Plasma Bulk Velocity Research Articles

Radial and Vertical Structures of Plasma Disk in Jupiter's Middle Magnetosphere

AbstractThe Juno mission flew through the plasma disk near the equator in Jupiter's magnetosphere frequently. We identify 274 plasma disk crossings of Juno between 10 and 40 RJ from PJ5 to PJ44. Using a forward modeling method that combines the JADE‐I time‐of‐flight and SPECIES data sets, we perform a survey of ion properties in the plasma disk. Ions are heated from 1.5 to 6 keV between 15 and 30 RJ. Density and temperature are locally anti‐correlated. Assumed to be related to centrifugal instabilities, cold, dense plasma are commonly observed near midnight. Plasma corotates around Jupiter and the rigid corotation breaks down outside 15–20 RJ. The plasma bulk velocity increases from the post‐dusk sector to the pre‐dawn sector featuring injection flows in the pre‐dawn sector, which is consistent with the Vasyliunas cycle. Strong outflows (>100 km/s) are commonly observed outside 20 RJ and the average radial velocity increases with radial distance. The ion abundance changes between 10 and 18 RJ and that might indicate plasma sources and/or sinks near Europa and Ganymede. The vertical distribution of ions is controlled by the balance between centrifugal, pressure gradient, and ambipolar electric field forces. An example near the M‐shell of 13.5 shows that average plasma temperature increases by a factor of 10 from the disk center to edge, because cold ions are more confined near the equator. Lighter ions with higher charge states have more mobility along the field line and have larger scale heights. The observations are compared with multi‐species diffusive equilibrium model.

Quadruple Langmuir probe characterization of different fuel gases in a plasma deflagration accelerator

Astrophysical flows may be studied by reproducing similar conditions using a coaxial plasma accelerator operating in the deflagration regime (or plasma deflagration accelerator). This allows for the recreation and investigation of dynamics present in complex highly coupled plasma systems at the laboratory scale. We report on measurements of the plasma density, temperature, plasma potential and velocity found using a quadruple Langmuir probe (QLP) on such a deflagration accelerator in the form of the Stanford Coaxial High ENerGy (CHENG) device operating with multiple gases – specifically argon, nitrogen and hydrogen. Experiments show a general decrease in bulk plasma velocity with gas atomic mass from upwards of $120\ {\rm km}\ {\rm s}^{-1}$ with hydrogen to less than $30\ {\rm km}\ {\rm s}^{-1}$ with argon. There was an accompanying increase in peak plasma density with increasing atomic mass from ${\sim }3\times 10^{20}\ {\rm m}^{-3}$ with hydrogen to ${\sim }1.5 \times 10^{21}\ {\rm m}^{-3}$ with argon. It was found that the momentum flux and internal energy density also generally increase with atomic mass while the particle flux is constant between shots. Further investigation is needed to understand these correlations and the underlying physics. Lastly, comparisons with scaling laws show that while the CHENG device may be operated in such a way as to simulate the effects of bulk solar wind movement, it may not properly capture the thermal effects.

Radio continuum tails in ram pressure-stripped spiral galaxies: Experimenting with a semi-empirical model in Abell 2255

Context. Wide-field radio continuum observations of galaxy clusters are revealing an increasing number of spiral galaxies hosting tens of kiloparsec-length radio tails produced by the displacement of nonthermal interstellar medium (ISM) by ram pressure. Aims. We present a semi-empirical model for the multifrequency radio continuum emission from ram-pressure-stripped tails based on the pure synchrotron cooling of a radio plasma moving along the stripping direction with a uniform velocity. Methods. We combine LOFAR and uGMRT observations at 144 and 400 MHz to study the flux density and spectral index profiles of the radio tails of seven galaxies in Abell 2255, and use the model to reproduce the flux density and spectral index profiles, and infer the stripped radio plasma velocity. Results. For five out of these seven galaxies, we observe a monotonic decrease in both flux density and spectral index up to 30 kpc from their stellar disk. Our model reproduces the observed trends with a projected radio plasma bulk velocity of between 160 and 430 km s−1. This result represents the first indirect measurement of the stripped, nonthermal ISM velocity. The observed spectral index trends indicate that the synchrotron cooling is faster than the adiabatic expansion losses, suggesting that the stripped radio plasma can survive for a few tens of million years outside of the stellar disk. This provides a lower limit on the lifetime of the stripped ISM outside of the disk. As a proof of concept, we use the best-fit velocities to constrain the 3D velocity of the galaxies in the cluster to be in the range of 300−1300 km s−1. We estimate the ram pressure affecting these galaxies to be between 0.1 and 2.9 × 10−11 erg cm−3, and measure the inclination between their stellar disk and the ram pressure wind.

Anisotropic Vorticity Within Bursty Bulk Flow Turbulence

AbstractUtilizing Magnetospheric Multiscale (MMS) observation in the tail plasma sheet, we study the vorticity field (ω = ∇ × V) of the plasma bulk (convective) velocity within the bursty bulk flow (BBF) in detailed. Two typical events are presented. In the event on 25 June 2017, Ey is the main component. In the other event on 6 July 2017, Ez is the main component. For both cases, the BBF electric field is dominated by the convective electric field (Ec = − V × B). Our case studies show clearly the existence of the convective vorticity field within the BBF. The vorticity field has prominent anisotropy (quantified by the anisotropic angle (θaa = arctan(ω⊥/ω∥)). More often, the BBF ω field has stronger perpendicular vorticity (ω⊥) than the parallel vorticity (ω∥). The dominance of vorticity by ω⊥‐dominating BBF is confirmed in the statistical sense. In particular, event on 25 June shows the significant evolution of the ion flux energy with the strength of the ω field. The strong ω field corresponds to the ion flux enhancement at high energy (above 10 keV), while the weak ω field corresponds to the ion flux enhancement at medium energy (2–5 keV). Investigation of the subset of channel from fast plasma investigation partial moment measurement reveals that the ion behaviors in the strong and decayed BBFs are distinctly different. The channeled ions form the narrow band distribution in the strong BBF but the multiple‐layer distribution in the weak BBF. Finally, spectrum analysis indicates that the BBF ω⊥ and ω∥ have a similar scaling about −2.0 (below 0.2 Hz).

Generalized non-linear Compton-Getting transformation for energetic particles: Applications throughout the heliosphere

We develop the theoretical basis for a non-linear frame transformation of energetic ion intensity anisotropies (i.e., a generalized Compton-Getting transformation). Within the basic physics-motivated assumption that the anisotropies are gyrotropic in the transform frame, the rather general formulation allows us to examine a wide range of ion intensity distributions throughout the heliosphere. In addition to generalizing more familiar applications of the Compton-Getting transformation (that utilizes the bulk plasma velocity V for the transformation), we also apply the component (V^) transverse to the magnetic field (B). The physical justification is that (V^) is the ExB drift velocity with which the magnetic field lines “move”. In this paper, we have focused on two quite different areas: the elucidation of the strong field-aligned anisotropies and steep energy spectra being observed near the Sun from Parker Solar Probe (and Solar Orbiter), and the extraction of the bulk plasma velocity in the heliosheath at ∼100 AU using the anisotropies in energetic ion intensities observed on Voyager 1&2.

Optical fiber interferometer application for high electron density measurements on compact torus plasmas.

An optical fiber Mach-Zehnder interferometer at a wavelength of 1.55 µm has been developed for measurements of high electron density on compact torus (CT) plasmas with a high time resolution of 0.1 µs and high phase resolution of 6.4 × 10-4 rad. To improve density measurement accuracy, the phase noise of the interferometer has been investigated in detail and optimized. In the bench test, the interferometer was calibrated using a piezoelectric ceramic actuator with known stroke. Initial results on CT plasma show that the optical fiber interferometer provides reliable density measurements at two spatial locations and the bulk velocity of plasma can be determined by the method of time of flight.

A statistical study of the long-term evolution of coronal hole properties as observed by SDO

Context.Understanding the evolution of coronal holes is especially important when studying the high-speed solar wind streams that emanate from them. Slow- and high-speed stream interaction regions may deliver large amounts of energy into the Earth’s magnetosphere-ionosphere system, cause geomagnetic storms, and shape interplanetary space.Aims.By statistically investigating the long-term evolution of well-observed coronal holes we aim to reveal processes that drive the observed changes in the coronal hole parameters. By analyzing 16 long-living coronal holes observed by the Solar Dynamic Observatory, we focus on coronal, morphological, and underlying photospheric magnetic field characteristics, and investigate the evolution of the associated high-speed streams.Methods.We use the Collection of Analysis Tools for Coronal Holes to extract and analyze coronal holes using 193 Å EUV observations taken by the Atmospheric Imaging Assembly as well as line–of–sight magnetograms observed by the Helioseismic and Magnetic Imager. We derive changes in the coronal hole properties and look for correlations with coronal hole evolution. Further, we analyze the properties of the high–speed stream signatures near 1AU from OMNI data by manually extracting the peak bulk velocity of the solar wind plasma.Results.We find that the area evolution of coronal holes shows a general trend of growing to a maximum followed by a decay. We did not find any correlation between the area evolution and the evolution of the signed magnetic flux or signed magnetic flux density enclosed in the projected coronal hole area. From this we conclude that the magnetic flux within the extracted coronal hole boundaries is not the main cause for its area evolution. We derive coronal hole area change rates (growth and decay) of (14.2 ± 15.0)×108km2per day showing a reasonable anti-correlation (ccPearson = −0.48) to the solar activity, approximated by the sunspot number. The change rates of the signed mean magnetic flux density (27.3 ± 32.2 mG day−1) and the signed magnetic flux (30.3 ± 31.5 1018Mx day−1) were also found to be dependent on solar activity (ccPearson = 0.50 andccPearson = 0.69 respectively) rather than on the individual coronal hole evolutions. Further we find that the relation between coronal hole area and high-speed stream peak velocity is valid for each coronal hole over its evolution, but we see significant variations in the slopes of the regression lines.

On the Determination of Kappa Distribution Functions from Space Plasma Observations.

The velocities of space plasma particles, often follow kappa distribution functions. The kappa index, which labels and governs these distributions, is an important parameter in understanding the plasma dynamics. Space science missions often carry plasma instruments on board which observe the plasma particles and construct their velocity distribution functions. A proper analysis of the velocity distribution functions derives the plasma bulk parameters, such as the plasma density, speed, temperature, and kappa index. Commonly, the plasma bulk density, velocity, and temperature are determined from the velocity moments of the observed distribution function. Interestingly, recent studies demonstrated the calculation of the kappa index from the speed (kinetic energy) moments of the distribution function. Such a novel calculation could be very useful in future analyses and applications. This study examines the accuracy of the specific method using synthetic plasma proton observations by a typical electrostatic analyzer. We analyze the modeled observations in order to derive the plasma bulk parameters, which we compare with the parameters we used to model the observations in the first place. Through this comparison, we quantify the systematic and statistical errors in the derived moments, and we discuss their possible sources.

Interaction of a supernova remnant with a wind of young massive star: MHD simulations

Fast stellar winds of young massive stars and supernovae play an essential role in the evolution of the interstellar medium in the Milky Way. They dominate the dynamics of the active starforming regions in starburst galaxies and in particularly their non-thermal radiation. The non-thermal emission from starburst galaxies NGC 253, M82, NGC 1068 was detected from radio to gamma-rays. To model the observed radiation, one needs to know the complex structure of the flows and magnetic fields in the starforming regions. In this work a 2.5D magnetohydrodynamic (MHD) simulation (i.e., we consider 3D vector fields, but assume a 2D axisymmetric geometry) of the interaction between a stellar wind and a supernova remnant shock wave is carried out using the MHD module of the code PLUTO. The structure of the flows in the collision region is obtained taking into account the magnetic fields of the rotating source stars. We present the profiles of the bulk plasma velocity, density and magnetic field in the collision region. The amplification of regular magnetic fields in such systems is studied.

RADYNVERSION: Learning to Invert a Solar Flare Atmosphere with Invertible Neural Networks

Abstract During a solar flare, it is believed that reconnection takes place in the corona followed by fast energy transport to the chromosphere. The resulting intense heating strongly disturbs the chromospheric structure and induces complex radiation hydrodynamic effects. Interpreting the physics of the flaring solar atmosphere is one of the most challenging tasks in solar physics. Here we present a novel deep-learning approach, an invertible neural network, to understanding the chromospheric physics of a flaring solar atmosphere via the inversion of observed solar line profiles in Hα and Ca ii λ8542. Our network is trained using flare simulations from the 1D radiation hydrodynamic code RADYN as the expected atmosphere and line profile. This model is then applied to single pixels from an observation of an M1.1 solar flare taken with the Swedish 1 m Solar Telescope/CRisp Imaging SpectroPolarimeter instrument just after the flare onset. The inverted atmospheres obtained from observations provide physical information on the electron number density, temperature, and bulk velocity flow of the plasma throughout the solar atmosphere ranging from 0 to 10 Mm in height. The density and temperature profiles appear consistent with the expected atmospheric response, and the bulk plasma velocity provides the gradients needed to produce the broad spectral lines while also predicting the expected chromospheric evaporation from flare heating. We conclude that we have taught our novel algorithm the physics of a solar flare according to RADYN and that this can be confidently used for the analysis of flare data taken in these two wavelengths. This algorithm can also be adapted for a menagerie of inverse problems providing extremely fast (∼10 μs) inversion samples.

Generation of Noble and Refractory Metals Plasma Jets by Electrothermal Discharge for Surface Deposition Applications

Arc-dominated capillary discharges generate high-density plasmas from the ablation of the material of the inner lining sleeve, where ablation is determined by the radiant energy transport in such electrothermal (ET) plasma devices. Properties of sleeve material are an important factor on the generated plasma in ET devices. The 1-D time-dependent NC State University ET plasma code ETFLOW provides simulation of the capillary discharge and predicts the plasma parameters along the capillary axis. Of interest are the plasma parameters at the exit of the capillary which include temperature, pressure, velocity, heat flux and total ablated mass ejected from the source. The experimental discharge current was used in the code, and the plasma parameters at the source exit were calculated. Six materials were tested, namely, titanium, chromium, copper, silver, platinum, and gold. The code results show that the plasma temperature is between 2.4 and 2.57 eV for all tested materials. The lower heat of sublimation of gold and silver leads to increase the total ablated mass, density, electrical conductivity, and the exit pressure combined with reduction of the plasma bulk velocity. The gold has the highest ablated mass while titanium has the lowest values. The total ablated mass of copper material is about 168 mg, which is in a good agreement with previous experimental results.

Efficient shock drift acceleration in the collision of two asymmetric pair plasma shells

The shock drift acceleration (SDA) process in the collision between two asymmetric relativistic pair plasma shells was investigated. It is found that the density ratio (nL/nR) of the two different plasma shells plays a crucial role in the SDA process and determines the efficiency of the SDA process. By increasing this parameter, the plasma bulk velocity and so as to the convection electric field in the downstream region can be much enhanced compared with the symmetric case, in which the densities in the two plasma shells are the same. As a result, the particles are efficiently accelerated by the large convection electric field and the efficiency of the SDA process is much increased as the density ratio is increased. Our particle-in-cell simulations demonstrate that the SDA efficiency can be improved by three to four times in the asymmetric case. In this way, more high energy particles could enter into the phase of the diffusive shock acceleration process.

Effect of Changing Source Capillary Radius on Bulk Flow Parameter Scaling Laws for Hypersonically Expanding Arc-Ablated Polycarbonate Plasma for Fusion and Space Applications

Capillary pulsed electrothermal plasma source coupled to a linear converging–diverging nozzle was previously found to be adequate as a simulator for the supersonic bulk flow patterns of the aerosol particulates generated following a hard disruption event that damages the plasma facing components in a fusion reactor. A similar system can assume an important role in the computational studies related to space electric propulsion and electrostatic ion thrusters. Scaling laws for the prediction of bulk temperature, density, pressure, plasma bulk velocity and Mach number as a function of the axial length traversed measured from the source exit had been proposed previously for an arc-ablated polycarbonate plasma jet evolving into a vacuum chamber via the transition nozzle. In the present work, the source capillary radius has been varied and with the sectional lengths remaining fixed, the converging and diverging angles have also been varied with it, as the diverging exit Mach number was kept constant at a high-hypersonic value ~18. The effect of changing source capillary radius as well as the transition region geometry on the aforesaid bulk flow parameters has been studied computationally and new scaling laws have been proposed.

On the resolution of the phase space density required to obtain a specified accuracy of the solar wind bulk velocity

AbstractPlasma measurements with significantly improved accuracy and time resolution are needed to advance knowledge and understanding of plasma physical process in the magnetosphere and the solar wind. In this study, it is shown how small unresolved features in particle distribution functions can have significant impacts on the measurement of plasma bulk parameters such as the plasma bulk velocity. Errors are caused by the numerical integration scheme used to compute the phase space integrals and by errors or experimental uncertainties in measurements of the phase space distribution. Here the errors in the bulk velocity caused by the numerical integration procedure are investigated for both Gaussian (Maxwellian) velocity distributions and Kappa distributions. The particle distribution functions are perturbed by a small “bump” represented by a relatively small amplitude Gaussian with a thermal width that is much smaller than the thermal width of the unperturbed distribution. The goal is to find the grid spacing Δv in phase space required to obtain a specified accuracy of the mean flow velocity . For parameters encountered in the solar wind near the orbit of the Earth, it is shown that to determine the radial component of the solar wind bulk velocity with an accuracy less than 0.1 km/s requires a grid spacing less than Δv/v = 0.8% or, equivalently, an energy spacing less than ΔE/E = 1.6%. To accurately resolve the velocity components transverse to the heliocentric radial direction, it is found that the spacing of the angle bins must be less than Δθ = 0.4∘. The required spacing of the energy and angle bins are approximately 1 order of magnitude less than those of any solar wind ion instrument ever flown in space.

A STATISTICAL STUDY OF THE SOLAR WIND TURBULENCE AT ION KINETIC SCALES USING THEK-FILTERING TECHNIQUE AND CLUSTER DATA

Plasma turbulence at ion kinetic scales in the solar wind is investigated using the multi-point magnetometer data from the Cluster spacecraft. By applying the k-filtering method, we are able to estimate the full three-dimensional power spectral density P(?sc, k) at a certain spacecraft frequency ?sc in wavevector (k) space. By using the wavevector at the maximum power in P(?sc, k) at each sampling frequency ?sc and the Doppler shifted frequency ?pla in the solar wind frame, the dispersion plot ?pla = ?pla(k) is found. Previous studies have been limited to very few intervals and have been hampered by large errors, which motivates a statistical study of 52 intervals of solar wind. We find that the turbulence is predominantly highly oblique to the magnetic field k ? k ?, and propagates slowly in the plasma frame with most points having frequencies smaller than the proton gyrofrequency ?pla < ? p . Weak agreement is found that turbulence at the ion kinetic scales consists of kinetic Alfv?n waves and coherent structures advected with plasma bulk velocity plus some minor more compressible components. The results suggest that anti-sunward and sunward propagating magnetic fluctuations are of similar nature in both the fast and slow solar wind at ion kinetic scales. The fast wind has significantly more anti-sunward flux than sunward flux and the slow wind appears to be more balanced.

Scaling Laws of Bulk Plasma Parameters for a 1-D Flow through a Capillary with Extended Converging–Diverging Nozzle for Simulated Expansion into Fusion Reactor Chamber

A capillary-extended converging–diverging transition region was previously proposed to allow for the flow and expansion of plasma into a large volume simulating aerosol expansion and particle transport in the active volume of a fusion reactor. It has been shown that the pulsed electrothermal plasma source was adequate for the simulation, and the expansion into the chamber is at steady conditions for the main plasma parameters indicating a uniform expansion of the aerosol following a disruption event. These parameters are the bulk temperature, density, pressure, plasma bulk velocity and Mach number for the same system geometrical configuration. Scaling laws in 1-D for bulk plasma parameters have been developed for ranges of axial length traversed by the flow to predict these parameters along the axis of the expansion chamber.

Validation of the &amp;lt;i&amp;gt;k&amp;lt;/i&amp;gt;-filtering technique for a signal composed of random-phase plane waves and non-random coherent structures

Abstract. Recent observations of astrophysical magnetic fields have shown the presence of fluctuations being wave-like (propagating in the plasma frame) and those described as being structure-like (advected by the plasma bulk velocity). Typically with single-spacecraft missions it is impossible to differentiate between these two fluctuations, due to the inherent spatio-temporal ambiguity associated with a single point measurement. However missions such as Cluster which contain multiple spacecraft have allowed for temporal and spatial changes to be resolved, using techniques such as k filtering. While this technique does not assume Taylor's hypothesis it requires both weak stationarity of the time series and that the fluctuations can be described by a superposition of plane waves with random phases. In this paper we test whether the method can cope with a synthetic signal which is composed of a combination of non-random-phase coherent structures with a mean radius d and a mean separation λ, as well as plane waves with random phase.

Propellant Utilization Efficiency in a Pulsed Plasma Thruster

In a pulsed plasma thruster discharge, the propellant utilization efficiency is commonly seen as the weak point for its thrust performance, and for its low thrust efficiency respectively. However, the value depends strongly on operational settings and the design of the pulsed plasma thruster itself. To access this efficiency to understand further the influence of parameters, a method combining experimental and modeling efforts is proposed within this paper. By using various optical methods, including high-speed camera imaging, emission spectroscopy, and Mach–Zehnder interferometry, the plasma bulk velocity for different energy configurations of the SPPT-68 ADD SIMP-LEX is derived. Together with the total ablated mass and an improved slug model, the ratio of mass accelerated in the discharge process is computed and, thus, the propellant utilization efficiency is derived. Velocities in the range of 25 to were measured, leading to propellant utilization efficiencies of about 40 to 60% depending on the voltage and capacitance used in the pulsed plasma thruster.

Optical Thomson Scattering Measurements of Plasma Parameters in the Ablation Stage of Wire ArrayZPinches

A Thomson scattering diagnostic has been used to measure the parameters of cylindrical wire array Z pinch plasmas during the ablation phase. The scattering operates in the collective regime (α>1) allowing spatially localized measurements of the ion or electron plasma temperatures and of the plasma bulk velocity. The ablation flow is found to accelerate towards the axis reaching peak velocities of 1.2-1.3×10(7) cm/s in aluminium and ∼1×10(7) cm/s in tungsten arrays. Precursor ion temperature measurements made shortly after formation are found to correspond to the kinetic energy of the converging ablation flow.

Optical Thomson scattering measurements of cylindrical wire array parameters

A Thomson scattering diagnostic has been used to measure the parameters of cylindrical wire array Z pinch plasmas. The scattering operates in the collective regime (α&amp;gt;1) allowing spatially localised measurements of the ion or electron plasma temperatures and of the plasma bulk velocity. The ablation flow is found to accelerate towards the axis reaching peak velocities of 1.2–1.3 × 107 cm/s in aluminium and ∼1 × 107 cm/s in tungsten arrays. Measurements of the precursor ion temperature shortly after formation are found to correspond to the kinetic energy of the converging ablation flow. Measurements during the implosion phase of tungsten arrays show the main imploding mass reaches velocities of ∼1.4–1.7 × 107 cm/s and is non-zero even at large radii close to the start of the x-ray pulse indicating current flow in the trailing mass.