Identification of Pre-flare Processes and Their Possible Role in Driving a Large-scale Flux Rope Eruption with Complex M-class Flare in the Active Region NOAA 12371

morp The AR NOAA 12371 appeared on the eastern limb of the Sun during the last hours of 15 June 2015 as a moderately complex $\beta$-type sunspot. Flaring activity from the AR was noted to start from 16 June 2015. Interestingly, despite producing numerous B and C class flares alongside few M-class ones, it did not produce any X-class flare. By 18 June 2015, it gradually became a more complex $\beta\gamma$-type sunspot (Figure \irefar_eva) and produced its first M-class flare on the same day. The AR evolved into the most complex $\beta\gamma\delta$-type (Figure \irefar_evb) on the next day, i.e. 19 June 2015. The highest flaring activity from the AR was observed during 20–22 June 2015. In this duration, it produced 3 M-class flares along with many C and B-class flares. The AR started to decay after 22 June 2015. In Table \ireftable1, we summarise various evolutionary aspects of NOAA 12371 during its disk passage during 18–25 June 2015 which are collected from the NOAA Solar Region Summary (SRS) reports¹¹1see https://www.swpc.noaa.gov/products/solar-region-summary.. The magnetic configuration of the AR reduced to $\beta\gamma$-type on 25 June 2015 (Figure \irefar_evh) and even further to $\beta$-type on 29 June 2015 just before it went on to the far side of the Sun from the western limb. During its lifetime on the visible hemisphere of the Sun, it produced a total of 5 M-class flares which are also listed in Table \ireftable1. Notably, the AR produced the largest flare of class M7.9 on 25 June 2015 when it had entered into the declining phase. However, in terms of space weather manifestations, the most interesting eruptive flare from the AR occurred on 21 June 2015. This event followed the characteristics of a long duration flare with peak GOES flux reaching a level of M2.6 and a fast CME²²2see https://cdaw.gsfc.nasa.gov/CME_list/index.html. with linear speed of 1366 km s^-1.

pfmg To have a comprehensive understanding of the photospheric magnetic activities during the prolonged phase of energy build up and subsequent flaring activity on 21 June 2015, we have thoroughly examined large as well as small-scale changes in the LOS magnetograms. During this period, the AR was consisted of two major sunspot groups (Figure \irefar_phota). A comparison of the white light image of the AR with a co-temporal magnetogram (cf. Figures \irefar_phota and b) reveals that almost all of the leading sunspot group of the AR was consisted of negative polarity while the trailing sunspot group was of mixed polarities forming $\delta$-type configuration. In this manner, the overall photospheric configuration of magnetic polarities of the AR made it a $\beta\gamma\delta$ sunspot region.

In Figure \irefhmi_lightcurve, we plot the evolution of the LOS magnetic flux through the whole AR as well as only from the trailing sunspot region from 20 June 2015 12:00 UT to 21 June 2015 05:00 UT. The trailing sunspot group has been shown within the box in Figure \irefhmi_lightcurvea. Notably, the eruptive flare under investigation was triggered from this region. The evolution of the trailing sunspot group of the AR can be inferred by comparing Figures \irefhmi_lightcurveb1 and \irefhmi_lightcurveb2 which present enlarged view of the selected region at two different times. The area marked by dashed circles shows rapidly evolving magnetic elements and moving magnetic features (MMFs) which are discussed in the next subsection. From Figure \irefhmi_lightcurvec, we readily find that both the positive and negative flux from the AR increased during $\approx$13:30 UT–23:00 UT on 20 June 2015 and thereafter decreased till the onset of the flare at $\approx$01:05 UT on 21 June 2015. Based on the flux variations, we have divided the whole interval in two phases: “phase 1”; when the active region displayed flux enhancement in both the polarities, and “phase 2”; when flux of both polarities decayed. Evolution of magnetic flux in the trailing sunspot group, (i.e. the flaring region) displayed similar variation as the entire AR (Figure \irefhmi_lightcurved). In phase 1, flux of both polarities increased; however, increase of positive flux was more than that of negative flux. Flux of both polarities decayed at similar rate during phase 2. To have an understanding of flaring activity in the AR, we have plotted the variation of GOES SXR flux along with AIA 94 Å light curve from 20 June 2015 12:00 UT to 21 June 2015 05:00 UT in Figure \irefhmi_lightcurvee. We find that, during the selected interval, there was no appreciable enhancement of SXR and EUV fluxes prior to the onset of the M-class flare on 21 June 2015 at $\approx$01:05 UT.

The AR NOAA 12371 underwent significant morphological changes prior to the reported flare which include MMFs, emerging/cancelling flux elements, and sunspot rotation. To highlight these features, we have identified two sub-regions in the AR with most prominent photospheric changes which are marked by the red and blue coloured boxes in Figure \irefar_photb. The leading sunspot group of the AR (inside the red box in Figure \irefar_photb) was associated with a very interesting display of morphological evolution (see the animation associated with Figure \irefar_phot). We note that, the extension of the leading sunspot group along the east-west direction increased from $\approx$55^′′ at 20 June 2015 12:00 UT to $\approx$65^′′ prior to the onset of the flare. During the same time, the north-south extension of the leading sunspot group decreased from $\approx$40^′′ to $\approx$35^′′ (cf. Figures \irefmag_rota and f). Further, a small circular element of negative polarity (indicated by the blue arrows in Figure \irefmag_rot) exhibited continuous motion along the southern boundary of the leading sunspot group. The diagonal expansion of the region alongside the motions of the small patch suggest “clockwise rotation” of the overall leading sunspot group. We also identify a distinct patch of small magnetic region (indicated by the red arrows in Figure \irefmag_rot) which, after initially exhibiting constant converging motion toward the major sunspot, merged with it during the final hours of 20 June 2015 (Figure \irefmag_rote).

In Figure \irefmag_mov, we show the evolution of the region inside the blue box (Figure \irefar_photb) where we have indicated two particular features by red and green arrows. The red arrows indicate a striking MMF element of negative polarity which also shows significant morphological changes (see animation with Figure \irefar_phot). The green arrows mark persistent cancellation of a positive flux element as the negative MMF moves toward north-west direction.

overview The temporal evolution of the M-class flare on 21 June 2015 is depicted by the flux variation in the Geostationary Operational Environmental Satellite (GOES) 1–8 Å and 0.5–4 Å channels which are plotted between 21 June 2015 00:00 UT and 05:00 UT in Figure \ireflightcurvea. The GOES profiles suggest that the eruptive flare evolved in two phases. According to the GOES 1–8 Å time profile, the event started at $\approx$01:20 UT while the peaks of the two subsequent episodes of energy release were recorded at 01:42 and 02:36 UT during which the flux rose to the levels of M2.1 and M2.6, respectively. Further, the flare was associated with two brief pre-flare flux enhancements at $\approx$01:05 and $\approx$01:14 UT (indicated by the dashed and dotted lines, respectively, in Figure \ireflightcurve). Profile of GOES 0.5–4 Å channel clearly shows that, while the flux enhancement during the first pre-flare peak was quite impulsive and short-lived, it was relatively gradual during the second pre-flare peak. Based on the temporal and spatial characteristics (discussed in the next subsection) of the episodic pre-flare enhancements, we refer them as SXR precursors.

In Figure \ireflightcurveb, we display the intensity variation of AIA (E)UV channels on 21 June 2015 from 00:00 UT to 05:00 UT. We note that, none of the AIA intensity profiles exhibited enhancement during the first GOES precursor while a subtle enhancement was observed during the second precursor in the AIA 304 and 171 Å channels (cf. the dashed and dotted lines in Figure \ireflightcurvea and b). Intensity in all the AIA channels, except the 94 Å channel, displayed a sharp peak at $\approx$01:36 UT (indicated by the solid line in Figure \ireflightcurveb) while the second peak in these AIA channels was rather gradual. The intensity variation in the AIA 94 Å channel diverged from other AIA channels in the timing and extended duration of the peaks. Interestingly, the AIA 94 Å channel displayed three distinct peaks, first and second ones of which were consistent with the GOES M2.1 and the GOES M2.6 peaks, respectively. The highest of the three AIA 94 Å peaks occurred during the gradual phase of the M-class flare. The flare moved into the gradual phase after $\approx$03:00 UT in all the AIA (E)UV and GOES SXR channels.

flare In Figure \ireffl94, we plot a series of AIA 94 Å images displaying the AR NOAA 12371 during different phases of the M-class flare. We readily observe the presence of a prominent hot channel at the core of the AR during the pre-flare phase (indicated by the yellow arrow in Figure \ireffl94b). Comparison of the location of the hot channel with the HMI LOS magnetogram contours in Figure \ireffl94a confirms that the hot channel was lying over the PIL in the trailing sunspot of the AR. After $\approx$00:52 UT, we observed a localized yet prominent brightening from a location near to the hot channel (indicated by the red arrow in Figure \ireffl94b). We note that, hard X-ray (HXR) emission of energies up to $\approx$25 keV originated from this location of pre-flare EUV brightenings. The examination of series of AIA 94 Å images suggest that the brightness of this localized region initially increased up to $\approx$01:05 UT and then decreased till $\approx$01:10 UT before increasing again. These pre-flare episodic brightenings observed in AIA 94 Å images are exactly co-temporal with the GOES SXR precursors observed at $\approx$01:05 UT and $\approx$01:14 UT (cf. Figure \ireflightcurvea). A very interesting phenomena was observed around $\approx$01:28 UT in terms of anti-clockwise motion of brightness depicting a narrow semicircular path from the northern end of the region of precursor brightening to the northern leg of the hot channel. This moving flash in indicated by the red arrows in Figures \ireffl94c and d. The flare entered into the impulsive rise phase by $\approx$01:20 UT as the hot channel got activated. During this time, we noted HXR emission of energies up to $\approx$25 keV predominantly from the northern part of the hot channel. The progression of brightness from the adjacent precursor region to the northern leg of the hot channel was immediately followed by eruption of the hot channel, i.e. the eruption was triggered. In Figure \ireffl94d, we indicate the direction of the hot channel eruption by the blue arrows. The eruption phase was followed by formation of post flare arcade in the trailing sunspot (Figures \ireffl94e and f) which are associated with HXR emission of energies up to $\approx$25 keV. A second phase of the eruption was observed between $\approx$01:53 UT and $\approx$02:05 UT followed by further restructuring of the AR loops at even larger scales, as inferred from the formation of large post flare arcade connecting the trailing sunspot with the leading sunspot. The large post-flare arcade was associated with strong diffused emission till $\approx$02:40 UT (Figure \ireffl94g) when the flare reached its second peak. The active region did not show any significant morphological change afterwards till the end of our studied period except the brightness of the large post flare arcade slowly reduced as the flare had moved into the gradual phase (Figures \ireffl94h, i). The above observations led Joshi et al. (2018) to conclude that two distinct phases of magnetic reconnection occurred successively at two separate locations and heights of the AR corona in the wake of single, large hot channel eruption. Based on the temporal and spatial proximity of the two distinct energy release phases along with spectral characteristics of emission, Lee et al. (2018) has termed the two-phases of energy release as the signature of a “composite flare” triggered by the flux rope eruption.

To have a further clarification on the triggering of the hot channel eruption, we selected three slits and computed time-slice diagrams along them (Figure \ireftriggering). These time-slice diagrams can be effectively used to observe the time evolution of plasma eruption and brightness progression along the selected slits (see also the animation associated with Figure \ireftriggering). From Figure \ireftriggeringb, we find that the motion of brightness (apparent signatures of slipping reconnection) from the precursor location started at $\approx$01:26 UT. We have highlighted the motion of the brightness in Figure \ireftriggeringb by a dotted curve. The estimated time of the arrival of the brightness at the T₂ point is indicated by the dashed vertical line in Figures \ireftriggeringb–c. From Figures \ireftriggeringc and d, it becomes evident that the eruption started immediately after the progression of brightness reached the core of the AR, i.e. the eruption was triggered by the processes linked with the moving brightness.

In Figure \ireffl304, we display a series of AIA 304 Å channel showing the evolution of the AR during the M-class flare. A filament (marked by the blue arrow in Figure \ireffl304b) was observed to lie along the PIL in the trailing sunspot region (cf. Figure \ireffl304b with HMI LOS contours in Figure \ireffl304a) which is co-spatial with the location of the hot channel observed in the AIA 94 Å channel images (cf. Figures \ireffl304b and \ireffl94b). The adjacent precursor activity was observed in the AIA 304 Å images also (Figure \ireffl304b and c). During the impulsive phase of the M-class flare, a clear set of flare ribbons formed in the trailing sunspot region (indicated by the arrows in Figure \ireffl304d). As expected from the standard flare scenario, the separation between the two ribbons increased, albeit rather slowly, with time and a dense post flare arcade was eventually formed connecting the two ribbons (Figures \ireffl304d–h).

preflare From AIA EUV images (Figures \ireffl94 and \ireffl304) it is clearly understood that the earliest flare brightening occurred at the west of the pre-existing hot channel, (i.e. MFR) which evolved with time but remained within a localized region. As discussed earlier, we identify this brightening as GOES SXR precursor (see Figure \ireflightcurvea). This was followed by the activation and eruption of the hot channel as a sequence of activities. We recall that this region region of precursor brightening was associated with rapidly evolving dispersed magnetic field of positive and negative polarities in which MMFs were observed besides emergence and cancellation of magnetic flux (Figure \irefmag_mov). The precursor activities and its relation to the small-scale magnetic field changes are further analysed in Figure \irefact. In Figure \irefacta, we highlight the region of precursor brightening over the HMI magnetogram by the box and show co-temporal overplots of the AIA 94 Å images with the magnetograms in Figures \irefactb1–b3. We identify several instances of flux emergence and cancellation of both polarities which are indicated by the arrows of different colors and the boxes in Figures \irefactb1–b3. In Figure \irefactc, we show the evolution of LOS magnetic flux from the region shown within the box in Figure \irefacta on 21 June 2015 from 00:00 UT till the onset of the impulsive phase of the flare at $\approx$01:20 UT. We find that magnetic flux of both positive and negative polarities exhibited episodic increase and decrease from the region, further implying significant small-scale flux variations. Notably, the overall trend of positive flux in this region displayed a slow decay while the negative flux increased initially up to $\approx$00:25 UT and decayed gradually thereafter.

ph_curr_fe Electric current density on the photosphere, being a direct consequence of flux emergence and decay as well as photospheric motions, is expected to provide important insights toward understanding the onset of flares. The longitudinal component of current density ($j_{\textrm{z}}$) on the photosphere can be calculated from horizontal components of magnetic field ($B_{\textrm{x}}$ and $B_{\textrm{y}}$) using the Ampere’s law (Tan et al., 2006; Kontogiannis et al., 2017):

From current density ($j_{\textrm{z}}$), we derive current ($I_{\textrm{z}}$) by multiplying $j_{\textrm{z}}$ with the area of one pixel, i.e. $\approx$13.14$\times$10¹⁰ m². In Figure \irefj_var, we plot the temporal evolution of average $I_{\textrm{z}}$ for the overall active region (Figure \irefj_varb) as well as the location of precursor brightening (Figure \irefj_varc). From the spatial distribution of $I_{\textrm{z}}$ prior to the hot channel activation (Figure \irefj_vara), we find large concentration of positive and negative currents along the narrow strip delineated by the PIL with the maximum and minimum values of $I_{\textrm{z}}$ in the active region being 2.09$\times$10¹⁰ A and -2.24$\times$10¹⁰ A, respectively. It is noteworthy that the region displaying precursor brightenings in the corona and underlying MMFs in the photosphere exhibited a complex distribution of $I_{\textrm{z}}$. We have indicated this region in the box in Figure \irefj_vara and identify this as the triggering region. We find that in the overall AR, both positive and negative currents experienced a slow variation. Both the positive and negative components of $I_{\textrm{z}}$ slowly increased during the pre-flare phase of the flare and decreased once the flare onset took place.

In Figure \irefj_img, we show the evolution of the spatial distribution of $I_{\textrm{z}}$ in the triggering region of the AR (within the box in Figure \irefj_vara). For convenience, we have plotted GOES SXR light curves in Figure \irefj_imga where the timings of the Figures \irefj_imgb–g are indicated by the vertical lines. We find that, in the triggering location, small-scale regions of both positive and negative $I_{\textrm{z}}$ were mostly distributed randomly. However, an interesting structure of the shape “A” formed by $I_{\textrm{z}}$ was very clear in the region during the pre-flare phase (outlined by pink-black dashed lines in Figure \irefj_imgb). In the “A” shaped distribution, the left arm was completely made of negative $I_{\textrm{z}}$ while the right arm was consisted of the positive $I_{\textrm{z}}$ in the northern part and negative $I_{\textrm{z}}$ in the southern part. The connecting part of the arms in the “A” was consisted of positive $I_{\textrm{z}}$. During the SXR precursor enhancement, in the northern tip of the structure (outlined by the oval shape in Figures \irefj_imgb–g), $I_{\textrm{z}}$ of opposite polarities became very close to each other (Figures \irefj_imgb and c) which may be ideal for dissipation of current in the form of magnetic reconnection. As Figures \irefj_imgd–g suggest, with the evolution of the flare, strength of $I_{\textrm{z}}$ at the tip of the “A” significantly decreased and the left arm of the “A” became fragmented (see region inside the box in Figures \irefj_imgb–g). We spotted another interesting feature from Figures \irefj_imgb–g in the form of appearance and decay of a positive current region which we indicate by the black arrows.

sec_extrp In order to understand the coronal connectivities between the photospheric magnetic polarities of the AR, we performed a non-linear force free field (NLFFF) extrapolation. For the purpose, we selected an HMI magnetogram at 01:00 UT on 21 June 2015 which represents the photospheric configuration prior to the onset of eruption (Figure \irefnlfffa). In Figures \irefnlfffb and c, we display different sets of NLFFF lines associated with the trailing sunspot group, from top and side views, respectively. The NLFFF extrapolation results readily suggest the presence of an extended flux rope (shown by sky-coloured lines) over the PIL which was enveloped by a set of low coronal closed loops (shown by blue lines) connecting the opposite polarity regions of the trailing sunspot group. Importantly, the modelled flux rope is situated at the same location where the hot coronal channel was identified in the AIA images (cf. Figure \irefnlfffb and \ireffl94a). The coronal configuration associated with the precursor brightening region is displayed by the pink lines in Figure \irefnlfff. Further, we identified a set of closed field lines (shown by the yellow lines) connecting the opposite polarity regions in the trailing sunspot group, a part of which are situated at a very close proximity to the pink lines (see Figure \irefnlfffc). Notably, the anti-clockwise motion of the brightness from the precursor location, clearly revealed by the time-slice diagram shown in Figure \ireftriggeringa, matches reasonably well with the footpoints of the yellow lines. We also note that, a few of the yellow and pink lines displayed a drastic change in the field-line linkage. In literature, such structures are termed as quasi-separatrix layers (QSLs) and are characterised by high squashing factor (Q; see, Priest and Démoulin, 1995). The Q value of this region is found to be higher than 10⁹ (shown by the neon-green coloured patch and the black arrow in Figure \irefnlfffc).

To have a further insight of the anti-clockwise motion of the brightening prior to the eruption of the hot channel, we show the photospheric regions with a Q value greater than 10⁷ by red colour in Figures \irefnlfffb–e. We find that, the footpoints of the yellow lines perfectly match with regions of high Q values (Figure \irefnlfffc) which implies that the anti-clockwise motion of brightness from the precursor location was a slipping reconnection (see e.g., Démoulin et al., 1997; Craig and Effenberger, 2014; Janvier et al., 2016).

In Figures \irefnlfffd and e, we display the coronal connectvities in the whole AR from top and side views, respectively. We note the presence of large coronal loops that connected the positive regions of the trailing sunspot group to the leading negative sunspot group (shown by green lines in Figure \irefnlfffd–e). Also, a part of the green lines originating at the positive polarity region were connected with the adjacent negative polarity region of the same sunspot group. These two sets of green lines constituted a second QSL (log(Q)$>$8) which we have indicated by a black arrow and the blue coloured patch in Figure \irefnlfffd. The observations of spatial progression of brightness within the same region (Figures \ireffl94f–g, \ireffl304f–h and animation associated with Figure \ireflightcurve) are consistent with the scenario of slipping reconnection. Interestingly, the same region of high Q value was associated with a large-scale slipping reconnection event during the M-class flare on 22 June 2015 (Jing et al., 2017) which implies that the large-scale magnetic structure associated with the active region remained preserved on the next day despite the eruption of a halo CME on 21 June 2015.

sec_fe The magnetic free energy ($E_{\textrm{F}}$) associated with an AR is important in order to understand the energy budget of the flares originated from that AR. $E_{\textrm{F}}$ can be estimated by the formula:

where $E_{\textrm{N}}$ and $E_{\textrm{P}}$ are non-potential energy and potential energy, respectively. We have calculated magnetic energy stored in the active region NOAA 12371 by employing the magnetic virial theorem (Klimchuk, Canfield, and Rhoads, 1992). According to this theorem, the magnetic energy stored in a coronal force-free magnetic field is given by the surface integral at the photospheric boundary involving the three vector magnetic field components, i.e.

where $B_{\textrm{x}}$, $B_{\textrm{y}}$, and $B_{\textrm{z}}$ are the $x$-, $y$-, and $z$-components of the photospheric magnetic field, respectively. We obtained the 3 components of photospheric magnetic field from the vector magnetograms of the “hmi.sharp_cea_720s” series. The magnetograms were then pre-processed as described in Wiegelmann, Inhester, and Sakurai (2006). We plot the evolution of the free magnetic energy stored in the AR NOAA 12371 normalised by the corresponding potential energy in Figure \irefimfea during the most active phase of AR 12371 (20-22 June, 2015). For reference, we have plotted GOES 1–8 Å SXR flux for the whole duration in Figure \irefimfeb. In this duration, the AR produced 3 M-class flares: class M1.0 on 20 June, class M2.6 on 21 June and class M6.5 on 22 June (see Table \ireftable1). From Figure \irefimfea, we find that prior to the M2.6 and M6.5 class flares, magnetic free energy in the AR 12371 was over 80% of the corresponding potential energy. After both the flares, free energy decreased and reached local minima at $\approx$57% and $\approx$65% of the corresponding potential energies, respectively. Interestingly, we did not find any significant decrease in magnetic free energy during and after the M-class flare on 20 June, neither free energy increased prior to the flare.