Observation of a Short Period Quasi-Periodic Pulsation in Solar X-ray, Microwave and EUV Emissions

2.6 MB·

by  Pankaj Kumar

Draftversion January10,2017
PreprinttypesetusingLATEXstyleemulateapjv.04/17/13
OBSERVATION OF A SHORT PERIOD QUASI-PERIODIC PULSATION IN SOLAR X-RAY, MICROWAVE
AND EUV EMISSIONS
PANKAJ KUMAR1,2, VALERY M. NAKARIAKOV3,4,5, KYUNG-SUK CHO1,6
1KoreaAstronomyandSpaceScienceInstitute (KASI),Daejeon,305-348, RepublicofKorea
2HeliophysicsScienceDivision,NASAGoddardSpaceFlightCenter,GreenbeltMD20771
3CentreforFusion,SpaceandAstrophysics,DepartmentofPhysics,UniversityofWarwick,CV47AL,UK
4School ofSpaceResearch,KyungHeeUniversity,Yongin,446-701, Gyeonggi, RepublicofKorea
5 StPetersburgBranch,SpecialAstrophysicalObservatory,RussianAcademyofSciences, 196140, StPetersburg,Russiaand
7 6UniversityofScienceandTechnology, Daejeon305-348,RepublicofKorea
1 Draft version January 10, 2017
0
2 ABSTRACT
n Thispaperpresentsthemulti-wavelengthanalysisofa13squasi-periodicpulsation(QPP)observed
a in hard X-ray (12–300 keV) and microwave (4.9–34 GHz) emissions during a C-class flare occurred
J on 21 September 2015. AIA 304 and 171 ˚A images show an emerging loop/flux tube (L1) moving

9 radially outward, which interacts with preexisting structures within the active region. The QPP was
observed during the expansion and rising motion of L1. The Nobeyama Radioheliograph (NoRH)

] microwave images in 17/34 GHz channels reveal a single radio source, which was co-spatial with a
R
neighboringloop(L2). Inaddition, usingAIA304˚A images,wedetected intensityoscillationsinthe
S legs of loop L2 with a period of about 26 s. A similar oscillation period was observed in the GOES
h. soft X-ray flux derivative. This oscillation period seems to increase with time. We suggest that the
p observed QPP is most likely generated by the interaction between loops L2 and L3 observed in the
- AIA hot channels (131 and 94 ˚A). The merging speed of loops L2 and L3 was 35 km s−1. Loop
o ∼
L1 destroyedpossibly by its interactionwith preexistingstructures in the active regionandproduced
tr a cool jet with the speed 106–118 km s−1associated with a narrow CME ( 770 km s−1). Another
s mechanism of the QPP in∼terms of a sausage oscillation of the loop (L2) is a∼lso possible.
a
[ Subject headings: Sun: flares—Sun: corona—Sun: oscillations— Sun: UV radiation

1
v 1. INTRODUCTION vationally. For example, the leakage of umbral oscilla-
9 tions from sunspots as slow magnetoacoustic waves was
Quasi-periodic pulsations (QPPs) with typical pe-
5 found to produce 3–5 minute QPP in small-scale explo-
riods ranging from a few seconds to several min-
1 sive events and flares (Ning et al. 2004; Chen & Priest
utes are often observed in the light curves of solar
2 2006; Sych et al. 2009). Recently, Kumar et al. (2015,
and stellar flares, taken in optical, EUV, X-ray, and
0
2016) reported two interesting 3-min QPPs in X-ray,ra-
. radio wavelengths (e.g. Nakariakov & Melnikov 2009;
1 Pandey & Srivastava 2009; Kupriyanova et al. 2010; dio and EUV channels. These QPPs were found to cor-
0 relate with the leakage of umbral waves from a nearby
Sim˜oes et al. 2015; Pugh et al. 2015; Inglis et al. 2015;
7 sunspot, and small untwisting filament interacting with
Cho et al.2016). The physicalmechanismforthe gener-
1 the ambientfield. Therefore,leakageof the sunspot um-
ationofQPPsisimportantforunderstandingthedrivers
: bral oscillation or an untwisting filament could trigger
v of the flaring energy releases,particle acceleration,asso-
the repetitive reconnectionat the magnetic null point in
i ciated plasma heating, and may be useful for the flare
X a fan-spine topology.
forecasting.
QPPs observed in hard X-ray (HXR) associated with
r QPPs in solar flares are usually associated with sev-
a the non-thermal electrons with the energy greater than
eral mechanisms: (i) periodic regime of spontaneous
several tens of keV, and microwave, with the energies
magnetic reconnection, associated with tearing of the
greater than hundreds of keV, are generally associated
current sheet and/or the formation/ejection of multiple
with the periodic acceleration of nonthermal electrons
plasmoids (“magnetic dripping”, see, e.g. Kliem et al.
thatthenemittheobservedradiationbybremsstrahlung
2000; Karlicky´ & B´arta 2007; B´arta et al. 2007, 2008;
andgyrosynchrotronmechanisms,respectively(e.g.Dulk
Kumar & Cho2013),(ii)directmodulationoftheflaring
1985; Aschwanden 1987). The time profiles of HXR and
plasmaorchargedparticlekinematicsbymagnetohydro-
microwave fluxes are usually seen to be well-correlated
dynamic (MHD) waves (e.g. Zaitsev & Stepanov 1982;
with each other, including QPP. It suggests that the
Nakariakov & Melnikov 2006), (iii) magnetic reconnec-
same population of the energetic electrons is responsi-
tioninducedperiodicallybyMHDwaves(Chen & Priest
ble for these signals (Nakajima et al. 1983; Asai et al.
2006; Nakariakovet al. 2006), (iv) alternate current in
2001; Grechnev et al. 2003; Inglis & Nakariakov 2009).
an equivalent LCR-circuit (e.g. Zaitsev et al. 1998), and
In particular, the kink oscillation caused by a fast mag-
(v) oscillatory regime of the coalescence of current car-
netoacoustic mode, of a coronal loop near the recon-
rying loops (Tajima et al. 1987; Kolotkov et al. 2016).
nection site, could periodically induce magnetic recon-
Some of these mechanisms have been confirmed obser-
nection. Hence it could periodically modulate the as-
sociated periodic acceleration of charged particles, driv-
[email protected]
2
ing HXR and microwaveQPPs (e.g. Foullon et al. 2005; Gamma-ray Burst Monitor (GBM, Meegan et al. 2009),
Nakariakov et al. 2006; Inglis & Nakariakov 2009). In the NobeyamaRadioheliograph(NoRH, Nakajima et al.
particular,Nakariakov et al.(2010)reporteda40-sQPP 1994) and Radio Polarimeters (NoRP, Nakajima et al.
detectedsimultaneouslyinphaseatHXR,microwaveand 1985) observations to investigate the HXR and mi-
gamma-ray,andsuggestedtriggeringofrepetitiverecon- crowaveemissions in different energy bands.
nection by a kink oscillation of a nearby loop. Like- On 21 September 2015, AR NOAA 12420 (of the βγ
wise,slowmagnetoacousticwavescouldperiodicallytrig- magnetic configuration, S08E80) was located close to
germagneticreconnection,which,inparticular,explains the eastern limb. The QPP, reported here, was associ-
well the progression of the energy release site along the ated with an impulsive/compact C4.2 flare that started
neutral line of the plasma arcade, typical for two-ribbon in this AR at about 01:57 UT, reached its peak at
flares (Nakariakov & Zimovets 2011). about 02:04 UT, and ended at 02:08 UT. The QPP
Sausage oscillations of thick/dense flare loops may wasobservedduring02:01:45-02:02:45UT,foraboutone
cause the density perturbation and associated modula- minute.
tion of HXR and microwave emission during solar flares
(Zaitsev & Stepanov1982;Roberts et al.1984). Almost- 2.1. X-ray and radio observation of the QPP
decaylessQPPswith a periodof 0.5-60s maybe excited
TheleftpanelofFig.1displaystheFermiGBM4-sca-
by the sausage oscillation of thick/fat loops. The theo-
dence X-ray flux profiles in different energy bands. The
retical condition for the existence of the trapped global
QPPwasobservedintheHXRchannels,from12–25keV
sausage-mode oscillation is n /n > 2.4(l/w)2, where
o e upto100–300keVenergy. Itsuggeststheperiodicaccel-
n /n is the density contrast and l/w is the ratio of the
o e eration of non-thermal electrons up to 300 keV energy.
length and width of the loop. Therefore, high-density
The 100–300 keV channel reveals a decaying oscillatory
contrast is required in the oscillating loop to satisfy the
pattern with three consecutive bursts of the energy re-
condition for the existence of a fundamental sausage
lease.
mode with a high quality factor, which can be achieved
The right panel of Fig. 1) shows the Radio Solar Tele-
in the case of flaring loops (Aschwanden et al. 2004).
scope Network (RSTN) 1-s cadence flux density profiles
Using NoRH observations, Nakariakov et al. (2003) de-
observed at the Learmonth observatory in 245, 2695,
tected 14–17 s QPPs, oscillating in phase at the loop
4995, 8800, and 15400 MHz frequencies. Interestingly,
apex and at its legs, and interpreted it in terms of the
the decaying oscillation is observed in the microwave,
fundamental sausage mode. In loops with lower density
i.e., 4995, 8800, and 15400 MHz. The 245 MHz shows a
contrastssausage oscillations can exist too, but they are
singleburstsuggestingthetypeIIIradioburstexcitedby
subject to leakage to the external medium, and hence
anelectronbeamescapingupwardfromthereconnection
have lower quality factors (Nakariakov et al. 2012).
region into the interplanetary medium.
Previous studies of short period QPPs (in the
The left panel of Fig. 2 displays the NoRP flux pro-
SOHO/EIT era) generally lack the detailed study of the
files obtained at 3.75,9.4, and 17 GHz channels. We see
spatial structure of the QPP source, because of the un-
clearlyfourconsecutivedecayingpeaksofthemicrowave
availability of high-resolution imaging data. At present,
emission in 9.4 GHz, while the 17 GHz flux profile is
the analysis of AIA (12-s cadence) extreme ultraviolet
consistent with the RSTN 15.4 GHz flux. The top right
(EUV) images together with the hard X-ray (HXR) and
panelshowstheNoRHfluxprofile34GHzsignalwiththe
microwavedata providesan excellent opportunity to ex-
1-s cadence. The decaying oscillatory pattern is clearly
plore the relevant drivers of QPPs in more detail.
observed in the 34 GHz channel. Although the QPP is
Inthispaper,wereportthedetectionofa13-sQPPin
clearly seen in the original light curves, we detrended
ashortdurationC4.2flareoccurredinactiveregion(AR)
the original signal by subtracting a 70-s smoothed sig-
NOAA 12420 on 21 September 2015. The pulsation was
nal shown by the dashed blue line. We adopted wavelet
observedmainlyintheHXRandmicrowavechannels. In
analysis(Torrence & Compo1998)ofthedetrendedlight
addition, a longer period, 26-s QPP was detected in the
curve to determine the oscillation period. The bottom
EUV intensity and GOES soft X-ray flux derivative. In
panel of Fig. 2 displays the wavelet power spectrum of
section 2, we present the observations, and in the last
thedetrended34GHzsignalandthe99%confidencelevel
section, we discuss and summarise the results.
contours. The oscillation period is estimated as about
2. OBSERVATIONSANDRESULTS 13-s.
Itis evidentthatthe decayingQPPwasobservedonly
The Atmospheric Image Assembly (AIA; Lemen et al. inthehardX-rayandmicrowavechannels,therefore,the
2012) onboard the Solar Dynamics Observatory (SDO) periodicprecipitation/trappingoftheenergeticelectrons
records full disk images of the Sun (field-of-view 1.3 in the high-density loop structures should be the main
R⊙) with a spatial resolution of 1.2′′ (0.6′′ pixel−1)∼and source of the observed QPP.
the time cadence of 12 s. For the present study, we
utilised the sequences of images taken at 304 ˚A (He II, 2.2. Spatial location of the microwave sources
T 0.05 MK), 171 ˚A (Fe IX, T 0.7 MK), 94 ˚A
Todeterminethespatiallocationofthe17and34GHz
≈ ≈
(FeX,FeXVIII, T 1 MK, T 6.3 MK), 131 ˚A (FeVIII, sources, we overplotted 17 and 34 GHz emission inten-
≈ ≈
Fe XXI, Fe XXIII, i.e., 0.4, 10, 16 MK), 1600 ˚A (C IV + sity contours over the HMI magnetogram and the AIA
continuum, T 0.01MK),and1700˚A(continuum,5000 304, 1600, and 1700 ˚A images during the peak time of
≈
K). We also used the Helioseismic and Magnetic Imager each burst (Fig. 3). The HMI magnetogram image at
(HMI, Schou et al. 2012) magnetograms to determine 02:01:18 UT (Fig. 3a) shows the distribution of photo-
the magnetic topology of the AR. We utilised the Fermi sphericmagneticpolaritiesintheactiveregionandloca-
3
Fermi GBM 6-12 keV 245 MHz
12-25 keV 2695 MHz
25-50 keV 4995 MHz
50-100 keV 8800 MHz
100-300 keV 15400 MHz
Fig.1.— Left: 4-s cadence X-ray flux profiles from Fermi GBM in 6-12, 12-25, 25-50, 50-100, and 100-300 keV channels. Right: 1-s
cadenceradiofluxprofiles(insfuunit,1sfu=10−22 Wm−2 Hz−1)in245,2695,4995,8800,and15400MHzfrequencybandsobservedat
theRSTNLearmonthradioobservatory. Theshort-perioddecaying oscillationismainlyobservedinthemicrowavechannels (4995, 8800,
and15400MHz).
NoRP 3.75 GHz NoRH 34 GHz
9.4 GHz detrended
17 GHz
13-s
Fig.2.— Left: 1-s cadence radio flux profiles (in sfu unit, 1 sfu=10−22 W m−2 Hz−1) in the 3.75, 9.4, and 17 GHz frequency bands
observed by Nobeyama radio polarimeters (NoRP). The short-period decaying oscillation is observed in the hard X-ray and microwave
channels. Fourcycleswithadecayingpatternareclearlyobservedinthe9.4GHzchannel. Right: 1-scadenceNoRH34GHzfluxprofiles.
Waveletpowerspectrumofthe34GHzdetrendedsignal. Theperiodofoscillationisabout13s. Thestarttimeis02:01:48UT.
4
(a)
L1
(d)
L1 L1
(e)
Fig.3.—HMImagnetogram (-300to+300Gauss)andAIA 304,1600, and1700˚A imagesoverlaidbyNoRH17(green) and34(blue)
GHz contours. The contour levels are 50%, 70%, and 90% of the peak intensity. Labels f1, f2, and f3 indicate the footpoints of loops L1
andL2. f1belongstooneofthefootpointsoftherisingloopL1,whereasf2andf3arethefootpoints ofloopL2.
tion of the 17 and 34 GHz sources at 02:01:55 UT near ture,L1,andformationofthreebrightkernelsf1,f2,and
the edge of a big sunspot. f3,whicharethefootpointsofloopsL1andL2(Fig.3(e-
Fig. 3 (b-d) display the AIA 304 ˚A (chromo- f)). Footpoint f1 belongs to the rising loop L1 whereas
sphere/transition region) images during the observed f2 and f3 are the footpoints of loop L2. The microwave
QPP, which are overlaid by 17 and 34 GHz microwave sources are located above footpoints f2 and f3.
sources at 02:01:55 UT, 02:02:07 UT, and 02:02:28 UT,
respectively. We see a radially upward rising thick loop, 2.3. The trigger of the QPP
L1, and the microwave sources lie over the loop L2 that To investigate the trigger mechanism of the QPP, we
was heated during the rising motion of loop L1. analyseAIA304,171,and131˚A images. Fig.4displays
The AIA 1600 and 1700 images show the rising struc-
selectedAIA304and171˚A imagesduringthe observed
5
(a) (b)
brightening
(d) (e)
f(cid:0)
f(cid:1)
L1 f(cid:2)
L1
(i)
L1
(l)
Fig.4.— AIA 304 and 171 ˚A image sequence during the flare showing a thick loop, L1, moving radiallyoutward and the appearance
of acusp-shaped loop, L2, nearby. The yellow sketch at the corner of panel (b) indicates the initial structure of the bright loopL1. The
13-sQPPwasobservedduringtherisingmotionofL1(∼02:01:45-02:02:45UT).LabelsS1,S2,andS3showtheslicecutsacrossandalong
the loops,taken forthetime-distance plots. AIA 1700 ˚A contours (white) areoverlaidinthe AIA 304˚A image(panel (d)) toshow the
emission from the footpoints (f1, f2, f3) of the interacting loops. The contour levels are 35%, 55%, 75%, 95% of the peak intensity (An
animation of this figure isavailable).
QPP. Interestingly, we see brightening associated with with bright-dark patches along the flux tube legs. Dur-
an expanding flux tube (L1) in the AIA 304 ˚A images ing the rise of L1, we see appearance of another loop,
at 02:01:43 UT. The plasma along the loop looks like a L2, in the southwarddirection. It may be interpreted as
sausage-like structure drawn in panel (b). Taking a cut the interaction of expanding loop L1 with a preexisting
along L1, we observe a bright-dark-bright pattern along structures in the active region, which makes L2 brighter
L1. A similar expanding flux tube was also detected during the rise of L1. One of the legs of loop L1 discon-
duringaflarebySrivastava et al.(2013). Theyobserved nected during the rising motion, leading to a surge-like
bright knots over an entire flux tube existing for about ejection of the plasma. We are still observing structure
60 s, suggesting it was a morphological signature of the L2 while loop L1 got destroyed and produced the surge-
sausage instability. However, we see a clear separation likeejection(Fig.4(h-i)). Thestructuressimilartoloops
between the legs of L1 during its expansion associated L1 and L2 are observed in the AIA 171 ˚A images dur-
6
(a)
GOES 1-8 Å
(b)
(d)
26-s
Fig.5.— Left: Time-distance plots of the AIA 304 ˚A intensity along slices S1, S2, and S3. The green curve is the average smoothed
counts extracted between the two horizontal dotted lines in the time-distance plot of AIA 304 ˚A intensity. The yellow curve shows the
NoRH34GHzfluxinsfu. Thedynamicradiospectrum (25–180 MHz)showingquasi-periodictype IIIradiobursts (markedby1,2, and
3) was observed at the Learmonth solar observatory. A vertical dotted line indicates the onset of QPP associated with a rising loop in
AIA304˚A channel. Right: GOESsoftX-rayflux(2-scadence)in1–8˚A channel,itsderivative,smoothed/detrended, andwaveletpower
spectrum. Thedash-dottedcurveindicateda60-ssmoothingofthesignal.
ing the flare (Fig. 4 (j-l)). The 13-s QPP was observed seems to be a sausage-like plasma structure. Further-
during the rising motion of L1. The loops L1/L2 were more, the QPP was observed during the expansion of
clearly observed in the AIA 304 and 171 ˚A channels si- loop L1 (Fig. 5(a)). The expansion of the loop seems to
multaneously, which seems to indicate that these loops cause a brightening of nearby loop L2. The interaction
arefilledinwithadensechromospheric/transitionregion ofL1 withthe preexisting structuresinthe activeregion
plasma. producedacooljetseenat304and171˚A,directedalong
WeselectedslicecutsS1,S2,andS3(Fig.4(e))tocre- theopenstructuresabovetheflaresite. Fig.5(a)reveals
atetime-distanceintensityplotsusingtheAIA304˚A im- an intensity oscillation in the legs of L2. The average
ages. The purpose of the stack plots is to show the dy- smoothed counts extracted from between two horizontal
namicsofL1andL2duringtheQPP.Fig. 5displaysthe lines (see panel (a)) indicates the presence of a 26-s os-
stack plots along S1, S2, and S3 during 02:00-02:07UT. cillation in the leg of loop L2 during the observed QPP.
Cut S1 is taken across the loop, whereas S2 and S3 However, after the QPP the oscillation period seems to
are along the rising loop L1. The NoRH 34 GHz flux become longer. Unfortunately, we cannot detect the in-
and Learmonth dynamic radio spectrum are included to tensity oscillation with the period shorter than 26 s, be-
show the temporal evolution of L1 during the observed cause of the limited temporal resolution (12 s) of the
QPP.We seethe simultaneousevolution(i.e.,rise)ofL1 AIA. The estimated speed of the jet, determined from
and observed QPP in the 34 GHz channel. We noticed the linear fit, is 106–118km s−1(Fig. 5(b-c)).
blob-like structures in the loop L1 whose morphology Also, we observed a repetitive type III radio burst
7
(a) (b)
L3
(d)
L1
(e)
Fig.6.—AIA131and94˚A imagesduringtheflareshowinginteractingtheloops(L2andL3). AIA131˚A image(panel(c))isoverlaid
byNoRH17(green)and34(blue)GHzcontours. Thecontourlevelsare50%,70%,and90%ofthepeakintensity. S4andS5aretheslice
cutsusedtoconstructthetime-distanceintensityplots. Arrowsindicatetherisingloop,L1. AIA1700˚A contours(red,02:02:30UT)are
overlaidontheAIA131˚A image(panel(d))toshowtheemissionfromthefootpoints(f1,f2andf3)oftheloops. Thecontourlevelsare
35%,55%,75%,95%ofthepeakintensity(An animation of thisfigure is available).
(Fig. 5(d)) that was co-temporal with the microwave in the SOHO/LASCO C2 coronagraphimages.
QPP. It suggests the periodic injection of non-thermal Interestingly,the oscillations were also detected in the
electrons along the open field lines, accelerated in the time derivative of the soft X-ray flux obtained with
vicinity of the magnetic reconnection site in the flare. GOES at 1–8 and 0.5–4 ˚A. The derivative signal was
ThesimultaneousobservationofthetypeIIIradio,HXR smoothed by 6 s, and detrended by subtracting a 40-s
andmicrowaveburstssuggesttheinjection/precipitation smoothed curve. The right panel of Fig. 5 displays the
of the electrons bidirectionally. The downward moving waveletpowerspectrumofthesmoothed/detrendedtime
non-thermalelectrons generate the HXR and microwave derivative of the GOES 1-8 ˚A flux signal taken with
QPP,whereasthe upwardmovingelectronsgeneratethe the 2-s cadence, using the three-point (quadratic) La-
type III radio bursts. In addition, we noticed a jet-like grangianinterpolationtechnique. The waveletspectrum
successfuleruptionofloopL1,generatinganarrowCME
8
(a) tionofL3withL2atthesametimeastheQPP.However,
the location of the microwavesources suggests that loop
L1 L2 is the source of the QPP observed in the microwave
emission. Very likely, the non-thermal electrons trapped
L2 in loop L2 generated the microwave emission.
2.4. DEM analysis of the loops
To determine the temperature and emission measure
in the interacting loops, we performed differential emis-
L3
sion measure (DEM) analysis using an automatic code
developed by Aschwanden et al. (2013). We utilised six
S4 near simultaneous AIA images taken in the 171, 335,
NoRH 34 GHz
211,193,131,94˚A channelsatabout02:01:59UT.The
(b) co-alignment of AIA images was done by using the limb
fitting method, with the accuracy less than 1 pixel. The
code fits a DEM solution in each pixel, which can be
fitted by a single Gaussian function that has three free
parameters: thepeakemissionmeasure(EM ),thepeak
p
temperature (T ), and the temperature width sigma
p
(σ ). The images are selected just before the QPP oc-
T
currenceatHXRandmicrowave,toavoidtheunwelcome
35 km/s
artefacts due to the image saturation during the flare.
The peak temperature (T ) and emission measure maps
p
(EM ) are shown in Fig. 8. Loops L2 and L3 are clearly
p
observed in the EM maps, suggesting the high plasma
S5
density in the loops. Interestingly, we do not see loop
L2 to appear complete in the T map, as only a part of
the loop near the footpoint region is seen. On the other
hand, loopL3 is well observedandrevealsthe high tem-
perature of about 6 MK. Loop L2 was observed in both
Fig. 7.— Time-distance intensity plots along the slices S4 and the cool (304, 171 ˚A) and hot (131 and 94 ˚A) channels
S5usingAIA 131and94˚A images. ThebluecurveistheNoRH suggesting the presence of a multi-thermal plasma. On
34GHzfluxinsfu. the other hand, loop L2 was best observed only in the
reveals the 26-s periodicity above the 99% confidence hot(131,94˚A)channels,andtheinteractionoftheloops
level. This oscillatory component persists for longer du- was observed in the hot channels too.
ration in the SXR flux derivative in comparison to the We determine the mean values of the peak T , EM ,
p p
HXR/microwave emission. σ by selecting a region (box) near the top of loops L2
T
To study the evolution of the structures in the hot and L3. We estimate the total emissionmeasure (TEM)
channel ( 7–10 MK), we analysed the AIA 131 and by the integral of the Gaussian DEM distribution over
∼
94 ˚A images. Fig. 6 displays the AIA 131 and 94 ˚A theentiretemperaturerange. Theaveragevaluesofpeak
imagestakenduringthe flareonset,impulsiveanddecay T and EM in the boxes are found to be about 1.58 MK,
phases(02:00:08–02:05:49UT).Interestingly,weobserve 2.2 1022cm−5K−1,respectively,forloopL2;andabout
two bright loops (L2 and L3) during the flare initiation 5.4×MK,4.58 1022 cm−5K−1,respectively,for loopL3.
(Fig. 6(a)). Later on, we see the rise of loop L1 (indi- Theestimated×TEMareabout1.92 1028 cm−5 forloop
cated by the arrow) near one of the footpoints of loop L2, and 3.90 1029 cm−5 for loop L×3. Using the values
×
L2. We see the rising motion of L1 during the flare im- of TEM, we calculated the densities at the loop top re-
pulsive phase. In addition, we also observe the merging gion, assuming that the column depth of the structure
of loop L3 into L1. The contours of NoRH 17 (green) along the line of sight is nearly equal to its width. The
and 34 (blue) GHz sources on the AIA 131 ˚A image apparent widths (w) of loops L2 and L3 are about 4.5′′
suggestthatthe microwaveemissionisgeneratedinloop and 4.2′′, respectively. Therefore, the densities of the
L2 (Fig. 6(c)). After the merging of L3 with L2, we see selected regions of loops L2 and L3 are estimated with
a single loop (AIA 94 ˚A) during the flare decay phase the use of (TEM/w)1/2 to be about 7.6 109 cm−3 and
(Fig. 6(f)). 3.5 1010 cm−3, respectively. ×
×
To investigate the time evolution of loops L1 and L3
2.5. An Associated Coronal Mass Ejection
with L2, we chose slice cuts S4 and S5 in the AIA
131 and 94 ˚A images (Fig. 6(b,e)). Fig. 7 shows the The cool jet-like ejection from the chromosphere was
time-distance intensity plot along cuts S4 and S5 dur- seentoproduceanarrowCMEthatwasobservedbythe
ing 02:00–02:05 UT. We see the merging of L3 with L2. LargeAngleandSpectrometricCoronagraphontheSolar
The approaching speed of loop L3 toward L2 was about and Heliospheric Observatory (SOHO/LASCO). Fig. 9
35kms−1. We overplottedthe NoRH34GHz radioflux displays an AIA 304 ˚A image taken after the QPP,
to show the evolution of the loop interaction with the showingajetandtheassociatedCME,andthefollowing
observedQPP.Thetiming ofthe QPPcoincidedexactly CME. According to the SOHO/LASCO catalogue, the
during the rise of L1. However, we also see the interac- first appearance of the CME was at 02:12:04 UT in the
9
Fig.8.—Peaktemperature(T)andemissionmeasure(EM)mapsoftheflaresitederivedfromthenearsimultaneousimagesinsixAIA
channels. Thegreenandblueboxes areusedtoestimatethemeanT andEMoftheloopsL2andL3.
(a)
Fig. 9.— (a) AIA 304 ˚A image showing the C4.2 flare and associated jet. (b-c) CME associated with EUV jet was observed by
SOHO/LASCOC2coronagraph.
f1
rising loop f2
L1 S1 ati
f3
L2 breaking of leg L2
MW MW
L3 L3
S2
1
S3
merging of L3 into L2
(~35 km/s)
1 =possible interaction site
MW=source of microwave QPP (L2)
Fig.10.—Schematic cartoon showingthepossibletriggerofQPPintermsofloops’interaction. L1istherisingloop. LoopL3merges
into L2 during the flare. Loop L2 is the mainsource of the microwave QPP. f1, f2, f3are the footpoints (bright kernels) observed inthe
AIA1600/1700 ˚A channels. S1,S2,S3indicatetheslicecuts usedtocreatethetime-distanceintensityplotsusingAIA304˚A images.
10
LASCO C2 field of view. The linear speed and accelera- be smaller than the resolution of the instrument. More-
tion of the CME were about 769 km s−1and -8.4 ms−2, over,thedetectedperiodisabouttheAIAcadencetime,
respectively. The observation of a metric (Learmonth) which also makes the possible oscillation undetectable.
and interplanetary type III (Wind/WAVES) radio burst In addition, According to the numerical simulation of
duringthe QPPalsoindicates the motionofthe acceler- a global sausage mode (Pascoe et al. 2007), the density
ated particles along the open field lines during the flare. contrastof5–10requiresa20Mmlooplengthtosupport
a13strappedsausageoscillation. ThelengthofloopL2
3. DISCUSSIONANDCONCLUSION is 20 Mm, and hence it would be able to generate the
sausage oscillation with the observed period.
We analysed a short period, 13-s QPP observed in
We also detected an intensity oscillation with the pe-
HXR and microwave emissions during the impulsive
riod of about 26 s in the legs of the flare loop (L2) ob-
phase ofa short-durationC4.2 flare. The possible mech-
served in the AIA 304 ˚A channel. The oscillation con-
anisms for the QPP are discussed below.
tinued even after the HXR and microwave QPP. This
We observed a radially rising small loop, L1, with a
speed ofabout 100km s−1 atthe time ofthe flare initi- oscillation could also be interpreted as a magnetohy-
drodynamic mode of loop L2. In addition, AIA 304
ation. Duringtheloopmovingfromthechromosphereto
˚A and GOES SXR derivative signals reveal increase
corona,we observedbreakingofoneofits legsduringits
in the oscillation period from 26 s to 35-40 s during
rising motion. Loop L1 may interact with the ambient
the flare maximum (02:03–02:05:30 UT). If the oscilla-
preexisting field located south to L1. The interaction
tion is caused by the sausage mode, the increase in the
could be in a similar way, e.g., interaction of a small
period may be explained by the increase in the density
filament with preexisting open structures in the active
of the flare loop caused by the chromospheric evapora-
region (Sterling et al. 2015; Kumar et al. 2016), which
tion after the precipitation of nonthermal electrons. For
produces a cool jet. The breaking of the leg of L1 and
the plasma beta β <1 and a constant magnetic field,
simultaneousappearance/heatingofL2couldbearesult
the period of the sausage mode depends on the den-
of the interaction of L1 with the preexisting structures
sity of the loop (P √n, Stepanov et al. 2012). This
in the active region. The AIA time-distance intensity
∼
scenario favours global sausage-mode oscillation of the
plots revealthatthe QPPis occurringatthe time ofthe
flare loop as a driver of the 26-s QPP. In this case the
expansion and rising motion of L1. The SDO/AIA and
13-s QPP in HXR/microwave could be interpreted as a
NoRH 17 and 34 GHz observationssuggestthat loop L2
second longitudinal harmonic of the sausage mode, with
isthemainsourceoftheobservedQPPinthemicrowave
thewavelengthequaltothe looplength. Anotheroption
emission. Thus, the question is what triggers the QPP
is that the 26-s oscillation is a subharmonic of the 13-s
in loop L2. Furthermore, we also see the appearance
oscillation, pumped by the parametric resonance (e.g.,
of loops L2 and L3 in the hot AIA channels (131/94
Zaitsev et al. 2008).
˚A) before the rising motion of L1 and triggering of the
On the other hand, the interaction of the loops ob-
microwave/HXR QPP. Therefore, the coalescence of L3
servedinthiseventmayleadtotheoscillatoryreconnec-
intoL2seemstobethemostlikelycauseoftheobserved
tion and acceleration of nonthermal electrons from the
microwave QPP.
loop coalescence site to the footpoint region, producing
The converging speed of loops L2 and L3 is about
therefore the QPP detected in the microwave and HXR
35 km s−1, which is consistent with the previous re-
emissions. The loop coalescence model (Tajima et al.
port ( 30 km s−1) of loop-loop interaction observed by
∼ 1987; Sakai & Ohsawa1987) suggests the presence of (i)
Kumar et al. (2010). In that study, the loop length was
quasi-periodicoscillationsoffieldsandthenumberofac-
estimatedasabout100Mm, andthe periodofQPPwas
celerated charged particles (ii) double peak structure of
measuredin the HXR andmicrowavechannelsas 40s.
∼ the oscillation profile. In our case, we indeed observed
Therefore, a fundamental standing wave, e.g. the kink
a double peak structure in the first HXR burst in the
orsausagemodeoftheloop,wouldhavethephasespeed
12–25keV,25–50keV,50–100keVenergychannels,and
of about 5000 km s−1. However, it was not possible to
in the first peak of the 9.4 GHz radio flux. Moreover,
detectakinkorsausageoscillationoftheloopduringthe
in loop–loop interaction events, the loops’ morphologi-
QPPdirectly,becauseofthelackofthenecessaryresolu-
cal changes are expected, that result usually in confined
tion. Inourcase,QPPisassociatedwithamuchsmaller
flares. Indeed, in this event we observed a complete
loop, L2. The distance between its footpoints was 18′′
(AIA 94 ˚A, Fig. 6(e)) Assuming the semi-circular s∼hape disruption of loop L3, and a cool jet-like eruption after
merging of L3 into L2. The observation of type III ra-
oftheloop,weestimatethelooplengthas 20Mm. For
dio bursts (25–180 MHz) during the QPP indicates the
∼
the 13-s period it gives us the phase speed (2L/P) of a
injection of energetic electrons upward from the recon-
fundamental mode as 3000 km s−1. This estimation nection site along the open field lines. Therefore, the si-
∼
is consistent with the typical values of the fast magne- multaneousobservationofQPPinhardX-ray,microwave
toacoustic speed in coronal active regions. Thus, the and type III radio burst suggests the quasi-periodic in-
short period QPPs could be excited by kink or sausage jectionofthe non-thermalelectronsbidirectionally(e.g.,
fast magnetoacoustic oscillations. However, we do not Kumar et al.2016). Thus,ourobservationsuggeststhat
observe any transverse oscillation of the loops at the the oscillatory regime of the loop–loopinteraction is the
flaresite,andthereforetheinterpretationoftheobserved likely driver of the detected QPP.
QPPintermsofMHDoscillationsdoesnothaveanyfur- Recently, M´esza´rosov´aet al. (2016) reported a sub-
therobservationalsupportinthediscussedcase. But,the second pulsation (0.07–1.49 s period) in broadband mi-
short period of the possible oscillation would mean the crowave (4–7 GHz) emission for 70 s during a C-class
small transverse displacement of the loop, which could