Cool transition region loops observed by the Interface Region Imaging Spectrograph

We report on the first Interface Region Imaging Spectrograph (IRIS) study of cool transition region loops. This class of loops has received little attention in the literature. A cluster of such loops was observed on the solar disk in active region NOAA11934, in the Si IV 1402.8 \AA\ spectral raster and 1400 \AA\ slit-jaw (SJ) images. We divide the loops into three groups and study their dynamics and interaction. The first group comprises relatively stable loops, with 382--626\,km cross-sections. Observed Doppler velocities are suggestive of siphon flows, gradually changing from -10 km/s at one end to 20 km/s at the other end of the loops. Nonthermal velocities from 15 to 25 km/s were determined. These physical properties suggest that these loops are impulsively heated by magnetic reconnection occurring at the blue-shifted footpoints where magnetic cancellation with a rate of $10^{15}$ Mx/s is found. The released magnetic energy is redistributed by the siphon flows. The second group corresponds to two footpoints rooted in mixed-magnetic-polarity regions, where magnetic cancellation occurred at a rate of $10^{15}$ Mx/s and line profiles with enhanced wings of up to 200 km/s were observed. These are suggestive of explosive-like events. The Doppler velocities combined with the SJ images suggest possible anti-parallel flows in finer loop strands. In the third group, interaction between two cool loop systems is observed. Evidence for magnetic reconnection between the two loop systems is reflected in the line profiles of explosive events, and a magnetic cancellation rate of $3\times10^{15}$ Mx/s observed in the corresponding area. The IRIS observations have thus opened a new window of opportunity for in-depth investigations of cool transition region loops. Further numerical experiments are crucial for understanding their physics and their role in the coronal heating processes.Comment: Accepted for publication in Ap

Investigating the Transition Region Explosive Events and Their Relationship to Network Jets

Recent imaging observations with the Interface Region Imaging Spectrograp (IRIS) have revealed prevalent intermittent jets with apparent speeds of 80--250 km~s$^{-1}$ from the network lanes in the solar transition region (TR). On the other hand, spectroscopic observations of the TR lines have revealed the frequent presence of highly non-Gaussian line profiles with enhanced emission at the line wings, often referred as explosive events (EEs). Using simultaneous imaging and spectroscopic observations from IRIS, we investigate the relationship between EEs and network jets. We first identify EEs from the Si~{\sc{iv}}~1393.755 {\AA} line profiles in our observations, then examine related features in the 1330 {\AA} slit-jaw images. Our analysis suggests that EEs with double peaks or enhancements in both wings appear to be located at either the footpoints of network jets, or transient compact brightenings. These EEs are most likely produced by magnetic reconnection. We also find that EEs with enhancements only at the blue wing are mainly located on network jets, away from the footpoints. These EEs clearly result from the superposition of the high-speed network jets on the TR background. In addition, EEs showing enhancement only at the red wing of the line are often located around the jet footpoints, possibly caused by the superposition of reconnection downflows on the background emission. Moreover, we find some network jets that are not associated with any detectable EEs. Our analysis suggests that some EEs are related to the birth or propagation of network jets, and that others are not connected to network jets.Comment: 9 figures; to appear in Ap

Explosive events on sub-arcsecond scale in IRIS observations: a case study

We present study of a typical explosive event (EE) at sub-arcsecond scale witnessed by strong non-Gaussian profiles with blue- and red-shifted emission of up to 150 km/s seen in the transition-region Si IV 1402.8 \AA, and the chromospheric Mg II k 2796.4 \AA\ and C II 1334.5 \AA\ observed by the Interface Region Imaging Spectrograph at unprecedented spatial and spectral resolution. For the first time a EE is found to be associated with very small-scale ($\sim$120 km wide) plasma ejection followed by retraction in the chromosphere. These small-scale jets originate from a compact bright-point-like structure of $\sim$1.5" size as seen in the IRIS 1330 \AA\ images. SDO/AIA and SDO/HMI co-observations show that the EE lies in the footpoint of a complex loop-like brightening system. The EE is detected in the higher temperature channels of AIA 171 \AA, 193 \AA\ and 131 \AA\ suggesting that it reaches a higher temperature of log T$=5.36\pm0.06$ (K). Brightenings observed in the AIA channels with durations 90--120 seconds are probably caused by the plasma ejections seen in the chromosphere. The wings of the C II line behave in a similar manner as the Si IV's indicating close formation temperatures, while the Mg II k wings show additional Doppler-shifted emission. Magnetic convergence or emergence followed by cancellation at a rate of $5\times10^{14}$ Mx s$^{-1}$ is associated with the EE region. The combined changes of the locations and the flux of different magnetic patches suggest that magnetic reconnection must have taken place. Our results challenge several theories put forward in the past to explain non-Gaussian line profiles, i.e. EEs. Our case study on its own, however, cannot reject these theories, thus further in-depth studies on the phenomena producing EEs are required.Comment: 16 figures, accepted for publication in Ap

Narrow-line-width UV bursts in the transition region above Sunspots observed by IRIS

Various small-scale structures abound in the solar atmosphere above active regions, playing an important role in the dynamics and evolution therein. We report on a new class of small-scale transition region structures in active regions, characterized by strong emissions but extremely narrow Si IV line profiles as found in observations taken with the Interface Region Imaging Spectrograph (IRIS). Tentatively named as Narrow-line-width UV bursts (NUBs), these structures are located above sunspots and comprise of one or multiple compact bright cores at sub-arcsecond scales. We found six NUBs in two datasets (a raster and a sit-and-stare dataset). Among these, four events are short-living with a duration of $\sim$10 mins while two last for more than 36 mins. All NUBs have Doppler shifts of 15--18 km/s, while the NUB found in sit-and-stare data possesses an additional component at $\sim$50 km/s found only in the C II and Mg II lines. Given that these events are found to play a role in the local dynamics, it is important to further investigate the physical mechanisms that generate these phenomena and their role in the mass transport in sunspots.Comment: 8 pages, 4 figures and 1 table, accepted for publication in ApJ