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Register a new userSpectro-Polarimetric Properties of Small-Scale Plasma Eruptions Driven by Magnetic Vortex Tubes
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Irina N. Kitiashvili
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Highly turbulent nature of convection on the Sun causes strong multi-scale interaction of subsurface layers with the photosphere and chromosphere. According to realistic 3D radiative MHD numerical simulations ubiquitous small-scale vortex tubes are generated by turbulent flows below the visible surface and concentrated in the intergranular lanes. The vortex tubes can capture and amplify magnetic field, penetrate into chromospheric layers and initiate quasi-periodic flow eruptions that generates Alfvenic waves, transport mass and energy into the solar atmosphere. The simulations revealed high-speed flow patterns, and complicated thermodynamic and magnetic structures in the erupting vortex tubes. The spontaneous eruptions are initiated and driven by strong pressure gradients in the near-surface layers, and accelerated by the Lorentz force in the low chromosphere. In this paper, the simulation data are used to further investigate the dynamics of the eruptions, their spectro-polarimetric characteristics for the Fe I 6301.5 and 6302.5 A spectral lines, and demonstrate expected signatures of the eruptions in the Hinode SP data. We found that the complex dynamical structure of vortex tubes (downflows in the vortex core and upflows on periphery) can be captured by the Stokes I profiles. During an eruption, the ratio of down and upflows can suddenly change, and this effect can be observed in the Stokes V profile. Also, during the eruption the linear polarization signal increases, and this also can be detected with Hinode SP.
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The solar surface is covered by high-speed jets transporting mass and energy into the solar corona and feeding the solar wind. The most prominent of these jets have been known as spicules. However, the mechanism initiating these eruptions events is still unknown. Using realistic numerical simulations we find that small-scale eruptions are produced by ubiquitous magnetized vortex tubes generated by the Suns turbulent convection in subsurface layers. The swirling vortex tubes (resembling tornadoes) penetrate into the solar atmosphere, capture and stretch background magnetic field, and push surrounding material up, generating quasiperiodic shocks. Our simulations reveal a complicated high-speed flow patterns, and thermodynamic and magnetic structure in the erupting vortex tubes. We found that the eruptions are initiated in the subsurface layers and are driven by the high-pressure gradients in the subphotosphere and photosphere, and by the Lorentz force in the higher atmosphere layers.
Vortex-type motions have been measured by tracking bright points in high-resolution observations of the solar photosphere. These small-scale motions are thought to be determinant in the evolution of magnetic footpoints and their interaction with plasma and therefore likely to play a role in heating the upper solar atmosphere by twisting magnetic flux tubes. We report the observation of magnetic concentrations being dragged towards the center of a convective vortex motion in the solar photosphere from high-resolution ground-based and space-borne data. We describe this event by analyzing a series of images at different solar atmospheric layers. By computing horizontal proper motions, we detect a vortex whose center appears to be the draining point for the magnetic concentrations detected in magnetograms and well-correlated with the locations of bright points seen in G-band and CN images.
Filaments, the dense cooler plasma floating in the solar corona supported by magnetic fields, generally exhibit certain activations before they erupt. In our previous study (Seki et al. 2017 ), we observed that the standard deviation of the line-of-sight (LOS) velocities of the small-scale motions in a filament increased prior to its eruption. However, because that study only analyzed one event, it is unclear whether such an increase in the standard deviation of LOS velocities is common in filament eruptions. In this study, 12 filaments that vanished in H{alpha} line center images were analyzed in a manner similar to the one in our previous work; these included two quiescent filaments, four active region filaments, and six intermediate filaments. We verified that in all the 12 events, the standard deviation of the LOS velocities increased before the filaments vanished. Moreover, we observed that the quiescent filaments had approximately 10 times longer duration of an increase in the standard deviation than the other types of filaments. We concluded that the standard deviation of the LOS velocities of the small-scale motions in a filament can potentially be used as the precursor of a filament eruption.
Very massive stars occasionally expel material in colossal eruptions, driven by continuum radiation pressure rather than blast waves. Some of them rival supernovae in total radiative output, and the mass loss is crucial for subsequent evolution. Some are supernova impostors, including SN precursor outbursts, while others are true SN events shrouded by material that was ejected earlier. Luminous Blue Variable stars (LBVs) are traditionally cited in relation with giant eruptions, though this connection is not well established. After four decades of research, the fundamental causes of giant eruptions and LBV events remain elusive. This review outlines the basic relevant physics, with a brief summary of essential observational facts. Reasons are described for the spectrum and emergent radiation temperature of an opaque outflow. Many proposed mechanisms are noted for instabilities in the stars photosphere, in its iron opacity peak zones, and in its central region. Some of the remarks and conjectures here have not yet become familiar in the published literature.
Large-scale solar eruptions have been extensively explored over many years. However, the properties of small-scale events with associated shocks have been rarely investigated. We present the analyses of a small-scale short-duration event originating from a small region. The impulsive phase of the M1.9-class flare lasted only for four minutes. The kinematic evolution of the CME hot channel reveals some exceptional characteristics including a very short duration of the main acceleration phase ($<$ 2 minutes), a rather high maximal acceleration rate ($sim$50 km s$^{-2}$) and peak velocity ($sim$1800 km s$^{-1}$). The fast and impulsive kinematics subsequently results in a piston-driven shock related to a metric type II radio burst with a high starting frequency of $sim$320 MHz of the fundamental band. The type II source is formed at a low height of below $1.1~mathrm{R_{odot}}$ less than $sim2$ minutes after the onset of the main acceleration phase. Through the band split of the type II burst, the shock compression ratio decreases from 2.2 to 1.3, and the magnetic field strength of the shock upstream region decreases from 13 to 0.5 Gauss at heights of 1.1 to 2.3 $~mathrm{R_{odot}}$. We find that the CME ($sim4times10^{30},mathrm{erg}$) and flare ($sim1.6times10^{30},mathrm{erg}$) consume similar amount of magnetic energy. The same conclusion for large-scale eruptions implies that small- and large-scale events possibly share the similar relationship between CMEs and flares. The kinematic particularities of this event are possibly related to the small footpoint-separation distance of the associated magnetic flux rope, as predicted by the Erupting Flux Rope model.
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