir of magnetic flux and energy. Eachtime they reconnect, energy is released and the flux is eventuallyexchausted. Consequently, each reconnection event between thesame magnetic flux systems is less e ff ective than the previousone. As a result, the recurrent jets appear to have di ff erent phys-ical 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 andblue 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 lead-ing loop with the negative polarity of the following loop rep-resent the magnetic domain of the envelope field. The arcade magnetic field is shown by the red fieldlines. At the beginningof the emission, the jet is directed vertically above the current,but eventually it becomes collimated along the reconnected field-lines of the envelope field. The 3D visualization reveals that thejet adopts a double-peak structure (panel 2b). The spikes are de-veloped at the leading edge of the jet and are moving along paral-lel fieldlines that belong to the same ( envelope ) field. At t = ≈
90 degrees. No specific obser-vations to date have indicated this feature. We believe that thegeometry of the overall system plays an important role in deter-mining the final direction of the jets. If, for example, the emerg-ing field reconnects with a (constant and uniform) pre-existingvertical (or oblique) field the direction of all recurrent jets willbe vertical (or oblique). If on the other hand the pre-existing field evolves dynamically into the 3D space the direction of the jetsdepends on the relative orientation of the fieldlines of the mag-netic systems at the time of their contact.
Now we study the e ff ect of the reconnection process on the re-currency of the jets. Figure 3 shows the time evolution of themaximum current density J = |∇ × B | at the evolved currentstructure between the interacting magnetic fields. For the calcu-lation we measured the maximum J within the current structure.We also plot the maximum value of the parallel electric field E k (in the same region), which is a rough estimate of the reconnec-tion rate between the magnetic fields into contact. Based on thereversal of the flow topology around the di ff usion region, it ispossible to distinguish four reconnection phases (RP1 to RP4).In each phase, J first (initial stage) reaches a maximum valueand then (later stage) drops before the next flow reversal. Theduration of each phase is between 9 and 13 minutes. In RP1 andRP3 inflows bring the two emerged toroidal loops into the di ff u-sion region. In RP2 and RP4 the inflows to the current structureand the outflows from the di ff usion region have a reversed direc-tion. Figure 1 shows the emission of the reconnection outflowsat a time when the value of J is maximum in each reconnectionphase.Figure 3 shows that there is a good correlation between J and E k .During the initial stage of RP1 (130 < t < following loop is emerging and comes into contact with the pre-existingfield for first time. Thus, the magnetic stresses throughout theregion of the interface (mainly on the side of the following loop)are large, increasing the compression at the interface. As a re-sult, the current density is enhanced, reaching a peak value at t = E k follows a similar evolution. The reconnection rateis minimal during the initial stage, but thereafter it increases asthe current structure builds up at the interface. As the fieldlinesdi ff use in through the plasma and cancel, the fluid is expelledout of the ends of the current layer, which eventually dissipates(later stage). Consequently, the reconnection becomes less e ff ec-tive and the reconnection rate quickly drops to a low value. This V. Archontis et al.: Recurrent solar jets in active regions. Fig. 3. Top : Time evolution of the maximum J (black line)and E k (red line). Bottom : Time evolution of the Lorentz force(dashed) and the total pressure gradient (solid).is shown by the decrease of E k in the later stage of RP1.The next time that these two systems press against each other isduring RP3. Compared to RP1, the maximum J is smaller. Thesame trend in the evolution of the current is followed during theother two phases: J is larger when fieldlines from the arcade and envelope fields reconnect for the first time (RP2) and smallerduring the second time (RP4). Also, it is stronger during the re-connection between the main emerging fields (RP1 and RP3)and weaker during RP2 and RP4. In all phases, the reconnectionrate undergoes a parallel evolution to the current density. Theydevelop a similar trend and reach local maxima and minima atapproximately the same time. Their behavior after RP4, mightbe described as convergent evolution towards an equilibrium.To study the mechanism that drives the recurrency of jets weinvestigated the forces around the di ff usion region. More pre-cisely, we calculated the maximum values of the Lorentz forceand the total (magnetic and gas) pressure gradient within a 3dsub-volume of the arcade field, in a very close vicinity of thecurrent structure (e.g., 6 < x < , − < y < − , < z <
24 at t = ar-cade increases. This is mainly due to the tension force of the bentmagnetic fieldlines, which are accumulated on the arcade duringreconnection. The tension force is directed towards the outflowregion and, thus acts against a possible upward motion of the arcade field. The stretching of the fieldlines in the arcade is ap-parent in Fig. 1 (panel 1b). Reconnected fieldlines are added to both outflow regions ( arcade and envelope ), increasing also thecompression and the total pressure there. Thus, the total pres-sure gradient increases, although with a slower rate comparedto the Lorentz force. The pressure gradient is directed towardsthe inflow regions and their interface. At the later stage of RP1,the pressure gradient continues to increase and eventually be-comes larger than the Lorentz force. This change in the forcessignals the onset of the initial stage of RP2. During this phase,the pressure gradient overwhelms the Lorentz force and causesthe envelope and arcade fields to reconnect. The inflow regionsin RP1, were pulled apart and reconnection between them hasbeen stopped. After t = ff uses awayand the pressure gradient in the inflow (outflow) regions de-creases (increases). Consequently, the arcade field retreats. It isactually shoved aside by the toroidal loops, which have regainedenough stress to push against each other for the second time. There-joining of the loops occurs: firstly, because the outward act-ing pressure gradient force increases during RP2 and secondly,because the magnetic field of the following loop continues toemerge and expand laterally.A similar evolution of the forces occurs in the last two reconnec-tion phases. This suggests that the same mechanism underliesthe successive reconnection events and the recurrency of jets. Itis the work of the total pressure force against the work of theLorentz force on the four magnetic domains, which is respon-sible for the persistent behavior of the system. The di ff erencebetween the early and late reconnection phases is that the ampli-tude of the forces is smaller. This can be understood as follows:it has been shown (Archontis & T¨or¨ok 2008) that the expansionof the emerging field cannot continue for ever. Eventually, theamount of emerging flux is exhausted and the dynamical riseslows down and reaches an equilibrium. In the present experi-ments, this equilibrium occured for the leading tube at t ≈ following loop and the contact with thepre-existing field is an event that causes an initial disturbanceof this equilibrium. The disturbance leads successive reconnec-tion events, which occur with less e ffi ciency over time, and theoverall system approaches a new equilibrium stage.
4. Conclusion
Although our results are based on a single experiment as a firstapproach, it is clear in identifying the important processes ande ff ects and in establishing the connection between flux emer-gence, reconnection and recurrent jets in a 3D environment. Weexpect observations to test whether recurrent jets appear in ac-tive regions due to successive reconnection events triggered byflux emergence and whether such magnetic systems evolve to-wards equilibrium. In a forthcoming study, we will investigatethe recurrency of jets in a broader range of interacting systems. Acknowledgements.
Financial support by the European Comission throughthe SOLAIRE network (MTRM-CT-2006-035484) is gratefully acknowledged.UKMHD consortium cluster funded by STFC and a SRF grant to the Universityof St Andrews.
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