Transient Formation of Loops in the Core of an Active Region

The existence of multi-million degree hot plasma in the solar corona has puzzled astronomers since its discovery in the 1940s. In the last several decades, there has been a great advancement in observational techniques and computation resources, which has led to tremendous progress in understanding this issue. However, the final solution remains elusive (Klimchuk, 2006; Reale, 2014). In general terms, the on-disk corona is covered by three different areas, viz. active regions, quiet Sun, and coronal holes. Of which, active regions have higher temperature followed by the quiet Sun and then coronal holes (see, e.g., Del Zanna & Mason, 2018).

Active regions show pronounced heating in the observations taken so far with imaging and spectroscopic instruments. Therefore, they have been used as an excellent target of opportunity to address solar coronal heating. The high resolution observation show that active regions are comprised of loops observed at different length scales (see e.g. Warren et al., 2008; Tripathi et al., 2009; Del Zanna, 2013a; Reale, 2014; Ghosh et al., 2017, and references/citations therein): such as fan loops (formed at $<$1 MK), warm loops (formed at $\sim$1 MK) and hot core loops ($\sim$3–5 MK). In addition, there is significant significant amount of diffuse emission (Del Zanna & Mason, 2003; Viall & Klimchuk, 2011), which has a peak emission at $\log\,T=6.2$ (Subramanian et al., 2014; Brooks, 2019). Observations of such loop structures along with the diffuse emission add further complication for the modeling.

With modern spectroscopic instruments observing in the spectral bands, which are sensitive to solar coronal plasma, the fan loops and warm loop were resolved (Warren et al., 2008; Tripathi et al., 2009; Guarrasi et al., 2010; Ghosh et al., 2017, and references therein). Therefore, it has been possible to directly measure plasma parameters such as electron density, temperature, filling factors in those loops and compare with hydrodynamic modelling (see e.g. Del Zanna & Mason, 2003; Warren et al., 2008; Tripathi et al., 2009; Gupta et al., 2015; Ghosh et al., 2017). Based on the observations from spectroscopic and imaging instruments, it has been shown to a great extent that the properties of warm and fan loops may be explained by low-frequency impulsive heating occurring in the corona.

However, in the core of the active region, structures are not well resolved, and therefore such analysis is not possible. Thus, the formation of hot plasma in the active region’s core remains a highly debatable topic. Due to the unresolved nature, the authors have relied on the measurements such as Doppler shifts as well as Differential Emission Measures (DEM) (see, e.g., Tripathi et al., 2010, 2011; Warren et al., 2012a; Del Zanna et al., 2015) that did not require the structures to be resolved.

With the high-resolution observations recorded by IRIS and AIA, it has now been possible to detect individual transient events, including in the core of active regions (see, e.g. Testa et al., 2014, 2016; Chitta et al., 2017, 2018). A detailed study of the formation and thermodynamics evolution of such transients will help us comprehend the existence of the hot plasma in the core of active regions.

Testa et al. (2014, 2016) suggested that these transients in the core of active regions result from coronal nanoflares with non-thermal electron beams. However, Chitta et al. (2017, 2018, 2020) suggest that these events are due to the magnetic reconnection at the footpoints driven by photospheric magnetic flux cancellation occurring within the mixed polarity regions (Priest et al., 2018; Syntelis et al., 2019). However, these studies did not focus on the thermodynamic evolution of these transients (Gupta et al., 2018).

In this paper, we study the transient formation of hot loops in the core of an active region. For this purpose, we have used the observations recorded with Interface Region Imaging Spectrometer (IRIS; De Pontieu et al., 2014), Atmospheric Imaging Assembly (AIA; Lemen et al., 2012) and Helioseismic and Magnetic Imager (HMI; Scherrer et al., 2012). We perform the full thermodynamic study of the transient loops by obtaining the differential emission measure. We also study the possible cause of the transient occurrence by using the line-of-sight (LOS) magnetograms. The rest of the paper is structures as follows. In § 2 we describe the observations. In § 3 we describe data analysis and results. We provide a summary of the results and conclude in § 4.

For this work, we have primarily used the observations recorded by IRIS and AIA. We have also used the line-of-sight (LOS) magnetograms obtained from HMI to infer the underlying magnetic field structure. Both AIA and HMI are telescopes onboard the Solar Dynamics Observatory (SDO). All three instruments simultaneously observed the active region AR 11890 and detected the transient formation of a loop system within the core of the active region. We note that the transient loop system studied here resembles the structure similar to those described by Hanaoka (1997) using the observations recorded by the Soft X-ray Telescope (SXT; Tsuneta et al., 1991) onboard Yohkoh. These were referred to as “double loop configuration of solar flares”. We emphasize that Hanaoka (1997), however, only studied the morphology of these structures and did not perform the study of their thermodynamical evolution.

The background image shown in Fig. 1 displays a portion of the Sun’s disk covering the active region recorded at the peak of the transient loops as observed in AIA 94 Å ¹¹194 Å image shown in the background is obtained after removing the contribution of cooler plasma.. The over-plotted pink box covers the region we have selected from the full disk AIA observations for further study. The area enclosed with the blue and white boxes locates the IRIS field-of-view (FOV) in SJI and spectroscopic mode. As is conspicuous, in SJI mode, IRIS covered the full extent of the feature. Unfortunately, however, the transient loops were not observed in the spectroscopic mode. However, there is still important information that warrants a detailed study using AIA and IRIS-SJI.

In SJI modes, IRIS observes the Sun in one near UV (NUV; 2796 Å) and two far-UV (FUV; 1330 Å, and 1400 Å) channels with a spatial resolution of 0.33 ″. These three filters cover the solar chromosphere and transition regions, respectively. For more details of the IRIS and its capabilities, see De Pontieu et al. (2014).

The AIA, being the full disk imager, observed this event in all its filters with a pixel size of $\sim$0.6 ″. In the current study, for the purpose of dynamic evolution, we have used the observations recorded in 1600 Å, 304 Å, 171 Å, 193 Å, 211 Å, 335 Å, 94 Å and 131 Å. Together, these filters cover a temperature range of $\log\,T=4.8$ to $\log\,T=20$ MK. For more details on AIA and its capabilities see Lemen et al. (2012) and information on the sensitivity of different passbands to temperature (O’Dwyer et al., 2010; Lemen et al., 2012; Boerner et al., 2012).

As demonstrated by O’Dwyer et al. (2010), the different AIA channels have contributions from plasma at different temperatures. The three channels, namely 94 Å, 335 Å and 131 Å, which are designed with a primary aim of studying the evolution of hot plasma, suffer from the fact that they have contributions from plasmas at lower temperatures. A couple of empirical relations, giving similar results, have been suggested (see Del Zanna, 2013b; Warren et al., 2012b) to estimate and remove the contribution of lower temperature plasma. In the present work, we have used the method proposed by Del Zanna (2013b) to remove the cooler temperature emission from 94 Å channel and studied the evolution of the transient in the Fe XVIII line.

To study the thermodynamic evolution of plasma within different temperature bins simultaneously, we have employed Differential Emission Measure (DEM) technique developed by Cheung et al. (2015). The method developed by Cheung et al. (2015) is considerably faster than the other existing codes (see also Hannah & Kontar, 2012). Hence, this technique allows us to study the thermodynamic evolution of the plasma in transient events. To derive the emission measure, we have used the original level 1 AIA observations for all the six EUV channels and applied aia_prep.pro.

In this paper, we present a multi-wavelength study of the formation of a transient event in the core of an active region. For this purpose, we have used slit-jaw images from IRIS and all the EUV and 1600 Å images of AIA. To probe the transient’s thermodynamic nature, we have employed the DEM technique on the EUV observations of AIA taken in 6 coronal channels. To shed light on the transient’s possible origin, we have also studied the LOS magnetograms obtained by HMI.

The transient is observed in the slit-jaw images of Mg II, C II and Si IV recorded using IRIS as well as in the UV and EUV channels of AIA. The two footpoints of the transient brightens-up nearly simultaneously, as is noted from SJI’s recorded in Si IV. Such simultaneous brightenings at both the footpoints have also been observed by, e.g., Testa et al. (2014); Chitta et al. (2017); Gupta et al. (2018).

The complete structure of the transient is best observed in the 94 Å images of the AIA (see Figs. 4 &  5). The transient comprises short loops located at the base and longer loops connecting the other main polarity of the active region. Transient events with a similar morphology have been reported earlier by Hanaoka (1997) and have been referred to as “double loop configuration of solar flares.” The AIA 304 Å images also reveals the presence of a twisted filament (panel D of Fig 3). By combining the observations recorded in cool channels such as 304 Å (see the corresponding animation Figure3D_animation.mp4) with those in 193 or 94 Å images, we find that the transient brightening appears in different strands. Observations also reveal that these strands are winding around the filament. Moreover, the transient event is highly filamentary (see Figs. 5 and 6), which is suggestive of multi-stranded structure. The highest emission is observed at the base of the transient within the temperature bin of $\log\,T=6.65-6.95$ (see panel D of Fig. 6), whereas the long loop-like structure is best seen, albeit very faint, in higher temperature $\log\,T=6.95-7.55$ (panels E & F of Fig. 6). However, we note that such emission in high temperatures should be interpreted with caution due to limited constraints at higher temperatures (e.g. Winebarger et al., 2012). Moreover, the structures observed in the EM maps obtained at such high temperatures may also be due to the presence of unaccounted cooler lines in the passbands of 94 Å and 131 Å passbands as was suggested by Young & Muglach (2014).

Combining the AIA observations with those from HMI line-of-sight magnetograms, we find that the base of the transient is rooted in primary negative polarity and the parasitic positive polarity, whereas the long loop connects to the main positive polarity of the active region (see Fig. 8). The evolution study of the photospheric LOS magnetic field throughout the existence of the transient, i.e., for 19 mins, did not show any noticeable change in the structure of the magnetic field at the base of the transient (see Fig. 9). However, over a long time scale, we observed noticeable changes in the field configuration (see Figure9_animation.mp4) as well as a change in the total signed flux over a small boxed region located at the base of the transient. Therefore, it is plausible to conclude that changes in the magnetic flux may have resulted in the transient. However, the data do now allow a conclusive determination of whether flux changes caused the observed transient.

Motivated by the observations, we provide a schematic diagram in Fig. 11, which is similar to that of Hanaoka (1997) and depicts the evolution of the transient studied here. We envision that there are small loops structures rooted within the main polarity forming between P4 and P5. These loops are due to the presence of parasitic polarities. There are also other loop systems connecting the two main polarities from P0 to P5. In Fig. 11, we show these loops in blue. Our observations suggest that reconnection occurs between the blue loops, leading to loops forming with green color. The loops coming out from the reconnection region will have a kink, which is also supported by the observational results of the presence of the kink in Fig. 4 marked with an arrow. Moreover, the EM curves suggest a substantial enhancement in temperature at the kink location (see Fig. 7, P3).

It is important to note that the physics of the formation of these transients may be similar to those of jets in different topological scenarios (see Raouafi et al., 2016, for a review on jets). In the typical coronal jet scenario, the reconnection occurs among closed field lines and open field lines giving rise to a collimated structure. However, the observation reported here suggests reconnection between two closed field lines that leads to the formation of a longer loop, which is analogous to the spire of typical coronal jets, similar to those observed in the simulations of Gontikakis et al. (2009); Archontis et al. (2010). Moreover, the properties such as lifetime (15 min), multi-thermal nature, and DEM at the base of the transient observed here are similar to those obtained for jets observed at the periphery of active regions (see, e.g., Mulay et al., 2016; Raouafi et al., 2016). We further note that the presence of filament at the base of the transients is also similar to jet events reported by many authors (see, e.g. Hong et al., 2011; Sterling et al., 2015; Hong et al., 2016; Panesar et al., 2017; Kumar et al., 2018, and references therein).

Although we have qualitatively described the formation of the transient reported here, further observational work, primarily with spectroscopic measurements combined with modeling, is necessary for a complete understanding of the physics of such transients within the core of active regions. Such studies will help comprehend the formation of hot plasma in the core of active regions.