Plume-like Structures Research Articles

Marine magnetotellurics imaged no distinct plume beneath the Tristan da Cunha hotspot in the southern Atlantic Ocean

The Tristan da Cunha (TDC) is a volcanic island located above a prominent hotspot in the Atlantic Ocean. Many geological and geochemical evidences support a deep origin of the mantle material feeding the hotspot. However, the existence of a plume has not been confirmed as an anomalous structure in the mantle resolved by geophysical data because of lack of the observations in the area. Marine magnetotelluric and seismological observations were conducted in 2012–2013 to examine the upper mantle structure adjacent to TDC. The electrical conductivity structure of the upper mantle beneath the area was investigated in this study. Three-dimensional inversion analysis depicted a high conductive layer at ~120km depth but no distinct plume-like vertical structure. The conductive layer is mostly flat independently on seafloor age and bulges upward beneath the lithospheric segment where the TDC islands are located compared to younger segment south of the TDC Fracture Zone, while the bathymetry is rather deeper than prediction for the northern segment. The apparent inconsistency between the absence of vertical structure in this study and geochemical evidences on deep origin materials suggests that either the upwelling is too small and/or weak to be resolved by the current data set or that the upwelling takes place elsewhere outside of the study area. Other observations suggest that 1) the conductivity of the upper mantle can be explained by the fact that the mantle above the high conductivity layer is depleted in volatiles as the result of partial melting beneath the spreading ridge, 2) the potential temperature of the segments north of the TDC Fracture Zone is lower than that of the southern segment at least during the past ~30Myr.

The fertile physics of chemical gardens

Many a child has enjoyed watching the gardens grow; many a physicist has puzzled over the transformation of self-organized, nonequilibrium patterns into permanent structures.

Convective instability in sedimentation: 3-D numerical study

To provide a probable explanation on the field observed rapid sedimentation process near river mouths, we investigate the convective sedimentation in stably stratified saltwater using 3-D numerical simulations. Guided by the linear stability analysis, this study focuses on the nonlinear interactions of several mechanisms, which lead to various sediment finger patterns, and the effective settling velocity for sediment ranging from clay (single-particle settling velocity V0 = 0.0036 and 0.0144 mm/s, or particle diameter d = 2 and 4 μm) to silt (V0 = 0.36 mm/s, or d = 20 μm). For very fine sediment with V0 = 0.0036 mm/s, the convective instability is dominated by double diffusion, characterized by millimeter-scale fingers. Gravitational settling slightly increases the growth rate; however, it has notable effect on the downward development of vertical mixing shortly after the sediment interface migrates below the salt interface. For sediment with V0 = 0.0144 mm/s, Rayleigh-Taylor instabilities become dominant before double-diffusive modes grow sufficiently large. Centimeter-scale and highly asymmetric sediment fingers are obtained due to nonlinear interactions between different modes. For sediment with V0 = 0.36 mm/s, Rayleigh-Taylor mechanism dominates and the resulting centimeter-scale sediment fingers show a plume-like structure. The flow pattern is similar to that without ambient salt stratification. Rapid sedimentation with effective settling velocity on the order of 1 cm/s is likely driven by convective sedimentation for sediment with V0 greater than 0.1 mm/s at concentration greater than 10–20 g/L.

Electromagnetic outline of the Solfatara–Pisciarelli hydrothermal system, Campi Flegrei (Southern Italy)

We describe the results from a combined controlled source audio magnetotelluric (CSAMT) and natural source magnetotelluric (MT) survey carried out in the Solfatara–Pisciarelli (S–P) area, located in the central part of the Campi Flegrei (CF) composite caldera, west of Naples, Southern Italy. The S–P area represents the most active zone within the CF caldera, in terms of hydrothermal manifestations and local seismicity. Since 1969, the CF caldera is experiencing ground deformation, seismicity and geochemical fluid changes, which are particularly evident in the S–P area. A 1km long, nearly W–E directed CSAMT–MT profile crossing the fumarole field was carried out in the S–P area with the aim of deducting a resistivity model of the structural setting of the hydrothermal system in the first 3km depth. An interpretation of the modelled section across the profile is given in this paper, taking advantage from already existing seismic, gravity and geochemical data in the same area. Three well distinct zones have been outlined. The first zone is a very shallow, electrically conductive body localized beneath the westernmost segment of the profile, which, within a short distance of about 100m, dips westwards from near surface down to some hundred metres in depth. Mostly accounting for the very low resistivity (1–10Ωm) and the exceedingly high values of vP/vS (>4), this shallow zone has been ascribed to a water-saturated, high-pressurized geothermal reservoir. The second zone, which has been localized below the west-central portion of the CSAMT–MT transect, appears as a composite body made up of a nearly vertical plumelike structure that escapes at about 2.25km depth from the top edge of the east side of a presumably horizontal platelike body. The plumelike structure rises up to the free surface in correspondence of the fumarole field, whereas the platelike structure deepens at least down to the 3km of maximum exploration depth. The combined interpretation of resistivity (50–100Ωm), body wave velocity ratio (vP/vS<2.0), mass density contrast (Δσ<0g/cm3), and geochemical data indicates that the plumelike portion can likely be associated with a steam/gas-saturated column and the platelike portion with a high temperature (>300°C), over-pressurized, gas-saturated reservoir. Finally, the third zone, which has been localized beneath the eastern half of the transect, from about 1.2km down to about 3km of depth, is also characterized by very low resistivity values (1–10Ωm). Jointly interpreted with seismic (vP/vS<1.73) and gravity (⨂⌠>0g/cm3) data, this last electrically conductive structure appears to be associated with a hydrothermally mineralized, clay-rich body.

Sheet-like and plume-like thermal flow in a spherical convection experiment performed under microgravity

AbstractWe introduce, in spherical geometry, experiments on electro-hydrodynamic driven Rayleigh–Bénard convection that have been performed for both temperature-independent (‘GeoFlow I’) and temperature-dependent fluid viscosity properties (‘GeoFlow II’) with a measured viscosity contrast up to 1.5. To set up a self-gravitating force field, we use a high-voltage potential between the inner and outer boundaries and a dielectric insulating liquid; the experiments were performed under microgravity conditions on the International Space Station. We further run numerical simulations in three-dimensional spherical geometry to reproduce the results obtained in the ‘GeoFlow’ experiments. We use Wollaston prism shearing interferometry for flow visualization – an optical method producing fringe pattern images. The flow patterns differ between our two experiments. In ‘GeoFlow I’, we see a sheet-like thermal flow. In this case convection patterns have been successfully reproduced by three-dimensional numerical simulations using two different and independently developed codes. In contrast, in ‘GeoFlow II’, we obtain plume-like structures. Interestingly, numerical simulations do not yield this type of solution for the low viscosity contrast realized in the experiment. However, using a viscosity contrast of two orders of magnitude or higher, we can reproduce the patterns obtained in the ‘GeoFlow II’ experiment, from which we conclude that nonlinear effects shift the effective viscosity ratio.

Waveform Tomography Reveals Channeled Flow at the Base of the Oceanic Asthenosphere

Understanding the relationship between different scales of convection that drive plate motions and hotspot volcanism still eludes geophysicists. Using full-waveform seismic tomography, we imaged a pattern of horizontally elongated bands of low shear velocity, most prominent between 200 and 350 kilometers depth, which extends below the well-developed low-velocity zone. These quasi-periodic fingerlike structures of wavelength ~2000 kilometers align parallel to the direction of absolute plate motion for thousands of kilometers. Below 400 kilometers depth, velocity structure is organized into fewer, undulating but vertically coherent, low-velocity plumelike features, which appear rooted in the lower mantle. This suggests the presence of a dynamic interplay between plate-driven flow in the low-velocity zone and active influx of low-rigidity material from deep mantle sources deflected horizontally beneath the moving top boundary layer.

Multiwavelength Observations of an Eruptive Flare: Evidence for Blast Waves and Break-Out

Images of an east-limb flare on 3 November 2010 taken in the 131 \AA\ channel of the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory provide a convincing example of a long current sheet below an erupting plasmoid, as predicted by the standard magnetic reconnection model of eruptive flares. However, the 171 \AA\ and 193 \AA\ channel images hint at an alternative scenario. These images reveal that large-scale waves with velocity greater than 1000 km/s propagated alongside and ahead of the erupting plasmoid. Just south of the plasmoid, the waves coincided with type-II radio emission, and to the north, where the waves propagated along plume-like structures, there was increased decimetric emission. Initially the cavity around the hot plasmoid expanded. Later, when the erupting plasmoid reached the height of an overlying arcade system, the plasmoid structure changed, and the lower parts of the cavity collapsed inwards. Hot loops appeared alongside and below the erupting plasmoid. We consider a scenario in which the fast waves and the type-II emission were a consequence of a flare blast wave, and the cavity collapse and the hot loops resulted from the break-out of the flux rope through an overlying coronal arcade.

Omnipresent long-period intensity oscillations in open coronal structures

Quasi-periodic propagating disturbances in coronal structures have been interpreted as slow magneto-acoustic waves and/or periodic upflows. Here we aim to understand their nature from the observed properties using a three-hour imaging sequence from AIA/SDO in two different temperature channels. We also compare the characteristics with a simple wave model. We searched for propagating disturbances in open-loop structures at three different locations; a fan loop-structure off-limb, an on-disk plume-like structure and the plume/interplume regions in the north pole of the sun. In each of the subfield regions chosen to cover these structures, the time series at each pixel location was subjected to wavelet analysis to find the different periodicities. We then constructed powermaps in three different period ranges. We also constructed space-time maps for the on-disk plume structure to estimate the propagation speeds in different channels. We find propagating disturbances in all three structures. Powermaps indicate that the power in the long-period range is significant up to comparatively longer distances along the loop than that in the shorter periods. This nature is observed in all three structures. A detailed analysis on the on-disk plume structure gives consistently higher propagation speeds in the 193 \AA channel and also reveals spatial damping along the loop. The amplitude and the damping length values are lower in hotter channels, indicating their acoustic dependence. These properties can be explained very well with a propagating slow-wave model. We suggest that these disturbances are more likely to be caused by propagating slow magneto-acoustic waves than by high-speed quasi-periodic upflows. We find that intensity oscillations in longer periods are omnipresent at larger heights even in active regions.

Retraction notice to A hybrid CFD framework for fluidized bed ozonation reactors coupling interface tracking and discrete particle methods

highlights An IT-DP framework was successfully developed for the catalytic ozonation of phenol-like pollutants. The effect of physicochemical properties was investigated in terms of bubble diameter and detachment time profiles. The hybrid CFD model handled agreeably the influence of ozone velocity on multiphase flow patterns. graphical a bstract Interstitial flow snapshots of hydrodynamic flow patterns resulting from IT-DP simulations for bubble abstract In this work, a fluidized bed reactor was investigated theoretically and experimentally for the catalytic ozonation of phenol-like pollutants. First, an interface tracking sub-model dealing with the individual motion of ozone bubbles in the continuous phase, and a discrete particle sub-model accounting for the trajectories of the solid particles were embedded accordingly into a hybrid CFD framework. Second, the effect of physicochemical properties including the surface tension and fluid viscosity was thoroughly evaluated at different hydrodynamic flow regimes. Afterwards, the influence of ozone velocity was quan- tified both on the gas and liquid superficial velocities and on the detoxification efficiency of liquid pollu- tants. Here, the interface tracking-discrete particle hybrid model was found to slightly overestimate radial bubble velocities being almost negligible in the center region of the fluidized bed reactor. As the surface tension increased, the mineralization efficiency was considerably lower and became higher when low-viscosity conditions were used for the ozonation of organic pollutants. Finally, the morphological features of interstitial flow maps at different hydrodynamic and reactive catalytic ozonation conditions highlighted the occurrence of bubble plume-like structures which affected the overall decontamination efficiency of phenol-like pollutants within the gas-liquid-solid ozonation reactor.

Yellowstone's Deep Roots

Geophysics![Figure][1] CREDIT: THINKSTOCK Yellowstone National Park in the western United States, renowned for its geothermal features, including geysers and hot springs, rests on a ∼4000-km2 caldera attributed to a series of supervolcano eruptions over the past 2 million years. The volcanic and geothermal activity is caused by the presence of a hot spot currently resting below the park that migrated eastward over the course of millions of years. Conflicting geological evidence implies that the source of this hotspot volcanism is either a deep plume originating hundreds of km below Earth's surface in the lower mantle or more shallow upper mantle melting caused primarily by extension of the continental lithosphere—a process that occurs elsewhere in the western United States. Schmandt et al. , analyzing data collected by the dense USArray seismic network while it was deployed across the region, addressed this debate through seismic imaging of the mantle below Yellowstone. Comparing the temperature-sensitive depth variation of mantle discontinuities at 410 and 660 km to global averages indicates the presence of a high-temperature upwelling that is vertically heterogeneous. The shallowing of the 660-km discontinuity, which marks the boundary between the upper and lower mantle, suggests that this plume-like structure originates in the lower mantle, but just how deep down remains unclear. Earth Planet. Sci. Lett. 10.1016/j.epsl.2012.03.025 (2012). [1]: pending:yes

The influence of finite Larmor radius effects on the radial interchange motions of plasma filaments

The influence of finite Larmor radius (FLR) effects on the perpendicular convection of isolated particle density filaments driven by interchange motions in magnetized plasmas is investigated using a two-moment gyrofluid model. By means of numerical simulations on a two-dimensional, bi-periodic domain perpendicular to the magnetic field, it is demonstrated that the radial velocities of the blob-like filaments are roughly described by the inertial scaling, which prescribes a velocity proportional to the square root of the summed electron and ion pressures times the square root of the blob width. Due to FLR effects, the poloidal up-down symmetry in the particle density field observed in the zero Larmor radius limit is broken. The symmetry breaking implies a poloidal motion of the blobs in the B×∇B direction. At later times, the direction of the poloidal motion is reversed when the blob is decelerated. It is shown that the spatial structure of the blobs depends on the ratio of the ion gyroradius to the initial filament size ρi/σ. Blobs with ρi/σ≳0.2 remain coherent as they move through the scrape-off layer, whereas blobs with ρi/σ≲0.1 form plume-like structures that loose their coherence and eventually become fragmented. After having traveled approximately five times their initial widths, coherent blobs carry ∼2–3 times the particle density of fragmented blobs. It is shown that FLR effects reduce mixing, stretching, and generation of small spatial scales in the particle density field by setting up a sheared flow surrounding the blob.

OBSERVATION OF HIGH-SPEED OUTFLOW ON PLUME-LIKE STRUCTURES OF THE QUIET SUN AND CORONAL HOLES WITHSOLAR DYNAMICS OBSERVATORY/ATMOSPHERIC IMAGING ASSEMBLY

Observations from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) reveal ubiquitous episodic outflows (jets) with an average speed around 120 km s-1 at temperatures often exceeding a million degree in plume-like structures, rooted in magnetized regions of the quiet solar atmosphere. These outflows are not restricted to the well-known plumes visible in polar coronal holes, but are also present in plume-like structures originating from equatorial coronal holes and quiet-Sun regions. Outflows are also visible in the "interplume" regions throughout the atmosphere. Furthermore, the structures traced out by these flows in both plume and inter-plume regions continually exhibit transverse (Alfvenic) motion. Our finding suggests that high-speed outflows originate mainly from the magnetic network of the quiet Sun and coronal holes, and that the plume flows observed are highlighted by the denser plasma contained therein. These outflows might be an efficient means to provide heated mass into the corona and serve as an important source of mass supply to the solar wind. We demonstrate that the quiet-Sun plume flows can sometimes significantly contaminate the spectroscopic observations of the adjacent coronal holes - greatly affecting the Doppler shifts observed, thus potentially impacting significant investigations of such regions.

Intermittency in the turbulent Ekman layer

The structure of the turbulent Ekman layer is explored by examining the three-dimensional velocity fields generated by means of direct numerical simulation (DNS). Emphasis is placed on determining the relation between the instantaneous structure of the velocity field and the turbulent dissipation. The results of our simulation show the following: (1) The pseudo-dissipation exhibits lognormal behavior, with a magnitude range spanning many orders of magnitude, (2) in horizontal planes within the Ekman layer, spatially localized regions of high and low pseudo-dissipation are found, with a magnitude ratio of about 104 between low and high regions, (3) the Ekman layer is found to be composed of a series of quasi-periodic plume-like structures, and (4) the pseudo-dissipation is found to be large at the outer edge of a typical plume, with much lower levels in the plume interior. Conjectures are put forth regarding the relevance of this work to known observations of clear air radar scattering, and suggestions are made for possible future efforts.

Subduction-triggered magmatic pulses: A new class of plumes?

A variety of atypical plume-like structures and focused upwellings that are not rooted in the lower mantle have recently been discussed, and seismological imaging has shown ubiquitous small-scale convection in the uppermost mantle in regions such as the Mediterranean region, the western US, and around the western Pacific. We argue that the three-dimensional return flow and slab fragmentation associated with complex oceanic subduction trajectories within the upper mantle can generate focused upwellings and that these may play a significant role in regional tectonics. The testable surface expressions of this process are the outside-arc alkaline volcanism, topographic swell, and low-velocity seismic anomalies associated with partial melt. Using three-dimensional, simplified numerical subduction models, we show that focused upwellings can be generated both ahead of the slab in the back-arc region (though ~ five times further inward from the trench than arc-volcanism) and around the lateral edges of the slab (in the order of 100 km away from slab edges). Vertical mass transport, and by inference the associated decompression melting, in these regions appears strongly correlated with the interplay between relative trench motion and subduction velocities. The upward flux of material from the depths is expected to be most pronounced during the first phase of slab descent into the upper mantle or during slab fragmentation. We discuss representative case histories from the Pacific and the Mediterranean where we find possible evidence for such slab-related volcanism.

Extended Application of a Novel Phase Calibration Approach of Multiple-Frequency Range Imaging to the Chung-Li and MU VHF Radars

Abstract Multiple-frequency range imaging (RIM), designed to improve the range resolution of radar echo distribution, is now available for the recently upgraded Chung-Li VHF radar (24.9°N, 121.1°E). To complete the RIM technique of this radar, a novel phase calibration approach, proposed initially for the Ostsee Wind (OSWIN) VHF radar, was employed to examine the effects of phase bias and the range-weighting function on the received radar echoes. The estimated phase bias indicated a time delay of ∼1.83 μs for the signal in the radar system. In contrast, such a time delay is more difficult to determine from the phase distribution of two-frequency cross-correlation functions. The same calibration approach was also applied successfully to the middle and upper atmosphere (MU) radar (34.85°N, 136.11°E) and revealed a time delay of ∼0.33 μs for the radar parameters employed. These calibration results for various radars demonstrate the general usability of the proposed calibration approach. With the high-resolution performance of RIM, some small-scale Kelvin–Helmholtz (KH) billows, double-layer structures, and plumelike structures in the troposphere that cannot be seen in height–time intensity plots have been recognized in present observations. The billows and double layers were found to be closely related to strong vertical wind shear and small Richardson number, supporting the hypothesis of a dynamic process of KH instability. On the other hand, the plumelike structures were observed to grow out of a wavy layer and could be attributed to saturation and breaking of gravity waves. These fine structures have shown some remarkable features resolved by the RIM method applied to VHF radars in the lower atmosphere.

Experimental and numerical investigation of shock-induced distortion of a spherical gas inhomogeneity

Results are presented from a series of experiments and simulations, studying the interaction of a planar shock wave (2.0⩽M⩽5.0) with a discrete gas inhomogeneity. Experiments and computations confirm that the phenomenology of shock–bubble interactions is fundamentally altered by the changes in the Atwood number. In the case of a low Atwood number, the late time flow field is dominated by coherent vortical structures, whereas in the case of a high Atwood number, the shocked bubble is effectively reduced to a small core of compressed fluid, which trails behind a plume-like structure indicative of a well-developed mixing region.

Vertical Temperature Structures of the Solar Corona Derived with the Hinode X-Ray Telescope

Abstract We obtained temperature structures in faint coronal features above and near the solar limb with the X-Ray Telescope aboard the Hinode satellite by accurately correcting the scattered X-rays from surrounding bright regions with occulted images during the solar eclipses. Our analysis yields a polar coronal hole temperature of about 1.0 MK and an emission measure in the range of 10$^{25.5}$–10$^{26.0}$cm$^{-5}$. In addition, our methods allow us to measure the temperature and emission measure of two distinct quiet-Sun structures: radial (plume-like) structures near the boundary of the coronal-hole and diffuse quiet Sun regions at mid-latitudes. The radial structures appear to have increasing temperature with height during the first 100 Mm, and constant temperatures above 100 Mm. For the diffuse quiet-Sun region the temperatures are the highest just above the limb, and appear to decrease with height. These differences may be due to different magnetic configurations.

Evidence for Polar Jets as Precursors of Polar Plume Formation

Observations from the Hinode/XRT telescope and STEREO/SECCHI/EUVI are utilized to study polar coronal jets and plumes. The study focuses on the temporal evolution of both structures and their relationship. The data sample, spanning 2007 April 7-8, shows that over 90% of the 28 observed jet events are associated with polar plumes. EUV images (STEREO/SECCHI) show plume haze rising from the location of approximately 70% of the polar X-ray (Hinode/XRT) and EUV jets, with the plume haze appearing minutes to hours after the jet was observed. The remaining jets occurred in areas where plume material previously existed, causing a brightness enhancement of the latter after the jet event. Short-lived, jetlike events and small transient bright points are seen (one at a time) at different locations within the base of preexisting long-lived plumes. X-ray images also show instances (at least two events) of collimated thin jets rapidly evolving into significantly wider plumelike structures that are followed by the delayed appearance of plume haze in the EUV. These observations provide evidence that X-ray jets are precursors of polar plumes and in some cases cause brightenings of plumes. Possible mechanisms to explain the observed jet and plume relationship are discussed.

J. R. R. Ritter &amp; U. R. Christensen (eds) 2007. Mantle Plumes. A Multidisciplinary Approach. viii + 501 pp. Berlin, Heidelberg, Dordrecht: Springer-Verlag. Price Euros 149.95, SFr 246.00, US $199.00, £115.50 (hard covers). ISBN 9783 540 68045 1.

J. R. R. Ritter &amp; U. R. Christensen (eds) 2007. Mantle Plumes. A Multidisciplinary Approach. viii + 501 pp. Berlin, Heidelberg, Dordrecht: Springer-Verlag. Price Euros 149.95, SFr 246.00, US $199.00, £115.50 (hard covers). ISBN 9783 540 68045 1. - Volume 145 Issue 4

On the Correlation between Convective Plume Updrafts and Downdrafts, Lidar Reflectivity and Depolarization Ratio

We question the correlation between vertical velocity (w) on the one hand and the occurrence of convective plumes in lidar reflectivity (i.e. range corrected backscatter signal Pz 2) and depolarization ratio (Δ) on the other hand in the convective boundary layer (CBL). Thermal vertical motion is directly investigated using vertical velocities measured by a ground-based Doppler lidar operating at 2 μm. This lidar provides also simultaneous measurements of lidar reflectivity. In addition, a second lidar 200 m away provides reflectivities at 0.53 and 1 μm and depolarization ratio at 0.53 μm. The time series from the two lidars are analyzed in terms of linear correlation coefficient (ρ). The main result is that the plume-like structures provided by lidar reflectivity within the CBL as well as the CBL height are not a clear signature of updrafts. It is shown that the lidar reflectivity within the CBL is frequently anti-correlated (ρ (w, Pz 2 )) with the vertical velocity. On the contrary, the correlation coefficient between the depolarization ratio and the vertical velocity ρ (w, Δ ) is always positive, showing that the depolarization ratio is a fair tracer of updrafts. The importance of relative humidity on the correlation coefficient is discussed.