Origin of Macrospicule and Jet in Polar Corona by A Small-scale Kinked Flux-Tube
aa r X i v : . [ a s t r o - ph . S R ] M a y Origin of Macrospicule and Jet in Polar Corona by A Small-scaleKinked Flux-Tube
P. Kayshap Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak,Nainital-263 129, IndiaAbhishek K. Srivastava Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak,Nainital-263 129, IndiaK. Murawski Group of Astrophysics,UMCS, ul. Radziszewskiego 10, 20-031 Lublin, Poland [email protected]
Durgesh Tripathi Inter-University Centre for Astrophysics, Post Bag - 4, Ganeshkhind, Pune-411007, India [email protected]
Received ; acceptedAstrophysical Journal Letters 2 –
ABSTRACT
We report an observation of a small scale flux-tube that undergoes kinkingand triggers the macrospicule and a jet on November 11, 2010 in the northpolar corona. The small-scale flux-tube emerged well before the triggering ofmacrospicule and as the time progresses the two opposite halves of this omegashaped flux-tube bent transversely and approached towards each other. After ∼ ∼
40 Mm in the solar atmosphere with a projected speed of ∼
95 km s − . Weperform 2-D numerical simulation by considering the VAL-C initial atmosphericconditions to understand the physical scenario of the observed macrospicule andassociated jet. The simulation results show that reconnection generated velocitypulse in the lower solar atmosphere steepens into slow shock and the cool plasmais driven behind it in form of macrospicule. The horizontal surface waves alsoappeared with the shock fronts at different heights, which most likely drove andspread the large-scale jet associated with the macrospicule. Subject headings: magnetohydrodynamics (MHD) : sun — chromosphere: sun —corona
1. Introduction
Macrospicules are giant spicules, mostly observed in polar coronal holes, reaching uptoa height between 7–45 Mm above the solar-limb with a life-time of 3–45 minutes (e.g., Bohlinet al. 1975; Sterling 2000; Wilhelm 2000). A number of mechanisms have been proposed forthe formation of such plasma ejecta, e.g., gas pressure pulse (Hollweg 1982), velocity pulse(Suematsu et al. 1982; Murawski et al. 2011). Shibata (1982) suggested that if reconnectiontakes place in upper chromosphere/lower corona (lower chromosphere/photosphere)macrospicule can be triggered due to magnetic reconnection (evolution of slow shocks).Alternative mechanisms have also been reported for the formation of such plasma ejecta(e.g., Moore at al. 1977; Habbal & Gonzalez 1991; Mustsevoi & Solovev 1997; De Pontieuet al. 2004; Kamio et al. 2010, and references cited there)Apart from the spicules and macrospicules in the lower solar atmosphere, the large-scalejets have significant role in mass and energy transport upto the higher corona, as well asin destabilizing large-scale coronal magnetic fields leading the eruptions (e.g., Innes et al.1997; Isobe & Tripathi 2006; Chifor et al. 2006; Culhane et al. 2007; 2007; Filippov et al.2009; Tripathi et al. 2009; de Pontieu et al. 2010; De Pontieu et al. 2011; Judge et al.2012). Using STEREO/EUVI (Kaiser et al. 2008), a variety of solar jets on the basis oftheir sizes, life-times have been reported (Nistic´o et al. 2009). The magnetic reconnectionwas found to be one of the drivers of coronal jets (Yokoyama et al. 1995; Innes et al. 1997;Culhane et al. 2007; Chifor et al. 2008; Filippov et al. 2009; Nishizuka et al. 2009). Pariatet al. (2009) have shown that reconnection generated nonlinear Alfv´en waves can producethe polar coronal jets. Alternatively, the MHD pulse-driven models are also employed forthe triggering of various large-scale solar jets (e.g., Srivastava & Murawski 2011; Srivastavaet al. 2012; Kayshap et al. 2013, and references cited therein), and supported by recentobservations (Morton et al. 2012). 4 –The inter-relationship of spicules and jets is also important to understand theirformation processes. Kamio et al. (2010) have shown the association of jet with themacrospicule, which was triggered by a twisted magnetic flux-rope. Close association ofthe polar surges with macrospicules is also reported by Georgakilas et al. (2001) . Mooreet al. (2011) have suggested that granule-size emerging bi-poles (EBs) can trigger thelonger spicules and associated Alfv´en waves, while the larger EBs can form X-ray jets. Inthis letter, we report firstly the evidence of the activation of a small-scale bipolar twistedflux-tube in the lower polar corona, which undergoes internal reconnection and triggers amacrospicule and associated coronal jet. We study the relationship between the formationof macrospicule and associated jet, as well their most likely triggering mechanism usingSDO/AIA observations and numerical simulation. In Section 2, we discuss the observationsof macrospicule and associated jet. In Section 3, we present the results of numericalsimulations. We outline discussion and conclusions in last section.
2. Observations of the Macrospicule and Associated Jet
High resolution observations (0.6 ′′ per pixel with cadence 12 s) from the AtmosphericImaging Assembly on board the Solar Dynamic Observatory (SDO) , using its various filterssensitive to the plasma at different temperatures (AIA; Cheimets et al. 2009, DelZanna etal. 2011, Lemen et al. 2012, O’Dwyer et al. 2012), recorded the origin of macrospicule andassociated polar jet on 11 Nov 2010 (cf., Figure 1 & MS-Jet-304.mpeg). For this study,we have used 304 ˚A image sequence to study the triggering of macrospicule and jet thatoccurred at the north pole of the Sun.Figure 1 (top-panel) displays some selected snapshots of SDO/AIA 304 ˚A during theevolution of macrospicule and associated polar jet. A bipolar, omega-shaped, small-scaleflux-tube is emerged at the polar cap around 00:58:08 UT (top-left panel). During the 5 –evolution of this flux-tube between 00:58:08–00:59:44 UT ( ∼
95 sec), the flux-tube showssome transverse bending of magnetic surfaces on its both the halves that may be thesignature of the evolution of kink perturbations (cf., blue arrows in the snapshot 00:58:44 –00:58:56 UT, and cartoon in bottom panel). The combined effect of apparent rotation andtransverse bending, generates the kinked flux-tube at smaller spatial-scales, which undergoesinternal reconnection and further leads the macrospicule and jet (cf., MS-Jet-304.mpeg).To the best of our knowledge, this is the first direct observation of the evolution of kinkperturbations in the small-scale flux-tube, which further enables internal reconnection andleads the formation of macrospicule and associated jet that move along ambient open fieldlines (cf., Figure 2, and cartoon in Figure 1). The two halves and legs of the flux-tubecome closer to each other and merge to produce a brightening (cf., snapshot on 00:59:44UT), which is very likely a signature of internal magnetic reconnection between the twoopposite halves of a small-scale flux-tube. A macrospicule is triggered at the same time (cf.,01:00:56 UT snapshot in Figure 2, and cartoon in Figure 1). The macrospicule reaches upto a height of ∼
12 Mm with a speed of about 80 km s − , and faded within ∼ ∼
40 Mm with a speed of ∼
95 km s − , while its some fainter traces are evidentupto ∼ ∼
10 minutes, the large-scale jet plasma material seen to be 6 –moving upward and later started to fall back towards the surface within total life-time of ∼
24 minutes. The average outflow speed and acceleration were ∼
95 km s − and 89.015 ms − , while the average down-flow speed and downward acceleration are respectively 154 kms − and -649.99 m s − .Small-scale flux-tube in the polar corona, which is most-likely imposed by the kinkperturbations due to asymmetric bending of magnetic surfaces on its both the halves,creates a reconnection diffusion region on the temporal scale of ∼
95 sec. This region haslength (2L) ∼
12 Mm, that is the height of the emerged flux-tube at 00:58:08 UT. Thehalf-width (S) is ∼ ∼
76 km s − , (soundspeed in choromosphere is 15 km s − ) and reconnected. The height of the reconnection siteis around ∼
3. Numerical Model of the Macrospicule and Jet
We perform a 2-D MHD numerical simulation to understand the physics of the observedmacrospicule and jet using FLASH code (Lee & Deane 2009) with an assumption that thepolar corona is gravitationally stratified. The set of equations solved using FLASH code areas follows: 7 – ∂̺∂t + ∇ · ( ̺ V ) = 0 , (1) ̺ ∂ V ∂t + ̺ ( V · ∇ ) V = −∇ p + 1 µ ( ∇ × B ) × B + ̺ g , (2) ∂p∂t + ∇ · ( p V ) = (1 − γ ) p ∇ · V , (3) ∂ B ∂t = ∇ × ( V × B ) , ∇ · B = 0 . (4)Here ̺ , V , B , p = k B m ̺T , T , γ = 5 / g = (0 , − g ) with its value g = 274 m s − , m , k B ,are respectively the mass density, flow velocity, magnetic field, gas pressure, temperature,adiabatic index, solar gravitational acceleration, mean particle mass, and Boltzmann’sconstant. Radiative cooling as well as thermal conduction are not included in our model aswe are only interested in the dynamics at this instance. We assume that the solar atmosphere is in the static equilibrium ( V e = ) with a forcefree magnetic field, ( ∇ × B e ) × B e = , (5)such that it satisfies the current free condition, ∇ × B e = , and it is specified by themagnetic flux function, A , as B e = ∇ × ( A ˆ z ) . (6)Here the subscript e corresponds to equilibrium quantities. 8 –We set a weakly curved arcade type magnetic field configuration by choosing A ( x, y ) = B Λ B cos ( x/ Λ B )exp[ − ( y − y r ) / Λ B ] . (7)Here, B is the magnetic field at the reference level, which is initial location of the pulse y = y r , and the magnetic scale-height isΛ B = 2 L/π . (8)We set and hold fixed L = 200 Mm and y r = 10 Mm.As a result of Eq. (5) the pressure gradient is balanced by the gravity force, − ∇ p e + ̺ e g = . (9)With the ideal gas law and the y -component of Eq. (9), we arrive at p e ( y ) = p exp (cid:20) − Z yy r dy ′ Λ( y ′ ) (cid:21) , ̺ e ( y ) = p e ( y ) g Λ( y ) , (10)where Λ( y ) = k B T e ( y ) / ( mg ) (11)is the pressure scale-height, and p denotes the gas pressure at the reference level that wechoose in the solar corona at y r = 10 Mm.We take an equilibrium temperature profile T e ( z ) (cf., bottom-left panel of Figure3) for the solar atmosphere that consists of VAL-C atmospheric model of Vernazza et al.(1981) and obtain the corresponding gas pressure and mass density (not shown) usingEq. (10) . In our simulation the transition region is located at y ≃ . y ≃ . We impulsively perturb the system in equilibrium by a Gaussian velocity pulse V thatis nearly parallel to the ambient magnetic field lines, viz., V k ( x, y, t = 0) = A v exp (cid:20) − ( x − x ) + ( y − y ) w (cid:21) . (12)Here A v is the amplitude of the pulse, ( x , y ) is its initial position and w is its width. Weset and hold fixed A v = 7 . − , x = 0 Mm, y = 0 . w = 0 . We set the simulation box as (-10,10) Mm × (0,40) Mm and impose the boundaryconditions by fixing all plasma quantities to their equilibrium values in time for x- andy-directions, while all plasma quantities remain invariant along the z-direction. In the study,we use AMR grid with minimum (maximum) level of refinement set to 3(8) (cf., Top-leftpanel of Figure 3). We launch the velocity pulse in the lower solar atmosphere, which isconsidered to be excited by the chromospheric reconnection generated energy between theopposite halves of the omega-shaped bipolar flux-tube (cf., Eq. 12). As the pulse propagatesin gravitationally stratified atmosphere, it converts into a slow shock at higher altitudes.As a result, a low pressure region develops behind it and drives cool chromospheric plasmaupwards. This lagging plasma exhibits the properties of the observed macrospicule.Figure 5, displays key snapshots of the simulation results as the temperature-map(color) and velocity (arrows) of the plasma. At 100 s (first image), the shock front of theinitially launched pulse has reached upto ∼ ∼ ∼
10 Mm at 250 s (see the right-top panel), the shock fronthas reached a height more than 30 Mm. Our observational finding shows that macrospiculegoes up-to ∼
12 Mm with its width ∼ ∼ ∼
11 Mm while the central part suppressed up-to ∼
4. Discussion and Conclusions
Using the high resolution observations of SDO/AIA at 304 ˚A , we studied the detailedevolution of a macrospicule and associated jet recorded on 11 November, 2010. In addition,we performed numerical simulation to qualitatively match with the observed macrospiculeand jet using VAL-C model of the solar atmosphere and FLASH code. To the best ofour knowledge, this is the first direct evidence of formation of a macrospicule due to themagnetic reconnection between two opposite halves of an emerging small-scale, kinkedbipolar loop at lower altitude in the polar corona. The observed kinked small-scale flux-tubemay also be a rotating helical structure that undergo in internal reconnection to trigger thejet (e.g., Patsourakos et al. 2008; Nistic´o et al. 2009; Pariat et al. 2009). The triggeringof the macrospicule was followed by the evolution of a large-scale jet. These types ofkinked flux-rope, internal reconnection, and related plasma dynamics were only observedin the large-scale active regions, sometime leading to large-scale coronal eruptions, e.g.prominences, CMEs etc. (e.g., T¨or¨ok & Kliem 2004; Tripathi et al. 2007; Tripathi et al.2009; Srivastava et al. 2010; Kliem et al. 2010; Srivastava et al. 2013a; Srivastava etal. 2013b, and references there in) . However, the analogous conditions of the evolutionof kinked bipolar-loop at small spatio-temporal scale in the polar corona is observed forthe first time as episodic mechanism for the triggering of macrospicule and jet. Themacrospicule goes up-to ∼
12 Mm with the projected speed ∼
80 km s − , and had a life 12 –time of ∼ ∼ ∼
95 km s − , and its life time was ∼
24 minute. The standard jet models deal with the direct reconnection driven forces (j × B)between open and closed fields lines in the corona leading to the jet plasma propulsion(Yokoyama et al. 1995; Nishizuka et al. 2009; Pariat et al. 2009). However, the present newepisodic mechanism suggests that the internal reconnection in a small-scale loop in lowerchromosphere further generates a velocity pulse that steepens in slow-shock wave trainspropagating through ambient open field lines and triggering the dynamics of macrospiculeand jet.Depending upon the height of reconnection site inside the chromosphere and amountof energy release during reconnection within small-scale flux-tube (e.g., Shibata 1982;Murawski et al. 2011; Kayshap et al. 2013, and references therein) , it most likely generatesthe velocity pulse that further converts into a slow shock and exhibits the features ofmacrospicule and associated jet. The excitation of surface waves and the motion of the shockfronts and associated plasma may be responsible for the formation of the observed solarjet. Our numerical results, therefore, approximately and qualitatively match the observedplasma dynamics. We conclude that the kinking and chromospheric reconnection in thesmall-scale flux-tube can be an episodic mechanism to drive the observed macrospicule andassociated jet via secondary consequences in form of the evolution of velocity pulse andassociated slow shocks.
5. Acknowledgments
We thanks reviewer for his/her valuable suggestions that improved the manuscriptconsiderably. We acknowledge the use of the SDO/AIA observations for this study. Thedata are provided courtesy of NASA/SDO, LMSAL, and the AIA, EVE, and HMI science 13 –teams. The FLASH code has been developed by the DOE-supported ASC/Alliance Centerfor Astrophysical Thermonuclear Flashes at the University of Chicago. AKS acknowledgesShobhna Srivastava for patient encouragement. KM thanks Kamil Murawski for hisassistance in drawing numerical data. DT acknowledge the support from DST under FastTrack Scheme (SERB/F/3369/2012-2013), while AKS and PK acknowledge DST-RFBRProjet (INT/RFBR/P-117).The computational resources were provided by the HPCInfrastructure for Grand Challenges of Science and Engineering Project, co-financed bythe European Regional Development Fund under the Innovative Economy OperationalProgram. PK acknowledges Jai Bhagwan for his encouragement and supports. 14 –
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Plasma Motion ✲ Spicule Fading ❅❅❅❘
Jet Plasma ❅❅❅❘
Down-falling PlasmaFig. 2.—
The macrospicule triggers on 00:59:44 UT due to the reconnection between oppositehalves of flux-tube as shown in Figure 1, and finally converts into a jet (cf., 01:02:08-01:06:08 UTsnapshot). The comprehensive dynamics of the macrospicule and jet are shown in the attachedmovie MS-Jet-304.mpeg. The cartoon in Figure 1 also depicts the scenario of the formation ofmacrospicule and jet along ambient open field lines.
20 –Fig. 3.— Left image shows the location of the slit along the jet to measure the height-timeprofile. Right snapshot shows the height-time profile of the jet. Two different paths onthis image show respectively the acceleration of the jet (red path) as well as its deceleration(yellow path). The bright and denser part of the jet goes up-to ∼
40 Mm with projectedspeed of ∼
95 km s − , while some of its fainter traces are also observed upto 50-60 Mm. 21 –Fig. 4.— Top-left panel shows the grid-scheme and blocks of the simulation set-up, whiletop-right panel displays the pattern of the magnetic field vectors. Bottom-left and bottom-middle panels respectively demonstrate the temperature and Alfv´en speed profiles used inthe numerical simulation.Bottom right most panel shows the variation of the normalizedmass density collected at a height of 5 Mm in the transition region showing the arrival ofshock wave trains. 22 –Fig. 5.— Temperature (color maps) as well as velocity (arrows) profiles are shown at t= 100s, t=200 s, t=250 s, t=400 s, t=450 s, t=500. Temperature drawn in the MK while the arrowsrepresent the velocity expressed in the units of 50 km s −1