scholarly journals Electron cyclotron maser instability (ECMI) in strong magnetic guide field reconnection

2017 ◽  
Vol 35 (4) ◽  
pp. 999-1013 ◽  
Author(s):  
Rudolf A. Treumann ◽  
Wolfgang Baumjohann
Keyword(s):  
Source Region ◽  
Turbulent Energy ◽  
Compact Objects ◽  
Small Scale ◽  
Hot Electron ◽  
Plasma Properties ◽  
Localized Source ◽  
Guide Field ◽  
Electron Holes ◽  
Current Filaments

Abstract. The ECMI model of electromagnetic radiation from electron holes is shown to be applicable to spontaneous magnetic reconnection. We apply it to reconnection in strong current-aligned magnetic guide fields. Such guide fields participate only passively in reconnection, which occurs in the antiparallel components to both sides of the guide-field-aligned current sheets with current carried by kinetic Alfvén waves. Reconnection generates long (the order of hundreds of electron inertial scales) electron exhaust regions at the reconnection site X point, which are extended perpendicular to the current and the guide fields. Exhausts contain a strongly density-depleted hot electron component and have properties similar to electron holes. Exhaust electron momentum space distributions are highly deformed, exhibiting steep gradients transverse to both the reconnecting and guide fields. Such properties suggest application of the ECMI mechanism with the fundamental ECMI X-mode emission beneath the nonrelativistic guide field cyclotron frequency in localized source regions. An outline of the mechanism and its prospects is given. Potential applications are the kilometric radiation (AKR) in auroral physics, solar radio emissions during flares, planetary emissions and astrophysical scenarios (radiation from stars and compact objects) involving the presence of strong magnetic fields and field-aligned currents. Drift of the exhausts along the guide field maps the local field and plasma properties. Escape of radiation from the exhaust and radiation source region still poses a problem. The mechanism can be studied in 2-D particle simulations of strong guide field reconnection which favours 2-D, mapping the deformation of the electron distribution perpendicular to the guide field, and using it in the numerical calculation of the ECMI growth rate. The mechanism suggests also that reconnection in general may become a source of the ECMI with or without guide fields. This is of particular interest in extended turbulent plasmas where reconnection serves as an integral dissipation mechanism of turbulent energy in myriads of small-scale current filaments.

scholarly journals Radiation in the neighbourhood of a double layer

2014 ◽  
Vol 32 (6) ◽  
pp. 677-687 ◽  
Author(s):  
R. Pottelette ◽  
M. Berthomier ◽  
J. Pickett
Keyword(s):  
Phase Space ◽  
Electron Distribution ◽  
Electron Distribution Function ◽  
Source Region ◽  
Double Layers ◽  
Small Scale ◽  
High Time Resolution ◽  
Local Electron ◽  
Nonlinear Phase ◽  
Electron Holes

Abstract. In the auroral kilometric radiation (AKR) source region, acceleration layers narrow in altitude and associated with parallel field-aligned potential drops of several kV can be identified by using both particles and wave-field high time-resolution measurements from the Fast Auroral SnapshoT explorer spacecraft (FAST). These so-called double layers (DLs) are recorded around density enhancements in the auroral cavity, where the enhancement can be at the edge of the cavity or even within the cavity at a small scale. Once immersed in the plasma, DLs necessarily accelerate particles along the magnetic field lines, thereby generating locally strong turbulent processes leading to the formation of nonlinear phase space holes. The FAST data reveal the asymmetric character of the turbulence: the regions located on the high-potential side of the DLs are characterized by the presence of electron holes, while on the low-potential side, ion holes are recorded. The existence of these nonlinear phase space holes may affect the AKR radiation pattern in the neighbourhood of a DL where the electron distribution function is drastically different from a horseshoe shape. We present some observations which illustrate the systematic generation of elementary radiation events occurring significantly above the local electron gyrofrequency in the presence of electron holes. These fine-scale AKR radiators are associated with a local electron distribution which presents a pronounced beam-like shape.

scholarly journals Plasma properties of suprathermal electrons near comet 67P/Churyumov-Gerasimenko with Rosetta

2019 ◽  
Vol 630 ◽  
pp. A42 ◽  
Author(s):  
M. Myllys ◽  
P. Henri ◽  
M. Galand ◽  
K. L. Heritier ◽  
N. Gilet ◽  
...  
Keyword(s):  
Solar Wind ◽  
Heliocentric Distance ◽  
Radial Distance ◽  
Hot Electron ◽  
Plasma Properties ◽  
Measured Velocity ◽  
Electron Population ◽  
Suprathermal Electrons ◽  
Comet 67P ◽  
Cometary Activity

Context. The Rosetta spacecraft escorted comet 67P/Churyumov-Gerasimenko from 2014 to September 2016. The mission provided in situ observations of the cometary plasma during different phases of the cometary activity, which enabled us to better understand its evolution as a function of heliocentric distance. Aims. In this study, different electron populations, called warm and hot, observed by the Ion and Electron Sensor (IES) of the Rosetta Plasma Consortium (RPC) are investigated near the comet during the escorting phase of the Rosetta mission. Methods. The estimates for the suprathermal electron densities and temperatures were extracted using IES electron data by fitting a double-kappa function to the measured velocity distributions. The fitting results were validated using observations from other RPC instruments. We give upgraded estimates for the warm and hot population densities compared to values previously shown in literature. Results. The fitted density and temperature estimates for both electron populations seen by IES are expressed as a function of heliocentric distance to study their evolution with the cometary activity. In addition, we studied the dependence between the electron properties and cometocentric distance. Conclusions. We observed that when the neutral outgassing rate of the nucleus is high (i.e., near perihelion) the suprathermal electrons are well characterized by a double-kappa distribution. In addition, warm and hot populations show a significant dependence with the heliocentric distance. The populations become clearly denser near perihelion while their temperatures are observed to remain almost constant. Moreover, the warm electron population density is shown to be strongly dependent on the radial distance from the comet. Finally, based on our results we reject the hypothesis that hot electron population seen by IES consists of solely suprathermal (halo) solar wind electrons, while we suggest that the hot electron population mainly consists of solar wind thermal electrons that have undergone acceleration near the comet.

The Fully Developed Wake of a Circular Cylinder

1949 ◽  
Vol 2 (4) ◽  
pp. 451 ◽  
Author(s):  
AA Townsend
Keyword(s):  
Circular Cylinder ◽  
Diffusion Coefficients ◽  
Large Scale ◽  
Turbulent Transport ◽  
Turbulent Energy ◽  
Small Scale ◽  
Mean Velocity ◽  
Turbulent Fluid ◽  
Air Stream ◽  
Scale Motion

Extending previous work on turbulent diffusion in the wake of a circular-cylinder, a series of measurements have been made of the turbulent transport of mean stream momentum, turbulent energy, and heat in the wake of a cylinder of 0.169 cm. diameter, placed in an air-stream of velocity 1280 cm. sec.-1. It has been possible to extend the measurements to 960 diameters down-stream from the cylinder, and it 1s found that, at distances in excess of 600 diameters, the requirements of dynamical similarity are very nearly satisfied. To account for the observed rates of transport of turbulent energy and heat, it is necessary that only part of this transport be due to bulk convection by the slow large-scale motion of the jets of turbulent fluid emitted by the central, fully turbulent core of the wake, which had been supposed previously to perform most of the transport. The remainder of the transport is carried out by the small-scale diffusive motion of the turbulent eddies within the jets, and may be described by assigning diffusion coefficients to the turbulent fluid. It is found that the diffusion coefficients for momentum and heat are approximately equal, but that for turbulent energy is considerably smaller. On the basis of these hypotheses, it is possible to calculate $he form of the mean velocity distribution in good agreement with experiment, and to give a qualitative explanation of the apparently more rapid diffusion of heat.

scholarly journals Ionosphere Influenced From Lower-Lying Atmospheric Regions

2021 ◽  
Vol 8 ◽  
Author(s):  
Petra Koucká Knížová ◽  
Jan Laštovička ◽  
Daniel Kouba ◽  
Zbyšek Mošna ◽  
Katerina Podolská ◽  
...  
Keyword(s):  
Current Knowledge ◽  
Source Region ◽  
Upper Atmosphere ◽  
Lower Atmosphere ◽  
Small Scale ◽  
Atmospheric Waves ◽  
Ionospheric Variability ◽  
Secular Variations ◽  
Wide Scale ◽  
Academy Of Sciences

The ionosphere represents part of the upper atmosphere. Its variability is observed on a wide-scale temporal range from minutes, or even shorter, up to scales of the solar cycle and secular variations of solar energy input. Ionosphere behavior is predominantly determined by solar and geomagnetic forcing. However, the lower-lying atmospheric regions can contribute significantly to the resulting energy budget. The energy transfer between distant atmospheric parts happens due to atmospheric waves that propagate from their source region up to ionospheric heights. Experimental observations show the importance of the involvement of the lower atmosphere in ionospheric variability studies in order to accurately capture small-scale features of the upper atmosphere. In the Part I Coupling, we provide a brief overview of the influence of the lower atmosphere on the ionosphere and summarize the current knowledge. In the Part II Coupling Evidences Within Ionospheric Plasma—Experiments in Midlatitudes, we demonstrate experimental evidence from mid-latitudes, particularly those based on observations by instruments operated by the Institute of Atmospheric Physics, Czech Academy of Sciences. The focus will mainly be on coupling by atmospheric waves.

The power distribution between the symmetric and anti-symmetric components of the tropical wavenumber-frequency spectrum

2021 ◽  
Author(s):  
Ofer Shamir ◽  
Chen Schwartz ◽  
Chaim Garfinkel ◽  
Nathan Paldor
Keyword(s):  
Frequency Spectrum ◽  
Power Distribution ◽  
Large Scale ◽  
Spectral Power ◽  
Spatial Scales ◽  
Turbulent Energy ◽  
Heat Fluxes ◽  
Small Scale ◽  
Ample Evidence ◽  
Parity Distribution

<p>A yet unexplained feature of the tropical wavenumber-frequency spectrum is its parity distributions, i.e., the distribution of power between the meridionally symmetric and anti-symmetric components of the spectrum. Due to the linearity of the decomposition to symmetric and anti-symmetric components and the Fourier analysis, the total spectral power equals the sum of the power contained in each of these two components. However, the spectral power need not be evenly distributed between the two components. Satellite observations and reanalysis data provide ample evidence that the parity distribution of the tropical wavenumber-frequency spectrum is biased towards its symmetric component. Using an intermediate-complexity model of an idealized moist atmosphere, we find that the parity distribution of the tropical spectrum is nearly insensitive to large-scale forcing, including topography, ocean heat fluxes, and land-sea contrast. On the other hand, by adding a small-scale (stochastic) forcing, we find that the parity distribution of the tropical spectrum is sensitive to asymmetries on small spatial scales compared to the observed large-scale spectrum. Physically, such forcing can be thought of as small-scale convection, which is believed to trigger some of the Tropics' large-scale features via an upscale (inverse) turbulent energy cascade. These results are qualitatively explained by considering the effects of triad interactions on the parity distribution. According to the proposed mechanism, any small-scale asymmetry (symmetric or anti-symmetric) in the forcing leads to symmetric bias in the spectrum, regardless of the source of variability providing the forcing.</p>

scholarly journals Anisotropic Characteristics of Turbulence Dissipation in Swirling Flow: A Direct Numerical Simulation Study

2015 ◽  
Vol 2015 ◽  
pp. 1-9 ◽  
Author(s):  
Xingtuan Yang ◽  
Nan Gui ◽  
Gongnan Xie ◽  
Jie Yan ◽  
Jiyuan Tu ◽  
...  
Keyword(s):  
Numerical Simulation ◽  
Energy Dissipation ◽  
Direct Numerical Simulation ◽  
Large Scale ◽  
Isotropic Turbulence ◽  
Vortex Breakdown ◽  
Swirling Flow ◽  
Turbulent Energy ◽  
Swirling Flows ◽  
Small Scale

This study investigates the anisotropic characteristics of turbulent energy dissipation rate in a rotating jet flow via direct numerical simulation. The turbulent energy dissipation tensor, including its eigenvalues in the swirling flows with different rotating velocities, is analyzed to investigate the anisotropic characteristics of turbulence and dissipation. In addition, the probability density function of the eigenvalues of turbulence dissipation tensor is presented. The isotropic subrange of PDF always exists in swirling flows relevant to small-scale vortex structure. Thus, with remarkable large-scale vortex breakdown, the isotropic subrange of PDF is reduced in strongly swirling flows, and anisotropic energy dissipation is proven to exist in the core region of the vortex breakdown. More specifically, strong anisotropic turbulence dissipation occurs concentratively in the vortex breakdown region, whereas nearly isotropic turbulence dissipation occurs dispersively in the peripheral region of the strong swirling flows.

scholarly journals Structure formation in a colliding flow: The Herschel view of the Draco nebula

2017 ◽  
Vol 599 ◽  
pp. A109 ◽  
Author(s):  
M.-A. Miville-Deschênes ◽  
Q. Salomé ◽  
P. G. Martin ◽  
G. Joncas ◽  
K. Blagrave ◽  
...  
Keyword(s):  
Energy Dissipation ◽  
Molecular Clouds ◽  
Turbulent Energy ◽  
Small Scale ◽  
Neutral Medium ◽  
Galactic Latitude ◽  
High Galactic Latitude ◽  
Dissipation Scale ◽  
Scale Structures ◽  
Small Scale Structures

Context. The Draco nebula is a high Galactic latitude interstellar cloud observed at velocities corresponding to the intermediate velocity cloud regime. This nebula shows unusually strong CO emission and remarkably high-contrast small-scale structures for such a diffuse high Galactic latitude cloud. The 21 cm emission of the Draco nebula reveals that it is likely to have been formed by the collision of a cloud entering the disk of the Milky Way. Such physical conditions are ideal to study the formation of cold and dense gas in colliding flows of diffuse and warm gas. Aims. The objective of this study is to better understand the process of structure formation in a colliding flow and to describe the effects of matter entering the disk on the interstellar medium. Methods. We conducted Herschel-SPIRE observations of the Draco nebula. The clumpfind algorithm was used to identify and characterize the small-scale structures of the cloud. Results. The high-resolution SPIRE map reveals the fragmented structure of the interface between the infalling cloud and the Galactic layer. This front is characterized by a Rayleigh-Taylor (RT) instability structure. From the determination of the typical length of the periodic structure (2.2 pc) we estimated the gas kinematic viscosity. This allowed us to estimate the dissipation scale of the warm neutral medium (0.1 pc), which was found to be compatible with that expected if ambipolar diffusion were the main mechanism of turbulent energy dissipation. The statistical properties of the small-scale structures identified with clumpfind are found to be typical of that seen in molecular clouds and hydrodynamical turbulence in general. The density of the gas has a log-normal distribution with an average value of 103 cm-3. The typical size of the structures is 0.1−0.2 pc, but this estimate is limited by the resolution of the observations. The mass of these structures ranges from 0.2 to 20 M⊙ and the distribution of the more massive structures follows a power-law dN/ dlog (M) ~ M-1.4. We identify a mass-size relation with the same exponent as that found in molecular clouds (M ~ L2.3). On the other hand, we found that only 15% of the mass of the cloud is in gravitationally bound structures. Conclusions. We conclude that the collision of diffuse gas from the Galactic halo with the diffuse interstellar medium of the outer layer of the disk is an efficient mechanism for producing dense structures. The increase of pressure induced by the collision is strong enough to trigger the formation of cold neutral medium out of the warm gas. It is likely that ambipolar diffusion is the mechanism dominating the turbulent energy dissipation. In that case the cold structures are a few times larger than the energy dissipation scale. The dense structures of Draco are the result of the interplay between magnetohydrodynamical turbulence and thermal instability as self-gravity is not dominating the dynamics. Interestingly they have properties typical of those found in more classical molecular clouds.

Study of the Reduction of Turbulent Drag by Photon Correlation Spectroscopy

1991 ◽  
Vol 248 ◽  
Author(s):  
P. Tong ◽  
W. I. Goldburg ◽  
J. S. Huang
Keyword(s):  
Large Scale ◽  
Functional Form ◽  
Photon Correlation Spectroscopy ◽  
Turbulent Energy ◽  
Correlation Spectroscopy ◽  
Small Scale ◽  
Concentric Cylinder ◽  
Photon Correlation ◽  
Suppression Effect ◽  
Turbulent Drag

AbstractTurbulent drag reduction in a dilute polymer solution has been studied using the technique of photon-correlation homodyne spectroscopy to measure velocity differences in a concentric cylinder cell, in which the inner cylinder rotates. A large anisotropic suppression of turbulent velocity differences is found in the bulk region of the turbulent fluid. The suppression effect occurs at various length scales up to ∼ 1 mm, which is far beyond the Kolmogorov dissipation length ℓd (∼ 0.04 mm). The large-scale velocity fluctuations are suppressed, but their statistical properties remain unchanged. The small-scale fluctuations, on the other hand, are damped out much more strongly, resulting in a different functional form for the velocity density function. The latter observation is consistent with the notion that the polymer-turbulence interaction causes a truncation of the turbulent energy cascade at small scales.

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