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The jet structure when radiative cooling is included in the solution starting from uniform density. On the left is temperature and velocity and on the right density and magnetic field.
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Small-scale explosive events or microflares occur throughout the chromospheric network of the Sun. They are seen as sudden bursts of highly Doppler-shifted spectral lines of ions formed at temperatures in the range 2104–5105 K. They tend to occur near regions of cancelling photospheric magnetic fields and are thought to be directly associated with...
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Context 1
... the case with uniform temperature initially (left hand panels) the narrowing of the jet is not obvious because after 50 s the jet length is hardly longer than the cooling length. The temperature and density structure of the cooling jet and their relationship to the jet flow and magnetic field are shown in Figure 4. As in the case without radiative losses, the slow magnetoa- coustic shocks accelerate and heat plasma along the boundary of the jet. ...
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Citations
... This presentation is different than the typical explosive event line profiles typically observed by IRIS in Si IV 1394 Å, where small transient events often show smaller bulk flows, dominant line core emission, and nonthermal broadening (Innes et al. 2015;Chitta et al. 2017). The bimodal profile and bulk flows exceeding the sound speed (∼78 km s −1 for O V at 225,000 K) suggest magnetic reconnection occurring in a small region with little to no stationary emitting plasma, most reminiscent of a Petschek-type reconnection (Innes & Tóth 1999). Though we hypothesize that both jets originate at a single reconnection site, we admit that it is puzzling that the observed fluctuations of the red and blue jets are not temporally correlated, most notably in explosive event d ( Figure 14). ...
The Extreme-ultraviolet Snapshot Imaging Spectrograph (ESIS) launched on a sounding rocket from White Sands Missile Range on 2019 September 30. ESIS is a computed tomography imaging spectrograph (CTIS) designed to map emission line profiles across a wide field of view, revealing the structure and dynamics of small-scale transient events that are prevalent at transition region temperatures. In this paper, we review the ESIS instrument, mission, and data captured. We demonstrate how this unique data set can be interpreted qualitatively and further present some initial quantitative inversions of the data. Using a multiplicative algebraic reconstruction technique, we combine information from all four ESIS channels into a single spatial–spectral cube at every exposure. We analyze two small explosive events in the O v 629.7 Å spectral line with jets near ±100 km s ⁻¹ that evolve on 10 s timescales and vary significantly over small spatial scales. Intriguingly, each of these events turns out to be a bimodal (red+blue) jet with outflows that are asymmetric and unsynchronized. We also present a qualitative analysis of a small jetlike eruption captured by ESIS and draw comparisons to previously observed mini-filament eruptions. In 5 minutes of observing time, ESIS captured the spatial and temporal evolution of tens of these small events across the ∼11.′5 field of view, as well as several larger extended eruptions, demonstrating the advantage of CTIS instruments over traditional slit spectrographs in capturing the spatial and spectral information of dynamic solar features across large fields of view.
... Most of the models so far have used a Harris-type current sheet (Harris 1962) as an initial setup. For example, Innes & Tóth (1999) used a magnetic field that changes as tanh across the current sheet. To initiate reconnection, these early models used an anomalously enhanced resistivity at a single location along the current sheet. ...
... To initiate reconnection, these early models used an anomalously enhanced resistivity at a single location along the current sheet. The resulting reconnection outflow speeds and temperatures are consistent with observations (Innes & Tóth 1999) in that they provide an explanation for the enhanced emission in the line wings often showing up as separate spectral components (Roussev et al. 2001b,c,a). When also considering gravitational stratification, Roussev & Galsgaard (2002) were able to find asymmetric line profiles (as seen in observations; Teriaca et al. 2004), but they could still not recover the increase of the intensity in the line core that is associated with most (but not all) transient events. ...
UV bursts are transients in the solar atmosphere with an increased impulsive emission in the extreme UV lasting for one to several tens of minutes. They often show spectral profiles indicative of a bi-directional outflow in response to magnetic reconnection. To understand UV bursts, we study how motions of magnetic elements at the surface can drive the self-consistent formation of a current sheet resulting in plasmoid-mediated reconnection. In particular, we want to study the role of the height of the reconnection in the atmosphere. We conducted numerical experiments solving the 2D MHD equations from the solar surface to the upper atmosphere. Motivated by observations, we drove a small magnetic patch embedded in a larger system of magnetic field of opposite polarity. This configuration creates an X-type neutral point in the initial potential field. The models are characterized by the plasma-beta at the height of this X point. The driving at the surface stretches the X-point into a current sheet, where plasmoids appear, and a bi-directional jet forms. This is consistent with what is expected for UV bursts or explosive events, and we provide a self-consistent model of the formation of the reconnection region in such events. The gravitational stratification gives an explanation for why explosive events are restricted to a temperature range around a few 0.1 MK, and the presence of plasmoids in the reconnection process provides an understanding of the observed variability during the transient events on a timescale of minutes. Our numerical experiments provide a comprehensive understanding of UV bursts and explosive events, in particular of how the atmospheric response changes if the reconnection happens at different plasma-beta, that is, at different heights in the atmosphere. This analysis also gives new insight into how UV bursts might be related to the photospheric Ellerman bombs.
... Most of the models so far have used a Harris-type current sheet (Harris 1962) as an initial setup. For example, Innes & Tóth (1999) used a magnetic field that changes as tanh across the current sheet. To initiate reconnection, these early models used an anomalously enhanced resistivity at a single location along the current sheet. ...
... To initiate reconnection, these early models used an anomalously enhanced resistivity at a single location along the current sheet. The resulting reconnection outflow speeds and temperatures are consistent with observations (Innes & Tóth 1999) in that they provide an explanation for the enhanced emission in the line wings often showing up as separate spectral components (Roussev et al. 2001a,b,c). When also considering gravitational stratification, Roussev & Galsgaard (2002) were able to find asymmetric line profiles (as seen in observations; Teriaca et al. 2004), but they could still not recover the increase of the intensity in the line core that is associated with most (but not all) transient events. ...
Context . Ultraviolet bursts are transients in the solar atmosphere with an increased impulsive emission in the extreme UV lasting for one to several tens of minutes. They often show spectral profiles indicative of a bi-directional outflow in response to magnetic reconnection.
Aims . To understand UV bursts, we study how motions of magnetic elements at the surface can drive the self-consistent formation of a current sheet resulting in plasmoid-mediated reconnection. In particular, we want to study the role of the height of the reconnection in the atmosphere.
Methods . We conducted numerical experiments solving the 2D magnetohydrodynamic equations from the solar surface to the upper atmosphere. Motivated by observations, we drove a small magnetic patch embedded in a larger system of magnetic field of opposite polarity. This type of configuration creates an X-type neutral point in the initial potential field. The models are characterized by the (average) plasma- β at the height of this X point.
Results . The driving at the surface stretches the X-point into a thin current sheet, where plasmoids appear, accelerating the reconnection, and a bi-directional jet forms. This is consistent with what is expected for UV bursts or explosive events, and we provide a self-consistent model of the formation of the reconnection region in such events. The gravitational stratification gives a natural explanation for why explosive events are restricted to a temperature range around a few 0.1 MK, and the presence of plasmoids in the reconnection process provides an understanding of the observed variability during the transient events on a timescale of minutes.
Conclusions . Our numerical experiments provide a comprehensive understanding of UV bursts and explosive events, in particular of how the atmospheric response changes if the reconnection happens at different plasma- β , that is, at different heights in the atmosphere. This analysis also gives new insight into how UV bursts might be related to the photospheric Ellerman bombs.
... First, an acceleration phase lasting 60 s shows plasma being accelerated from rest into bidirectional jets at the characteristic Alfvén speed. The next phase lasts approximately 150 s and is similar to the steady-state Petschek model of Innes & Tóth (1999): wing emission at the Alfvén speed and no emission from low-velocity plasma, i.e., no line core. Core brightening begins after 300 s, with core emission dominating the profiles through the end of the simulation (400 s). ...
... Given that explosive events occur rather frequently, there is great potential to study reconnection itself under transition region conditions. The particular reconnection models of Innes & Tóth (1999) andInnes et al. (2015) suggest that the observed strength of emission in the line core is a vital clue to help distinguish which reconnection process is at work. ...
... The example MOSES-06explosive events have weak line core emission and show no indication of core brightening over the observation period. The bidirectional jet in particular supports the Petschek reconnection explosive event model of Innes & Tóth (1999) rather than the tearing mode model of Innes et al. (2015). The difference between the He II and Si IV line profiles-core dominated versus wing dominated -may indicate that He II and Si IV explosive events fall in different regions of the reconnection phase diagram of Ji & Daughton (2011). ...
In this paper the unique data from the Multi-Order Solar Extreme-Ultraviolet Spectrograph ( MOSES ) are used to investigate transition region explosive events in the He ii λ 304 spectral line. Particular attention is paid to two example events: one blueshifted jet and one bidirectional jet. Observations suggest that these events consist exclusively of high-velocity (∼100 km s ⁻¹ ) plasma. These two and other examples presented here exhibit a striking lack of emission in the line core. No acceleration phase is observed at the onset of either event. In total, 41 examples of explosive events are identified, including 5 blueshifted jets, 2 redshifted jets, and 10 bidirectional jets. The remaining 24 events resist simple classification, but observations indicate compact, highly Doppler-shifted emission. Event spatial scales and lifetimes are consistent with published explosive event characteristics. Data from the Michelson Doppler Imager provide magnetic context to the MOSES observations. Bidirectional jets lacking line core emission are interesting because they are predicted in models of Petschek reconnection in the transition region.
... MHD numerical simulations have been applied to magnetic reconnection in the corona (Shen et al. 2011;Ni et al. 2012), transition region (e.g., Innes & Tóth 1999;Sarro et al. 1999;Roussev et al. 2001) and chromosphere or even photosphere (Ni et al. 2015(Ni et al. , 2016 on the Sun. Previous simulations of the transition-region explosive events were often based on the Petschek mechanism (Petschek 1964). ...
... Further simulation (Roussev et al. 2001) indicated that the reconnection with an X-point in the transition region produced a large blue shift (∼100 km s −1 ) but a small red shift. Meanwhile, Innes & Tóth (1999) performed compressible MHD simulations of small-scale explosive events based on the Petschek model, and they found that these simulations could reproduce well the blue or red shifts with high velocities, but failed to explain the bright core near the line center with low velocities observed in the spectra (e.g., Dere et al. 1991;Innes 2001). Based on large-scale MHD simulations (e.g., Bhattacharjee et al. 2009;Heggland et al. 2009;Huang & Bhattacharjee 2010;Huang et al. 2017), magnetic reconnection that proceeds via plasmoid instability has been proposed. ...
... The explosive events with bi-directional jets observed in the transition regions are thought to be produced by small-scale magnetic reconnection on the Sun (Innes et al. 1997). They have been studied by many authors based on spectroscopic observations (e.g., Dere 1994;Innes et al. 1997;Ning et al. 2004;Innes & Teriaca 2013;Huang et al. 2014) and MHD simulations (Jin et al. 1996;Innes & Tóth 1999;Sarro et al. 1999;Roussev et al. 2001). However, there is always a contradiction between the observations and simulations of these line profiles. ...
Magnetic reconnection is thought to be a key process in most solar eruptions. Thanks to highresolution observations and simulations, the studied scale of the reconnection process has become smaller and smaller. Spectroscopic observations show that the reconnection site can be very small, which always exhibits a bright core and two extended wings with fast speeds, i.e., transition-region explosive events. In this paper, using the PLUTO code, we perform a 2-D magnetohydrodynamic simulation to investigate small-scale reconnection in double current sheets. Based on our simulation results, such as the line-of-sight velocity, number density and plasma temperature, we can synthesize the line profile of Si IV 1402.77 Å which is a well known emission line used to study transition-region explosive events on the Sun. The synthetic line profile of Si IV 1402.77 Å is complex with a bright core and two broad wings which can extend to nearly 200 km s ⁻¹ . Our simulation results suggest that the transition-region explosive events on the Sun are produced by plasmoid instability during small-scale magnetic reconnection.
... MHD numerical simulations have been applied to the magnetic reconnection in the corona (Shen et al., 2011;Ni et al., 2012), the transition region (e.g., Innes & Tóth, 1999;Sarro et al., 1999;Roussev et al., 2001), and the chromosphere or even the photosphere (Ni et al., 2015(Ni et al., , 2016 on the Sun. Previous simulations of the transition-region explosive events were often based on the Petschek mechanism (Petschek, 1964). ...
... Further simulation (Roussev et al., 2001) indicated that the reconnection with an X-point in the transition region produced a large blue shift (∼100 km s −1 ), but a small red shift. Meanwhile, Innes & Tóth (1999) performed a compressible MHD simulations of the small-scale explosive events based on the Petschek model, and they found that the simulations could well reproduce the blue or red shifts with high velocities, but failed to explain the bright core near the line center with low velocities observed by the spectra (e.g., Dere et al., 1991;Innes, 2001). Based on the large-scale MHD simulations (e.g., Bhattacharjee et al., 2009;Heggland et al., 2009;Huang & Bhattacharjee, 2010;, the magnetic reconnection proceeds via the plasmoid instability has been proposed. ...
... The explosive evens with bi-directional jets observed in the transition regions are thought to be produced by the small-scale magnetic reconnection on the Sun (Innes et al., 1997). They have been studied by many authors based on the spectroscopic observations (e.g., Dere, 1994;Innes et al., 1997;Ning et al., 2004;Innes & Teriaca, 2013;Huang et al., 2014) and MHD simulations (Jin et al., 1996;Innes & Tóth, 1999;Sarro et al., 1999;Roussev et al., 2001). However, there is always a contradiction between the observations and the simulations of these line profiles. ...
Magnetic reconnection is thought to be a key process in most of solar eruptions. Thanks to high-resolution observations and simulations, the studied scale of reconnection process has become smaller and smaller. Spectroscopic observations show that the reconnection site can be very small, which always exhibits a bright core and two extended wings with fast speeds, i.e., transition-region explosive events. In this paper, using the PLUTO code, we perform a 2-D magnetohydrodynamic simulation to investigate the small-scale reconnection in double current sheets. Based on our simulation results, such as the line-of-sight velocity, number density and plasma temperature, we can synthesize the line profile of Si IV 1402.77 A which is a well known emission line to study the transition-region explosive events on the Sun. The synthetic line profile of Si IV 1402.77 A is complex with a bright core and two broad wings which can extend to be nearly 200 km/s. Our simulation results suggest that the transition-region explosive events on the Sun are produced by plasmoid instability during the small-scale magnetic reconnection.
... Chae et al. (1998aestablished that the majority of explosive events are associated with the cancellation of photospheric magnetic flux, which was recently confirmed by Huang et al. (2014) and Gupta & Tripathi (2015). EEs were modelled by Innes & Tóth (1999), Roussev & Galsgaard (2002), Roussev et al. (2001c), Roussev et al. (2001b), Roussev et al. (2001a), and Innes et al. (2015 in twodimensional numerical simulations as the product of magnetic reconnection. Non-Gaussian line profiles in the solar transition region are intensively investigated since the first flight of the Naval Research Laboratory (NRL) High Resolution Telescope and Spectrograph (HRTS) in 1975 . ...
Transition-region explosive events (EEs) are characterized by non-Gaussian line profiles with enhanced wings at Doppler velocities of 50-150 km/s. They are believed to be the signature of solar phenomena that are one of the main contributors to coronal heating. The aim of this study is to investigate the link of EEs to dynamic phenomena in the transition region and chromosphere in an active region. We analyze observations simultaneously taken by the Interface Region Imaging Spectrograph (IRIS) in the Si IV 1394\AA\ line and the slit-jaw (SJ) 1400\AA\ images, and the Swedish 1-m Solar Telescope (SST) in the H$\alpha$ line. In total 24 events were found. They are associated with small-scale loop brightenings in SJ 1400\AA\ images. Only four events show a counterpart in the H$\alpha$-35 km/s and H$\alpha$+35 km/s images. Two of them represent brightenings in the conjunction region of several loops that are also related to a bright region (granular lane) in the H$\alpha$-35km/s and H$\alpha$+35 km/s images. Sixteen are general loop brightenings that do not show any discernible response in the H$\alpha$ images. Six EEs appear as propagating loop brightenings, from which two are associated with dark jet-like features clearly seen in the H$\alpha$-35 km/s images. We found that chromospheric events with jet-like appearance seen in the wings of the H$\alpha$ line can trigger EEs in the transition region and in this case the IRIS Si IV 1394\AA\ line profiles are seeded with absorption components resulting from Fe II and Ni II. Our study indicates that EEs occurring in active regions have mostly upper-chromosphere/transition-region origin. We suggest that magnetic reconnection resulting from the braidings of small-scale transition region loops is one of the possible mechanisms of energy release that are responsible for the EEs reported in this paper.
... However , these phenomena are mostly in the shorttime scale, such as explosive events (e.g., Innes et al. 1997; Pérez et al. 1999; Innes & Teriaca 2013), chromospheric upflow events (Chae et al. 1998), and blinkers (Chae et al. 2000; Brkovi´cBrkovi´c & Peter 2004; Bewsher et al. 2005 ). Following the observation results, the MHD simulations is applied to build up the models of these events based on the magnetic reconnection (see., Innes & Tóth 1999; Sarro et al. 1999; Roussev et al. 2001a,b). Until now, investigation on the bi-directional outflows in the CBP is still poorly done. ...
... Innes et al. (1997) had indicated that the bi-directional outflows in the transition explosive events can be as the direct observational evidence of the magnetic reconnection. This is consistent with the MHD results, which suggest that the bi-directional outflows in the explosive evens are from magnetic reconnection regions (Innes & Tóth 1999; Sarro et al. 1999; Roussev et al. 2001a,b). Recently, Ning & Guo (2014) detected the similar bi-directional outflows in a CBP, and considered them as the direct observational evidence of the magnetic reconnection. ...
... They could be explained by the reconnection model. This is similar as the bi-directional outflows in the explosive events which also interpreted as the reconnection jets (e.g., Innes et al. 1997; Innes & Tóth 1999; Sarro et al. 1999; Pérez et al. 1999; Roussev et al. 2001a,b; Innes & Teriaca 2013; Innes et al. 2015). As indicated by Chae et al. (1998) , the explosive events may be the manifestation of hot plasma materials flowing out of the transition region. ...
We report the bi-directional moving structures in a coronal bright point (CBP) on 2015 July 14. It is observed by the Atmospheric Imaging Assembly (AIA) onboard Solar Dynamics Observatory (SDO). This CBP has a lifetime of about 10 minutes, and a curved shape. The observations show that many bright structures are moving intermittently outward from the CBP brightness core. Such moving structures are clearly seen at AIA 171, 193, 211, 131, 94, 335 and 304 A, slit-jaw (SJI) 1330 and 1400 A. In order to analyze these moving structures, the CBP is cut along the moving direction with a curved slit from the AIA and SJI images. Then we can obtain the time-distance slices, including the intensity and intensity-derivative diagrams, from which, the moving structures are recognized as the oblique streaks, and they are characterized by the bi-direction, simultaneity, symmetry, and periodicity. The average speed is around 300 km/s, while the typically period is about 90 s. All these features (including the bi-directional flows and their periodicity) can be detected simultaneously at all the 9 wavelengths. This CBP takes place at the site between a small pair of magnetic polarities. High time resolution observations show that they are moving close to each other during its lifetime. These facts support the magnetic reconnection model of the CBP and the bi-directional moving structures could be the observational outflows after the reconnection. Therefore, they can be as the direct observation evidence of the magnetic reconnection.
... Dere et al. (1991) used explosive events to examine magnetic reconnection by assuming that they are all the result of reconnection. Innes and Tóth (1999) conducted simulations of explosive events to examine the behaviour of temperature emission lines from reconnection. Winebarger et al. (2002) explored the energetics of explosive events and found that individual events were not energetically significant with regard to coronal and chromospheric heating. ...
A comprehensive understanding of the different transient events is necessary for any eventual solution of the coronal heating problem. We present a cold loop whose heating caused a short-lived small-scale brightening that was observed by AIA. The loop was simulated using an adaptive hydrodynamic radiation code that considers the ions to be in a state of non-equilibrium. Forward modelling was used to create synthetic AIA intensity plots, which were tested against the observational data to confirm the simulated properties of the event. The hydrodynamic properties of the loop were determined. We found that the energy released by the heating event is within the canonical energy range of a nanoflare.
... Explosive events seem to concentrate along the boundaries of the magnetic network near sites of cancelling/evolving magnetic flux (Porter and Dere, 1991;Chae et al., 1998;Muglach, 2008;Aiouaz, 2008) and are thus believed to result from magnetic reconnection at the Sun. Although the line profiles, energetics and magnetic field evolution can be explained by energy release at a reconnection site in the transition region (Sterling, Shibata, and Mariska, 1993;Dere et al., 1991;Innes et al., 1997a;Innes and Tóth, 1999;Roussev et al., 2001), it has been argued that the transition region reacts with similar signatures of explosive energy release to microflaring in the corona (Krucker and Benz, 2000) or reconnection in the photosphere ( Tarbell et al., 1999;Ryutova and Tarbell, 2000). It is also possible that explosive events are related to chromospheric jets that cause jet-like brightenings in transition region and coronal images (De Pontieu et al., 2011). ...
Explosive events are small transition region phenomena characterised by broad
non-Gaussian wings in their line profiles. Images from the Solar Dynamics
Observatory (SDO) give a first view of the plasma dynamics at the sites of
explosive events seen in O VI spectra of a region of quiet Sun, taken with the
ultraviolet spectrometer SUMER/SOHO. Distinct event bursts were seen either at
the junction of supergranular network cells or near emerging flux. Three are
described in the context of their surrounding transition region (304 A) and
coronal (171 A) activity. One showed plasma ejected from one footpoint of a
small loop which resulted in a `splash' at the other footpoint. The second was
related to flux cancellation, inferred from SDO/HMI magnetograms, and a coronal
dimming surrounded by a ring of bright EUV emission with explosive events at
positions where the spectrometer slit crossed the bright ring. The third series
of events occurred at the base of a slow mini-CME. All events studied here
imply jet-like flows probably triggered by magnetic reconnection at
supergranular junctions. Events come from sites close to the footpoints of jets
seen in AIA images, and possibly from the landing site of induced high velocity
flows. They are not caused by rapid rotation in spicules.
