Acceleration Region Research Articles

Juno's Multi‐Instruments Observations During the Flybys of Auroral Bright Spots in Jupiter's Polar Aurorae

AbstractJuno's arrival at Jupiter in 2016 revealed unprecedented details about Jupiter's ultraviolet aurorae thanks to its unique suite of remote sensing and in situ instruments. Here we present results from in situ observations during Juno flybys above specific bright auroral spots in Jupiter's polar aurora. We compare data observed by Juno‐UVS, JEDI, JADE, Waves, and MAG instruments when Juno was magnetically connected to bright polar auroral spots (or their immediate vicinity) during perijove 3 (PJ3), PJ15, and PJ33. The highly energetic particles observed by JEDI show enhancements dominated by upward electrons, which suggests that the particle acceleration region takes place below the spacecraft. Moreover, brightness and upward particle flux were higher for the northern bright spot in PJ3 than the southern spots found in PJ15 and PJ33. In addition, we notice the intensification of whistler‐mode waves at the time of the particle enhancements, suggesting that wave‐particle interactions contribute to the acceleration of particles that cause the UV aurorae. The MAG data reveal magnetic perturbations during the PJ3 spot detection by Juno, which suggests the presence of significant field‐aligned electric currents. While the stable positions of the bright spots in System III suggest that the phenomenon is fixed to the planet's rotation, the presence of field‐aligned currents leaves the possibility of an origin rooted much farther in the magnetosphere.

Impact of Magnetic Focusing on the Transport of Energetic Electrons in the Solar Corona

Observations of Type III radio bursts discovered that electron beams with power-law energy spectra are commonly produced during solar flares. The locations of these electron beams are ~ 300 Mm above the particle acceleration region of the photosphere, and the velocities range from 3 to 10 times the local background electron thermal velocity. However, the mechanism that can commonly produce electron beams during the propagation of energetic electrons with power-law energy spectra in the corona remains unclear. In this paper, using kinetic transport theory, we find for the first time that the magnetic focusing effect governs the formation of electron beams over the observational desired distance in the corona. The magnetic focusing effect can sharply increase the bulk velocity of energetic electrons to the observed electron beam velocity within 0.4 solar radii (300 Mm) as they escape from the acceleration region and propagate upward along magnetic field lines. In more rapidly decreasing magnetic fields, energetic electrons with a harder power-law energy spectrum can generate a higher bulk velocity, producing type III radio bursts at a location much closer to the acceleration region. During propagation, the spectral index of the energetic electrons is unchanged.

Particle Acceleration and Their Escape into the Heliosphere in Solar Flares with Open Magnetic Field

Energetic particle populations in the solar corona and in the heliosphere appear to have different characteristics even when produced in the same solar flare. It is not clear what causes this difference: properties of the acceleration region, the large-scale magnetic field configuration in the flare, or particle transport effects, such as scattering. In this study, we use a combination of magnetohydrodynamic and test-particle approaches to investigate magnetic reconnection, particle acceleration, and transport in two solar flares: an M-class flare on 2013 June 19, and an X-class flare on 2011 September 6. We show that in both events, the same regions are responsible for the acceleration of particles remaining in the coronal and being ejected toward the heliosphere. However, the magnetic field structure around the acceleration region acts as a filter, resulting in different characteristics (such as energy spectra) acquired by these two populations. We argue that this effect is an intrinsic property of particle acceleration in the current layers created by the interchange reconnection, and therefore, may be ubiquitous, particularly, in noneruptive solar flares with substantial particle emission into the heliosphere.

Characteristics of the pressure profile in the accelerator on the RF negative ion source at ASIPP

Neutral beam injection (NBI) systems based on negative hydrogen ion sources—rather than the positive ion sources that have typically been used to date—will be used in the future magnetically confined nuclear fusion experiments to heat the plasma. The collisions between the fast negative ions and neutral background gas result in a significant number of high-energy positive ions being produced in the acceleration area, and for the high-power long-pulse operation of NBI systems, this acceleration of positive ions back to the ion source creates heat load and material sputtering on the source backplate. This difficulty cannot be ignored, with the neutral gas density in the acceleration region having a significant impact on the flux density of the backstreaming positive ions. In the work reported here, the pressure gradient in the acceleration region was estimated using an ionization gauge and a straightforward 1D computation, and it was found that once gas traveled through the acceleration region, the pressure dropped by nearly one order of magnitude, with the largest pressure drop occurring at the plasma grid. The computation also revealed that the pressure drop in the grid gaps was substantially smaller than that in the grid apertures.

The Effects of Geomagnetic Activities on Acceleration Regions of Radiation Belt Electrons

AbstractSolar wind and magnetospheric processes play important roles in relativistic electron acceleration in the Earth's outer radiation belt. However, how geomagnetic activity affects the acceleration region of relativistic electrons is still not clear. Here we first report the local acceleration regions during the strong storm of 1 June 2013. The first local acceleration region occurs at the lower L shell (L* ∼ 4.0) during the early recovery phase, which is associated with non‐continuous substorm activities. The second local acceleration region occurs at the higher L shell (L* ∼ 5.0) during the late recovery phase, which is associated with continuous substorm activities. To provide a more comprehensive picture, we additionally analyze two storm events with different geomagnetic activities. These events suggest that (a) the strong storm makes it possible for the local acceleration regions to occur at different L shells; (b) the timing, intensity, and duration of substorms during the recovery phase are crucial to the location and dynamics of the local acceleration region. These results can help us further understand the dynamics of relativistic electrons in the Earth's outer radiation belt.

On the Possibility of Cross-Flow Vortex Cancellation by Plasma Actuators

Cancellation of the cross-flow vortices in a swept-wing boundary layer is attempted by plasma actuator array in numerical simulation. The response of the boundary layer to the stationary excitation by a single actuator section is measured experimentally and compared to the response obtained from the solution to the parabolized stability equations. A linear approach is shown to be held within the peak-to-peak magnitude of the stationary cross-flow vortices below 10% of the local potential flow velocity. Within the linear model, an optimal control strategy and a faster suboptimal one are developed to calculate voltage amplitude distribution across the electrodes, taking into account the forcing constraints. Simulation of the cancellation process is performed, showing up to a 20 dB reduction in the initial spanwise velocity modulation in the boundary layer. The minimal actuator resolution required for the successive implementation of the control is shown to be in the order of a quarter of the most amplified wavelength, or 3–4 displacement thickness of the boundary layer. Linear estimates predict up to a 150 mm (22% of flow acceleration region length) transition delay for an actuator momentum coefficient of 0.005%.

Film cooling and aerodynamic loss performance of turbine vanes with fan-shaped and wave-trenched holes

Experimental measurement and numerical simulation were carried out to study the film cooling and aerodynamic loss performances of turbine vanes with cylindrical, fan-shaped, and wave-trenched holes. Tests were performed on a linear cascade. The mainstream inlet Reynolds number based on the vane chord length was 1.03 × 105, and three coolant local blowing ratios were conducted. A single row of film cooling holes was arranged in the acceleration region upstream of the pressure and suction surfaces of the test vanes, respectively. The effects of the hole shape and position of the hole row on the film cooling effectiveness and total pressure loss under various blowing ratios were comprehensively analyzed. Results show that on the pressure surface of the vane, at low blowing ratios, the film cooling effectiveness of wave-trenched holes is higher than that of fan-shaped holes. The wave-trenched hole exhibits obvious double peaks of the film cooling effectiveness. A pair of vortices and a coolant core are observed downstream of the trough wall and crest wall of the wave-trenched hole, respectively. At high blowing ratios, the film cooling effectiveness of them is comparable. On the suction surface of the vane, at low blowing ratios, the film cooling effectiveness of wave-trenched holes is lower than that of fan-shaped holes, and the trend is opposite at high blowing ratios. Two pairs of vortices are observed downstream of the crest wall of the wave-trenched hole. Wave-trenched holes have the highest shear stress rate and total pressure loss, followed by the cylindrical holes and fan-shaped holes at high blowing ratios. To the best of our knowledge, this is one of the few studies available on the systematic analysis of the film cooling and aerodynamic loss performance of turbine vanes with the three shapes of film cooling holes. The conclusions could provide effective guidance for the application of fan-shaped and wave-trenched holes during the elaborate design of film cooling structures on turbine vanes.

Experimental and numerical studies of low power Hall thruster performance

The low power Hall thruster has the potential to be applied in multiple apace missions. In order to study the performance of the low power Hall thruster, a 200W-class Hall thruster is designed and manufactured. The plasma properties in the discharge channel are investigated via numerical simulation. The ionic composition and ion current density are respectively measured by a Wien filter and a Faraday probe. The generated thrust is measured by a thrust target. The simulation results show that the maximal ion number density is more than 1×1018 m-3. The ionization and acceleration regions are located in the midstream and downstream of the discharge channel. The experimental results reveal that thrust, anode specific impulse and anode efficiency are 12.13 mN, 1380 s, and 41% at 200 W. The ion current density decreases along the radial direction. According to the results of simulation and experiment, the thruster is reasonable and feasible. Our research provides a reference for the design of low power Hall thruster. Structural optimization and performance improvement will be carried out further.

Study on the ionization and acceleration of a microwave discharge cusped field thruster

The microwave discharge cusped field thruster is a novel concept electric micro propulsion device, also a candidate thruster for the gravitational detection mission. A coaxial transmission line resonator is utilized to feed the microwave into the thruster to generate Xe plasma steadily with a mass flow rate as low as 0.1 sccm. Due to the separation of ionization and acceleration, the thruster performs high operation mode stability over a wide range of voltage in low mass flow conditions. Experimental and simulation methods are carried out to study the ionization and ion acceleration of the thruster. The results show that in operating conditions with a mass flow rate of 0.1 sccm, an anode voltage of 0 V to 1000 V, and a microwave power of 2 W, the right-hand circularly polarized wave (R wave) and the ordinary wave (O wave) play the most important role in the ionization process. The ion acceleration region locates around the exit magnetic separatrix, and the acceleration region tends to converge toward the separatrix as the anode voltage increases, resulting in an increased focus of the thruster plume and concentration of the ion energy distribution. Due to the separation of the ionization and acceleration regions, the thruster performs a divergence efficiency of 0.5–0.8, and an acceleration efficiency of 0.9.

Theoretical study of general principle of high-resolution MALDI-linear time-of-flight mass spectrometry in middle to high m/z ranges

This work discusses general principles of high-resolution linear time-of-flight (TOF) mass spectrometers across a wide m/z range. Conventional instrument designs are unable to offer adequate performance across a wide m/z range due to inappropriate calculation. Based on big data analyses conducted using comprehensive flight-time topological calculations, high mass resolving power (Rm) is available when instrument dimensions and extraction parameters satisfy several preconditions. The result indicates that a longer flight distance does not necessarily provide higher Rm. The possibility to achieve high Rm for ions of m/z range 100–100,000 are high when the relative length of extraction and acceleration regions to overall instrument length satisfies specific ratios that distinct from conventional designs. With adequate instrument dimensions, achieving optimal Rm needs to optimize the voltages and extraction delay in the ion source. Simulation shows that a 900 mm-long instrument of ideal design can enhance Rm by more than 250 times (to the order of >100,000) in comparison to the extreme condition of conventional design for ions of m/z 100,000. This general principle can be applied to other TOF instruments of different sizes. The method to finding the ultimate parameters is thoroughly described.

Estimating the Evolution of Sparks in the Partially Screened Gap of Pulsars from Subpulse Drifting

A novel scheme has been developed to show that the observed phase behavior associated with subpulse drifting from two pulsars, J1034−3224 and J1720−2933, can be used to obtain the magnetic field configuration in the partially screened gap (PSG). The outflowing plasma along the open magnetic field line region of pulsars is generated as a result of spark discharges in an inner acceleration region (IAR) above the polar cap. The IAR has been modeled as a PSG with a steady supply of positively charged ions emitted from the heated polar cap surface dominated by strong non-dipolar magnetic fields. In a PSG the sparks are tightly packed and constrained to be present along the polar cap boundary. The sparks lag behind the rotation of the star during their lifetimes. As a result, the sparking pattern evolves along two different directions in a clockwise and counterclockwise manner around a stationary central spark and can be associated with the observed phenomenon of subpulse drifting. PSR J1034−3224 has four prominent components and exhibits bi-drifting where alternate components show the opposite sense of drifting, while PSR J1720−2933 has a single component profile and shows systematically coherent drift bands. We show that the differences in their drifting behavior can be directly linked to different natures of the non-dipolar surface magnetic field configurations.

The Efficiency of Electron Acceleration during the Impulsive Phase of a Solar Flare

Solar flares are known to be prolific electron accelerators, yet identifying the mechanism(s) for such efficient electron acceleration in solar flare (and similar astrophysical settings) presents a major challenge. This is due in part to a lack of observational constraints related to conditions in the primary acceleration region itself. Accelerated electrons with energies above ∼20 keV are revealed by hard X-ray (HXR) bremsstrahlung emission, while accelerated electrons with even higher energies manifest themselves through radio gyrosynchrotron emission. Here, we show, for a well-observed flare on 2017 September 10, that a combination of RHESSI HXR and and the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) EUV observations provides a robust estimate of the fraction of the ambient electron population that is accelerated at a given time, with an upper limit of ≲10−2 on the number density of nonthermal (≥20 keV) electrons, expressed as a fraction of the number density of ambient protons in the same volume. This upper limit is about 2 orders of magnitude lower than previously inferred from microwave observations of the same event. Our results strongly indicate that the fraction of accelerated electrons in the coronal region at any given time is relatively small but also that the overall duration of the HXR emission requires a steady resupply of electrons to the acceleration site. Simultaneous measurements of the instantaneous accelerated electron number density and the associated specific electron acceleration rate provide key constraints for a quantitative study of the mechanisms leading to electron acceleration in magnetic reconnection events.

New mechanisms for forming multiple hotspots in radio jets

ABSTRACTHotspots of radio galaxies are regions of shock-driven particle acceleration. Multiple hotspots have long been identified as potential indicators of jet movement or precession. Two frequent explanations describe a secondary hotspot as either the location of a prior jet termination point or a deflected backflow-driven shock: the so-called dentist’s drill and splatter-spot models. We created high-resolution simulations of precessing jets with a range of parameters. In addition to the existing mechanisms, our results show three additional mechanisms for multiple hotspot formation: (1) the splitting of a large terminal hotspot into passive and active components; (2) jet stream splitting resulting in two active hotspots; and (3) dynamic multiple hotspot complexes that form as a result of jet termination in a turbulent cocoon, linked here to rapid precession. We show that these distinct types of multiple hotspots are difficult to differentiate in synthetic radio maps, particularly hotspot complexes that can easily be mistaken for the jet itself. We discuss the implication for hypothesized binary supermassive black hole systems where jet precession is a key component of the morphology, and show a selection of potential precession candidates found using the Low-Frequency Array Two-metre Sky Survey Data Release 2.

Spectral and Imaging Diagnostics of Spatially Extended Turbulent Electron Acceleration and Transport in Solar Flares

Solar flares are efficient particle accelerators with a large fraction of released magnetic energy (10%–50%) converted into energetic particles such as hard X-ray producing electrons. This energy transfer process is not well constrained, with competing theories regarding the acceleration mechanism(s), including MHD turbulence. We perform a detailed parameter study examining how various properties of the acceleration region, including its spatial extent and the spatial distribution of turbulence, affect the observed electron properties, such as those routinely determined from X-ray imaging and spectroscopy. Here, a time-independent Fokker–Planck equation is used to describe the acceleration and transport of flare electrons through a coronal plasma of finite temperature. Motivated by recent nonthermal line broadening observations that suggested extended regions of turbulence in coronal loops, an extended turbulent acceleration region is incorporated into the model. We produce outputs for the density-weighted electron flux, a quantity directly related to observed X-rays, modeled in energy and space from the corona to chromosphere. We find that by combining several spectral and imaging diagnostics (such as spectral index differences or ratios, energy or spatial-dependent flux ratios, and electron depths into the chromosphere) the acceleration properties, including the timescale and velocity dependence, can be constrained alongside the spatial properties. Our diagnostics provide a foundation for constraining the properties of acceleration in an individual flare from X-ray imaging spectroscopy alone, and can be applied to past, current, and future observations including those from RHESSI and Solar Orbiter.

Stationary expansion of gas mixture from a spherical source into vacuum

Sublimation under solar illumination of diverse ices composing the cometary nucleus forms multispecies gas coma. Depending on the heliocentric distance, the gas production of the nucleus varies in a broad range that results in circumnucleus flow regimes ranging from free-molecular to fluid. Therefore it is important to consider kinetic effects occurring in the gas and their influence on the flow and boundary conditions.We consider stationary flows of pure gases (H2O, CO2, CO) and their mixtures from a spherical surface. It is assumed that gas production of the surface is a consequence of sublimation within the surface layer. The detailed study of the structure and parameters of the gas outflow is based on the kinetic approach. The study considers flow conditions from fluid to free molecular.The results are numerically computed quantities which make the link between production rate, gas composition and surface temperature and: (1) velocities and abundances of gas species beyond the acceleration region; (2) mass, momentum and energy fluxes from the gas environment towards the surface; (3) parameters of the gas flow at the sonic surface circumscribing the source. Also, we define regions and conditions where the fluid approximation of the flow can be correctly applied. These data are necessary for definitions of parameters in: (1) simplified “Haser-type” models; and (2) advanced models of cometary activity which take into consideration the influence of gas environment on nucleus surface; and (3) coma models based on the fluid description of the gas flow.

Multipoint Observations of Dispersive Scale Alfvénic Field‐Line Resonances Associated With Substorm Auroral Beads

AbstractWe present a case study of the field‐aligned current (FAC) systems that transpire within the high‐altitude auroral acceleration region of an “auroral bead” initiated double oval substorm observed on 23 February 2001 by the Cluster fleet. Conjunctive Cluster measurements and auroral images from IMAGE reveal that auroral bead current system formation and evolution is a multi‐scale, injection‐mediated process. The FACs at large scales vary on substorm evolution time scales (∼minutes) in response to the injection and evolution of hotter denser magnetospheric plasma. Embedded within the large‐scale FACs are intense short‐scale (≲ few 10s of km) currents comprising dispersive scale Alfvén wave (DAW) fluctuations. The DAWs are a complex mixture of ingoing and reflected components that regularly interfere to form a broad spectrum of kinetic (dispersive) scale Alfvénic field‐line resonances (KFLRs). The Alfvénic currents appear as a nested series of upward and downward FAC densities with amplitudes reaching a few 100 nA/m2. Energized field‐aligned or counterstreaming electrons near keV energies and below are observed with parallel skews that vary in concert with variations in the DAW current sense. Positive correlations between DAW electric field energy densities and the energies of energized H+, He+, and O+ outflow are observed, indicative of ion energization within the DAW fields. Due to their L‐shell location (L ∼5.8–7.0) and associations with injections, the KFLRs are interpreted as the high‐altitude auroral zone analog of KFLRs observed in the equatorial inner magnetosphere.

Deriving Large Coronal Magnetic Loop Parameters Using LOFAR J Burst Observations

Large coronal loops around one solar radius in altitude are an important connection between the solar wind and the low solar corona. However, their plasma properties are ill-defined, as standard X-ray and UV techniques are not suited to these low-density environments. Diagnostics from type J solar radio bursts at frequencies above 10 MHz are ideally suited to understand these coronal loops. Despite this, J-bursts are less frequently studied than their type III cousins, in part because the curvature of the coronal loop makes them unsuited for using standard coronal density models. We used LOw-Frequency-ARray (LOFAR) and Parker Solar Probe (PSP) solar radio dynamic spectrum to identify 27 type III bursts and 27 J-bursts during a solar radio noise storm observed on 10 April 2019. We found that their exciter velocities were similar, implying a common acceleration region that injects electrons along open and closed magnetic structures. We describe a novel technique to estimate the density model in coronal loops from J-burst dynamic spectra, finding typical loop apex altitudes around 1.3 solar radius. At this altitude, the average scale heights were 0.36 solar radius, the average temperature was around 1 MK, the average pressure was 0.7~text{mdyn},text{cm}^{-2}, and the average minimum magnetic field strength was 0.13 G. We discuss how these parameters compare with much smaller coronal loops.

Comparison of building structure performance with masonry and without masonry

In structural designing, commonly the brickwall is only considered as a gravity load acting on the beam. On the other words, the wall in the building structure is assumed as a non-structural component. In this paper, the structural performance of a three-story building with masonry and without masonry due to seismic loads will be analyzed using pushover analysis by using the ATC 40 standard. The modeling of the building uses SeismoStruct software and assumed that the building is located in the earthquake ground acceleration region in Padang, West Sumatra. Based on the results of the analysis, the performance level of the three-story building with masonry is different compared to the building without masonry. The building with masonry as a structural component shows a better performance than without. This explains that the presence of masonry can contribute to the strength of the structure in receiving lateral loads such as earthquakes.

DNN Partitioning for Inference Throughput Acceleration at the Edge

Deep neural network (DNN) inference on streaming data requires computing resources to satisfy inference throughput requirements. However, latency and privacy sensitive deep learning applications cannot afford to offload computation to remote clouds because of the implied transmission cost and lack of trust in third-party cloud providers. Among solutions to increase performance while keeping computation on a constrained environment, hardware acceleration can be onerous, and model optimization requires extensive design efforts while hindering accuracy. DNN partitioning is a third complementary approach, and consists of distributing the inference workload over several available edge devices, taking into account the edge network properties and the DNN structure, with the objective of maximizing the inference throughput (number of inferences per second). This paper introduces a method to predict inference and transmission latencies for multi-threaded distributed DNN deployments, and defines an optimization process to maximize the inference throughput. A branch and bound solver is then presented and analyzed to quantify the achieved performance and complexity. This analysis has led to the definition of the acceleration region, which describes deterministic conditions on the DNN and network properties under which DNN partitioning is beneficial. Finally, experimental results confirm the simulations and show inference throughput improvements in sample edge deployments.

Cosmic-Ray Boron Flux Measured from 8.4 GeV/n to 3.8 TeV/n with the Calorimetric Electron Telescope on the International Space Station.

We present the measurement of the energy dependence of the boron flux in cosmic rays and its ratio to the carbon flux in an energy interval from 8.4 GeV/n to 3.8 TeV/n based on the data collected by the Calorimetric Electron Telescope (CALET) during ∼6.4 yr of operation on the International Space Station. An update of the energy spectrum of carbon is also presented with an increase in statistics over our previous measurement. The observed boron flux shows a spectral hardening at the same transition energy E_{0}∼200 GeV/n of the C spectrum, though B and C fluxes have different energy dependences. The spectral index of the B spectrum is found to be γ=-3.047±0.024 in the interval 25<E<200 GeV/n. The B spectrum hardens by Δγ_{B}=0.25±0.12, while the best fit value for the spectral variation of C is Δγ_{C}=0.19±0.03. The B/C flux ratio is compatible with a hardening of 0.09±0.05, though a single power-law energy dependence cannot be ruled out given the current statistical uncertainties. A break in the B/C ratio energy dependence would support the recent AMS-02 observations that secondary cosmic rays exhibit a stronger hardening than primary ones. We also perform a fit to the B/C ratio with a leaky-box model of the cosmic-ray propagation in the Galaxy in order to probe a possible residual value λ_{0} of the mean escape path length λ at high energy. We find that our B/C data are compatible with a nonzero value of λ_{0}, which can be interpreted as the column density of matter that cosmic rays cross within the acceleration region.