scholarly journals An In Situ Study of Turbulence near Stellar Bow Shocks

2021 ◽  
Vol 922 (2) ◽  
pp. 233
Author(s):  
Stella Koch Ocker ◽  
James M. Cordes ◽  
Shami Chatterjee ◽  
Timothy Dolch
Keyword(s):  
Hubble Space Telescope ◽  
Surrounding Medium ◽  
Density Fluctuations ◽  
Wave System ◽  
Selection Effects ◽  
Shock Interactions ◽  
Bow Shocks ◽  
Wavenumber Spectrum ◽  
Warm Ionized Medium

Abstract Stellar bow shocks are observed in a variety of interstellar environments and shaped by the conditions of gas in the interstellar medium (ISM). In situ measurements of turbulent density fluctuations near stellar bow shocks are only achievable with a few observational probes, including Hα-emitting bow shocks and the Voyager Interstellar Mission (VIM). In this paper, we examine density variations around the Guitar Nebula, an Hα bow shock associated with PSR B2224+65, in tandem with density variations probed by VIM near the boundary of the solar wind and ISM. High-resolution Hubble Space Telescope observations of the Guitar Nebula taken between 1994 and 2006 trace density variations over scales from hundreds to thousands of au, while VIM density measurements made with the Voyager 1 Plasma Wave System constrain variations from thousands of meters to tens of au. The power spectrum of density fluctuations constrains the amplitude of the turbulence wavenumber spectrum near the Guitar Nebula to log 10 C n 2 = − 0.8 ± 0.2 m−20/3 and for the very local ISM probed by Voyager to log 10 C n 2 = − 1.57 ± 0.02 m−20/3. Spectral amplitudes obtained from multiepoch observations of four other Hα bow shocks also show significant enhancements from values that are considered typical for the diffuse, warm ionized medium, suggesting that density fluctuations near these bow shocks may be amplified by shock interactions with the surrounding medium or selection effects that favor Hα emission from bow shocks embedded in denser media.

scholarly journals On the short-term variability of turbulence and temperature in the winter mesosphere

2018 ◽  
Vol 36 (4) ◽  
pp. 1099-1116
Author(s):  
Gerald A. Lehmacher ◽  
Miguel F. Larsen ◽  
Richard L. Collins ◽  
Aroh Barjatya ◽  
Boris Strelnikov
Keyword(s):  
Attitude Control ◽  
Large Scale ◽  
Lower Thermosphere ◽  
Density Fluctuations ◽  
Large Temperature ◽  
Small Scale ◽  
Spatial And Temporal Variability ◽  
Temperature Structure ◽  
Wind Shears

Abstract. Four mesosphere–lower thermosphere temperature and turbulence profiles were obtained in situ within ∼30 min and over an area of about 100 by 100 km during a sounding rocket experiment conducted on 26 January 2015 at Poker Flat Research Range in Alaska. In this paper we examine the spatial and temporal variability of mesospheric turbulence in relationship to the static stability of the background atmosphere. Using active payload attitude control, neutral density fluctuations, a tracer for turbulence, were observed with very little interference from the payload spin motion, and with high precision (<0.01 %) at sub-meter resolution. The large-scale vertical temperature structure was very consistent between the four soundings. The mesosphere was almost isothermal, which means more stratified, between 60 and 80 km, and again between 88 and 95 km. The stratified regions adjoined quasi-adiabatic regions assumed to be well mixed. Additional evidence of vertical transport and convective activity comes from sodium densities and trimethyl aluminum trail development, respectively, which were both observed simultaneously with the in situ measurements. We found considerable kilometer-scale temperature variability with amplitudes of 20 K in the stratified region below 80 km. Several thin turbulent layers were embedded in this region, differing in width and altitude for each profile. Energy dissipation rates varied between 0.1 and 10 mW kg−1, which is typical for the winter mesosphere. Very little turbulence was observed above 82 km, consistent with very weak small-scale gravity wave activity in the upper mesosphere during the launch night. On the other hand, above the cold and prominent mesopause at 102 km, large temperature excursions of +40 to +70 K were observed. Simultaneous wind measurements revealed extreme wind shears near 108 km, and combined with the observed temperature gradient, isolated regions of unstable Richardson numbers (0<Ri<0.25) were detected in the lower thermosphere. The experiment was launched into a bright auroral arc under moderately disturbed conditions (Kp∼5).

Observations on the Performance of Vibration-Isolated Railway Track

1997 ◽  
Author(s):  
Hugh E. M. Hunt
Keyword(s):  
Travelling Waves ◽  
Surrounding Medium ◽  
Railway Track ◽  
Relative Speed ◽  
Tunnel Wall ◽  
Infinite Beam ◽  
Slab Track ◽  
Torsional Response ◽  
Made In

Abstract Vibration generated by underground railways is difficult to control because of the very limited space available in a tunnel. A popular approach is to use ‘floating slab track’ whereby the rails are fixed to a large concrete foundation separated from the tunnel wall by a resilient material. This paper investigates some of the vibration characteristics of floating slab track from measurements made in situ. These measurements are compared with the theoretical response of an infinite beam on an elastic foundation taking into account the torsional response of the beam. The transmission of vibration through the tunnel wall and into the surrounding medium is discussed with reference to the relative speed of travelling waves and the coincidence frequencies.

Plasmalemma Invaginations as Characteristic Constituents of Plasmodia of Physarum Polycephalum

1974 ◽  
Vol 16 (1) ◽  
pp. 23-37
Author(s):  
K. E. WOHLFARTH-BOTTERMANN
Keyword(s):  
Physarum Polycephalum ◽  
Close Contact ◽  
Surrounding Medium ◽  
Defined Medium ◽  
Subsequent Paper ◽  
Outer Border ◽  
Shuttle Streaming ◽  
Food Absorption ◽  
Plasmalemma Invaginations

Plasmodia of Physarum polycephalum grown on agar or filter paper and fed with rolled oats as food or with a partially defined medium were morphologically analysed in the living state and after fixation. Observation of the living plasmodium growing on agar reveals plasmalemma indentations in the outer regions of protoplasmic strands, which were studied in more detail by phase-contrast microscopy of unstained 1-µm sections. Plasmodia fixed and embedded in situ, i.e. in close contact to their substrate, exhibit an extensive system of plasmalemma invaginations as characteristic constituents throughout all regions. In plasmodial strands measuring between 40 µm and 1.5 mm in diameter and involved in shuttle streaming, the plasmalemma invaginations are found within the outer ectoplasmic wall. Rounded-up parts of this branched extracellular labyrinth limit the endoplasmic core engaged in the mass transport of protoplasm by shuttle streaming. Despite this clearcut borderline, the central endoplasmic core and the ectoplasmic cortex are connected by occasional protoplasmic bridges. The extracellular phase within the ectoplasmic regions of the strands can be interpreted either as a result of plasmalemma invaginations from the outer border of the strand, or as a consequence of pseudopodial-like processes originating from the central core and extending into the surrounding medium. The invagination system provides an extensive enlargement of the surface area within the multinucleate protoplasmic mass, probably important for food absorption, excretion processes and motility phenomena. In thick protoplasmic strands with diameters between 0.2 and 1.5 mm, there is an intimate connexion between the actomyosin fibrils and the invagination system. The fibrils are attached to the plasmalemma invaginations and/or run parallel to the invaginated plasmalemma sheets. The close relations between the invagination system and actomyosin fibrils will be described in detail in a subsequent paper.

Design and In Situ Validation of a Guided Wave System for Corrosion Monitoring in Coated Buried Steel Pipes

2019 ◽  
Vol 38 (3) ◽  
Author(s):  
Ricardo C. Jacques ◽  
Henrique H. de Oliveira ◽  
Rafael W. F. dos Santos ◽  
Thomas G. R. Clarke
Keyword(s):  
Guided Wave ◽  
Corrosion Monitoring ◽  
Wave System ◽  
Steel Pipes

scholarly journals First in-situ observations of neutral and plasma density fluctuations within a PMSE layer

1993 ◽  
Vol 20 (20) ◽  
pp. 2311-2314 ◽  
Author(s):  
Franz-Josef Lübken ◽  
Gerald Lehmacher ◽  
Tom Blix ◽  
Ulf-Peter Hoppe ◽  
Eivind Thrane ◽  
...  
Keyword(s):  
Plasma Density ◽  
Density Fluctuations ◽  
In Situ Observations

scholarly journals Simultaneous in situ measurements of small-scale structures in neutral, plasma, and atomic oxygen densities during WADIS sounding rocket project

2019 ◽  
Author(s):  
Boris Strelnikov ◽  
Martin Eberhart ◽  
Martin Friedrich ◽  
Jonas Hedin ◽  
Mikhail Khaplanov ◽  
...  
Keyword(s):  
Gravity Waves ◽  
Atomic Oxygen ◽  
Density Fluctuations ◽  
Sounding Rocket ◽  
Small Scale ◽  
Scale Structures ◽  
Small Scale Structures ◽  
Mlt Region ◽  
Diffusion Properties

Abstract. In this paper we present an overview of measurements conducted during the WADIS-2 rocket campaign. We investigate the effect of small-scale processes like gravity waves and turbulence on the distribution of atomic oxygen and other species in the MLT region. Our analysis suggests that density fluctuations of atomic oxygen are coupled to fluctuations of other constituents, i.e., plasma and neutrals. Our measurements show that all measured quantities, including winds, densities, and temperatures, reveal signatures of both waves and turbulence. We show observations of gravity wave saturation and breakdown together with simultaneous measurements of generated turbulence. Atomic oxygen inside turbulence layers shows two different spectral behaviors, which might imply change of its diffusion properties.

A small-scale numerical study of fault slip mechanisms using DEM

2020 ◽  
Author(s):  
Nathalie Casas ◽  
Guilhem Mollon ◽  
Ali Daouadji
Keyword(s):  
Numerical Study ◽  
Surrounding Medium ◽  
Computational Cost ◽  
Static Friction ◽  
Discrete Element Modelling ◽  
Small Scale ◽  
In Situ Experiments ◽  
Fault Gouges ◽  
Fault Network

&lt;p&gt;How do earthquakes start? What are the parameters influencing fault evolutions? What are the local parameters controlling the seismic or aseismic character of slip?&lt;/p&gt;&lt;p&gt;To predict the dynamic behaviour of faults, it is important to understand slip mechanisms and their source. Lab or in-situ experiments can be very helpful, but tribological experience has shown that it is complicated to install local sensors inside a mechanical contact, and that they could disturb the behaviour of the sheared medium. Even with technical improvements on lab tools, some interesting data regarding gouge kinematics and rheology remains very difficult or impossible to obtain. Numerical modelling seems to be another way of understanding physics of earthquakes.&lt;/p&gt;&lt;p&gt;Fault zone usually present a granular gouge, coming from the wear material of previous slips. That is why, in this study, we present a numerical model to observe the evolution and behaviours of fault gouges. We chose to focus on physics of contacts inside a granular gouge at a millimetre-scale, studying contact interactions and friction coefficient between the different bodies. In order to get access to this kind of information, we implement a 2D granular fault gouge with Discrete Element Modelling in the software MELODY (Mollon, 2016). The gouge model involves two rough surfaces representing the rock walls separated by the granular gouge.&lt;/p&gt;&lt;p&gt;One of the interests of this code is its ability to represent realistic non-circular grain shapes with a Fourier-Vorono&amp;#239; method (Mollon et al., 2012). As most of the simulations reported in the literature use circular (2D)/spherical (3D) grains, we wanted to analyse numerically the contribution of angular grains. We confirm that they lead to higher friction coefficients and different global behaviours (Mair et al., 2002), (Guo et al., 2004).&lt;/p&gt;&lt;p&gt;In a first model, we investigate dry contacts to spotlight the influence of inter-particular cohesion and small particles on slip behaviour and static friction. A second model is carried out to observe aseismic and seismic slips occurring within the gouge. As stability depends on the interplay between the peak of static friction and the stiffness of the surrounding medium, the model includes the stiffness of the loading apparatus on the rock walls.&lt;/p&gt;&lt;p&gt;The work presented here focuses on millimetre-scale phenomena, but the employed model cannot be extended to the scale of the entire fault network, for computational cost reasons. It is expected, however, that it will lead to a better understanding of local behaviours that may be injected as simplified interface laws in larger-scale simulations.&lt;/p&gt;

scholarly journals Pulsar wind nebulae created by fast-moving pulsars

2017 ◽  
Vol 83 (5) ◽  
Author(s):  
O. Kargaltsev ◽  
G. G. Pavlov ◽  
N. Klingler ◽  
B. Rangelov
Keyword(s):  
Hubble Space Telescope ◽  
Ambient Medium ◽  
Pulsar Wind Nebulae ◽  
The Past ◽  
Recent Developments ◽  
Pulsar Velocity ◽  
Bow Shocks ◽  
Observational Status ◽  
Pulsar Wind ◽  
Made In

We review multiwavelength properties of pulsar wind nebulae created by supersonically moving pulsars and the effects of pulsar motion on the pulsar wind nebulae morphologies and the ambient medium. Supersonic pulsar wind nebulae are characterized by bow-shaped shocks around the pulsar and/or cometary tails filled with the shocked pulsar wind. In the past several years significant advances in supersonic pulsar wind nebula studies have been made in deep observations with the Chandra and XMM-Newton X-ray observatories and the Hubble Space Telescope. In particular, these observations have revealed very diverse supersonic pulsar wind nebula morphologies in the pulsar vicinity, different spectral behaviours of long pulsar tails, the presence of puzzling outflows misaligned with the pulsar velocity and far-UV bow shocks. Here we review the current observational status focusing on recent developments and their implications.

scholarly journals An Observing System Simulation Experiment for the Calibration and Validation of the Surface Water Ocean Topography Sea Surface Height Measurement Using In Situ Platforms

2018 ◽  
Vol 35 (2) ◽  
pp. 281-297 ◽  
Author(s):  
Jinbo Wang ◽  
Lee-Lueng Fu ◽  
Bo Qiu ◽  
Dimitris Menemenlis ◽  
J. Thomas Farrar ◽  
...  
Keyword(s):  
Surface Water ◽  
Simulation Experiment ◽  
System Simulation ◽  
Sea Surface ◽  
Station Keeping ◽  
Calibration And Validation ◽  
Wavenumber Spectrum ◽  
Ocean Topography ◽  
Observing System Simulation Experiment

AbstractThe wavenumber spectrum of sea surface height (SSH) is an important indicator of the dynamics of the ocean interior. While the SSH wavenumber spectrum has been well studied at mesoscale wavelengths and longer, using both in situ oceanographic measurements and satellite altimetry, it remains largely unknown for wavelengths less than ~70 km. The Surface Water Ocean Topography (SWOT) satellite mission aims to resolve the SSH wavenumber spectrum at 15–150-km wavelengths, which is specified as one of the mission requirements. The mission calibration and validation (CalVal) requires the ground truth of a synoptic SSH field to resolve the targeted wavelengths, but no existing observational network is able to fulfill the task. A high-resolution global ocean simulation is used to conduct an observing system simulation experiment (OSSE) to identify the suitable oceanographic in situ measurements for SWOT SSH CalVal. After fixing 20 measuring locations (the minimum number for resolving 15–150-km wavelengths) along the SWOT swath, four instrument platforms were tested: pressure-sensor-equipped inverted echo sounders (PIES), underway conductivity–temperature–depth (UCTD) sensors, instrumented moorings, and underwater gliders. In the context of the OSSE, PIES was found to be an unsuitable tool for the target region and for SSH scales 15–70 km; the slowness of a single UCTD leads to significant aliasing by high-frequency motions at short wavelengths below ~30 km; an array of station-keeping gliders may meet the requirement; and an array of moorings is the most effective system among the four tested instruments for meeting the mission’s requirement. The results shown here warrant a prelaunch field campaign to further test the performance of station-keeping gliders.

scholarly journals Simultaneous ground-based and in situ Swarm observations of equatorial F-region irregularities over Jicamarca

2020 ◽  
Vol 38 (5) ◽  
pp. 1063-1080
Author(s):  
Sharon Aol ◽  
Stephan Buchert ◽  
Edward Jurua ◽  
Marco Milla
Keyword(s):  
Electron Density ◽  
Seasonal Pattern ◽  
Density Fluctuations ◽  
Ionospheric Irregularities ◽  
Plasma Plumes ◽  
December Solstice ◽  
Spread F ◽  
Field Lines ◽  
Statistical Trends

Abstract. Ionospheric irregularities are a common phenomenon in the low-latitude ionosphere. They can be seen in situ as depletions of plasma density, radar plasma plumes, or ionogram spread F by ionosondes. In this paper, we compared simultaneous observations of plasma plumes by the Jicamarca Unattended Long-term Investigations of the Ionosphere and Atmosphere (JULIA) radar, ionogram spread F generated from ionosonde observations installed at the Jicamarca Radio Observatory (JRO), and irregularities observed in situ by Swarm in order to determine whether Swarm in situ observations can be used as indicators of the presence of plasma plumes and spread F on the ground. The study covered the years from 2014 to 2018, as this was the period for which JULIA, Swarm, and ionosonde data sets were available. Overall, the results showed that Swarm's in situ density fluctuations on magnetic flux tubes passing over (or near) the JRO may be used as indicators of plasma plumes and spread F over (or near) the observatory. For Swarm and the ground-based observations, a classification procedure was conducted based on the presence or absence of ionospheric irregularities. There was a strong consensus between ground-based observations of ionospheric irregularities and Swarm's depth of disturbance of electron density for most passes. Cases, where ionospheric irregularities were observed on the ground with no apparent variation in the in situ electron density or vice versa, suggest that irregularities may either be localized horizontally or restricted to particular height intervals. The results also showed that the Swarm and ground-based observations of ionospheric irregularities had similar local time statistical trends with the highest occurrence obtained between 20:00 and 22:00 LT. Moreover, similar seasonal patterns of the occurrence of in situ and ground-based ionospheric irregularities were observed with the highest percentage occurrence at the December solstice and the equinoxes and low occurrence at the June solstice. The observed seasonal pattern was explained in terms of the pre-reversal enhancement (PRE) of the vertical plasma drift. Initial findings from this research indicate that fluctuations in the in situ density observed meridionally along magnetic field lines passing through the JRO can be used as an indication of the existence of well-developed plasma plumes.

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