High-Cadence and High-Resolution Halpha Imaging Spectroscopy of a Circular Flare's Remote Ribbon with IBIS
Na Deng, Alexandra Tritschler, Ju Jing, Xin Chen, Chang Liu, Kevin Reardon, Carsten Denker, Yan Xu, Haimin Wang
aa r X i v : . [ a s t r o - ph . S R ] A p r High-Cadence and High-Resolution H a Imaging Spectroscopy of a CircularFlare’s Remote Ribbon with IBIS
NA DENG , ALEXANDRA TRITSCHLER , JU JING , XIN CHEN , CHANG LIU , KEVINREARDON , , , CARSTEN DENKER , YAN XU , AND HAIMIN WANG
1. Space Weather Research Laboratory, New Jersey Institute of Technology, University Heights,Newark, NJ 07102-1982, USA; [email protected]. National Solar Observatory, Sacramento Peak, Sunspot, NM 88349-0062, USA;3. INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy;4. Astrophysics Research Centre, Queen’s University, Belfast, BT7 1NN, Northern Ireland, UK;5. Leibniz-Institut f¨ur Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
ABSTRACT
We present an unprecedented high-resolution H a imaging spectroscopic observa-tion of a C4.1 flare taken with the Interferometric Bidimensional Spectrometer (IBIS)in conjunction with the adaptive optics system at the 76 cm Dunn Solar Telescopeon 2011 October 22 in active region NOAA 11324. Such a two-dimensional spec-troscopic observation covering the entire evolution of a flare ribbon with high spatial(0.1 ′′ pixel − image scale), cadence (4.8 s) and spectral (0.1 ˚A stepsize) resolution israrely reported. The flare consists of a main circular ribbon that occurred in a parasiticmagnetic configuration and a remote ribbon that was observed by the IBIS. Such acircular-ribbon flare with a remote brightening is predicted in 3D fan-spine reconnec-tion but so far has been rarely observed.During the flare impulsive phase, we define “core” and “halo” structures in the ob-served ribbon based on IBIS narrowband images in the H a line wing and line center.Examining the H a emission spectra averaged in the flare core and halo areas, we findthat only those from the flare cores show typical nonthermal electron beam heatingcharacteristics that have been revealed by previous theoretical simulations and obser-vations of flaring H a line profiles. These characteristics include: broad and centrallyreversed emission spectra, excess emission in the red wing with regard to the bluewing (i.e., red asymmetry), and redshifted bisectors of the emission spectra. We alsoobserve rather quick timescales for the heating ( ∼
30 s) and cooling ( ∼ − ) between discrete magneticelements implying reconnection involving different flux tubes. We observe a very hightemporal correlation ( & .
9) between the integrated H a and HXR emission during theflare impulsive phase. A short time delay (4.6 s) is also found in the H a emissionspikes relative to HXR bursts. The ionization timescale of the cool chromosphere andthe extra time taken for the electrons to travel to the remote ribbon site may contributeto this delay. Subject headings:
Sun: flares — Sun: atmospheric motions — Sun: chromosphere— Sun: magnetic fields — line: profiles
1. INTRODUCTION
Flares are three-dimensional (3D) phenomena involving a variety of physical processes suchas magnetic reconnection, energy release, particle acceleration, plasma heating and mass motion.These processes give rise to an enhanced emission at various wavelengths throughout the solarspectrum covering different atmospheric layers, which give us opportunities to scrutinize the flarephenomena from different perspectives (see reviews by, e.g., Benz 2008; Fletcher et al. 2011). Forthe longest time (since the 1930s), the strong H a line has been an important diagnostic tool toexamine the signature of flares in the chromosphere (e.g., Hudson 2007, and references therein).A flare often shows bright expanding ribbons or patches in H a images (e.g., Zirin 1988). TheH a emission in those bright ribbons could be due to various chromospheric heating mechanismsduring flares. The impulsive heating of H a flare kernels is predominantly due to precipitation ofenergetic (nonthermal) electron beams (e.g., Canfield et al. 1990b; Rubio da Costa 2011) that areaccelerated in the coronal reconnection region and responsible for the hard X-rays (HXR) emissionat the flare loop footpoints via collisional thick-target bremsstrahlung (Brown 1971; Lin & Hudson1976; Emslie 1978). The chromospheric brightening can also be heated by thermal conduc-tion from the overlying flared hot coronal loops (Zarro & Lemen 1988; Czaykowska et al. 2001;Battaglia et al. 2009), or indirectly by soft X-rays (SXR) and EUV irradiation emitted from heatedcoronal and transition region plasmas (Somov 1975; Henoux & Nakagawa 1977; Hawley & Fisher1992; Allred et al. 2005).Superior to H a filtergram observations that only reveal dynamic morphology and intensity,the complete H a spectra contain much more diagnostic capabilities. The observed H a spectra offlares usually show a variety of shapes and spectral signatures that can be compared with theoretical 3 –calculations of H a line profiles based on various physical conditions of the chromosphere duringflares. For example, Canfield et al. (1984) compute the H a profiles for modeled flare chromo-spheres in response to three specific excitation processes: nonthermal electron beam precipitation,heat conduction, and high coronal pressure. They find that H a spectra respond sensitively to theseprocesses. In particular, a central reversal with broad enhanced wings is a remarkable signatureof H a line profile when the chromosphere is bombarded by an intense flux of nonthermal elec-trons. The broadened and centrally reversed H a emission profile is also linked to nonthermaleffects by precipitating energetic electrons in other numerical simulations of flare chromospheres(Canfield & Gayley 1987; Fang et al. 1993).The H a emission profiles have been observed to show such theoretically predicted enhancedbroad wing and central reversal signatures in flare kernels that are spatially and temporally wellcorrelated with HXR emissions during the impulsive phase of flares (Canfield & Gunkler 1985;Lee et al. 1996), which justified the nonthermal electron beam heating mechanism in those ar-eas and the characteristic H a line profile corresponding to such heating mechanism. Based onthis remarkable spectral characteristics, the observed impulsive-phase H a emission profiles havebeen used as a diagnostic tool to identify the sites of nonthermal electron beam precipitation intothe chromosphere and to compare them with HXR emissions that originate from the same effect(Canfield et al. 1990b, 1993).It is noted, however, other effects and heating mechanisms can also produce centrally reversedH a line profiles (e.g., Abbett & Hawley 1999; Kaˇsparov´a et al. 2009; Rubio da Costa 2011). There-fore, this characteristic profile shape itself can not solely determine the heating mechanism. Never-theless, the close temporal correlation between variations of HXR flux and H a emission (with shorttime lag in H a emission) and fast heating and cooling time for the chromosphere can add strong ev-idence to the nonthermal electron beam heating mechanism (Heinzel 1991; Kaˇsparov´a et al. 2009).The existence of nonthermal heating processes does not exclude other heating mechanisms. Inmany cases, different heating mechanisms work together during flares (Cheng et al. 2006).Besides the characteristic shapes, the shift and asymmetry of the line profile can also revealdynamic phenomena of flare-related mass motions. During the impulsive phase of flares, red-shifted H a emission spectra along with conspicuous red asymmetry (i.e., excess red wing emissionand the line wing bisectors shift more toward red) are most commonly observed in small brightflare kernels (Tang 1983; Ichimoto & Kurokawa 1984) although blueshifts and blue asymmetryare occasionally found in the early impulsive phase (Canfield et al. 1990b). The redshifts and redasymmetry of H a emission spectra are found closely associated with intense HXR or microwaveemissions in space and time (Wuelser & Marti 1989; Canfield et al. 1990b). Those redshifted andred asymmetric H a emissions are attributed to downward moving chromospheric condensation(Canfield et al. 1990a; Gan et al. 1993; Ding & Fang 1996), which is driven by explosive chromo- 4 –spheric evaporation caused by impulsive heating of the upper chromosphere due to bombardmentof energetic electrons (Fisher et al. 1985a,b,c). Therefore, the redshifts and red asymmetry couldbe additional characteristics for H a emission spectra during nonthermal electron beam heating.Compared to extensive H a imaging observations, two-dimensional (2D) H a spectral obser-vations that spatially and temporally cover a flare are relatively scarce. The aforementioned ob-servations of 2D H a spectra during flares were mainly acquired by traditional one-dimensional(1D) single-slit scanning spectrographs that usually have low spatial resolution ( > ′′ ) and lowcadence (e.g., >
10 s for a small region) due to the time consumed to scan a 2D region with the1D slit (e.g., Canfield et al. 1990b; Wuelser et al. 1994; Ding et al. 1995). Recently, modern 2Dimaging spectroscopic instruments based on tunable narrowband filters (e.g., Fabry-P´erot etalons)are routinely available and capable of high-spatial (sub arcsecond) and high-temporal (a few sec-onds) resolution spectroscopy or spectropolarimetry, for example, the Interferometric Bidimen-sional Spectrometer (IBIS; Cavallini 2006). They are especially useful for observing small-scaleor highly dynamic phenomena, such as the chromosphere, jets, and flares (e.g., Cauzzi et al. 2008,2009). However, flare spectra taken by these kinds of instruments are rarely published so far.Kleint (2012) reported a spectropolarimetric observation of a C-class flare taken with IBIS duringonly the decaying phase of the flare. Moving or suddenly appeared brightenings inside the flareribbons are observed to show strong emission in the chromospheric Ca II a spectra of two sympathetic mini-flaresobserved by the G¨ottingen Fabry-P´erot spectrometer. In the present paper, we will present an un-precedented 2D H a spectroscopic observation carried out by IBIS covering the morphology andentire evolution of a flare ribbon with high spatial (0.1 ′′ pixel − image scale), temporal (4.8 s), andspectral (0.1 ˚A sampling stepsize) resolution, which provides detailed morphological, dynamic,and spectral diagnostics of the flare.It is noteworthy that 3D spectroscopy (i.e., the x , y , l data cube is simultaneously captured inone exposure although with limited field-of-view and spectral sampling) provided by MultichannelSubtractive Double Pass (MSDP) imaging spectrograph (Mein 1991, 2002) has played an impor-tant role in very high temporal resolution (sub second) studies of H a spectra of flare kernels aswell as the spatial and temporal correlation between H a and HXR emissions (Radziszewski et al.2006, 2007, 2011).While the classical 2D flare model (Carmichael 1964; Sturrock 1966; Hirayama 1974; Kopp & Pneuman1976) can explain most of the well defined two-ribbon flares, some observed flares differ from thetypical two-ribbon ones, such as J -shaped ribbon flares, multi-ribbon flares, etc. Recent 3D numer-ical simulations that involve 3D coronal null point reconnection do expect flares having a closedcircular ribbon associated with a jet or a remote brightening occurring in a fan-spine magnetictopology (Masson et al. 2009; Pariat et al. 2010). However, observations of such circular-ribbon 5 –flares with a jet or a remote bright ribbon are very rare and have only been reported in a few papers(Ugarte-Urra et al. 2007; Masson et al. 2009; Reid et al. 2012; Wang & Liu 2012; Liu et al. 2013).The present paper adds another example of a confined circular-ribbon flare with a concurrent re-mote bright ribbon (SOL2011-10-22T15:20(C4.1)).
2. OBSERVATIONS
We carried out an observing campaign to study chromospheric jets from 2011 October 17 toOctober 23 in National Solar Observatory/Sacramento Peak using the IBIS instrument at the 76 cmDunn Solar Telescope that is equipped with high-order adaptive optics (AO) system (Rimmele & Marino2011). This ground-based observation was coordinated with space-based observation by the Hin-ode satellite (Kosugi et al. 2007). On October 22, we targeted active region NOAA 11324 (N11 ◦ E18 ◦ ) and captured the remote ribbon of a C4.1 flare SOL2011-10-22T15:20 from onset to decayduring a good seeing period.IBIS consists of two synchronized channels: a narrowband (0.022 ˚A FWHM around 6563 ˚A)channel with two tunable Fabry-P´erot interferometers (FPI) to take 2D spectral data and a broad-band ( ∼
100 ˚A) channel to take white-light reference images (at 6600 ˚A in our case) for calibrationand post-facto image processing purpose (Cavallini 2006; Reardon & Cavallini 2008). An H a pre-filter of 2.6 ˚A FWHM centered around 6562.8 ˚A was used in the narrowband channel to isolateone of the periodic transmission peaks of the FPIs. The IBIS with a round field-of-view (FOV) of90 ′′ diameter and 0.1 ′′ pixel − detector image scale repetitively scanned the H a line from 6561.1to 6563.8 ˚A (i.e., from − + a line center 6562.8 ˚A) using 28 equidistantsteps of 0.1 ˚A stepsize. The observing campaign was aimed at studying high-speed upflowing jets,so we sampled more wavelength points toward the blue wing. Each spectral scan took about 4.8 swith a frame rate of ∼ a spectra acquisition was never saturated throughout the observation.The Solar Optical Telescope (SOT; Tsuneta et al. 2008) of Hinode targeted the same regionduring the same time period. High resolution (0.16 ′′ pixel − , 64 s cadence) Na I D a and HXR emission during the flaresimpulsive phase can provide important clues on energy transport mechanisms. The evolution of 6 –the flare HXR emission was entirely registered by the Reuven Ramaty High Energy Solar Spectro-scopic Imager (RHESSI; Lin et al. 2002). CLEAN images (Hurford et al. 2002) in the nonthermalenergy range (12–25 keV) were reconstructed using the front segments of detectors 2–8 with 48 sintegration time throughout the event. The cadence of RHESSI X-ray light curves is 4 s, which issimilar to the cadence (4.8 s) of our H a line scans.To provide the observational context for the entire flare region, we also use LOS magne-tograms taken by the Helioseismic and Magnetic Imager (HMI; Schou et al. 2012) and imagestaken by the Atmospheric Imaging Assembly (AIA; Lemen et al. 2012) aboard the Solar Dynam-ics Observatory (SDO).
3. DATA REDUCTION
We first performed the standard calibration procedures for IBIS observations, which includedark and flat-field correction, alignment and destretch of images in each spectral scan using broad-band white-light images as a reference, and correction of blue shifts across the FOV because ofthe collimated mount of IBIS FPIs. We then aligned a 45-minute time sequence consisting of 550spectral scans that cover the entire flare duration, again using broadband white-light images as areference.To obtain the prefilter transmission curve that is superimposed on all observed spectral pro-files (see Figure 1), we simply divide an average flat-field line profile by a well registered pro-file cropped from the Kitt Peak FTS (Fourier Transform Spectrometer) atlas of the disk center(Neckel & Labs 1984; Neckel 1999) and smooth the resulting curve with a high degree polynomialfit. The flat-field spectral scans were taken at disk center with the telescope guider performing arandom motion. The average flat-field line profile has been obtained from the calibrated flat-fieldspectra after averaging over all flat-field scans taken in about 10 minutes and subsequently cor-recting for the systematic wavelength shift across the FOV. Finally, all the observed spectra arecorrected by dividing them with the acquired prefilter transmission curve. This procedure alsonaturally normalizes the observed H a spectra to the continuum intensity as shown in Figure 1.We display the observed H a spectra in two ways: original spectra I ( l , t ) and emission spectra D I ( l , t ) = I ( l , t ) − I ( l , t ) . The emission spectrum D I ( l , t ) is defined as the difference betweenan enhanced line profile during a flare (i.e., the original spectra I ( l , t ) ) and a reference line pro-file I ( l , t ) that is averaged in the ribbon area but before the onset of the flare. This kind ofdifference spectra has been widely used to study the net emission component, especially for emis-sions that are not strong enough to flip over the line profile (e.g., Canfield et al. 1990b; Ding et al.1995; Johns-Krull et al. 1997). The original spectra of optically thick chromospheric lines during 7 –flares are often complicated in shape and sometimes self-reversed. It is thus difficult and error-prone to deduce Doppler velocities from those original profiles using the bisector method (e.g.,Heinzel et al. 1994; Berlicki et al. 2005, 2006). Instead the net emission spectra are more regularin shape. Following previous studies (e.g., Canfield et al. 1990b; Ding et al. 1995), we derive bi-sectors (the middle points of horizontal chords intersecting the line profile at different levels) fromthe emission spectra to qualitatively illustrate the motion of H a emitting materials.
4. RESULTS AND ANALYSIS4.1. The Circular Flare and its Remote Brightening
Figure 2 and the accompanying online movie depict the entire C4.1 flare that occurred in ac-tive region NOAA 11324 based on SDO observations. This confined flare shows five conspicuousfootpoints (F1–F5) in the low chromosphere (AIA 1700 ˚A image). The flare consists of a main part(circular-like ribbon) in the east (F1–F4) and a remote brightening in the west (F5). The circular-shaped ribbon is most prominent in the AIA 335 ˚A image, where F1 sits inside the circle and fansout to the outer circle connecting F2–F4. In the magnetogram, F1–F4 form a so-called “parasiticmagnetic configuration”. The negative polarity F1 is encompassed by the positive polarities (F2–F4). Therefore a circular-like magnetic polarity inversion line (PIL) is naturally formed betweenF1 and F2–F4. F5 resides in a remote region outside of the parasitic configuration and sharesthe same polarity (i.e., negative polarity) as the parasitic F1. This kind of magnetic configurationseems to be a common feature for all observed circular flares involving 3D fan-spine reconnection(Ugarte-Urra et al. 2007; Masson et al. 2009; Reid et al. 2012; Wang & Liu 2012; Liu et al. 2013).The AIA 131 ˚A image clearly shows that the remote footpoint (F5) is connected to the circularflaring fan by a long spine loop.The RHESSI 12–25 keV light curve indicates that the C4.1 flare has two impulsive phasesfrom 15:17:00 to 15:20:40 UT. The first and second impulsive phases peak around 15:18 and15:20 UT, respectively. The left leg of F5 is brightened locally during the first impulsive phasethen the right leg of F5 shows brightening and fast expansion during the second impulsive phase.Interestingly, the circular-ribbon flare studied by Masson et al. (2009) and Reid et al. (2012) alsoindicated two impulsive phases in the HXR light curve. 8 – a Imaging Spectroscopy of the Flare’s Remote Ribbon
IBIS captured the remote ribbon (F5) of the circular flare and fully covered its temporal evo-lution. Figure 3 illustrates the impulsive-phase evolution of the brightening ribbon in narrowbandfiltergrams of five wavelengths spanning the H a line. The ribbon lies in the upper-middle of theIBIS FOV and close to the AO lock point (the small pore in the lower-middle of the panels). TheLOS magnetograms obtained by Hinode NFI provide the detailed magnetic field configuration.The entire ribbon (F5) resides in the negative polarity.The ribbon area appears largest and has the highest contrast in the H a line center imagesfrom which we outline the entire flaring area (blue contours) with an intensity threshold of 1.55times the quiet region intensity. Toward the line wings that form at progressively deeper layers, theribbon area and contrast decrease, finally only the most powerful flare kernels and ribbon frontsthat penetrate to the deepest layer can be seen. We define these strongest flare kernels that survivein the observed reddest (6563.8 ˚A, i.e., H a + . ′′ .As mentioned in Section 4.1, the dynamic morphology of the entire ribbon and the motions ofthe ribbon cores show a distinct two-stage evolution during the two impulsive phases correspond-ingly. During the first impulsive phase from ∼ ∼ ∼ ∼ − during the secondimpulsive phase. This jumping strong core can even be seen in the bluest wing image observed(6561.1 ˚A, i.e., H a − . − . The ribbon cores disappeared in the decay phase of the flare. Similar two-stage evolutionof flare ribbons has also been observed and studied in details for typical two-ribbon flares (e.g.,Moore et al. 2001; Qiu 2009)Figure 4 displays the IBIS observed H a spectra averaged over the flare ribbon halo and coreareas in two ways: original spectra and emission spectra, at a few times during the impulsive phase.The flare causes enhanced H a profiles which, however, are not strong enough to directly show theemission profile due to the small magnitude of the flare (C4.1). In this case, the defined emissionspectra (i.e., the difference spectra between the flaring enhanced spectra and the reference pre-flarespectrum) become more useful to reveal the net emission. Comparing the emission spectra in halo 9 –and core areas, we find that those in core areas are stronger and wider. Moreover, the emissionspectra in core areas show obvious central reversal signatures, which is not the case for those inhalo areas. The bisectors of emission spectra in core areas show clear redshifts in contrast to thosein halo areas. The redshifts of bisectors tend to increase from the line center to the line wing. TheH a emission is generally stronger in the red wing than in the blue wing, especially for the coreareas, giving rise to the red asymmetry. Combining all the observed morphological, dynamical, andspectral characteristics shown in Figures 3 and 4, it is reasonable to suggest that the cores representthe sites of intense nonthermal electron beam precipitation. Thus, the motion of cores tracksmagnetic reconnection processes involving different flux tubes. In contrast, the heating in the haloareas is most likely due to other mechanisms, such as thermal conduction and SXR/EUV radiation.Such a core-halo structure of flare ribbons was also found in Neidig et al. (1993) and Xu et al.(2006) for white-light flares based on continuum or H a wing images. The authors used directheating by nonthermal particle beam and indirect heating such as chromospheric back-warming tointerpret the white-light emissions in the core and halo areas, respectively.Examining the temporal evolution of H a spectra at a fixed position where a core once ap-peared can provide clues on the heating and cooling processes of the chromosphere which is di-rectly bombarded by energetic electrons. Figure 5 illustrates how the local spectra evolve at threefixed positions when a strong core moves into and leaves from the positions. The local emissionincreases as the core appears then decreases as the core disappears in those locations. Only whenthe core lies in the position, the emission becomes wider and centrally reversed. The redshifts ofemission spectra bisectors also tend to be largest then. During the core’s existence, the emissionin the red wing becomes stronger than that in the blue wing, which is more conspicuous in theoriginal spectra. The excess red-wing emission gives rise to the red asymmetry. All these spectralcharacteristics and their temporal evolution are consistent with previous studies of electron beamprecipitation sites where the intense heating of the chromosphere by nonthermal electrons causesexplosive evaporation in the upper chromosphere and consequently downward moving conden-sation in the lower chromosphere (e.g., Ichimoto & Kurokawa 1984; Canfield & Gunkler 1985;Canfield et al. 1990b, 1993; Lee et al. 1996).The light curves of H a emission in the three core positions are plotted in Figure 6 as a functionof time. They illustrate the heating and cooling time profile of the regions bombarded by energeticelectrons. The maxima of these light curves prior to fast cool-down are denoted with black boxes.It is interesting that these maxima occurred at different locations correlate well with the sub-peaksof the RHESSI HXR light curve with only a slight delay. This implies that the sub-bursts of HXRmay correspond to different locations. The multiple peaks at the location of core 1 suggest that onelocation can also be bombarded for several episodes. Amongst the three core locations, the locationof core 2 shows the fastest heating and cooling time profile, because the core moved quickly andstayed there for only short time (less than 30 s). In contrast, the core persisted in location 1 for 10 –more than one minute, so the heating timescale is not clear for the location of core 1. From thelight curves (the shaded areas), we estimate that the heating times for the locations of core 2 and3 are about 24 and 33 s, respectively. The cooling times (the time taken for the emission intensityto decrease to half of the maximum value) for the three core positions are about 33, 14, and 24 s,respectively. a and HXR Emission As shown in the top two panels of Figure 7, we construct dynamic spectra using a time seriesof IBIS observed H a original and emission profiles that are spatially averaged over the entirebrightening ribbon. Both types of dynamic spectra show two major H a emission phases that arecotemporal with the two impulsive phases of the HXR light curve. Embedded in the two majorimpulsive phases, several episodes of H a emission can be seen that seem to correlate with thesub-peaks of HXR emission.For a quantitative comparison between H a and HXR emission, we compute the time profilesof the H a emission intensity based on the dynamic emission spectra. The H a emission spectrumat each time is averaged symmetrically over the red wing (6562.8–6563.8 ˚A), the entire line core(6561.8–6563.8 ˚A), and the blue wing (6561.8–6562.8 ˚A), giving the light curves for the threewavelength bands shown in the 3rd panel. These light curves clearly show that the H a emissionin the red wing exceeds that in the blue wing during the impulsive phase by about 15%. Otherthan that, the temporal variations for the three wavelength bands are quite similar. The H a emis-sion light curves closely resemble that of the HXR emission in the rising and impulsive phases,but decay slower during the gradual (decaying) phase, where they resemble more that of the SXRemission that originates from hot coronal loops. This kind of behavior is consistent with the knowncharacteristics of flare optical emissions (e.g., Zirin & Tang 1990). It also implies that in the im-pulsive phase the H a emission is dominated by nonthermal electron beam heating, while in thegradual phase it is mainly due to coronal thermal conduction and/or radiation. Our observationshows a very high temporal correlation ( & .
9) between H a and HXR emission during the im-pulsive phase, with higher correlation in the red wing emission than in the blue wing emission.Note that the RHESSI HXR light curve is measured from the entire flaring region, while the H a emission is only measured from the remote ribbon of the flare. Their close temporal correlationimplies that the remote brightening is actively involved in the 3D reconnection process and directlyheated by reconnection-accelerated precipitating electron beams.Both HXR and H a emission light curves show short timescale fluctuations (spikes or episodes)superposed on the general evolution curve. These fast fluctuations are likely signatures of elemen-tary reconnection and energy transfer processes. Seeking the correlation between the HXR and H a
11 –emission fluctuations can provide clues on their emission timings and the chromospheric heatingmechanism during a flare (Wang et al. 2000). The 4th panel of Figure 7 illustrates the fluctuationsof H a and HXR emission by removing their general evolution curves. Shifting the H a fluctu-ations back and forth, we find that the correlations between H a and HXR fluctuations reach aunique maximum value (linear correlation = 0.48) when the H a fluctuations curve is shifted for-ward by 4.6 s. This provides a quantitative assessment of the time delay of H a emission relativeto HXR emission. At the current temporal resolution (HXR 4 s, H a a emission with re-spect to HXR emission have been statistically studied by Radziszewski et al. (2011) for many flarekernels and their sub-bursts. The authors found two distinct groups of time delays. The short de-lay (1–6 s) group is associated with fast heating by nonthermal electrons, while the longer delay(10–18 s) group is ascribed to a slower energy transfer mechanism such as a moving conductionfront (Trottet et al. 2000). Our observed 4.6 s delay falls into the short delay group, thus reinforc-ing the electron beam heating mechanism for the remote brightening during the impulsive phase.Wang et al. (2000) attributed their observed 2–3 s time delay between the H a blue wing and HXRbursts in the early impulsive phase of a C5.7 flare to the ionization timescale of the cool chromo-sphere (Canfield & Gayley 1987). The time lags due to temperature structure evolution, hydrogenionization and emission timescales, and energy deposit rate as proposed by Kaˇsparov´a et al. (2009)may explain the delay of H a bursts relative to HXR bursts in our observation. In addition, the longspine loop for the electrons to travel from the 3D null point reconnection site (most probably some-where in the corona above the main circular ribbon) to this remote chromospheric precipitation sitemay also contribute to the time delay, since the majority of HXR emission comes from the maincircular ribbon site. We estimate the spine loop length to be about 100 ′′ from Figure 2. The speedof nonthermal electrons of 17 keV is about a quarter of the speed of light. So it will take thoseelectrons about 1 s to travel along the spine loop.
5. SUMMARY AND PERSPECTIVE
We present unprecedented 2D H a imaging spectroscopy of a circular flare’s remote ribbonobserved by IBIS with high spatial (0.1 ′′ pixel − image scale), temporal (4.8 s cadence), andspectral (0.1 ˚A stepsize) resolution. The main circular ribbon part of the flare occurred in a parasiticmagnetic configuration, which seems to be a common feature for all observed circular-ribbon flaresaccompanied by a jet and/or a remote brightening involving a 3D fan-spine magnetic topology.We defined “core” and “halo” structures in the impulsive-phase flare ribbon based on nar-rowband images in the H a line wing and line center. Only the emission spectra in the core ar- 12 –eas showed typical spectral characteristics of theoretically simulated H a line profiles of flaringchromosphere heated by nonthermal electron beam precipitation. These spectral characteristicsinclude: broad and centrally reversed emission spectra, excess emission in the red wing than in theblue wing (i.e., red asymmetry), and redshifted bisectors of the emission spectra. In addition, weobserved rather quick heating ( ∼
30 s) and cooling ( ∼ − ) between discrete magnetic elements implying reconnectioninvolving different flux tubes. The cores disappeared in the decay phase of the flare. The heatingof the halo is more likely due to thermal conduction and/or SXR/EUV radiation.Our observation shows a very high temporal correlation ( & .
9) between the integrated H a and HXR emission light curves during the flare impulsive phase, with higher correlation in the redwing emission than in the blue wing emission. Further comparison of the sub-bursts between theH a and HXR emission light curves revealed a 4.6 s time delay in the H a emission compared tothe HXR emission. The short time delay reinforces the electron beam heating mechanism for theremote ribbon of the circular flare during the impulsive phase. The ionization timescale of the coolchromosphere and the long spine loop for the electrons to travel may contribute to the short delayin the H a emission of the remote flare ribbon with respect to the HXR emission of the entire flare.The sub-bursts of HXR/H a emission may correspond to different locations. One location can alsobe bombarded for several episodes.Besides IBIS, other FPI-based 2D imaging spectrometers are either being operated or com-missioned around the world, such as the Telecentric Etalon SOlar Spectrometer (TESOS; Kentischer et al.1998; Tritschler et al. 2002) at the 75 cm Vacuum Tower Telescope (VTT) in Tenerife, the CRispImaging SpectroPolarimeter (CRISP; Scharmer et al. 2008) at the 1 m Swedish Solar Telescope(SST), the GREGOR Fabry-P´erot Interferometer (GFPI; Denker et al. 2010; Puschmann et al. 2012)at the 1.5 m GREGOR telescope in Canary Islands, and the Fabry-P´erot system at the 1.6 m NewSolar Telescope (NST) in Big Bear Solar Observatory (BBSO). Moreover, a spectral imager basedon two FPIs for studying the lower solar atmosphere is planned at the focal plane of the 50 cmtelescope onboard the ADAHELI (ADvanced Astronomy for HELIophysics) mission, an ItalianSpace Agency small satellite mission (Berrilli et al. 2010). All these instruments are anticipatedto provide valuable spectroscopy or spectropolarimetry with high spatial, spectral, and temporalresolution for scrutinizing flares and other highly dynamic or small-scale phenomena on the Sun. 13 –This work was supported by NASA under grants NNX08AQ32G, NNX11AQ55G, NNX13AF76G& NNX13AG13G and by NSF under grants AGS 0936665, 1153226, 0839216 & 1153424. CDwas supported by grant DE 787/3-1 of the Deutsche Forschungsgemeinschaft (DFG). We thank Dr.Etienne Pariat for carefully reading the paper and providing valuable comments. IBIS is a projectof INAF/OAA with additional contributions from Univ. of Florence and Rome and NSO. We thankthe NSO/SP observers (Doug Gilliam, Joe Elrod, and Mike Bradford) for their professional and ex-cellent observing support. We also appreciate the great support from Hinode and communicationswith Dr. Toshifumi Shimizu and other Hinode team members before and during the successfulIBIS-Hinode coordinated observing campaign. The National Solar Observatory is operated by theAssociation of Universities for Research in Astronomy under a cooperative agreement with theNational Science Foundation, for the benefit of the astronomical community. Hinode is a Japanesemission developed and launched by ISAS/JAXA, with NAOJ as domestic partner and NASA andSTFC (UK) as international partners. It is operated by these agencies in co-operation with ESAand NSC (Norway). This research has made use of NASA’s Astrophysics Data System (ADS). REFERENCES
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This preprint was prepared with the AAS L A TEX macros v5.2.
19 – O b s er v e d H a L i n e P r o f il e H a P re f il t er C u r v e C o rrec t e d H a L i n e P r o f il e Fig. 1.— From the top down: An observed H a line profile with prefilter transmission curvesuperposed, the H a prefilter transmission curve for our observation, and the corrected H a lineprofile with prefilter transmission curve removed and normalized to the continuum intensity. Thediamond symbols represent the IBIS spectral sampling of our observation. The yellow diamondsymbols highlight several wavelengths that will be used to display the narrowband filtergrams inFigures 3 and 5. 20 – -6 -6 -6 -6 [ W a tt s m - ] -6 -6 -6 -6 GOES 1-8 A SXRRHESSI 3-6 keVRHESSI 6-12 keVRHESSI 12-25 keV C oun t R a t e [ s - de t - ] Y ( a r cs e cs ) -400 -350 -300X (arcsecs)50100150 Y ( a r cs e cs ) -400 -350 -300X (arcsecs)
94A (6.3MK) 15:19:50 UT -400 -350 -300X (arcsecs) Y ( a r cs e cs ) LOS HMI 15:19:16 UTF2 F1F3 F4 F5
Fig. 2.— Upper Panel: GOES and RHESSI X-ray light curves of the C4.1 flare. Lower Panels:SDO HMI and AIA images show the magnetic field configuration and flare emissions at differenttemperatures (i.e., different atmospheric heights) around 15:20 UT when the HXR (RHESSI 12–25 keV) light curve has a second peak during the impulsive plateau. The RHESSI contours areplotted on the 94 ˚A image. The green contour on the HMI image indicates the circular-like PIL ofthe parasitic magnetic configuration. F1–F5 denote the 5 flare footpoints in the low chromosphere.An animation of this figure is provided as online material to depict the complete evolution of theflare. 21 – H a −1.1 Å6561.7 H a −0.7 Å6562.1 H a H a +0.6 Å6563.4 H a +1.0 Å6563.8
10" B
LOS
Fig. 3.— IBIS narrowband filtergrams show the evolution (top to bottom) of the flare remoteribbon (F5 in Figure 2) at different wavelengths (left to right) crossing the H a line. Only a 30 ′′ × ′′ sub-region in the upper-middle of the IBIS FOV is shown. The right-most column showscorresponding Hinode NFI Na I D LOS magnetograms. The blue dotted contours encompassthe entire brightening ribbon appearing in the H a line center image (6562.8 ˚A). The magentasolid contours outline the most powerful flare “core” structures that are obtained by selecting thebrightest areas in the red wing image at 6563.8 ˚A (i.e., H a + . a lineprofile. The wavelengthes are labeled on top of each narrowband image. 22 – M e a n H a l o Sp ec t r a I ( l , t) M e a n H a l o E m i ss i o n D I ( l , t) M e a n C o re Sp ec t r a I ( l , t) M e a n C o re E m i ss i o n D I ( l , t) Fig. 4.— The H a original and emission spectra averaged in the flare ribbon halo (blue curves)and core (magenta curves) areas at several times during the flare impulsive phase. The blackdotted profiles plotted along with original enhanced spectra are the reference H a line profile thatis averaged over the ribbon area but before the flare. The difference between the original enhancedspectra due to flare and the reference spectrum gives the emission spectra. The vertical dash-dottedline marks the H a line center at rest. The red diamond symbols are the bisectors of the emissionspectra. The morphology of the ribbon at each time is shown on the top with H a line center(6562.8 ˚A) images. 23 – M e a n Sp ec t r a I ( l , t) M e a n E m i ss i o n D I ( l , t) M e a n Sp ec t r a I ( l , t) M e a n E m i ss i o n D I ( l , t) M e a n Sp ec t r a I ( l , t) M e a n E m i ss i o n D I ( l , t) C o re C o re C o re Fig. 5.— The temporal evolution of H a original and emission spectra averaged in three fixedpositions (the colored boxes) where a strong core passes by. We simply name the three corepositions as Core 1, 2, and 3. The red wing images at 6563.6 ˚A (i.e., H a + . H a E m i ss i o n I n t e n s i t y Core 1 Core 2 Core 3 R H E SS I − k e V F l u x ( ph o t o n s s − c m − k e V − ) Fig. 6.— The light curves of H a emission intensity in the three core positions defined in Figure 5showing the heating and cooling time profiles. The emission intensity is averaged over the entireH a line observed. As comparison, the RHESSI HXR (12–25 keV) light curve is also plotted inmagenta. 25 – H a Dynamic Spectra av e l e n g t h [ Å ] H a Dynamic Spectra av e l e n g t h [ Å ] Spectra Intensity I/I C GO E S . − . k e V F l u x ( W a tt s m − ) R H E SS I − k e V F l u x ( ph o t o n s s − c m − k e V − ) H a Emission Dynamic Spectra av e l e n g t h [ Å ] H a Emission Dynamic Spectra av e l e n g t h [ Å ] Emission Intensity D I/I C GO E S . − . k e V F l u x ( W a tt s m − ) R H E SS I − k e V F l u x ( ph o t o n s s − c m − k e V − ) H a E m i ss i o n A v er ag e d i n E n t i re R i bb o n Correlation = 0.96Correlation = 0.94Correlation = 0.89 H a Emission Averaged in Red WingH a Emission Averaged in Entire LineH a Emission Averaged in Blue Wing GO E S . − . k e V F l u x ( W a tt s m − ) R H E SS I − k e V F l u x ( ph o t o n s s − c m − k e V − ) H a E m i ss i o n Sp i k e s Correlation = 0.48H a Delay Time = 4.60 s −10−50510152025 R H E SS I − k e V F l u x Sp i k e s ( ph o t o n s s − c m − k e V − ) Fig. 7.— Top two panels: H a dynamic spectra and emission dynamic spectra, i.e., color-coded H a original or emission spectrum (along the y -axis) as a function of time. Each spectrum is spatiallyaveraged over the entire brightening ribbon. The H a line center at rest is marked with a white line.The GOES SXR and RHESSI HXR light curves are overplotted as purple and magenta curves. 3rdpanel: light curves of H a emission averaged symmetrically over the red wing, the entire line core,and the blue wing are compared with GOES and RHESSI light curves. The correlation coefficientsbetween H a and RHESSI HXR light curves within the impulsive time interval (inbetween greenvertical lines) are labeled. 4th panel: the light curves of H aa