We investigated mean properties and the spatiotemporal variability of eddies in the South China Sea (SCS) by analyzing more than 7000 eddies corresponding to 827 eddy tracks, identified using the ...winding angle method and 17 years of satellite altimetry data. Eddies are mainly generated in a northeast‐southwest direction and southwest of Luzon Strait. There is no significant difference between the numbers of two types of eddies (anticyclonic and cyclonic) in most regions. The mean radius and lifetime of eddies are 132 km and 8.8 weeks, respectively, both depending on where the eddies are formed. Anticyclonic and cyclonic eddies tend to deform during their lifetimes in different ways. Furthermore, eddy propagation and evolution characteristics are examined. In the northern SCS, eddies mainly propagate southwestward along the continental slope with velocities of 5.0–9.0 cm s−1, while in the central SCS, eddies tend to move with slight divergence but still in a quasi‐westward direction with velocities of 2.0–6.4 cm s−1. Eddy propagation in the western basin to the east of Vietnam is quite random, with no uniform propagate direction. Investigation of 38 long‐lived eddies shows that eddies have a swift growing phase during the first 12 weeks and then a slow decaying phase that affects the eddy radii and eddy energy densities. Nevertheless, vorticity has less variability. In addition, the effect of eddies on the thermocline and halocline is analyzed using 763 Argo temperature profile data. Cyclonic eddies drive the thermocline shallower and thinner and significantly strengthen the thermocline intensity, whereas anticyclonic eddies cause the thermocline to deepen and thicken and weaken the thermocline intensity to a certain degree. The halocline impacted by cyclonic eddies is also shallower and thinner than that impacted by anticyclonic eddies. Finally, eddy temporal variations are examined at seasonal and interannual scales. Eddy activity is sensitive to the wind stress curl and in the northern SCS it is also related with the strength of the background flows.
Key Points
Mean properties of eddies in the South China Sea
Spatiotemporal variability of eddies
The effect of eddies on the temperature profile
We analyze a unique event with an M1.8 confined circular-ribbon flare on 2016 February 13, with successive formations of two circular ribbons at the same location. The flare had two distinct phases ...of UV and extreme ultraviolet emissions with an interval of about 270 s, of which the second peak was energetically more important. The first episode was accompanied by the eruption of a mini-filament and the fast elongation motion of a thin circular ribbon (CR1) along the counterclockwise direction at a speed of about 220 km s−1. Two elongated spine-related ribbons were also observed, with the inner ribbon co-temporal with CR1 and the remote brightenings forming ∼20 s later. In the second episode, another mini-filament erupted and formed a blowout jet. The second circular ribbon and two spine-related ribbons showed similar elongation motions with that during the first episode. The extrapolated three-dimensional coronal magnetic fields reveal the existence of a fan-spine topology, together with a quasi-separatrix layer (QSL) halo surrounding the fan plane and another QSL structure outlining the inner spine. We suggest that continuous null-point reconnection between the filament and ambient open field occurs in each episode, leading to the sequential opening of the filament and significant shifts of the fan plane footprint. For the first time, we propose a compound eruption model of circular-ribbon flares consisting of two sets of successively formed ribbons and eruptions of multiple filaments in a fan-spine-type magnetic configuration.
Fan-spine is a special topology in solar atmosphere and is closely related to magnetic null point, as well as circular-ribbon flares, which can provide important information for understanding the ...intrinsic 3D nature of solar flares. However, the fine structure within the fan has rarely been investigated. In present paper, we investigate a secondary fan-spine (SFS) structure within the fan of a larger fan-spine topology. On 2013 November 18, this large fan-spine structure was traced out owing to the partial eruption of a filament, which caused a circular-ribbon flare in NOAA Active Region 11897. The extrapolated 3D magnetic fields and squashing factor Q maps depict distinctly this fan-spine topology, its surrounding quasi-separatrix layer (QSL) halo, and a smaller quasi-circular ribbon with high Q located in the center, which implies the existence of fine structure within the fan. The imaging observations, extrapolated 3D fields, and Q maps on November 17 show that there indeed exists an SFS surrounded by a QSL, which is enveloped by another QSL halo corresponding to the overlying larger dome-shaped fan. Moreover, the material flows caused by the null-point reconnection are also detected along this SFS. After checking the evolution of the underneath magnetic fields, we suggest that the continuous emergence of magnetic flux within the central parasitic region encompassed by the opposite-polarity fields results in the formation of the SFS under the large fan.
Two Types of Confined Solar Flares Li, Ting; Liu, Lijuan; Hou, Yijun ...
Astrophysical journal/The Astrophysical journal,
08/2019, Volume:
881, Issue:
2
Journal Article
Peer reviewed
Open access
With the aim of understanding the physical mechanisms of confined flares, we selected 18 confined flares during 2011-2017, and first classified them into two types based on their different dynamic ...properties and magnetic configurations. "Type I" confined flares are characterized by slipping reconnection, strong shear, and a stable filament. "Type II" flares have almost no slipping reconnection, and have a configuration in potential state after the flare. A filament erupts but is confined by a strong strapping field. "Type II" flares could be explained by 2D MHD models, while "type I" flares need 3D MHD models. Seven of 18 confined flares (∼39%) belong to "type I" and 11 (∼61%) are "type II." The post-flare loops (PFLs) of "type I" flares have a stronger non-potentiality, but the PFLs in "type II" flares are weakly sheared. All the "type I" flares exhibit ribbon elongations parallel to the polarity inversion line (PIL) at speeds of several tens of km s−1. Only a small proportion of "type II" flares show ribbon elongations along the PIL. We suggest that different magnetic topologies and reconnection scenarios dictate the distinct properties for the two types of flares. Slipping magnetic reconnections between multiple magnetic systems result in "type I" flares. For "type II" flares, magnetic reconnections occur in antiparallel magnetic fields underlying the erupting filament. Our study shows that "type I" flares account for more than one third of all the large confined flares, and should not be neglected in further studies.
Eddy activity has been examined in many regions of the world ocean. However, the mechanism behind interannual variability of mesoscale eddies remains uncertain. Although eddies can be frequently ...observed in the Bay of Bengal (BOB), there is no thorough statistical assessment of eddy activity in the bay. Here, we use altimeter data and ocean reanalysis data to show the genesis, propagation, mean properties and the spatiotemporal variability of eddies in the BOB. Energy analysis suggests the interannual variation of eddy kinetic energy (EKE) in the BOB depends on that of perturbation kinetic energy induced by mesoscale eddies. Eddy activity at interannual scale is sensitive to the baroclinic instability of the background flow. When the baroclinic instability is stronger, more energy is converted from the mean flow to the eddy energy, which makes eddies stronger and more stable but fewer in number. The west coast of the BOB is a critical region for eddy-mean flow interaction.
► Eddies are mainly generated along the coasts, especially at the eastern boundary. ► Eddy activity at interannual scale is sensitive to the baroclinic instability. ► The converted energy from the mean flow makes eddies stronger but fewer in number.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
This study describes characteristics of eddy (turbulent) heat and salt transports, in the basin‐scale circulation as well as in the embedded mesoscale eddy found in the South China Sea (SCS). We ...first showed the features of turbulent heat and salt transports in mesoscale eddies using sea level anomaly (SLA) data, in situ hydrographic data, and 375 Argo profiles. We found that the transports were horizontally variable due to asymmetric distributions of temperature and salinity anomalies and that they were vertically correlated with the thermocline and halocline depths in the eddies. An existing barrier layer caused the halocline and eddy salt transport to be relatively shallow. We then analyzed the transports in the basin‐scale circulation using an eddy diffusivity method and the sea surface height data, the Argo profiles, and the climatological hydrographic data. We found that relatively large poleward eddy heat transports occurred to the east of Vietnam (EOV) in summer and to the west of the Luzon Islands (WOL) in winter, while a large equatorward heat transport was located to the west of the Luzon Strait (WLS) in winter. The eddy salt transports were mostly similar to the heat transports but in the equatorward direction due to the fact that the mean salinity in the upper layer in the SCS tended to decrease toward the equator. Using a 21/2‐layer reduced‐gravity model, we conducted a baroclinic instability study and showed that the baroclinic instability was critical to the seasonal variation of eddy kinetic energy (EKE) and thus the eddy transports. EOV, WLS, and WOL were regions with strong baroclinic instability, and, thus, with intensified eddy transports in the SCS. The combined effects of vertical velocity shear, latitude, and stratification determined the intensity of the baroclinic instability, which intensified the eddy transports EOV during summer and WLS and WOL during winter.
Key Points
The eddy heat and salt transport in the South China Sea basin
Eddy transports feature in the mesoscale eddy
Seasonal variations mechanism of eddy transports
We establish the largest eruptive/confined flare database to date and analyze 322 flares of Geostationary Operational Environmental Satellite class M1.0 and larger that occurred during 2010-2019, ...i.e., almost spanning all of solar cycle 24. We find that the total unsigned magnetic flux ( ) of active regions (ARs) is a key parameter governing the eruptive character of large flares, with the proportion of eruptive flares exhibiting a strong anticorrelation with . This means that an AR containing a large magnetic flux has a lower probability that the large flares it produces will be associated with a coronal mass ejection (CME). This finding is supported by the high positive correlation we obtained between the critical decay index height and , implying that ARs with a larger have a stronger magnetic confinement. Moreover, the confined flares originating from ARs larger than Mx have several characteristics in common: stable filament, slipping magnetic reconnection, and strongly sheared post-flare loops. Our findings reveal new relations between the magnetic flux of ARs and the occurrence of CMEs in association with large flares. The relations obtained here provide quantitative criteria for forecasting CMEs and adverse space weather, and have important implications for "superflares" on solar-type stars and stellar CMEs.
Abstract
Partial filament eruptions have often been observed; however, the physical mechanisms that lead to filament splitting are not yet fully understood. In this study, we present a unique event ...of a partial filament eruption that undergoes two distinct splitting processes. The first process involves vertical splitting and is accompanied by brightenings inside the filament, which may result from internal magnetic reconnection within the filament. Following the first splitting process, the filament is separated into an upper part and a lower part. Subsequently, the upper part undergoes a second splitting, which is accompanied by a coronal blowout jet. An extrapolation of the coronal magnetic field reveals a hyperbolic flux tube structure above the filament, indicating the occurrence of breakout reconnection that reduces the constraining field above. Consequently, the filament is lifted up, but at a nonuniform speed. The high-speed part reaches the breakout current sheet to generate the blowout jet, while the low-speed part falls back to the solar surface, resulting in the second splitting. In addition, continuous brightenings are observed along the flare ribbons, suggesting the occurrence of a slipping reconnection process. This study presents, for the first time, the unambiguous observation of a two-stage filament-splitting process, advancing our understanding of the complex dynamics of solar eruptions.
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
) between these two flows, and subsequently the smooth left boundary exhibits a sawtooth pattern, evidencing the onset of the instability.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Abstract Until now, how the magnetic fields in M/X-class flaring active regions (ARs) differ from C-class flaring ARs remains unclear. Here, we calculate the key magnetic field parameters within the ...area of high photospheric free energy density (HED region) for 323 ARs (217 C- and 106 M/X-flaring ARs), including total photospheric free magnetic energy density E free , total unsigned magnetic flux Φ HED , mean unsigned current helicity h c , length of the polarity inversion lines L PIL with a steep horizontal magnetic gradient, etc., and compare these with flare/coronal mass ejection (CME) properties. We first show the quantitative relations among the flare intensity, the eruptive character, and Φ HED . We reveal that Φ HED is a measure for the GOES flux upper limit of the flares in a given region. For a given Φ HED , there exists the lower limit of F SXR for eruptive flares. This means that only the relatively strong flares with the large fraction of energy release compared to the total free energy are likely to generate a CME. We also find that the combinations of E free – L PIL and E free – h c present a good ability to distinguish between C-class and M/X-class flaring ARs. Using determined critical values of E free and L PIL , one predicts correctly 93 out of 106 M/X-class flaring ARs and 159 out of 217 C-class flaring ARs. The large L PIL or h c for M/X-class flaring ARs probably implies the presence of a compact current with twisted magnetic fields winding about it.
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