ON THE RELATIONSHIP BETWEEN A HOT-CHANNEL-LIKE SOLAR MAGNETIC FLUX ROPE AND ITS EMBEDDED PROMINENCE

On 2011 September 12, a prominence erupted above the east limb of the Sun. Its early evolution was well captured by the SDO/AIA due to its high spatial resolution (12), high temporal cadence (12 s), and multi-temperature (0.06–20 MK; O'Dwyer et al. 2010) ability. Tripathi et al. (2013) studied this prominence but concentrated on its partial eruption. Here, we focus on the relationship between the prominence and the associated hot-channel-like MFR. The early activation can clearly be seen from the AIA 304 Å images (Figures 1(a) and (b)). Initially, the prominence lay low above the solar surface with the two footpoints anchored in the chromosphere (Figure 1(a)). Probably due to reconnection, indicated by EUV brightenings along the prominence and the slight enhancement of the GOES soft X-ray (SXR) 1–8 Å flux (also see Tripathi et al. 2013), the prominence started to rise slowly after ∼20:30 UT (Figure 1(b)). At ∼20:46 UT, the prominence suddenly exhibited an impulsive acceleration. The morphology evolved instantaneously from a Λ shape to a reverse-Y shape (Figures 1(c) and (d)), from which we infer the occurrence of kink instability (e.g., Török & Kliem 2005). However, probably due to the fact that some materials drained down to the chromosphere, the left part of the reversed-Y shape became invisible after ∼20:48 UT. Moreover, at ∼20:46 UT, the brightness at the projected crossing point of the two legs of the prominence increased to maximum in all AIA EUV passbands (Figure 1(c)). This implies that magnetic reconnection took place there. Subsequently, the EUV brightenings were also enhanced at the two footpoints of the prominence (Figure 1(d)), indicating that reconnection also heats the chromosphere.

We find that the erupted prominence was closely associated with an elongated hot-channel-like structure. In the first several minutes, some diffuse threads, similar to the prominence, were visible in the AIA 131 Å passbands (Figure 1(e)). As time elapsed, probably due to reconnection heating, more and more threads revealed themselves. At 20:36 UT (Figure 1(f)), all of the threads seemed to converge. At 20:46 UT, the whole system formed a well-shaped and coherent channel-like structure (Figure 1(g)). As has been revealed in previous studies (Zhang et al. 2012; Cheng et al. 2013a; Patsourakos et al. 2013), the structure can only be seen in the AIA high temperature passbands, i.e., 131 Å and 94 Å, but not in the other cooler wavelengths. This shows that the channel must have a temperature of ⩾8 MK.

An interesting discovery is that the hot channel was initially almost co-aligned with the prominence in space, but later on the top of the hot channel separated from that of the prominence (Figures 2(a)–(c)). In particular, at ∼20:48 UT, the prominence was only cospatial with the bottom of the right part of the hot channel (Figure 2(d)). The results can be interpreted as a general scenario wherein the hot channel is likely the MFR and the prominence is only the collection of cool materials at the bottom of the MFR; the eruption of the prominence was essentially followed by that of the MFR. Moreover, the hot channel also displayed a morphology transformation similar to the prominence. From ∼20:46 UT, the top of the hot channel started to arch upward. In the period of 20:46–20:48 UT, the arching first made the hot channel take on a Λ shape (Figure 2(c)), which then quickly evolved into a reverse-Y shape (Figures 1(h) and 2(d)).

As the morphology of the hot channel transitioned from Λ to reverse-Y shape, flare-related reconnection began to dominate the whole heating process. We calculate the differential emission measure (DEM) of the erupted structure using the SolarSoft routine "xrt_dem_iterative2.pro" (Cheng et al. 2012). With the DEM results, we then construct two-dimensional maps of the emission measure (EM) of the plasma in different temperature intervals (ΔT) through the formula EM(T)=$\int _{T-\Delta T}^{T}{\rm DEM}(T^\prime)$dT'.

Figure 3 shows the EM structures of the eruption in three temperature intervals. It can be seen that at ∼20:46 UT, the plasma in the space occupied by the prominence showed emission in all temperatures (0.5–5 MK) while the surrounding hot channel only exhibited emission from the hot plasma (10–15 MK). This reveals that the hot channel was heated to high temperatures at that time. We suggest that slow magnetic reconnection probably occurs inside or around the hot channel to heat and build up the channel before ∼20:46 UT; however, this reconnection is too weak to generate nonthermal particles, and thus is different from the fast flare reconnection in the later phase (also see Aulanier et al. 2010; Cheng et al. 2013b; Guo et al. 2013). After 20:46 UT, the hot channel still had a temperature of >8 MK and the prominence had a temperature of <5 MK; however, the EM of both the hot channel and the prominence tended to decrease, mainly due to expansion. On the other hand, the EM of the flare region underneath the hot channel was quickly enhanced. In particular, for the hot plasma, the EM increased from ∼10²⁸ cm⁻⁵ at 20:47 UT to ∼10²⁹ cm⁻⁵ at 20:50 UT in the low-lying flare region. This indicates that the morphology transformation of the hot channel is also associated with the triggering of the fast flare reconnection that may further help to heat and build up the hot channel.

In this section, we study the kinematics of the hot-channel-like MFR and the prominence in detail. We take a slice along the direction of the eruption (Figure 2(c)). The time sequence of the slice makes a stack plot. Using the stack plot, we measure the heights of the hot channel and the associated prominence. Applying a first-order piecewise numerical derivative to the height–time data, we derive the velocities of the hot channel and prominence. The uncertainties in the velocities arise from the uncertainties in the height measurements, which are estimated to be 4 pixel sizes (∼1.7 Mm).

Figures 2(e) and (f) display the height and velocity profiles of the hot channel and prominence. One can observe that the hot channel experienced two distinct phases: a slow rise phase of 20 minutes and an impulsive acceleration phase of only ∼5 minutes. During the first several minutes, the height of the hot channel increased from ∼10 Mm at 20:30 UT to ∼20 Mm at 20:36 UT, resulting in an average velocity of ∼30 km s⁻¹. The early rise process corresponds well to the brightening along the hot channel and prominence, confirming that their activation was most likely the result of slow reconnection. In the period ∼10 minutes after 20:36 UT, the rise of the hot channel tended to slow down; the height only increased to 25 Mm at ∼20:46 UT, corresponding to an average velocity of ∼10 km s⁻¹ (Figure 2(f)).

An important result is that the height evolution of the hot channel had an apparent jump (Figure 2(e)). Correspondingly, the velocity impulsively increased around the jump, e.g., from ∼10 km s⁻¹ at ∼20:46 UT to ∼250 km s⁻¹ 3 minutes later at ∼20:49 UT (Figure 4(b)). The average acceleration in this period is estimated to be ∼1300 m s⁻², which is much larger than the average acceleration (330 m s⁻²) of the impulsive CMEs (e.g., Zhang & Dere 2006). After ∼20:49 UT, the velocity started to decrease gradually and became to be ∼50 km s⁻¹ at ∼21:00 UT with an average deceleration of ∼300 m s⁻². This deceleration led to a failed eruption, as is evident from the lack of propagating CME in the white-light coronagraph images. In order to exactly estimate the onset time of the impulsive acceleration, we use a function consisting of both a linear and an exponential component to fit the height–time measurements of the hot channel from 20:36 UT to 20:48 UT. The details of the technique can be found in Cheng et al. (2013b). From Figures 4(a) and (b), one can see that the height of the hot channel is well described by a combination of the linear and exponential functions. The exponential component is believed to be a fundamental feature of the MFR eruption trajectory driven by kink instability (e.g., Török & Kliem 2005; Schrijver et al. 2008). Assuming that the hot channel is impulsively accelerated at the time when the velocity of the exponential component is equal to that of the linear component, the impulsive acceleration onset is found to be at 20:45:40 UT with an uncertainty of 1.7 minutes, almost perfectly coincident with the sudden transformation of the hot-channel morphology from the Λ shape to reverse-Y shape. These results indicate that the hot channel most likely underwent kink instability, thus triggering and driving impulsive acceleration in a very short period, simultaneously distorting the axis of the hot channel, revealing a transformation of the morphology.

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As for the prominence, the evolution of the height and velocity had exactly the same trends as the hot channel, however, with some difference in magnitude. In the slow rise phase, the prominence retained the same height and linear velocity as the hot channel. With impulsive acceleration commencing, the height of the prominence also exponentially increased (Figure 4(c)). Using the same technique, we are able to fit the height variation of the prominence very well with a combination of the linear and exponential functions. The onset of the prominence impulsive acceleration is determined to occur at 20:44:20 UT with an uncertainty of 5.0 minutes (Figure 4(d)), nearly coinciding with that of the hot channel. On the other hand, we find that the height and velocity of the prominence increased more slowly than those of the hot channel shortly after the beginning of the impulsive acceleration. These results suggest that the prominence and hot channel share the same MFR system throughout the course of the eruption. Most likely, the hot channel is the MFR and the prominence corresponds to the dips of the helical field lines of the MFR. Prior to and during the slow rise phase, the MFR remains small and compact, and thus almost cospatial with the prominence (Figure 2(a)); the distance between the MFR top and prominence is too small to be recognizable. In the acceleration phase, as a result of fast reconnection, represented by the peak of the time derivative of the GOES SXR flux (Figure 2(f)), the newly formed high temperature poloidal flux quickly envelops the MFR, resulting in heating of the MFR and a separation between the tops of the MFR and the prominence; thus, the upper part of the MFR is only visible in the AIA high-temperature passbands (131 Å and 94 Å), while the lower part is seen in all AIA passbands because this part consists of both the cool core and the hot shell of the MFR (Figures 2(c) and (d)).

Moreover, the onset of hot channel and prominence impulsive acceleration was almost coincident with the rapid enhancement of flare emission (Figures 4(b) and (d)). Through inspection of the AIA images, we find that the strongest brightening appeared at the crossing point of the two legs of the hot channel and prominence at the onset time (∼20:46 UT; Figure 1(c) and the left column of Figure 3). With the hot channel and prominence ascending, the brightening underneath them was also rapidly increased, showing that fast reconnection started which rapidly increased the flare emission and formed the flare loops (Figures 1(g) and (h), and 3(h) and (i)). The transition of brightening from the crossing part to underneath the hot channel and prominence implies that kink instability may also have a role in causing fast flare reconnection.