Solar Jets Research Articles

Magnetic Flux Cancelation as the Buildup and Trigger Mechanism for CME-producing Eruptions in Two Small Active Regions

Abstract We follow two small, magnetically isolated coronal mass ejection (CME)-producing solar active regions (ARs) from the time of their emergence until several days later, when their core regions erupt to produce the CMEs. In both cases, magnetograms show: (a) following an initial period where the poles of the emerging regions separate from each other, the poles then reverse direction and start to retract inward; (b) during the retraction period, flux cancelation occurs along the main neutral line of the regions; (c) this cancelation builds the sheared core field/flux rope that eventually erupts to make the CME. In the two cases, respectively 30% and 50% of the maximum flux of the region cancels prior to the eruption. Recent studies indicate that solar coronal jets frequently result from small-scale filament eruptions, with those “minifilament” eruptions also being built up and triggered by cancelation of magnetic flux. Together, the small-AR eruptions here and the coronal jet results suggest that isolated bipolar regions tend to erupt when some threshold fraction, perhaps in the range of 50%, of the region's maximum flux has canceled. Our observed erupting filaments/flux ropes form at sites of flux cancelation, in agreement with previous observations. Thus, the recent finding that minifilaments that erupt to form jets also form via flux cancelation is further evidence that minifilaments are small-scale versions of the long-studied full-sized filaments.

Rotating network jets in the quiet Sun as observed by IRIS

Aims. We perform a detailed observational analysis of network jets to understand their kinematics, rotational motion, and underlying triggering mechanism(s). We analyzed the quiet-Sun (QS) data. Methods. IRIS high-resolution imaging and spectral observations (slit-jaw images: Si IV 1400.0 Å; raster: Si IV 1393.75 Å) were used to analyze the omnipresent rotating network jets in the transition region (TR). In addition, we also used observations from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observation (SDO). Results. The statistical analysis of 51 network jets is performed to understand their various mean properties, e.g., apparent speed (140.16 ± 39.41 km s−1), length (3.16 ± 1.18 Mm), and lifetimes (105.49 ± 51.75 s). The Si IV 1393.75 Å line has a secondary component along with its main Gaussian, which is formed due to the high-speed plasma flows (i.e., network jets). The variation in Doppler velocity across these jets (i.e., blueshift on one edge and redshift on the other) signify the presence of inherited rotational motion. The statistical analysis predicts that the mean rotational velocity (i.e., ΔV) is 49.56 km s−1. The network jets have high-angular velocity in comparison to the other class of solar jets. Conclusions. The signature of network jets is inherited in TR spectral lines in terms of the secondary component of the Si IV 1393.75 Å line. The rotational motion of network jets is omnipresent, which is reported first for this class of jet-like features. The magnetic reconnection seems to be the most favorable mechanism for the formation of these network jets.

Solar Coronal Jets Extending to High Altitudes Observed during the 2017 August 21 Total Eclipse

Abstract Coronal jets, which extend from the solar surface to beyond 2 R ⊙, were observed in the polar coronal hole regions during the total solar eclipse on 2017 August 21. In a time-series of white-light images of the corona spanning 70 minutes taken with our multi-site observations of this eclipse, six jets were found as narrow structures upwardly ejected with an apparent speed of about 450 km s−1 in polar plumes. On the other hand, extreme-ultraviolet (EUV) images taken with the Atmospheric Image Assembly of the Solar Dynamics Observatory show that all of the eclipse jets were preceded by EUV jets. Conversely, all the EUV jets whose brightnesses are comparable to ordinary soft X-ray jets and that occurred in the polar regions near the eclipse period, were observed as eclipse jets. These results suggest that ordinary polar jets generally reach high altitudes and escape from the Sun as part of the solar wind.

Recent progress in Asia-Pacific solar physics and astrophysics

More than 40 participants from the solar/astrophysical community attended the First Asia-Pacific Conference on Plasma Physics. Among them, four colleagues presented invited talks in the plenary session. In the Solar/Astron session, there were 23 invited talks and 14 contributed talks, with another two posters. These talks cover recent progress obtained in a wide spectrum of topics, including solar and galactic dynamo, solar and stellar flares, solar and galactic filaments, solar and astrophysical jets, solar and accretion disk winds, plasma waves and coronal heating, solar coronal mass ejections, magnetic reconnection in non-relativistic and relativistic regimes, star and planetary formation, shock–medium interactions, and even gravitational waves. Laboratory laser experiments and some new rocket and space missions were also introduced.

Observing Kelvin\u2013Helmholtz instability in solar blowout jet

Kelvin–Helmholtz instability (KHI) is a basic physical process in fluids and magnetized plasmas, with applications successfully modelling e.g. exponentially growing instabilities observed at magnetospheric and heliospheric boundaries, in the solar or Earth’s atmosphere and within astrophysical jets. Here, we report the discovery of the KHI in solar blowout jets and analyse the detailed evolution by employing high-resolution data from the Interface Region Imaging Spectrograph (IRIS) satellite launched in 2013. The particular jet we focus on is rooted in the surrounding penumbra of the main negative polarity sunspot of Active Region 12365, where the main body of the jet is a super-penumbral structure. At its maximum, the jet has a length of 90 Mm, a width of 19.7 Mm, and its density is about 40 times higher than its surroundings. During the evolution of the jet, a cavity appears near the base of the jet, and bi-directional flows originated from the top and bottom of the cavity start to develop, indicating that magnetic reconnection takes place around the cavity. Two upward flows pass along the left boundary of the jet successively. Next, KHI develops due to a strong velocity shear (∼204 km s−1) between these two flows, and subsequently the smooth left boundary exhibits a sawtooth pattern, evidencing the onset of the instability.

Onset of the Magnetic Explosion in Solar Polar Coronal X-Ray Jets

Abstract We follow up on the Sterling et al. discovery that nearly all polar coronal X-ray jets are made by an explosive eruption of a closed magnetic field carrying a miniature filament in its core. In the same X-ray and EUV movies used by Sterling et al., we examine the onset and growth of the driving magnetic explosion in 15 of the 20 jets that they studied. We find evidence that (1) in a large majority of polar X-ray jets, the runaway internal/tether-cutting reconnection under the erupting minifilament flux rope starts after both the minifilament’s rise and the spire-producing external/breakout reconnection have started; and (2) in a large minority, (a) before the eruption starts, there is a current sheet between the explosive closed field and the ambient open field, and (b) the eruption starts with breakout reconnection at that current sheet. The variety of event sequences in the eruptions supports the idea that the magnetic explosions that make polar X-ray jets work the same way as the much larger magnetic explosions that make a flare and coronal mass ejection (CME). That idea and recent observations indicating that magnetic flux cancellation is the fundamental process that builds the field in and around the pre-jet minifilament and triggers that field’s jet-driving explosion together suggest that flux cancellation inside the magnetic arcade that explodes in a flare/CME eruption is usually the fundamental process that builds the explosive field in the core of the arcade and triggers that field’s explosion.

Solar jet on 2014 April 16 modeled by Kelvin–Helmholtz instability

We study here the arising of Kelvin–Helmholtz Instability (KHI) in one fast jet of 2014 April 16 observed by the Atmospheric Imaging Assembly (AIA) on board Solar Dynamics Observatory (SDO) in different UV and EUV wavelengths. The evolution of jet indicates the blob like structure at its boundary which could be one of the observable features of the KHI development. We model the jet as a moving cylindrical magnetic flux tube of radius a embedded in a magnetic field Bi and surrounded by rest magnetized plasma with magnetic field Be. We explore the propagation of the kink MHD mode along the jet that can become unstable against the KHI if its speed exceeds a critical value. Concerning magnetic fields topology we consider three different configurations, notably of (i) spatially homogeneous magnetic fields (untwisted magnetic flux tube), (ii) internal (label ‘i’) twisted magnetic field and external homogeneous one (label ‘e’) (single-twisted flux tube), and (iii) both internal and external twisted magnetic fields (double-twisted magnetic flux tube). Electron number densities in the two media ni and ne are assumed to be homogeneous. The density contrast is defined in two ways: first as ne/ni and second as ne/(ni+ne). Computations show that the KHI can occur at accessible flow velocities in all the cases of untwisted and single-twisted flux tubes. It turns out, however, that in the case of a double-twisted flux tube the KHI can merge at an accessible jet speed only when the density contrast is calculated from the ratio ne/(ni+ne). Evaluated KHI developing times and kink mode wave phase velocities at wavelength of 4 Mm lie in the ranges of 1–6.2 min and 202–271 km s−1, respectively—all being reasonable for the modeled jet.

Penumbral Waves Driving Solar Fan-shaped Chromospheric Jets

Abstract We use Hα imaging spectroscopy taken via the Swedish 1 m Solar Telescope to investigate the occurrence of fan-shaped jets at the solar limb. We show evidence for near-simultaneous photospheric reconnection at a sunspot edge leading to the jets appearance, with upward velocities of 30 km s−1, and extensions up to 8 Mm. The brightening at the base of the jets appears recurrent, with a periodicity matching that of the nearby sunspot penumbra, implying running penumbral waves could be the driver of the jets. The jets’ constant extension velocity implies that a driver counteracting solar gravity exists, possibly as a result of the recurrent reconnection erupting material into the chromosphere. These jets also show signatures in higher temperature lines captured from the Solar Dynamics Observatory, indicating a very hot jet front, leaving behind optically thick cool plasma in its wake.

Multiple Solar Jets from NOAA AR 12644

AbstractWe present here the observations of solar jets observed on April 04, 2017 from NOAA active region (AR) 12644 using high temporal and spatial resolution AIA instrument. We have observed around twelve recurring jets during the whole day. Magnetic flux emergence and cancellation have been observed at the jet location. The multi-band observations evidenced that these jets were triggered due to the magnetic reconnection at low coronal null–point.

Magnetic Flux Cancelation as the Trigger of Solar Coronal Jets in Coronal Holes

Abstract We investigate in detail the magnetic cause of minifilament eruptions that drive coronal-hole jets. We study 13 random on-disk coronal-hole jet eruptions, using high-resolution X-ray images from the Hinode/X-ray telescope(XRT), EUV images from the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA), and magnetograms from the SDO/Helioseismic and Magnetic Imager (HMI). For all 13 events, we track the evolution of the jet-base region and find that a minifilament of cool (transition-region-temperature) plasma is present prior to each jet eruption. HMI magnetograms show that the minifilaments reside along a magnetic neutral line between majority-polarity and minority-polarity magnetic flux patches. These patches converge and cancel with each other, with an average cancelation rate of ∼0.6 × 1018 Mx hr−1 for all 13 jets. Persistent flux cancelation at the neutral line eventually destabilizes the minifilament field, which erupts outward and produces the jet spire. Thus, we find that all 13 coronal-hole-jet-driving minifilament eruptions are triggered by flux cancelation at the neutral line. These results are in agreement with our recent findings for quiet-region jets, where flux cancelation at the underlying neutral line triggers the minifilament eruption that drives each jet. Thus, from that study of quiet-Sun jets and this study of coronal-hole jets, we conclude that flux cancelation is the main candidate for triggering quiet-region and coronal-hole jets.

Study of Stealth CMEs and associated ICMEs

AbstractGenerally Coronal Mass Ejections (CMEs) are large eruptions of plasma and magnetic field from the Sun into interplanetary space. CMEs are most frequently associated with a variety of phenomena occurring in the lower corona before, during and after onset of eruption and generally are visible in coronagraph observation. Stealth CMEs do not obviously exhibit any of the low-coronal signatures (LCS) like solar flares, flows, jets, coronal dimmings or brightenings, filament eruptions or the formation of flare loop arcades. In this study, five stealth CMEs are selected using LASCO/SOHO CME catalogue and associated ICMEs (Interplanetaty CMEs) are identified using data from STEREO, ACE and WIND.

Observational study on the fine structure and dynamics of a solar jet. II. Energy release process revealed by spectral analysis

Abstract We report on a solar jet phenomenon associated with the C5.4 class flare on 2014 November 11. The data of the jet was provided by the Solar Dynamics Observatory, the X-Ray Telescope (XRT) aboard Hinode, and the Interface Region Imaging Spectrograph and Domeless Solar Telescope (DST) at Hida Observatory, Kyoto University. These plentiful data enabled us to present this series of papers to discuss all the processes of the observed phenomena, including energy storage, event trigger, and energy release. In this paper, we focus on the energy release process of the observed jet, and mainly describe our spectral analysis on the Hα data of DST to investigate the internal structure of the Hα jet and its temporal evolution. This analysis reveals that in the physical quantity distributions of the Hα jet, such as line-of-sight velocity and optical thickness, there is a significant gradient in the direction crossing the jet. We interpret this internal structure as the consequence of the migration of the energy release site, based on the idea of ubiquitous reconnection. Moreover, by measuring the horizontal flow of the fine structures in the jet, we succeeded in deriving the three-dimensional velocity field and the line-of-sight acceleration field of the Hα jet. The analysis result indicates that part of the ejecta in the Hα jet experienced additional acceleration after it had been ejected from the lower atmosphere. This secondary acceleration was found to occur in the vicinity of the intersection between the trajectories of the Hα jet and the X-ray jet observed by Hinode/XRT. We propose that a fundamental cause of this phenomenon is magnetic reconnection involving the plasmoid in the observed jet.

A Breakout Model for Solar Coronal Jets with Filaments

Abstract Recent observations have revealed that many solar coronal jets involve the eruption of miniature versions of large-scale filaments. Such “mini-filaments” are observed to form along the polarity inversion lines of strong, magnetically bipolar regions embedded in open (or distantly closing) unipolar field. During the generation of the jet, the filament becomes unstable and erupts. Recently we described a model for these mini-filament jets, in which the well-known magnetic-breakout mechanism for large-scale coronal mass ejections is extended to these smaller events. In this work we use 3D magnetohydrodynamic simulations to study in detail three realizations of the model. We show that the breakout-jet generation mechanism is robust and that different realizations of the model can explain different observational features. The results are discussed in relation to recent observations and previous jet models.

Kelvin–Helmholtz Instability in a Cool Solar Jet in the Framework of Hall Magnetohydrodynamics: A Case Study

We investigate the conditions under which the magnetohydrodynamic (MHD) modes in a cylindrical magnetic flux tube moving along its axis become unstable against the Kelvin--Helmholtz (KH) instability. We \textbf{use} the dispersion relations of MHD modes \textbf{obtained} from the linearized Hall MHD equations for cool (zero beta) plasma \textbf{by assuming} real wave numbers and complex angular wave frequencies\textbf{/complex wave phase velocities}. The dispersion equations are solved numerically at fixed input parameters and varying values of the ratio $l_\mathrm{Hall}/a$, where $l_\mathrm{Hall} = c/\omega_\mathrm{pi}$ ($c$ being the speed of light, and $\omega_\mathrm{pi}$ the ion plasma frequency) and $a$ is the flux tube radius. It is shown that the stability of the MHD modes depends upon four parameters: the density contrast between the flux tube and its environment, the ratio of external and internal magnetic fields, the ratio $l_\mathrm{Hall}/a$, and the value of the Alfv\'en Mach number \textbf{defined as the ratio of the tube axial velocity to Alfv\'en speed inside the flux tube}. It is found that at high density contrasts, for small values of $l_\mathrm{Hall}/a$, the kink ($m = 1$) mode can become unstable against KH instability at some critical Alfv\'en Mach number (or equivalently at critical flow speed), but a threshold $l_\mathrm{Hall}/a$ can suppress the onset of the KH instability. At small density contrasts, however, the magnitude of $l_\mathrm{Hall}/a$ does not affect noticeably the condition for instability occurrence---even though it can reduce the critical Alfv\'en Mach number. It is established that the sausage mode ($m = 0$) is not subject to the KH instability.

Untwisting Jets Related to Magnetic Flux Cancellation

Abstract The rotational motion of solar jets is believed to be a signature of the untwisting process resulting from magnetic reconnection, which takes place between twisted closed magnetic loops (i.e., magnetic flux ropes) and open magnetic field lines. The identification of the pre-existing flux rope, and the relationship between the twist contained in the rope and the number of turns the jet experiences, are then vital in understanding the jet-triggering mechanism. In this paper, we will perform a detailed analysis of imaging, spectral, and magnetic field observations of four homologous jets, among which the fourth one releases a twist angle of 2.6π. Nonlinear force-free field extrapolation of the photospheric vector magnetic field before the jet eruption presents a magnetic configuration with a null point between twisted and open fields—a configuration highly in favor of the eruption of solar jets. The fact that the jet rotates in the opposite sense of handness to the twist contained in the pre-eruption photospheric magnetic field confirms the unwinding of the twist by the jet’s rotational motion. The temporal relationship between jets’ occurrence and the total negative flux at their source region, together with the enhanced magnetic submergence term of the photospheric Poynting flux, shows that these jets are highly associated with local magnetic flux cancellation.

A Three-dimensional Magnetohydrodynamic Simulation of the Formation of Solar Chromospheric Jets with Twisted Magnetic Field Lines

Abstract This paper presents a three-dimensional simulation of chromospheric jets with twisted magnetic field lines. Detailed treatments of the photospheric radiative transfer and the equations of state allow us to model realistic thermal convection near the solar surface, which excites various MHD waves and produces chromospheric jets in the simulation. A tall chromospheric jet with a maximum height of 10–11 Mm and lifetime of 8–10 minutes is formed above a strong magnetic field concentration. The magnetic field lines are strongly entangled in the chromosphere, which helps the chromospheric jet to be driven by the Lorentz force. The jet exhibits oscillatory motion as a natural consequence of its generation mechanism. We also find that the produced chromospheric jet forms a cluster with a diameter of several Mm with finer strands. These results imply a close relationship between the simulated jet and solar spicules.

The association of aJ-burst with a solar jet

Context. The Sun is an active star that produces large-scale energetic events such as solar flares and coronal mass ejections and numerous smaller-scale events such as solar jets. These events are often associated with accelerated particles that can cause emission at radio wavelengths. The reconfiguration of the solar magnetic field in the corona is believed to be the cause of the majority of solar energetic events and accelerated particles. Aims. Here, we investigate a bright J-burst that was associated with a solar jet and the possible emission mechanism causing these two phenomena. Methods. We used data from the Solar Dynamics Observatory (SDO) to observe a solar jet, and radio data from the Low Frequency Array (LOFAR) and the Nan\c{c}ay Radioheliograph (NRH) to observe a J-burst over a broad frequency range (33-173 MHz) on 9 July 2013 at ~11:06 UT. Results. The J-burst showed fundamental and harmonic components and it was associated with a solar jet observed at extreme ultraviolet wavelengths with SDO. The solar jet occurred at a time and location coincident with the radio burst, in the northern hemisphere, and not inside a group of complex active regions in the southern hemisphere. The jet occurred in the negative polarity region of an area of bipolar plage. Newly emerged positive flux in this region appeared to be the trigger of the jet. Conclusions. Magnetic reconnection between the overlying coronal field lines and the newly emerged positive field lines is most likely the cause of the solar jet. Radio imaging provides a clear association between the jet and the J-burst which shows the path of the accelerated electrons.

Slippage of Jets Explained by the Magnetic Topology of NOAA Active Region 12035

In this study, we present the investigation of eleven recurring solar jets originated from two different sites (site 1 and site 2) close to each other (~ 11 Mm) in the NOAA active region (AR) 12035 during 15--16 April 2014. The jets were observed by the Atmospheric Imaging Assembly (AIA) telescope onboard the Solar Dynamics Observatory (SDO) satellite. Two jets were observed by the Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital, India telescope in H-alpha. On 15 April flux emergence is important in site 1 while on 16 April flux emergence and cancellation mechanisms are involved in both sites. The jets of both sites have parallel trajectories and move to the south with a speed between 100 and 360 km/s. We observed some connection between the two sites with some transfer of brightening. The jets of site 2 occurred during the second day and have a tendency to move towards the jets of site 1 and merge with them. We conjecture that the slippage of the jets could be explained by the complex topology of the region with the presence of a few low-altitude null points and many quasi-separatrix layers (QSLs), which could intersect with one another.

Observational study on the fine structure and dynamics of a solar jet. I. Energy build-up process around a satellite spot

Abstract We report on a solar jet phenomenon associated with successive flares on 2014 November 10. These explosive events were involved with the satellite spots’ emergence around a δ-type sunspot in the decaying active region NOAA 12205. The data for this jet were provided by the Solar Dynamics Observatory, the X-Ray Telescope aboard Hinode, and the Interface Region Imaging Spectrograph and Domeless Solar Telescope at Hida Observatory, Kyoto University. These abundant data enabled us to present this series of papers to discuss the entire process of the observed phenomena, including the energy storage, event trigger, and energy release. In this paper, we focus on the energy build-up and trigger phases, by analyzing the photospheric horizontal flow field around the active region by an optical flow method. The analysis shows the following results: (1) The observed explosive phenomena involved three satellite spots, the magnetic fluxes of which successively reconnected with their pre-existing ambient fields; (2) All of these satellite spots emerged in the moat region of a pivotal δ-type sunspot, especially near its convergent boundary with the neighboring supergranules or moat regions of adjacent sunspots; (3) Around the jet ejection site, the positive polarities of the satellite spot and adjacent emerging flux encountered the global magnetic field with a negative polarity in the moat region of the pivotal δ-type sunspot, and thus the polarity inversion line was formed along the convergent boundary of the photospheric horizontal flow channels.

Coronal Jet Collimation by Nonlinear Induced Flows

Abstract Our objective is to study the collimation of solar jets by nonlinear forces corresponding to torsional Alfvén waves together with external forces. We consider a straight, initially non-rotating, untwisted magnetic cylinder embedded in a plasma with a straight magnetic field, where a shear between the internal and external flows exists. By implementing magnetohydrodynamic theory and taking into account the second-order thin flux tube approximation, the balance between the internal nonlinear forces is visualized. The nonlinear differential equation containing the ponderomotive, magnetic tension, and centrifugal forces in the presence of the shear flow is obtained. The solution presents the scale of influence of the propagating torsional Alfvén wave on compressive perturbations. Explicit expressions for the compressive perturbations caused by the forces connected to the torsional Alfvén wave show that, in the presence of a shear flow, the magnetic tension and centrifugal forces do not cancel each other’s effects as they did in its absence. This shear flow plays in favor of the magnetic tension force, resulting in a more efficient collimation. Regarding the ponderomotive force, the shear flow has no effect. The phase relations highlight the interplay of the shear flow and the plasma-β. As the shear flow and plasma-β increase, compressive perturbation amplitudes emerge. We conclude that the jet collimation due to the torsional Alfvén wave highly depends on the location of the jet. The shear flow tightens the collimation as the jet elevates up to the solar corona.