The Astrophysical Journal Letters, 782:L15 (5pp), 2014 February 20 doi:10.1088/2041-8205/782/2/L15C© 2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

HOMOLOGOUS CYCLONES IN THE QUIET SUN

Xinting Yu1,2, Jun Zhang1, Ting Li1, Yuzong Zhang1, and Shuhong Yang11 Key Laboratory of Solar Activity, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China;

yxt27272@mail.ustc.edu.cn, zjun@nao.cas.cn, liting@nao.cas.cn, yuzong@nao.cas.cn, shuhongyang@nao.cas.cn2 School of Earth and Space Science, University of Science and Technology of China, Hefei, Anhui 230026, China

Received 2013 November 14; accepted 2014 January 8; published 2014 January 30

ABSTRACT

Through observations with the Solar Dynamics Observatory Atmospheric Imaging Assembly (AIA) and Helioseis-mic and Magnetic Imager, we tracked one rotating network magnetic field (RNF) near the solar equator. It lastedfor more than 100 hr, from 2013 February 23 to 28. During its evolution, three cyclones were found to be rootedin this structure. Each cyclone event lasted for about 8 to 10 hr. While near the polar region, another RNF wasinvestigated. It lasted for a shorter time (∼70 hr), from 2013 July 7 to 9. There were two cyclones rooted in theRNF and each lasted for 8 and 11 hr, respectively. For the two given examples, the cyclones have a similar dynamicevolution, and thus we put forward a new term: homologous cyclones. The detected brightening in AIA 171 Åmaps indicates the release of energy, which is potentially available to heat the corona.

Key words: Sun: activity – Sun: corona – Sun: magnetic fields – Sun: photosphere

Online-only material: animations, color figures

1. INTRODUCTION

The different layers of the solar atmosphere are found to bevery dynamic, and many active phenomena, such as jets, surges,macroflares, and macrospicules, are spread everywhere. In thedynamic corona, a kind of rotation structure, named “cyclone,”has been reported by Zhang & Liu (2011) using observa-tions from the Atmospheric Imaging Assembly (AIA; Lemenet al. 2012) on board the Solar Dynamics Observatory (SDO;Pesnell et al. 2012). The cyclones last several hours and somecan last more than 10 hr; they are associated with extreme-ultraviolet (EUV) brightenings at the later phase. The typicalspatial extents for the cyclones are about 50′′. Cyclones are al-ways anchored in the network magnetic fields and appear whenthe underlying magnetic fields rotate. If there is a cyclone, theremust exist rotating network magnetic fields (RNFs) in the corre-sponding photosphere. However, RNFs may not correspond to acyclone because a cyclone could only be observed at the appro-priate temperature, density, and magnetic structures. There is adistinction between the terms “cyclone” and “tornado.” Torna-does usually rotate through several full circles and have a spiralstructure, and are often connected with large-scale activities,e.g., filament and prominence eruptions. On the other hand,cyclones usually rotate for less than 360◦ and have no spiralstructure, and are always related to small-scale loop evolutionand lead to EUV brightening. Giant tornadoes with larger scalehave also been studied in detail (Su et al. 2012; Li et al. 2012;Su & van Ballegooijen 2013; Wedemeyer et al. 2013). Mostof the giant tornadoes are found to be the legs of prominencesand rotate around the vertical axis. Tornadoes are suggestedto play a very important role in the eruption of solar promi-nences. Using Swedish 1 m Solar Telescope (Scharmer et al.2003) observations, tiny tornadoes were detected and analyzedby Wedemeyer-Bohm et al. (2012). They found direct evidencefor rotation in the chromospheric swirls and co-located emissionin the transition region and corona. Based on the observationsand more detailed analysis of three-dimensional simulations,magnetic tornadoes are thought to be energy channels in thesolar corona.

Cyclones are found to be rooted in the RNFs Zhang & Liu(2011). It seems that rotational motions in the photospherecreate rotating features in the overlying layers, such as cyclones,macrospicules, and surges (jets). Rotational motions in theSun’s photosphere have already been described by Brandt et al.(1988). Since then, swirling movements in jets (Patsourakoset al. 2008; Liu et al. 2009), surges (Zhang et al. 2000), brightpoints (Bonet et al. 2008), and macrospicules (Pike & Mason1998) in the solar atmosphere have been reported. Small-scalechromospheric swirls were detected by Wedemeyer-Bohm &Rouppe van der Voort (2009). These events have been provento originate from the same mechanism, “the bathtub effect”(Nordlund 1985). After cooling down at the surface, the plasmasinks down again into the solar convection zone. Due to theconservation of angular momentum, the sinking plasma formsvortex flows in the intergranular lanes (Stein & Nordlund 1998).So, the phenomena above are in fact responses to the effecton different layers in the Sun (Wedemeyer-Bohm et al. 2012)coming from below the photosphere to the chromosphere, andextending to the transition and even corona region (Innes et al.2009). However, although these vortices are different in size andlifetime, they usually swirl just once and then vanish. Recurrentrotational events for vortices that swirl several times during a31.6 minute time interval were discovered by Bonet et al. (2010)in the photospheric level.

Our study mainly concerns recurrent events in the solarcorona known as cyclones. There exist many homologousphenomena in the Sun, such as homologous flares (Woodgateet al. 1984), coronal mass ejections (Zhang & Wang 2002),flux ropes (Li & Zhang 2013), and coronal EUV waves (Zhenget al. 2012). Here, we define homologous cyclones as follows:(1) these cyclones are located at the same RNFs; (2) theyhave similar shapes; and (3) their rotational directions are thesame. We will display two examples of this phenomenon. Theobservations are obtained through the AIA and the Helioseismicand Magnetic Imager (HMI; Schou et al. 2012; Scherrer et al.2012), both on the SDO. This Letter is organized as follows. Wegive the observational data used for our study in Section 2.In Section 3, the two examples and the results are given.

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Figure 1. Panel (a): AIA 171 Å image showing the connectivity of the cyclone on 2013 February 26 (see the accompanying Animation 1). The two dotted curvesoutline the two loops. Panel (b): the corresponding HMI magnetogram. The two solid curves are duplicates of the curves in panel (a). Panel (c): the evolution of totalpositive magnetic flux (blue) and EUV brightness (red) within the black rectangle of panel (a). The pink, yellow, and green rectangles mark the corresponding cycloneevent from 1 to 3. Panels (d)–(g): sequence of magnetograms showing the cancelation of the opposite magnetic structure. The field of view of panels (d)–(g) is outlinedby the black square in panel (b). The arrow points to the negative magnetic polarities, which cancel with the corresponding positive one.

(An animation and a color version of this figure are available in the online journal.)

Finally, the conclusions and discussion are put forward inSection 4.

2. OBSERVATIONS

From 2013 February 26 to 28, SDO/AIA observed threehomologous cyclones (∼50′′) near the solar equator. Thefirst cyclone lasted from 07:00:00 UT to 17:00:00 UT onFebruary 26, varying from 160′′ to 210′′ west, and 20′′to 70′′ north. The second one started at 22:00:00 UT onFebruary 27, and ended at 10:00:00 UT the next day. It hadthe same spatial extent as the first one. The last cyclone wasobserved from 02:00:00 UT to 11:00:00 UT on February 28. Itmoved westward for about 20′′. SDO presents observations in10 wavelengths of the full solar disk with a 12 s cadence anda sampling rate of 0.′′6 pixel−1. The 171 channel shows the cy-clones best and we focus on this channel in this study. In orderto investigate the underlying magnetic fields where the cyclonesare rooted, the full-disk line-of-sight (LOS) magnetic field datafrom the HMI on board SDO are also used. The LOS magnetic

fields have a cadence of 45 s and a sampling of 0.′′5 pixel−1. Allthe AIA 171 Å and HMI images are co-aligned with the AIA171 Å image at 00:00:00 UT on 2013 February 26. To comparewith the event near the equator, another example of homologouscyclones near the north pole is selected. This event occurred on2013 July 7 to 9 and contains two homologous cyclones.

3. RESULTS

The loop-like structures of the first cyclone on February 26at 171 Å are revealed in Figure 1(a), tracked by the two dashedcurves. By examining the AIA and HMI LOS magnetograms,we find that these EUV loop-like structures are rooted ina common positive network magnetic field (black square inFigure 1(b)). The other two homologous cyclones near the equa-tor are also rooted in this positive network magnetic structure(see Animation 1). The main positive magnetic structure wherethe three homologous cyclones are anchored started to emergeat around 20:11:42 UT on February 23. It constantly convergedwith nearby smaller positive patches and became bigger.

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Figure 2. Panels (a)–(c): AIA 171 Å images showing the first homologous cyclone on 2013 February 26 (see the accompanying Animation 2). The three solid curvesoutline the loop-like structure of the cyclone, and the dot dashed curves in panels (b) and (c) are, respectively, the duplicates of the solid curves in panels (a) and (b).The three circles are the centers of the magnetic centroids (shown as the green plus sign in panels (d)–(f)). The two intersection points of the curves and the circle withthe center measure the rotational degrees of the cyclone. Panels (d)–(f): HMI magnetograms showing the rotation of the magnetic fields underlying the cyclone. Theblack contours denote the underlying magnetic fields (the white patches denote the positive rotating structure) at a level of 40 G. The solid lines, which denote thedirections of the longer axis of the contoured area, and the dot dashed lines in panels (e) and (f) are, respectively, duplicates of the solid lines in panels (d) and (e).

(An animation and a color version of this figure are available in the online journal.)

In Figure 1(c), the total brightness was calculated within theblack rectangle of panel (a). The total positive magnetic fieldswere calculated within regions chosen to follow the evolution ofthe magnetic fields under the following criteria: (1) regions arechosen approximately every 20 minutes; (2) the region coversonly the main magnetic field patch, which relates to the cyclone;(3) when there is new flux joining the main patch, we include theflux only when it is attaching to the main patch. Then we plottedboth the total brightness and total positive magnetic fields withtime. We only calculate the positive magnetic flux since it is themain positive magnetic structure that determines the underlyingstructure for the three cyclones on February 26 to 28. Timeintervals containing each of the three cyclone events are inthree different colors. Here we make the conclusion that themagnetic fields are relatively strong during the cyclone events,and when the magnetic field flux decreases, the cyclones nolonger occur. During each cyclone event, the brightness–timeprofile has a peak value. We believe that it is due to the magneticcancelation between positive and negative magnetic fields. Thecancelation process within the black square in panel (b) is shownin panels (d)–(g). The black arrows indicate the position ofthe negative magnetic structure. The converging motion of thenegative magnetic structure toward the positive one was clearlydisplayed in these panels.

The details of one series of homologous cyclone events fromFebruary 26 to 28 are shown in Figures 2–4. The first cycloneevent happened at 07:00:00 UT on February 26 and lasted forabout 10 hr (see Animation 2). Its spatial scale is about 50′′,varying from 160′′ to 210′′ west, and 20′′ to 70′′ north. Themeasurement of rotational angles for the cyclones is relativelyreliable. Since the borders of the loop system that constitutes acyclone are comparatively clear, it is relatively easy to track the

evolution of the cyclones. For the error margins, we measuredthe rotation angle 10 times and the standard deviation is thoughtto be the error margin. Figures 2(a)–(c) presents one rotatingloop-like structure tracked from 10:30:11 UT to 13:10:11 UT.During this time interval, the loop-like structure rotated counter-clockwise for 59◦ ± 6◦ in the first 75 minutes and then 32◦ ± 2◦in the last 85 minutes. This indicates that the rotational angularvelocity is not constant, 47◦ ± 5◦ and 23◦ ± 1◦, respectively.The corresponding network magnetic fields that the cyclone isanchored in also rotated counterclockwise during this interval.The RNFs extend from 180′′ to 200′′ west, and 35′′ to 55′′ north.For the underlying magnetic fields, we choose a relatively steadysalient structure, with the salient structure and the main bodyof the rotating magnetic structures forming an oval feature. Therotation of the long axis of the oval feature is determined tobe the rotational angle and angle speed. It rotated for about26◦ ± 3◦ from 10:28:57 UT to 11:47:42 UT (Figures 2(d)–(e)),a little slower than the loop-like structure. Afterward, it rotatedmuch faster and the rotation angle was about 40◦ ± 5◦ in about80 minutes (Figures 2(e)–(f)). The respective rotational speedsare 20◦ ± 2◦ and 30◦ ± 4◦. The reliability of the measurementof the angles for the rotating magnetic structures underlying thecyclones is lower. There are several reasons for this: (1) themeasurement of angles relies on the salient structure of rotatingmagnetic fields, but the spatial extent of the salient structure isvery small; (2) the salient structure usually evolves with time.If the rotational angle is measured through the change in thepositions of the salient structure, the evolution of the salientstructure is also included. The evolution of the salient structureis mainly manifested by the shape, and the change in position isslower. Thus, the uncertainty in the measurement of rotationalangles caused by the evolution of the salient structure is smaller.

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Figure 3. AIA 171 Å images and underlying magnetic fields showing the secondhomologous cyclone on 2013 February 27 (see the accompanying Animation3). The meanings of the different symbols are similar to those in Figure 2.

(An animation and a color version of this figure are available in the onlinejournal.)

About 5 hr after the end of the first cyclone, the secondcyclone occurred at 22:00:00 UT on February 26. This cycloneand the underlying RNFs have the same spatial scale as thefirst one. It lasted for 12 hr and stopped at 10:00:00 UT onFebruary 27, as shown in Figures 3(a) and (b); see Animation 3.A protruding structure of the cyclone is selected and followed.From 00:40:11 UT to 02:15:11 UT, the cyclone spun for 60◦ ±3◦, 38◦ ± 2◦. For the magnetograms in Figures 3(c) and (d), themain patch made a 35◦ ± 3◦ rotation counterclockwise from01:05:42 UT to 02:50:42 UT, 30◦ ± 2◦.

For the last event, the cyclone was no longer as conspicuousas the first two cyclones. It only rotated for not more than 180◦in total, beginning at 02:00:00 UT on February 28 and lastingfor 9 hr (see Animation 4). Its spatial scale also changed 20′′ inlongitude, varying from 180′′ to 230′′ west. However, it can stillbe observed that a structure rotated 58◦ ± 1◦ from 04:20:11 UTto 05:35:11 UT, 46◦ ± 1◦, as shown in Figures 4(a) and (b). Themagnetic patch also went westward for about 15′′, varying from195′′ to 215′′ west. It rotated for 67◦ ± 4◦ counterclockwise from03:46:57 UT to 05:37:12 UT in Figures 4(c) and (d), 36◦ ± 2◦.

To compare the homologous cyclones near the equator withthat in the polar region, another example of a homologouscyclone event near the north pole is given. This event containstwo homologous cyclones, as shown in Animation 5. Wepresent details only for the first cyclone in Figure 5. Thesecond cyclone is similar to the first one and has the samerotational direction. Due to SDO’s projection effect for thispolar cyclone, two protruding structures are marked to showthe swirling motion. As shown in Figures 5(a)–(c), the twostructures are first on the opposite sides of the rotating feature.With the whole patch spinning counterclockwise, the twostructures moved toward each other. The red feature movedwestward while the green feature moved eastward. The spinof the corresponding magnetograms can still be recognized,as demonstrated in Figures 5(d)–(f). The patch rotated 23◦both times from 16:52:25 UT to 17:44:55 UT and then to18:21:40 UT.

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Figure 4. Evolution of the third cyclone on 2013 February 28 (see theaccompanying Animation 4). The meanings of the different symbols are similarto those in Figure 2.

(An animation and a color version of this figure are available in the onlinejournal.)

4. DISCUSSION AND CONCLUSIONS

In this Letter, we report the observations of homologouscyclones appearing in the quiet Sun. The underlying magneticfield patch rotates recurrently for several days. The underlyingmagnetic fields are also responsible for the connected loopsin the cyclone, which rotates and connects to negative fieldpatches nearby. This evidently shows the correlation betweenthe solar corona and underlying photosphere. The magneticfields are relatively strong during the cyclone events, andwhen the magnetic field flux decreases, the cyclones no longeroccur.

Since the cyclones are rooted in the photosphere, we thinkthat the recurrence of cyclones is related to the underlyingmagnetic fields. Indeed, we find that the magnetic patches wherethe cyclones are rooted in exhibit rotational–stable–rotationalstructures. We here propose three possible reasons for the longlifetime of the main magnetic field patch. First, there are nonegative field patches nearby that is comparable to the size ofthe main patch and interacts with it. Second, coalescences of thesame polarity flux, converging with the main patch, are observedseveral times. Third, there may be a strong root underneaththe main patch although the magnetic flux of the patch is notsignificantly large compared with other “one-time” cyclones,which are about 1019 Mx.

The various phenomena such as small photospheric vortices(Bonet et al. 2010), large photospheric vortices (Brandt et al.1988), magnetic tornadoes (Wedemeyer-Bohm et al. 2012), andprominence-related giant tornadoes (Su et al. 2012; Li et al.2012) occur on rather different spatial and temporal scales.These phenomena with different spatial and temporal scalespossibly correspond to common (similar) physical processes inthe photosphere: due to the conservation of angular momentum,sinking plasma forms vortices and the vortices may cause therotation of magnetic fields. To determine this physical process,simultaneous photospheric white-light (G band), magnetic field,

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Figure 5. Panels (a)–(c): AIA 171 Å images showing the first homologous cyclone on 2013 July 7 (see the accompanying Animation 5). The red and green solidcurves, which outline the boundaries of the structures of two distinct cyclones, and the dot dashed ones in panels (b) and (c) are, respectively, duplicates of panels (a)and (b). Panels (d)–(f): corresponding HMI magnetograms showing the rotation of underlying magnetic fields. The green pluses, the contoured white patch, and thelines are similar to those in Figures 2(d)–(f).

(An animation and a color version of this figure are available in the online journal.)

chromospheric Hα, and coronal EUV observations with hightemporal and spatial resolution are needed.

Rotating magnetic structures play an important role in mag-netic reconnection and energy release (Zhang et al. 2007; Zhang& Liu 2011). The coronal loops connecting the positive and neg-ative magnetic field patches coincide with cyclone’s “arms.”Thisshows a direct connection between the photospheric magneticfields and the solar corona.

The EUV brightenings that indicate an energy release af-ter reconnection are often accompanied by those phenomena.Sometimes, waves are also observed accompanying the ro-tational events in different levels (De Pontieu et al. 2007;McIntosh et al. 2011; Zhang & Liu 2011). Due to the energy re-lease and waves that exist to propagate the energy, those swirlingmotions may serve as channels for corona heating. In this Let-ter, we also attribute each of those EUV brightenings to themagnetic cancelation process. With the magnetic patch swirlingand going downward, the negative field lines intertwine with themain patch, leading to magnetic reconnection. Thus, energy isreleased to provide energy for the brightenings and heating ofsolar corona.

The authors are indebted to the NASA/SDO program for boththe AIA and HMI observations. This work is supported by theOutstanding Young Scientist Project 11025315, the NationalBasic Research Program of China under grant 2011CB811403,the National Natural Science Foundations of China (11221063,11303050, 11303049, and 11203037), and the CAS ProjectKJCX2-EW-T07.

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