Radial Outflows Research Articles

On the Transition from Efficient to Inefficient Line Driving in Irradiated Flows

Observations of ionized outflows from active galactic nuclei (AGNs) provide evidence of energy and momentum transfer from the AGN radiation to the plasma. The AGN radiation is very energetic. Therefore, at distances of parsec scale, where gravity is relatively weak, energy transfer alone can lead to outflow. Much closer to the black hole, gravity dominates thermal energy and the gas is in the so-called “cold” regime. Only magnetic or radiation forces can lead to outflow. However, it is unclear when the radiation force is efficient in overcoming gravity because of its dependence on the spectral energy distribution (SED) of the radiation and opacity. In this work, we survey the parameter space of radiation forces due to spectral lines resulting from blackbody SEDs with temperatures ranging from ∼104 to 106 K. The objective was to identify the radiation temperature above which line driving becomes inefficient. We find that the temperature ≲4 × 105 K marks such a transition. We also self-consistently calculate heating and cooling balance to estimate gas temperature and identify the transition where thermal driving becomes comparable to line driving. We summarize hydrodynamical simulations of radial outflows to illustrate how wind properties change during the transition from line to thermal driving and their dependence on outflow parameters and SED.

Experimental translating downbursts immersed in the atmospheric boundary layer

Thunderstorm winds are cold descending gravity currents whose impingement on the ground creates strong radial outflows with maximum wind speeds in the near-ground region. They represent one of the greatest hazards for natural and built environment as well as one of the deadliest phenomena all over the world. This study carries on the post-processing analyses of the downburst experimental campaign performed at the WindEEE Dome, at Western University in Canada, in the context of the ERC project THUNDERR. While a former study presented the interaction between downburst and atmospheric boundary layer (ABL) winds, here the focus is on the influence exerted by the thunderstorm cloud translation. This was experimentally replicated at large scale by means of impinging jet technique where the jet axis was inclined to a non-zero angle with respect to the vertical. Finally, the inclusion of background ABL wind allowed to reconstruct the complete three-dimensional and non-stationary nature of the phenomenon. The outflow radial symmetry is lost in case of inclined jet axis. This leads to an intensification of the front-wind side and weakening of the rear-wind side, where the entrainment of the counter-directed ABL wind, and consequent flow speed-up, are not as pronounced as in the vertical-axis case. The non-linearity of the complex interaction between downburst, ABL flow and cloud translation is proven and quantified. Vertical profiles of mean wind speed and turbulence intensity are discussed in relation to the mutual interaction among flow components.

High-resolution imaging of solar pores

Context. Light bridges are bright, long, and narrow features that are typically connected to the formation or decay processes of sunspots and pores. Aims. The interaction of magnetic fields and plasma flows is investigated in the trailing part of an active region, where pores and magnetic knots evolve into a complex sunspot. The goal is to identify the photospheric and chromospheric processes, which transform the mainly vertical magnetic fields of pores into a sunspot with multiple umbral cores, light bridges, and rudimentary penumbrae. Methods. Conducting observations with a broad variety of telescopes and instruments provides access to different atmospheric layers and the changing morphology of features connected to strong magnetic fields. While the Helioseismic and Magnetic Imager (HMI) of the Solar Dynamics Observatory (SDO) provides full-disk continuum images and line-of-sight magnetograms, the fine structure and flows around a pore can be deduced from high-resolution observations in various wavelengths as provided by the Goode Solar Telescope (GST) at the Big Bear Solar Observatory (BBSO). Horizontal proper motions are evaluated applying local correlation tracking (LCT) to the available time series, whereas the connectivity of sunspot features can be established using the background-subtracted activity maps (BaSAMs). Results. Photospheric flow maps indicate radial outflows, where the light bridge connects to the surrounding granulation, whereas inflows are present at the border of the pores. In contrast, the chromospheric flow maps show strong radial outflows at superpenumbral scales, even in the absence of a penumbra in the photosphere. The region in between the two polarities is characterized by expanding granules creating strong divergence centers. Variations in BaSAMs follow locations of significant and persistent changes in and around pores. The resulting maps indicate low variations along the light bridge, as well as thin hairlines connecting the light bridge to the pores and strong variations at the border of pores. Various BaSAMs demonstrate the interaction of pores with the surrounding supergranular cell. The Hα line-of-sight velocity maps provide further insights into the flow structure, with twisted motions along some of the radial filaments around the pore with the light bridge. Furthermore, flows along filaments connecting the two polarities of the active region are pronounced in the line-of-sight velocity maps. Conclusions. The present observations reveal that even small-scale changes of plasma motions in and around pores are conducive to transform pores into sunspots. In addition, chromospheric counterparts of penumbral filaments appear much earlier than the penumbral filaments in the photosphere. Penumbra formation is aided by a stable magnetic feature that anchors the advection of magnetic flux and provides a connection to the surrounding supergranular cell, whereas continuously emerging flux and strong light bridges are counteragents that affect the appearance and complexity of sunspots and their penumbrae.

Experimental investigation of the near-surface flow dynamics in downburst-like impinging jets

Downbursts are strong downdrafts that originate from thunderstorm clouds and create vigorous radial outflows upon hitting the ground. This study is part of the comprehensive experimental research on downburst outflows produced as large-scale impinging jets in the WindEEE Dome simulator at Western University, Canada. The 2800 tests carried out form the largest database of experimental measurements on downburst winds developed thus far, which is made available to the public in its whole and described in detail in a complementary study. Therefore, the current manuscript merely focuses on the data post-processing outcomes and interpretation of results from a selected subset of measurements. Impinging jets are here simulated as transient phenomena in which velocity time series are characterized by a sudden ramp-up of velocity, followed by the velocity peak, a short statistically stationary region, and the final velocity slowdown, as it is expected to occur in the actual downbursts. A dominant velocity peak that was systematically observed in all velocity records is associated with the radial advection of the primary vortex in the outflow. Depending on the radial distance from the downdraft, the primary vortex was sometimes preceded by a secondary, much smaller, vortex close to the surface. Vertical profiles of mean velocity and turbulence intensity are for the first time characterized through the extent of a downburst-like event in the spatiotemporal domain. Particularly, these profiles rapidly change in relation to the passage of the primary vortex and consequent variation of the surface layer thickness. This study lays out a foundation for an experimental model of non-stationary downburst outflows to come.

Downburst-like experimental impinging jet measurements at the WindEEE Dome

This paper describes the dataset of measurements collected and published in the context of the comprehensive experimental campaign on downburst-like outflows that was performed at the WindEEE Dome at Western University, Canada. Downbursts are strong downdrafts of air that originate from thunderstorm clouds and create vigorous radial outflows upon hitting the ground. Downbursts are here simulated as transient phenomena produced by large-scale impinging jet. Two jet velocities were adopted in the experiments. The three-component velocity measurements were recorded using 7 Cobra probes mounted on a vertical stiff mast and displaced at 10 radial positions in respect to the downdraft centerline. For every radial position, each experiment with the same initial condition was repeated 20 times to inspect the deterministic features of the signal. Overall, the total of 2800 tests (2 jet velocities × 20 repetitions × 10 radial positions × 7 heights) represent one of the largest experimental campaigns on downburst winds carried out in a wind tunnel facility thus far.

Non-Gaussianity of optical emission lines in SDSS star-forming galaxies and its implications on galactic outflows

Abstract The shape of emission lines in the optical spectra of star-forming galaxies reveals the kinematics of the diffuse gaseous component. We analyse the shape of prominent emission lines in a sample of $\sim$ 53000 star-forming galaxies from the Sloan Digital Sky Survey, focusing on departures from gaussianity. Departures from a single gaussian profile allow us to probe the motion of gas and to assess the role of outflows. The sample is divided into groups according to their stellar velocity dispersion and star formation rate (SFR). The spectra within each group are stacked to improve the signal-to-noise ratio of the emission lines, to remove individual signatures, and to enhance the effect of SFR on the shapes of the emission lines. The moments of the emission lines, including kurtosis and skewness, are determined. We find that most of the emission lines in strong star-forming systems unequivocally feature negative kurtosis. This signature is present in $\mathrm{H}\unicode{x03B2}$ , $\mathrm{H}\unicode{x03B1}$ , [N ii], and [S ii] in massive galaxies with high SFRs. We attribute it as evidence of radial outflows of ionised gas driven by the star formation of the galaxies. Also, most of the emission lines in low-mass systems with high SFRs feature negative skewness, and we interpret it as evidence of dust obscuration in the galactic disk. These signatures are however absent in the [O iii] line, which is believed to trace a different gas component. The observed trend is significantly stronger in face-on galaxies, indicating that star formation drives the outflows along the galactic rotation axis, presumably the path of least resistance. The data suggest that outflows driven by star formation exert accumulated impacts on the interstellar medium, and the outflow signature is more evident in older galaxies as they have experienced a longer total duration of star formation.

Ionized gas kinematics of cluster AGN at z ∼ 0.8 with KMOS

ABSTRACT We present an integral field spectroscopy study of two active galactic nuclei (AGN) in the galaxy cluster RXJ0152–137 at z = 0.84. We have combined integrated and 3D spectra with HST images to perform a complete morpho-kinematical analysis of each galaxy. For G557, we detect narrow and broad H α and [N ii] emission. The broad H α component is redshifted by v ∼ 939 km s−1 concerning the narrow component. The kinematics analysis and VLT/FORS2 data indicate that G557 presents evidence of radial outflows. For G300, it was possible to detect only the broad H α emission. The spectral properties of both galaxies are equivalent to those of local AGN. We also compare the results obtained for each AGN with their location within the cluster. We found a possible connection of the environment with the properties inferred for each of them, and the possible interaction of G557 with an unconfirmed neighboring galaxy, for which a bridge-like structure is observed in the HST images. This work shows evidence of how the same type of AGN can show differences in their spectroscopic properties depending on the region of the cluster in which they develop, which is important to understand how the environment affects AGN and, consequently, the evolution of galaxies.

Experimental simulation of stationary and traveling density-driven thunderstorm downbursts using the two-fluid model

In the present study, novel release mechanisms were developed for laboratory simulations of density-driven downbursts using the two-fluid model. The ability of these new designs in producing repeatable outflows was examined and the influence of minimizing the release cylinder wall effect was tested. The entire experimental setup was also modified to simulate the effect of translation on the resultant downburst wind flows. Planar laser induced fluorescence (PLIF) technique was utilized to visualize the flows and to provide quantitative scalar (density) field data. The results from this study showed a clear difference between the flow fields arising from the two release mechanisms, namely, a higher degree of mixing with ambient fluid, lower downward speed and higher radial propagation rate at the early stages of the simulated downburst lifecycle for the porous wall cylinder. The outflows of stationary and traveling downbursts were compared and the differences between their flow fields and propagation rates were elaborated; for the shear flows tested, a linear relationship between the downburst displacement and storm translation velocity was obtained. The radial outflows of the simulated downbursts were compared to the empirical gravity currents and the mixing of downburst and ambient fluids was quantified using calibrated PLIF images.

Secondary Eyewall Formation and Concentric Eyewall Replacement in Association with Increased Low-Level Inner-Core Diabatic Cooling

Abstract The role of increased diabatic cooling in secondary eyewall formation (SEF) and eyewall replacement cycle (ERC) is examined using idealized numerical simulation. The experiment with the low-level inner-core diabatic cooling increased by 30% features the low-entropy air and downward motion in the inner-core region whereas the convergence and active convective updrafts are in the outer-core region. In collaboration with the favorable ambient dynamical conditions and boundary layer dynamical processes, the concentric convective ring is initiated with the aid of the outward expansion of strong wind field, and then contracts inward to replace the inner eyewall. Subsequently, the deep-tropospheric radial outflows driven by the large outward-directed agradient force related to the massive strong tangential wind generate a largely outward-tilted eyewall, eventually forming a large-eyed storm. The sensitivity to the strength and radial location of diabatic cooling shows that neither the 20% increase nor 10-km radially inward shift of the low-level cooling produces a pronounced SEF and ERC because of the lack of an evident moat region. In contrast, both the 40% increase and 10-km radially outward shift of cooling lead to the active outer rainbands occurring at a larger radius. In the former case, because of the deep-layer radial outflow above the boundary layer, the largely outward-tilted concentric eyewall shrinks slowly, directly creating a large-eyed structure. In the latter case, the formation of concentric eyewall is delayed because of the low inertial stability at a large radius, but experiences an expeditious ontraction because of the strong radial inflow.

Resolving the Nuclear Obscuring Disk in the Compton-thick Seyfert Galaxy NGC 5643 with ALMA

Abstract We present ALMA Band 6 12CO(2–1) line and rest-frame 232 GHz continuum observations of the nearby Compton-thick Seyfert galaxy NGC 5643 with angular resolutions 0.″11–0.″26 (9–21 pc). The CO(2–1) integrated line map reveals emission from the nuclear and circumnuclear region with a two-arm nuclear spiral extending ∼10″ on each side. The circumnuclear CO(2–1) kinematics can be fitted with a rotating disk, although there are regions with large residual velocities and/or velocity dispersions. The CO(2–1) line profiles of these regions show two different velocity components. One is ascribed to the circular component and the other to the interaction of the AGN outflow, as traced by the [O iii]λ5007 Å emission, with molecular gas in the disk a few hundred parsecs from the AGN. On nuclear scales, we detected an inclined CO(2–1) disk (diameter 26 pc, FWHM) oriented almost in a north–south direction. The CO(2–1) nuclear kinematics can be fitted with a rotating disk that appears to be tilted with respect to the large-scale disk. There are strong non-circular motions in the central 0.″2–0.″3 with velocities of up to 110 km s−1. In the absence of a nuclear bar, these motions could be explained as radial outflows in the nuclear disk. We estimate a total molecular gas mass for the nuclear disk of M(H2) = 1.1 × 107 M ⊙ and an H2 column density toward the location of the AGN of N(H2) ∼ 5 × 1023 cm−2, for a standard CO-to-H2 conversion factor. We interpret this nuclear molecular gas disk as the obscuring torus of NGC 5643 as well as the collimating structure of the ionization cone.

Stellar Winds and Dust Avalanches in the AU Mic Debris Disk

Abstract We explain the fast-moving, ripple-like features in the edge-on debris disk orbiting the young M dwarf AU Mic. The bright features are clouds of submicron dust repelled by the host star’s wind. The clouds are produced by avalanches: radial outflows of dust that gain exponentially more mass as they shatter background disk particles in collisional chain reactions. The avalanches are triggered from a region a few au across—the “avalanche zone”—located on AU Mic’s primary “birth” ring at a true distance of ∼35 au from the star but at a projected distance more than a factor of 10 smaller: the avalanche zone sits directly along the line of sight to the star, on the side of the ring nearest Earth, launching clouds that disk rotation sends wholly to the southeast, as observed. The avalanche zone marks where the primary ring intersects a secondary ring of debris left by the catastrophic disruption of a progenitor up to Varuna in size, less than tens of thousands of years ago. Only where the rings intersect are particle collisions sufficiently violent to spawn the submicron dust needed to seed the avalanches. We show that this picture works quantitatively, reproducing the masses, sizes, and velocities of the observed escaping clouds. The Lorentz force exerted by the wind’s magnetic field, whose polarity reverses periodically according to the stellar magnetic cycle, promises to explain the observed vertical undulations. The timescale between avalanches, about 10 yr, might be set by time variability of the wind mass loss rate or, more speculatively, by some self-regulating limit cycle.

The Grism Lens-Amplified Survey from Space (GLASS). X. Sub-kiloparsec Resolution Gas-phase Metallicity Maps at Cosmic Noon behind the Hubble Frontier Fields Cluster MACS1149.6+2223

Abstract We combine deep Hubble Space Telescope grism spectroscopy with a new Bayesian method to derive maps of gas-phase metallicity for 10 star-forming galaxies at high redshift ( ). Exploiting lensing magnification by the foreground cluster MACS1149.6+2223, we reach sub-kiloparsec spatial resolution and push the limit of stellar mass associated with such high-z spatially resolved measurements below for the first time. Our maps exhibit diverse morphologies, indicative of various effects such as efficient radial mixing from tidal torques, rapid accretion of low-metallicity gas, and other physical processes that can affect the gas and metallicity distributions in individual galaxies. Based upon an exhaustive sample of all existing sub-kiloparesec resolution metallicity gradient measurements at high z, we find that predictions given by analytical chemical evolution models assuming a relatively extended star-formation profile in the early disk-formation phase can explain the majority of observed metallicity gradients, without involving galactic feedback or radial outflows. We observe a tentative correlation between stellar mass and metallicity gradients, consistent with the “downsizing” galaxy formation picture that more massive galaxies are more evolved into a later phase of disk growth, where they experience more coherent mass assembly at all radii and thus show shallower metallicity gradients. In addition to the spatially resolved analysis, we compile a sample of homogeneously cross-calibrated integrated metallicity measurements spanning three orders of magnitude in stellar mass at z ∼ 1.8. We use this sample to study the mass–metallicity relation (MZR) and find that the slope of the observed MZR can rule out the momentum-driven wind model at a 3σ confidence level.

Relativistic jet feedback in high-redshift galaxies – I. Dynamics

We present the results of three dimensional relativistic hydrodynamic simulations of interaction of AGN jets with a dense turbulent two-phase interstellar medium, which would be typical of high redshift galaxies. We describe the effect of the jet on the evolution of the density of the turbulent ISM. The jet driven energy bubble affects the gas to distances up to several kiloparsecs from the injection region. The shocks resulting from such interactions create a multi-phase ISM and radial outflows. One of the striking result of this work is that low power jets (P_jet < 10^{43} erg/s) although less efficient in accelerating clouds, are trapped in the ISM for a longer time and hence affect the ISM over a larger volume. Jets of higher power drill through with relative ease. Although the relativistic jets launch strong outflows, there is little net mass ejection to very large distances, supporting a galactic fountain scenario for local feedback.

Anatomy of outbursts and quiescent activity of Comet 29P/Schwassmann–Wachmann

Detailed morphological and photometric characterisation of Comet 29P in the optical region is presented comprising: (a) multi-filter observations of the outburst coma in 2010–2012 with the 2.0-m Faulkes Telescopes (FT); (b) high-cadence, high-precision photometry in May–September 2014; and (c) HST observations in March 1996 (WFPC2/F702W filter). Outbursts appear to be explosive in that: the rise to maximum light is short-lived; the expanding coma fits a model in which ejecta are produced in a singular event and expand into space with uniform velocity; and the motion of condensations within the outburst coma indicate a common onset time and origin. The bright outburst of 2010 February 2 generated a dust coma exhibiting expansion speeds up to 0.257±0.013kms−1 consistent with acceleration of cometary grains close to the nucleus driven by sublimating CO ice and N2 ice at 24±6K. Material ejected ∼1d after this outburst exhibited a V–R colour gradient: redder towards the main outflow, bluer in the opposite sense; potentially arising from differences in spectral emission (from gas), light scattering (particle size), and spectral reflectance (composition). B–V, V–R and R–I colour images revealed colour/compositional differences in near-nucleus structures and the evolution of the expanding coma, which brightened by ∼30% within 5–6days of the outburst. Broadband photometry indicated a general reddening coma with time (change in B–V from +0.76 to +0.83; V–SDSS-r′ from 0.25 to +0.31 in 14d). SDSS-r′–SDSS-i′ photometry indicated gradual spectral attenuation at >700nm. Asymmetric, fan-shaped comae, characteristic of 29P, potentially form when the expanding cloud from an outburst is shielded by the nucleus. Rotational-gradient filtered HST and FT images show unusual 2-fold and 4-fold symmetry involving oppositely-directed radial outflows moving at up to 0.15kms−1: possibly an indication of material escaping from fissures along the perimeter of a crustal ‘plate’ when dislodged by pressure build-up in the subsurface. Pairs of outbursts separated in time by 52–65d took place in 2010, 2011, and 2012 exhibiting similar coma outflow patterns indicative of outbursts repeating a second time from the same source, and suggesting a nuclear rotation period of 59±4 (s.e.)d. The escape velocity of the nucleus is sufficiently high (0.013–0.023kms−1) that a significant fraction of ejecta falls back onto its surface, the action of which, we suggest, re-forms the crust and may trigger outbursts from nearby sites (e.g. triple events of February 2010, and May 2014). A short-lived (<1d) anomalous brightening of 0.36±0.12mag observed on 2014 July 21 during quiescence may have arisen from an especially weak mini-outburst in which most of the ejected material failed to reach escape velocity. During quiescence in 2014, Comet 29P fluctuated in brightness over time-scales of 2–10d by up to ±0.25mag, probably via local jet activity continuing to feed the faint persistent inner coma. 29P also exhibited weak outbursts of <1mag amplitude during quiescence and these will generally be missed by observers. Image analysis methodology tracking the locus of the coma photocentre with increasing photometric aperture size is used to quantify the directional motion of cometary haloes within 1–2d of an outburst.

Direct numerical simulation of Open Von Kármán Swirling Flow

Direct numerical simulations are used to investigate the Open Von Kármán Swirling Flow, a new type of unsteady three-dimensional flow that is formed between two counter-rotating coaxial disks with an axial extraction enclosed by a cylinder chamber. Solution verification shows that monotonic convergence is achieved on three systematically refined grids for average pressure at the disk periphery with a small grid uncertainty at 3.5%. Effects of the rotational speeds and flow rates on the flow field are examined. When the disks are rotating at the lowest speed, ±100 RPM, only circular vortices are formed regardless of the flow rates. When the disks are rotating at ±300 RPM and ±500 RPM, negative spiral vortex network is formed. The radial counterflow concept for such spiral vortex network is verified by examining various horizontal cuts and radial velocity component, which show radial outflows in two bands near the two disks and radial inflow in one band between them. Overall, the flow is similar to the Stewartson type flow but with significant differences for all three velocity components due to the axial suction at the upper disk center and gap between the disk periphery and chamber wall.

Properties of the HII Regions Derived Using Integral Field Spectroscopy

Here we review some of our more recent results on the observed properties of HIIregions using Integral Field Spectroscopy. In particular, we illustrate the use of this technique to study in detail the ionization conditions across the nebulae for galactic HIIregions (focused on the Orion Nebula) and the statistical study of large samples of extragalactic HIIregions. We review the reported new scaling relation between the local mass density and the oxygen abundance across the disk galaxies and the recently discovered universal gradient for oxygen abundances. We update our previous results the lack of a dependence of the Mass-Metallicity relation with the starformation rate, including new unpublished data. Finally we discuss on the relation between the ionization conditions in the nebulae and the underlying stellar population. All together our results indicate that disk galaxies present a chemical enrichment dominated by an inside-out growth scenario, with a less evident effect of radial migrations and/or outflows.

A Cloud-Resolving Simulation Study on the Merging Processes and Effects of Topography and Environmental Winds

Abstract The cloud-resolving fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) was used to study the cloud interactions and merging processes in the real case that generated a mesoscale convective system (MCS) on 23 August 2001 in the Beijing region. The merging processes can be grouped into three classes for the studied case: isolated nonprecipitating and precipitating cell merging, cloud cluster merging, and echo core or updraft core merging within cloud systems. The mechanisms responsible for the multiscale merging processes were investigated. The merging process between nonprecipitating cells and precipitating cells and that between clusters is initiated by forming an upper-level cloud bridge between two adjacent clouds due to upper-level radial outflows in one vigorous cloud. The cloud bridge is further enhanced by a favorable middle- and upper-level pressure gradient force directed from one cloud to its adjacent cloud by accelerating cloud particles being horizontally transported from the cloud to its adjacent cloud and induce the redistribution of condensational heating, which destabilizes the air at and below the cloud bridge and forms a favorable low-level pressure structure for low-level water vapor convergence and merging process. The merging of echo cores within the mesoscale cloud happens because of the interactions between low-level cold outflows associated with the downdrafts formed by these cores. Further sensitivity studies on the effects of topography and large-scale environmental winds suggest that the favorable pressure gradient force from one cloud to its adjacent cloud and stronger low-level water vapor convergence produced by the topographic lifting of large-scale low-level airflow determine further cloud merging processes over the mountain region.

SST/CRISP observations of convective flows in a sunspot penumbra

Context. Recent discoveries of intensity correlated downflows in the interior of a sunspot penumbra provide direct evidence for overturning convection, adding to earlier strong indications of convection from filament dynamics observed far from solar disk center, and supporting recent simulations of sunspots. Aims. Using spectropolarimetric observations obtained at a spatial resolution approaching 0'.'1 with the Swedish 1-m Solar Telescope (SST) and its spectropolarimeter CRISP, we investigate whether the convective downflows recently discovered in the C i line at 538.03 nm can also be detected in the wings of the Fe i line at 630.15 nm Methods. We make azimuthal fits of the measured LOS velocities in the core and wings of the 538 nm and 630 nm lines to disentangle the vertical and horizontal flows. To investigate how these depend on the continuum intensity, the azimuthal fits are made separately for each intensity bin. By using spatially high-pass filtered measurements of the LOS component of the magnetic field, the flow properties are determined separately for magnetic spines (relatively strong and vertical field) and inter-spines (weaker and more horizontal field). Results. The dark convective downflows discovered recently in the 538.03 nm line are evident also in the 630.15 nm line, and have similar strength. This convective signature is the same in spines and inter-spines. However, the strong radial (Evershed) outflows are found only in the inter-spines. Conclusions. At the spatial resolution of the present SST/CRISP data, the small-scale intensity pattern seen in continuum images is strongly related to a convective up/down flow pattern that exists everywhere in the penumbra. Earlier failures to detect the dark convective downflows in the interior penumbra can be explained by inadequate spatial resolution in the observed data.

Two-dimensional axisymmetric and three-dimensional helical equilibrium in the line-tied screw pinch

The line-tying condition at a conducting anode is shown to provide a localized modification to the well-understood 1-D screw pinch equilibrium in the presence of bulk plasma diamagnetism. Diamagnetic currents cannot flow near the conducting anode and are measured to disappear in a localized boundary layer, causing a weak mirror configuration that breaks 1-D equilibrium and causes large parallel pressure gradients suggestive of significant radial outflows. For sufficiently large plasma currents, the paramagnetic nature of parallel current drives the equilibrium to paramagnetism and destroys the mirror effect. At a critical plasma current, the axisymmetric equilibrium is found to transition to a long-lived, rotating, helical 3-D equilibrium state. Internal measurements of this state via multi-point correlation analysis techniques illustrate that it preserves the flux surfaces and pressure profile of the axisymmetric equilibrium. Measurements indicate that despite the fact that the flux surfaces wander at the anode, the line-tied boundary conditions are not necessarily violated.

PENUMBRAL FINE STRUCTURE AND DRIVING MECHANISMS OF LARGE-SCALE FLOWS IN SIMULATED SUNSPOTS

We analyze in detail the penumbral structure found in a recent radiative MHD simulation. Near tau=1, the simulation produces penumbral fine structure consistent with the observationally inferred interlocking comb structure. Fast outflows exceeding 8 km/s are present along almost horizontal stretches of the magnetic field; in the outer half of the penumbra, we see opposite polarity flux indicating flux returning beneath the surface. The bulk of the penumbral brightness is maintained by small-scale motions turning over on scales shorter than the length of a typical penumbral filament. The resulting vertical rms velocity at tau=1 is about half of that found in the quiet Sun. Radial outflows in the sunspot penumbra have two components. In the uppermost few 100 km, fast outflows are driven primarily through the horizontal component of the Lorentz force, which is confined to narrow boundary layers beneath tau=1, while the contribution from horizontal pressure gradients is reduced in comparison to granulation as a consequence of anisotropy. The resulting Evershed flow reaches its peak velocity near tau=1 and falls off rapidly with height. Outflows present in deeper layers result primarily from a preferred ring-like alignment of convection cells surrounding the sunspot. These flows reach amplitudes of about 50% of the convective rms velocity rather independent of depth. A preference for the outflow results from a combination of Lorentz force and pressure driving. While the Evershed flow dominates by velocity amplitude, most of the mass flux is present in deeper layers and likely related to a large-scale moat flow.