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Limb Event Brightenings and Fast Ejection Using IRIS Mission Observations

The Interface Region Imaging Spectrograph (IRIS) of the NASA small explorer mission provides significantly more complete and higher resolution spectral coverage of the dynamical conditions inside the chromosphere and transition region (TR) than has heretofore been available. Near the solar limb high temporal, spatial (0.3") and spectral resolution observations from the ultraviolet IRIS spectra reveal high-energy limb event brightenings (LEBs) at low chromospheric heights, around 1 Mm above the limb. They can be characterized as explosive events producing jets. We selected two events showing spectra of a confined eruption just off or near the quiet Sun limb, the jet part showing obvious moving material with short duration large Doppler shifts in three directions identified as macrospicules on slit-jaw (SJ) images in Si IV and He II 304 A. The events are analyzed from a sequence of very close rasters taken near the central meridian and the South pole limb. The processed SJ images and the simultaneously observed fast spectral sequences having large Doppler shifts, with a pair of red shifted elements together with a faster blue shifted element from almost the same position, are analyzed. Shifts correspond to velocities of up to 100 km/s in projection on the plane of the sky. The occurrence of erupting spicules and macrospicules from these regions is noticed from images taken before and after the spectra. The cool low first ionization potential (FIP) element simultaneous line emissions of the MgII h and k resonance lines do not clearly show a similar signature due to optical thickness effects but the Si IV broad-band SJ images do. The bidirectional plasma jets ejected from a small reconnection site are interpreted as the result of coronal loop-loop interactions leading to reconnection in nearby sites.

features (e.g. Yokoyama and Shibata, 1995;Moore et al., 2010). We note that regarding spicules, they are classically observed in: i) cool low excitation lines like Hα and Hβ of H I or in the He I lines, that are high first ionization potential (high FIP) lines above 10 eV and ii) low FIP lines, like the powerful resonance lines of CaII in the UV and in the infrared (IR). Sterling (2000) and more recently Tsiropoula et al. (2012) presented reviews of spicule morphological properties, including their derived physical parameters and dynamical behavior. Earlier Pasachoff, Jacobson, and Sterling (2009) showed that spicules appeared to have bidirectional plasma flows at their maximum heights consistent with the magnetic reconnection scenario introduced by Innes (1997) from much larger scale off limb spectrograms taken in hot transition region (TR) lines by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instrument onboard the SOHO. Tsiropoula, Alissandrakis, and Schmieder (1994) reported a similar behavior in mottles on-disk. Chromospheric bright points have been observed in cases where there are emerging arch-shaped mottles, and thus the bright points result from reconnection (Singh et al., 2012), where a current sheet provides a strong acceleration of charged particles as is the case for larger scale events (Filippov, Koutchmy, and Tavabi 2013). Emerging miniature arches in the chromosphere appear to play a key role in many X null-point reconnection sites (Shimojo et al., 1996). Cirtain et al. (2007) and Filippov, koutchmy, and Golub (2009b) reported plasma jets of various scales from X-Ray Telescope (XRT) observations of the Hinode mission, from giant X-ray jets to numerous _ 5 small jets with sizes typical of macrospicules. Macrospicules are now continually observed using the 304 Å resonance line emission of HeII from full disk imaging experiments such as the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory (SDO) mission.
It seems clear that near limb and even better, off-limb higher resolution observations, would permit a better characterization of all brightening events, as far as the height parameter is concerned, in case of observations with high enough resolution to resolve the reconnection regions involved. We believe the new IRIS observations will let us tackle this point.

Observations and Data Reduction
IRIS 0.3''-0.4'' spatial resolution observations, with a pixel size of 0.166'' (corresponding to 120 km at disk center) and high cadence, reveal the dynamical behavior of the chromosphere and of TR fine features (De Pontieu et al., 2014). The available high cadence allows the analysis of the evolution of these features and enables us to follow them from the photosphere to the low corona in channels (Figure 3) of different spectral lines, in the near ultraviolet (NUV, 2782-2835 Å with the MgII h and k resonance lines seen in absorption on-disk and in emission off-disk, similarly to the well-known H and K lines of CaII), and in the far ultraviolet (FUV, 1331-1358 Å with the CII line seen in emission) and in 1389-1407 Å (with the SiIV lines seen in emission). Note that the MgII lines are due to a low-FIP element typical of the low temperature plasma produced above the temperature minimum near the 500 km height above τ5 = 1 (optical deep in 5000 Å).
Their FIP is 7.65 eV. The CII line is from a high-FIP element with excitation potential corresponding to a higher temperature. The SiIV lines are produced at much higher temperatures with an ionization potential of Si +2 of 33.5 eV. The velocity resolution in IRIS spectra is 1 km s -1 . In addition SJ images with 175×175 arcsec 2 field-of-view (FOV) reflected off the slit/prism assembly through a filter wheel with four different filters (De Pontieu et al., 2014) are available.  shows the bright event adjacent to the limb seen in the TR lines (SiIV; CII) right under the slit of the spectrograph, shown by the vertical black line. The spicule "forest" above the limb is apparent in all images, with rather good correlation between them. Note the apparent absence of the bright event in the SJ image in the MgII cool line above the limb in this high-resolution image with minimal contribution from the adjacent continuum, the vertical and horizontal axes indicate the distance form Sun center.

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Our region is exactly above a solar pole (Figure 3), which is usually a quiet region and no active region is observed. It is not a coronal hole region. Small patches of concentrated magnetic field can however exist in polar region, Figure 3 provides a synoptic image of the region seen in 304 Å HeII emission from AIA observations during the IRIS observations reported here. A very large IRIS raster over the Sun's South pole was done on 10 October 2013 at 10:02 to 11:01 UT centered at [6 '' , -945 '' ]. Calibrated level 2 data were used. The raster cadence is 9 seconds with a spatial step of 0.35 '' . The 400pixel long slit covers a 141x174 arcsec 2 region. The SJ high-resolution images were obtained in three channels, showing: i) the low temperature chromosphere in MgII k (2796 Å, logT~3.6 to 4), and ii) the much hotter transition region in SiIV (lines near 1400 Å with logT~4.8) and iii) the CII low transition region filter (1330 Å, logT~4.0). Their FOV is 167×174 arcsec 2 with a 36 seconds cadence and pixel size is of the order of 0.1 '' (De Pontieu et al., 2014).
Because we want to precisely look at LEBs and evaluate the height of the event with respect to the photospheric limb, it is important to use a reference showing the limb in spectra and to align precisely the spectra where the limb is not clearly seen (SiIV spectra). This can be done with reference to fiducial mark or hair line (hl) put on the entrance slit of the spectrograph although it is not so easy to find the signature _ 8 of this hl in the spectra. A rather deep processing is needed, see Figure 4, where the method is illustrated using the example of a LEB taken at 10:34 UT that we believe clearly shows the LEB definitely above the photospheric limb seen on Mg II h and k line spectra.

Results
In classical observations taken above the limb, a bright background obscures or smears the signature of small events because of the line-of-sight (LOS) effects, i.e. a long integration path along the line of sight with superposition or overlap effects appearing in projection in the 2D images. Note that images taken near the disk center do not suffer as much from LOS effects. However, it is believed that TR structures are indeed of very small cross section, corresponding to a tiny value of the filling factor, 10 -2 to 10 -5 ( Judge, 2000). Thanks to the very high spatial resolution of FUV observations performed with IRIS the opportunity to observe the tiniest events that can be found in the region of the quiet Sun for the first time without suffering too much from LOS effect. To give a quantitative evaluation of the dynamical bright event (flashing LEB) using spectra, the bright feature should be observed with the slit of the spectrograph crossing it ( Figure 5). We emphasize that due to the short lifetime and the very small size of the bright knot, it is relatively rare so far to optimally observe such event, with the IRIS spectrograph slit pointed exactly "over" the event during the raster. Fortunately in our case, the slit crosses the limb at the right time when the bright dot flashes (see Figure 5). Doppler shifts can then be detected and measured. In case of a long and broader slit put on-disk, these events are easier to pick up as was done for the events reported by Brueckner and Bartoe (1983) but their fine structure could not be analyzed as shown on Figure 5. We evaluate the typical extension along the limb of this LEB to be three steps of the spectroscopic raster, or 1.05''. From Figure 4 the typical height of the SiIV LEB above the limb is evaluated to be 4 pixels or 0.67'' (see also Figure 7).  Dopplergrams were constructed by subtracting red and blue wing intensities at the same wavelength offset from the rest, the bright-dark (positive-negative) corresponds to towards (blue shift)-red shift (towards-away from the observer). The position of the LEB is marked by the green arrows as well as the very rapid blue and co-temporal red shifts, in 1403 Å SiIV raster Dopplergrams in the velocity space. Both SiIV channels show similar behaviors, the morphological properties in all ranges of Doppler velocities are similar; in both channels we possibly see an elongated typical spicule with an inclination angle having a high velocity value (blue arrows). The brightest knot-like feature of the LEB is not seen in the MgII k line ( Figure 6, third row), while the thicker spicules (more like a macrospicule or a spicule bush) appear definitely above the location of the LEB (bright means blue shifts). In these figures we see fewer features in the far wings than near the center of the line (panels from left to right).  Unlike what is seen in these lines, the MgII k cooler line shows a broad doubly peaked shape typical for a large and broad limb made of spicules and possibly macrospicules seen along the LOS with significant optical thicknesses at this position, but no sign of the strongly Doppler shifted components that were seen in SiIV. The red wing of the hot lines shows a pair of shifted components of material with a definite and small separation distance between them (0.5'' or 360 km). The shift is comparable in extension to the blue wing shift. The shifted components correspond to flows spanning the whole range of velocities from 0 to approximately 100 km s -1 without taking into account the LOS effect that could drastically reduce the values of the apparent velocities in case propagation is occurring in a direction with a large departure from the direction of the LOS. It is not excluded that some large flows also exist in the direction corresponding to the image plane without producing any Doppler shifted component. We note that the line profile above the peak of maximum intensity is broad over approximately 2''. The splitting of the two largely red shifted components is an important feature in case we deal with the so-called diffusion region where reconnections are taking place (Aschwanden 2014), the region extends over at least 500 km in height (see Figure 7).  for smearing). The total lifetime of the brightening event is not well known but assuming the LEB is similar to the EEs of Brueckner (1980), it is about 80 seconds, in agreement with the observed behavior resulting from the analysis of the SJ images that show a 1.5 min lifetime for the LEB at 10:34 UT.
This figure shows what we interpret as a reconnection site because there is evidence of a sudden large brightening recorded in the hot lines only and that a non-thermal heating occurs in this region. The very narrow ejection of plasma seen as a long and tiny thread shown in the SJ images (top of Figure 10 and Figure 11) is another piece of evidence that we interpret in favor of a reconnection event. The absence of evidence in the cool lines finally also favors this interpretation because in the hot but much lower density plasma, the magnetic field plays a more important role.
Another possibility to have a sudden heating would be to consider a shock, but we do not have any evidence of a wave occurring in this region; only some violent ejection phenomena are observed.   The strong intensity enhancement in the hot lines of SiIV provides some evidence of heating; in addition Figure 9 suggests some dispersion of velocities inside the LEB. To improve the signal/noise ratio for offlimb parts of the SJ images, we decided to use a logarithmic scale ( Figure 10) and process the images using an unsharp masking filter with a three pixle size window; an increased contrast was also used. The time evolution in SiIV is shown in Figure 11 with clear evidence of a thin long linear thread emerging from the bright knot region of the LEB. After 2 minutes the thread appears also noticeable in MgII k line at the same position but with a thicker and shorter length (Figure 12).  Note also the quasi-horizontal thread (crossing the whole FOV) probably related to a polar region filament, see The thin linear jet-like thread of Figure 11 seems to originate from the bright point that is marked by arrows in Figure 10 (top panel) that we called LEB. This thin linear long feature was not noticed in the original and unprocessed SJ images; it is seen only in the SJ hottest channel (1400 Å) after some image processing, see Figure 10. We hesitate to call the feature a macro-spicule; it is definitely longer and evidently hotter than the usual spicules that are shown using the cooler emissions of MgII, see Figure 12. It is noteworthy that the origin of this fast jet was probably detected in Dopplergrams of large amplitude velocities as a thicker blue shifted material appearing in Figure 6. Its counterpart in Figure 12 is not obvious although some feature(s) appear at the same location but later in time.

Discussion
The most significant aspect of spicules and especially of macrospicules is perhaps their potential ability to transfer mass and energy from the photosphere to the chromosphere and to the bottom of the corona. In a precursor study of Transition Region And Coronal Explorer (TRACE) Lyα and quasisimultaneous CIV broad band filtergrams, Alissandrakis, Zachariadis, and Gontikakis (2005) found a lot of similarity between spicules recorded in the rather cool but optically thick Lyα line and in the hotter CIV line (both are high-FIP elements and the CIV line behaves similarly to our SiIV lines).
The bright points at the feet of jets (Tsiropoula and Tziotziou 2004;Sterling, Moore, and DeForest 2010;Tavabi 2014 andSuematsu et al., 1995) have been seen in many chromospheric and coronal emission lines  and often interpreted as the result of magnetic reconnection, where the magnetic free energy is suddenly and partially converted to thermal and kinetic energy in the region of the null-point with much weaker magnetic field strength. Some of the magnetic field lines then open up and the hot plasma is readily able to move along these open field lines outwardly, being quickly accelerated. The gas pressure gradient works like a piston that pushes plasma through a nozzle (Filippov, Koutchmy, and Tavabi 2013).
Emission lines exhibit Doppler shifts towards red and blue as one would expect from a bidirectional jet (Innes et al., 1997) at TR temperatures.
In our LEB of 10:34 UT we saw evidence of a significant extension of the reconnection region, of order of 400 km in height and 800 km in azimuth (see Figures 4, 5 and 7). This is also suggested in Figure 8 where the FWHM of the LEB in SiIV lines extends over approximately four positions of the raster, which corresponds to a 1'' distance along the limb. At lower temperatures the signature is weak and possibly seen after, from our observation. These simultaneous Doppler red and blue shifts in the compact but definitely extended volume (extension of order of the event diameter) shown in Figure 3 and the splitting of the strongly separated components shown on Figure 5, make us believe that this phenomenon is similar to the explosive events (EEs) which were reported by Brueckner with large velocities (Brueckner, 1980) occurring close to the surface in directions rather parallel to the underlining surface but not necessarily parallel to the LOS. Accordingly, velocities could be of higher amplitude than what is deduced from the observed disk from 5 minutes duration sequences obtained during a rocket flight with few events showing only a predominantly blue shifted component, which could be the result of a peculiar positioning of the slit over a more turbulent explosive event. The components which we found split at heights which would not be seen on-disk.
After highly enhancing and sharpening our SJ images in logarithmic scale, a rather thin and long jet-like spicule was detected above the bright knot of the LEB over the background of the quiet Sun polar region (Figures 8,9,10,11 and 12). It is tempting to make the connection with the LEB, knowing that the Doppler shift value is about 100 km s -1 and the opposite wing shift similar in two split components, without taking into account the LOS effect. These values of LOS velocities could be corrected using the mean value of spicule inclination in polar regions in case we identify these components with spicules; the average tilt angle of spicules in polar areas is of order of 30 degrees or less (Tavabi, Koutchmy, and Ajabshirizadeh 2012) such that the corrected value would increase drastically by a rather large factor. Bidirectional jets in such events are accelerated in opposite directions from the reconnection sites, the first approximation X null-point reconnection geometry could imply the involvement of magnetic loops, separatrix surfaces, and spine (see e.g. Figure 14 for a possible scenario) for which outflow jets are actually expected to have a multidirectional structure, which indeed has been observed in the form of simultaneous blue and red Doppler shifts in the LEB taken at 10:34 UT, similar to the on-disk and larger scales events in Brueckner and Bartoe (1983), and Dere, Bartoe, and Brueckner (1989). In general, the topology of a jet resembles the geometry of field lines in the vicinity of a null point see Koutchmy (2009a), Filippov, Koutchmy, andGolub (2009b) and observations often show that a jet may start from the expansion of loops within a dome-like magnetic configuration.

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Another possibility is to start with a quadrupolar configuration (Filippov, Koutchmy, and Tavabi 2013) with a linear jet above resembling the Eiffel-tower configuration as was suggested for macrospicules and twin-spicules by Zirin and Cameron (1998). However with LEBs we deal with smaller energetic events at chromospheric temperatures and even a very fine thread like linear jet is seen in SiIV emissions (Figures 8,9,10,11 and even in Figures 2 and 13). Yokoyama and Shibata (1995) using Yohkoh soft X-ray observations showed two types of 2D magnetic configurations numerically simulated, the anemone jet type and the two-sided loop-jets type. The first one occurs when emerging flux appears in quiet regions, and two-sided loop interaction (or jet) occurs in predominantly unipolar field regions, in the horizontal direction on both sides of the emerging flux. In 3D it could resemble our naïve cartoon in Figure 14. A recent qualitative extension of this model is discussed in Singh et al. (2012) for events closer in size to our more energetic LEB that occur significantly above the limb. In Singh et al. (2012) the earlier 2D reconnection anemone model (Yokoyama and Shibata 1995) is extended to the case of a 3D twisted flux-tube reconnecting with an ambient already open field in order to explain their SOT observations of multiple knots appearing at the feet of several fine-scale jets with systematic motion. The occurrence of gradually more and more violent processing responsible for the launch of the jets was noted. Here the question "what temperatures do the LEBs reach?" arises. Would it be possible for them to reach coronal temperature? It is not possible to see the coronal counterpart of the LEB from IRIS data. The AIA (SDO) images do not have enough resolution to look at sub-arcsec events with a sufficient signal/noise ratio. In order to progress on this question we finally picked up another example of a LEB observed with the IRIS experiment at 10:24 UT. Briefly, we show in Figure 15 this LEB in different emission lines where the much hotter (De Pontieu et al., 2014;Peter et al., 2014) OIV line is seen in Figure 16 with the slit shifted by 0.35''. Although the OIV line is significantly weaker in IRIS data than the SiIV lines, we clearly see the signature of the LEB, suggesting they could reach at least the TR temperature. Note also the behavior of the velocities shown by the spectral shifts of the Si IV lines for this LEB when Figures 15 and 16 are compared. From an energetic point of view, it is also worth noting that the LEBs look different from the bombs described by Peter et al. (2014) because the solar regions involved are completely different, the height above the photosphere is higher and finally, the OIV emission is seen in LEB and not in bombs (Peter et al., 2014). We however suggest that the velocities from the spectra seen in the TR lines are localized along directions nearly parallel to the surface with LOS effect. Note that the model proposed by Roussev et al. (2001) does not extend the diffusion region in the horizontal plane and would have difficulty explaining our observed split components and the occurrence of both red and blue shifted components.

Conclusions
Thanks to IRIS's outstanding resolution data where an unprecedented spectro-spatial resolution (about 1 km s -1 precision,1/3 arcsec in space and 10 seconds in time) was obtained in spectral rasters with simultaneous SJ images, we discovered a good example of a short duration but complex jet related to a limb event brightening (LEB) directly above the South polar limb, confirming and extending the earlier speculative interpretations given from the HRTS (Brueckner and Bartoe, 1983) photographic rocket observations. Evidence of extensions of the reconnection region in both height and azimuth (horizontal) is given with reconnection sites existing inside the 700 to 800 km side volume. At the SiIV emission temperature we observe a pair of rapidly moving separated parts like two split branches with a narrow 400 km space between them in the radial direction and another part nearby rapidly moving in the opposite direction. The signature of a LEB is also evidenced from the OIV hotter emission line. This type of spectral signature could be interpreted as evidence of 3D magnetic reconnection releasing energy at the sites where it occurs. The geometry of the magnetic region could correspond to the Eiffel-tower or to the 3D anemone configuration which eventually produces a long collimated ray that we detected on processed images as a linear thread fading out without apparent motion. The cool, dense surrounding chromosphere shows only a marginal spectral signature of these LEBs.
Our observation seems compatible with many possible scenarios that have been suggested for coronal type interactions at low plasma β, but more details need to be introduced for this type of event occurring at rather low heights, deep in the chromospheric layers (typically at 1 Mm height). The situation at these heights is more complex because the cool and dense surrounding neutral medium increases the importance of collisions and of selective radiations in emission lines during the LEB. A clear distinction is evident in the temperature "space" in the sense that almost nothing is simultaneously seen at the lower chromospheric temperature, which seems to be described here for the first time. Some limitation however occurs in our interpretation: the inherent method of observation that makes it impossible to get all the spectral data simultaneously because of the need for scanning the region using a very narrow slit and collecting spectra in each line to make the raster. Some ambiguity inevitably exists regarding the temporal and the spatial variations at very small scales. More statistically significant events need to be analyzed before speculating on their importance for the loading of mass into the corona and even for the coronal heating issue.
Conversely, 3D models should be developed with a more extended reconnection diffusion region, made of several knots in a volume of typically 800 km size, than in the classical X-type reconnection scenario. the work of our referee for making an extended detailed report and for many interesting suggestions and requests. This work has been supported by the Institut d'Astrophysique de Paris-CNRS and UPMC. LG was supported by a contract from Lockheed Martin to SAO.