Downstream Electron Research Articles

Characteristics of the O(1S) to O(1D) 557.7 nm green emission observed in an argon plasma jet

An extensive study on the green auroral emission characterization is presented based on a single dielectric barrier discharge geometry argon plasma jet driven by a kHz sine voltage. The plasma was generated by using 99.999% pure argon and the observed 557.7 nm green line resulted from the excited O(1S) state. An optical emission spectroscopy method using line ratios of argon was used to obtain the electron density and electron temperature under different conditions in the downstream region. The characteristics of discharge and green emission with variations in interelectrode distance, applied voltage (power) and flow rate are discussed. The spatially diffuse distribution of O(1S), owing to its long lifetime, is shown by the short exposure imaging. Two discharge regimes are presented, accompanied by two distinct branches of the green emission intensity, with a clear conclusion that the 557.7 nm emission is favored in the low electron temperature environment. In this work, the intense and diffuse green plume only forms when the downstream electron density is approximately lower than 1 × 1014 cm−3 and the electron temperature is lower than 1.1 eV. By charging the two electrodes in two opposite ways, it is shown that the green emission from oxygen is favored in the case where the electric field and the electron drift are not continuous.

Exceptional Quantum Efficiency Powers Biomass Production in Halotolerant Algae Picochlorum sp.

The green algal genus Picochlorum is of biotechnological interest because of its robust response to multiple environmental stresses. We compared the metabolic performance of P. SE3 and P. oklahomense to diverse microbial phototrophs and observed exceptional performance of photosystem II (PSII) in light energy conversion in both Picochlorum species. The quantum yield (QY) for O2 evolution is the highest of any phototroph yet observed, 32% (20%) by P. SE3 (P. okl) when normalized to total PSII subunit PsbA (D1) protein, and 80% (75%) normalized per active PSII, respectively. Three factors contribute: (1) an efficient water oxidizing complex (WOC) with the fewest photochemical misses of any organism; (2) faster reoxidation of reduced (PQH2)B in P. SE3 than in P. okl. (period-2 Fourier amplitude); and (3) rapid reoxidation of the plastoquinol pool by downstream electron carriers (Cyt b6f/PETC) that regenerates PQ faster in P. SE3. This performance gain is achieved without significant residue changes around the QB site and thus points to a pull mechanism involving faster PQH2 reoxidation by Cyt b6f/PETC that offsets charge recombination. This high flux in P. SE3 may be explained by genomically encoded plastoquinol terminal oxidases 1 and 2, whereas P. oklahomense has neither. Our results suggest two distinct types of PSII centers exist, one specializing in linear electron flow and the other in PSII-cyclic electron flow. Several amino acids within D1 differ from those in the low-light-descended D1 sequences conserved in Viridiplantae, and more closely match those in cyanobacterial high-light D1 isoforms, including changes near tyrosine Yz and a water/proton channel near the WOC. These residue changes may contribute to the exceptional performance of Picochlorum at high-light intensities by increasing the water oxidation efficiency and the electron/proton flux through the PSII acceptors (QAQB).

Cryo-EM structure of HQNO-bound alternative complex III from the anoxygenic phototrophic bacterium Chloroflexus aurantiacus.

Alternative complex III (ACIII) couples quinol oxidation and electron acceptor reduction with potential transmembrane proton translocation. It is compositionally and structurally different from the cytochrome bc1/b6f complexes but functionally replaces these enzymes in the photosynthetic and/or respiratory electron transport chains (ETCs) of many bacteria. However, the true compositions and architectures of ACIIIs remain unclear, as do their structural and functional relevance in mediating the ETCs. We here determined cryogenic electron microscopy structures of photosynthetic ACIII isolated from Chloroflexus aurantiacus (CaACIIIp), in apo-form and in complexed form bound to a menadiol analog 2-heptyl-4-hydroxyquinoline-N-oxide. Besides 6 canonical subunits (ActABCDEF), the structures revealed conformations of 2 previously unresolved subunits, ActG and I, which contributed to the complex stability. We also elucidated the structural basis of menaquinol oxidation and subsequent electron transfer along the [3Fe-4S]-6 hemes wire to its periplasmic electron acceptors, using electron paramagnetic resonance, spectroelectrochemistry, enzymatic analyses, and molecular dynamics simulations. A unique insertion loop in ActE was shown to function in determining the binding specificity of CaACIIIp for downstream electron acceptors. This study broadens our understanding of the structural diversity and molecular evolution of ACIIIs, enabling further investigation of the (mena)quinol oxidoreductases-evolved coupling mechanism in bacterial energy conservation.

Electron Heating in Shocks: Statistics and Comparison

AbstractSupernova remnant (SNR) shocks are the highest Mach number non‐relativistic shocks in electron‐ion plasmas. These shocks are the most efficient particle accelerators in space. SNR shock parameters are inferred from measurements of electromagnetic radiation from heated and accelerated particles. Temperature of the shock heated electrons is one of the most important parameters in supernova remnant shocks. Knowledge of the downstream electron‐to‐ion temperature ratio or of the ratio of the downstream electron temperature to the incident ion energy is crucial for understanding physics of the very high‐Mach number SNR shocks. Heliospheric shocks have substantially lower Mach numbers than SNR shocks but can be extensively studied in in situ observations with further extrapolation of the findings to higher Mach numbers. Magnetospheric Multiscale mission observations of the Earth bow shock are used to analyze dependence of the electron heating on the shock Mach number. It is found that the ratio of the downstream electron temperature to the incident ion energy decreases with the increase of the Mach number. At high Mach numbers this ratio and stabilizes at about 2.5%. The electron‐to‐ion temperature ratio stabilizes at about 10%. The peak electron temperature occurs at the overshoot maximum, further downstream electrons cool down. The mean ratio of the 4.5 s averages of the downstream and maximum electron temperatures is 0.85. Electron heating does not follow the thermodynamic adiabatic law. The heating and cooling behavior implies that the energy is provided by the overall cross‐shock potential while small‐scale electric fields rapidly isotropize the electron distribution.

Restricting electron flow at cytochrome b6f when downstream electron acceptors are severely limited.

Photosynthetic organisms frequently experience abiotic stress that restricts their growth and development. Under such circumstances, most absorbed solar energy cannot be used for CO2 fixation and can cause the photoproduction of reactive oxygen species (ROS) that can damage the photosynthetic reaction centers of PSI and PSII, resulting in a decline in primary productivity. This work describes a biological "switch" in the green alga Chlamydomonas reinhardtii that reversibly restricts photosynthetic electron transport (PET) at the cytochrome b6f (Cyt b6f) complex when the capacity for accepting electrons downstream of PSI is severely limited. We specifically show this restriction in STARCHLESS6 (sta6) mutant cells, which cannot synthesize starch when they are limited for nitrogen (growth inhibition) and subjected to a dark-to-light transition. This restriction represents a form of photosynthetic control that causes diminished electron flow to PSI and thereby prevents PSI photodamage but does not appear to rely on a ΔpH. Furthermore, when electron flow is restricted, the plastid alternative oxidase (PTOX) becomes active, functioning as an electron valve that dissipates some excitation energy absorbed by PSII and allows the formation of a proton motive force (PMF) that would drive some ATP production (potentially sustaining PSII repair and nonphotochemical quenching [NPQ]). The restriction at the Cyt b6f complex can be gradually relieved with continued illumination. This study provides insights into how PET responds to a marked reduction in availability of downstream electron acceptors and the protective mechanisms involved.

Effect of edge ion temperature on the divertor tungsten sputtering in WEST

The influence of upstream ion temperature in the scrape-off layer (SOL) on the tungsten (W) sputtering in the divertor is studied in the WEST tokamak. For an almost constant power into the SOL, the upstream ion temperature and its ratio over the electron temperature gradually increase with the decrease of electron density in the SOL. This increment is observed to enhance the energy transfer from ions to electrons. This increases the downstream electron temperature and by coupling of electrons and ions, the impact energy of ions causing W sputtering in the divertor. This enhancement mechanism may become crucial to sputtering the W material for high upstream T i/T e ratio since the impact energy of ions in the divertor would increase accordingly.

Flocs enhance nitrous oxide reduction capacity in a denitrifying biofilm-based system: Mechanism of electron competition

Nitrous oxide (N2O) accumulation induced by insufficient electron donors and substrate diffusion restriction is inevitably occurred in denitrifying biofilm-based systems, yet rare mechanism is intensively explored. This study discovered the mechanism of electron competition affecting N2O reduction capacity in two denitrifying biofilm-based systems, and uncovered the role of flocs for mitigating N2O production with varying exogenous organic carbon levels. The results clearly delineated that substrate diffusion restriction of biofilms deteriorated electron donors availability, leading to fiercer electron competition (ratio of 1.95) in the single denitrifying biofilm system. Such competition also occurred with the decreasing carbon loading rates (from 180 to 30 mg-COD/g VSS/h), further resulting in a higher N2O accumulation of 0.95 mg-N/L. By contrast, flocs from the denitrifying biofilm/flocs system can efficiently reduce overall N2O accumulation by 32 %. Also, flocs promoted the higher proportion of the electrons distributed to nitrous oxide reductase (Nos), indicating a strong N2O-reducing capacity. In addition, electrons from Nos inclinedly distributed to nitrate reductase (Nar) with an increase of 21.4 % under co-occurrence of NO3−, NO2− and N2O in the single denitrifying biofilm system, suggesting the mechanism of electron competition was the competition in upstream electron pool. In contrast, electrons from Nos tended to nitrite reductase (Nir) (5.9 %) instead of Nar (2.1 %) in the denitrifying biofilm/flocs system implied the competition in downstream electron pool. This study unraveled that flocs can be applied in denitrifying biofilm-based systems to alleviate N2O emissions and total carbon footprints at wastewater treatment plants (WWTPs).

Real-world development challenges of the Clarkson University 3 meter ducted wind turbine

The enticing viability of ducted turbines continues to be demonstrated around the world, both numerically and experimentally, however real-world implementation of this technology has historically been a challenge. Promising wind tunnel results for a 2.5 meter rotor ducted turbine model at Clarkson University have led to the construction of a 3 meter prototype test bed, installed on the Clarkson campus. An overview of the development process and the resulting experimental performance measurements on the 3 meter unit is presented. Construction of a prototype for field tests, including component selection, fabrication details of the structure, and selection of downstream electronics is reviewed. Performance results from various system implementations and lessons learned are compared. The turbine demonstrated Cpt efficiencies, based on the duct area, of up to 0.45.

Axial and radial development of the hot electron distribution in a helicon plasma source, measured by a retarding field energy analyzer (RFEA)

The information about the electron population of a helicon source plasma that expands along a magnetic nozzle is important for understanding the plasma acceleration across the potential drop that forms in the nozzle. The electrons need an energy higher than the potential drop to escape from the source. At these energies the signal of a Langmuir probe is less accurate. An inverted RFEA measures the high-energy tail of the electrons. To reach the probe, they must have energies above the plasma potential V P, which can vary over the region of the measurement. By constructing a full distribution by applying the electron temperature T e obtained from the electron IV-curve and the V P obtained from the ion collecting RFEA or an emissive probe, a density measure of the hot electron distribution independent of V P can be obtained. The variation of the high-energy tail of the EEDF in both radial and axial directions, in the two different cases of (1) a purely expanding magnetic field nozzle, and (2) a more constricted one by applying current in a third, downstream coil was investigated. The electron densities and temperatures from the source are then compared to two analytic models of the downstream development of the electron density. The first model considers the development for a pure Boltzmann distribution while the second model takes an additional magnetic field expansion into account. A good match between the measured densities and the second model was found for both configurations. The RFEA probe also allows for directional measurement of the electron current to the probe. This property is used to compare the densities from the downstream and upstream directions, showing a much lower contribution of downstream electrons into the source for a purely expanding magnetic field in comparison to the confined magnetic field configuration.

Spatial evolution characteristics of ion and electron flow for 300 W class low-power Hall thruster

The Faraday probe and cylindrical Langmuir probe were used to characterize the downstream ion and electron spatial evolution of a 300 W class low-power Hall thruster. The time-averaged ion current density, electron energy probability function, plasma potential, electron temperature, and electron density measurements were conducted at discharge voltages of 200–400 V and anode mass flow rates of 0.65 and 0.95 mg s−1 in the range of 100–500 mm axially and −100 to 100 mm radially downstream of the thruster. The results show that the ion and electron flows exhibit a bipolar diffusion characteristic along the radial direction. Meanwhile, the radial diffusion rate of ions in the plume is greater than the axial diffusion rate. The plasma potential decreases from 18 V at 100 mm axially from the thruster exit to 4 V at 500 mm axially and 100 mm radially. Correspondingly, the electron temperature decreases from 4.2 to 1.0 eV. The electron number density decreases from 2.6 × 1016 to 4 × 1014 m−3. A variable exponential relationship between electron temperature and electron density was observed from the measurements of electron energy probability distribution functions, with an adiabatic factor γ ranging between 1.3 and 1.4 (below the adiabatic value of 5/3). The adiabatic factor γ is considered to correlate with the anode mass flux and the spatial location of plasma, which suggests a possible dependence on the collision rate. These data are of great importance for plume model validation, improvement, plume effect evaluation, and protection mechanisms.

Electron Acceleration at Rippled Low-mach-number Shocks in High-beta Collisionless Cosmic Plasmas

Abstract Using large-scale fully kinetic two-dimensional particle-in-cell simulations, we investigate the effects of shock rippling on electron acceleration at low-Mach-number shocks propagating in high-β plasmas, in application to merger shocks in galaxy clusters. We find that the electron-acceleration rate increases considerably when the rippling modes appear. The main acceleration mechanism is stochastic shock-drift acceleration, in which electrons are confined at the shock by pitch-angle scattering off turbulence and gain energy from the motional electric field. The presence of multiscale magnetic turbulence at the shock transition and the region immediately behind the main shock overshoot is essential for electron energization. Wide-energy non-thermal electron distributions are formed both upstream and downstream of the shock. The maximum energy of the electrons is sufficient for their injection into diffusive shock acceleration. We show for the first time that the downstream electron spectrum has a power-law form with index p ≈ 2.5, in agreement with observations.

Increased CO2 Relevant to Future Ocean Acidification Alleviates the Sensitivity of a Red Macroalgae to Solar Ultraviolet Irradiance by Modulating the Synergy Between Photosystems II and I.

While intertidal macroalgae are exposed to drastic changes in solar photosynthetically active radiation (PAR) and ultraviolet radiation (UVR) during a diel cycle, and to ocean acidification (OA) associated with increasing CO2 levels, little is known about their photosynthetic performance under the combined influences of these drivers. In this work, we examined the photoprotective strategies controlling electron flow through photosystems II (PSII) and photosystem I (PSI) in response to solar radiation with or without UVR and an elevated CO2 concentration in the intertidal, commercially important, red macroalgae Pyropia (previously Porphyra) yezoensis. By using chlorophyll fluorescence techniques, we found that high levels of PAR alone induced photoinhibition of the inter-photosystem electron transport carriers, as evidenced by the increase of chlorophyll fluorescence in both the J- and I-steps of Kautsky curves. In the presence of UVR, photoinduced inhibition was mainly identified in the O2-evolving complex (OEC) and PSII, as evidenced by a significant increase in the variable fluorescence at the K-step (Fk) of Kautsky curves relative to the amplitude of FJ−Fo (Wk) and a decrease of the maximum quantum yield of PSII (Fv/Fm). Such inhibition appeared to ameliorate the function of downstream electron acceptors, protecting PSI from over-reduction. In turn, the stable PSI activity increased the efficiency of cyclic electron transport (CET) around PSI, dissipating excess energy and supplying ATP for CO2 assimilation. When the algal thalli were grown under increased CO2 and OA conditions, the CET activity became further enhanced, which maintained the OEC stability and thus markedly alleviating the UVR-induced photoinhibition. In conclusion, the well-established coordination between PSII and PSI endows P. yezoensis with a highly efficient photochemical performance in response to UVR, especially under the scenario of future increased CO2 levels and OA.

Printed-Circuit-Board (PCB) Based Multiplex Sensor for Ion Sensing

Combining electrochemical sensing with microfluidic system presents an attractive approach for in vitro study of physiology and pathophysiology (e.g. organ-on-chip platform) [1] and for inline monitoring of environmental parameters (e.g. water quality) [2]. Ion-selective electrodes (ISE) are potentiometric sensors that measure activities of ions. They offer a simpler analytical method compare to voltammetric sensors or ISFETs. Solid-state ISEs differ from conventional ISEs in that the inner filling solution is replaced by a solid contact material. The removal of liquid compartment can make such sensor more durable and flexible, enabling it to be integrated with microfluidic device. In this work, we fabricated and evaluated a multiplex ion sensing platform by modifying the printed circuit board with solid-state ISEs. The choice of PCB platform allows straightforward multiplexing of electrodes and interfacing with downstream electronics by leveraging standardized PCB technology, which can also be cost effective in terms of fabrication. ISEs for potassium (K+) and nitrate (NO3-) approximately 1.5 mm in diameter were prepared by coating polymeric membrane and mesoporous carbon layers as the solid contact on a pre-patterned transfer tape. The electrode response was significantly stabilized by casting the membrane away from the underlying metal pads. The stabilizing effect was attributed to the increase of effective surface area of mesoporous carbon between membrane and conducting metal. The multiplex sensor was further integrated with a 3D printed microchamber for onchip analysis (Figure 1A). The K+ and NO3- electrodes displayed near-Nernstian slopes of 53 and -50 mV/dec (vs. Ag/AgCl/0.01 M Cl- reference) in KNO3 solutions, respectively. Despite some noise and baseline drift, both electrodes showed reasonable stability and reversibility upon switching solution (Figure 1B). The sensor can be stored dry between measurements. The slope of K+ electrode decayed over time, while NO3- electrode maintained sensitivity for at least one week. The slope decrease was likely due to the diffusion of ion-exchanger (tetraphenylborate) into the surrounding adhesive tape. During integration with the microchamber, the K+ electrode was also found subject to the bonding adhesive between the chamber and transfer tape, where possible reaction and/or degradation of the ion exchanger has caused complete loss of electrode response. Nevertheless, with further improvement, the PCB based multiplex ISEs provides a promising tool for ion sensing in many biomedical and environmental applications.

Generation of a Pulsed Electron Beam by a Forevacuum Plasma-Cathode Source Based on a Constricted Arc Discharge

We describe the features of electron emission from the plasma formed by a constricted arc discharge, and the generation of a pulsed (100–150 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{s}$ </tex-math></inline-formula> ) low-energy (up to 10 keV) electron beam by a plasma-cathode electron source based on this type of arc in the forevacuum pressure range (5–20 Pa). The electron beam current pulse is delayed relative to the arc discharge current pulse. Limitation of the discharge current to the cylindrical part of the anode, increasing the gas pressure, and increasing the beam accelerating voltage all serve to reduce this delay time and provide a sharper rise time of the electron beam current pulse. The emission of electrons from the plasma formed by the constricted arc discharge is accompanied by a decrease in arc voltage and a corresponding increase in arc current. These characteristics are due to the specifics of operating the constricted arc discharge in the forevacuum pressure range and also the influence of backstreaming ions penetrating into the discharge system from the downstream electron beam region.

Mildly relativistic magnetized shocks in electron–ion plasmas – II. Particle acceleration and heating

ABSTRACT Particle acceleration and heating at mildly relativistic magnetized shocks in electron–ion plasma are investigated with unprecedentedly high-resolution 2D particle-in-cell simulations that include ion-scale shock rippling. Electrons are super-adiabatically heated at the shock, and most of the energy transfer from protons to electrons takes place at or downstream of the shock. We are the first to demonstrate that shock rippling is crucial for the energization of electrons at the shock. They remain well below equipartition with the protons. The downstream electron spectra are approximately thermal with a limited supra-thermal power-law component. Our results are discussed in the context of wakefield acceleration and the modelling of electromagnetic radiation from blazar cores.

A photoexcited halogen-bonded EDA complex of the thiophenolate anion with iodobenzene for C(sp3)-H activation and thiolation.

Thiophenol was discovered to form an EDA complex with iodobenzene through halogen bonding interactions upon treatment with KOH. A direct photochemical thiolation of C(sp3)–H bond-containing etheric, allylic, and benzylic substrates with thiophenol was developed. The reaction proceeded on the basis of the in situ generation of a thiyl radical and aryl radical through single electron transfer between the photoexcited thiophenolate anion and aryl iodide EDA complex. Then a C(sp3) centred-radical was formed by aryl radical-mediated hydrogen atom transfer and the thiolation products were delivered via a radical–radical cross-coupling with the thiyl radical.

Alterations in Sub-Axonal Architecture Between Normal Aging and Parkinson's Diseased Human Brains Using Label-Free Cryogenic X-ray Nanotomography.

Gaining insight to pathologically relevant processes in continuous volumes of unstained brain tissue is important for a better understanding of neurological diseases. Many pathological processes in neurodegenerative disorders affect myelinated axons, which are a critical part of the neuronal circuitry. Cryo ptychographic X-ray computed tomography in the multi-keV energy range is an emerging technology providing phase contrast at high sensitivity, allowing label-free and non-destructive three dimensional imaging of large continuous volumes of tissue, currently spanning up to 400,000 μm3. This aspect makes the technique especially attractive for imaging complex biological material, especially neuronal tissues, in combination with downstream optical or electron microscopy techniques. A further advantage is that dehydration, additional contrast staining, and destructive sectioning/milling are not required for imaging. We have developed a pipeline for cryo ptychographic X-ray tomography of relatively large, hydrated and unstained biological tissue volumes beyond what is typical for the X-ray imaging, using human brain tissue and combining the technique with complementary methods. We present four imaged volumes of a Parkinson’s diseased human brain and five volumes from a non-diseased control human brain using cryo ptychographic X-ray tomography. In both cases, we distinguish neuromelanin-containing neurons, lipid and melanic pigment, blood vessels and red blood cells, and nuclei of other brain cells. In the diseased sample, we observed several swellings containing dense granular material resembling clustered vesicles between the myelin sheaths arising from the cytoplasm of the parent oligodendrocyte, rather than the axoplasm. We further investigated the pathological relevance of such swollen axons in adjacent tissue sections by immunofluorescence microscopy for phosphorylated alpha-synuclein combined with multispectral imaging. Since cryo ptychographic X-ray tomography is non-destructive, the large dataset volumes were used to guide further investigation of such swollen axons by correlative electron microscopy and immunogold labeling post X-ray imaging, a possibility demonstrated for the first time. Interestingly, we find that protein antigenicity and ultrastructure of the tissue are preserved after the X-ray measurement. As many pathological processes in neurodegeneration affect myelinated axons, our work sets an unprecedented foundation for studies addressing axonal integrity and disease-related changes in unstained brain tissues.

Detector Count Rate Nonlinearity Determination Using Signal Intermodulation

Abstract Detectors that use HgCdTe exhibit Count Rate Non-Linearity (CRNL), where the integrated photon-generated charge in a pixel has an undesirable variation with photon arrival rate, at fixed delivered photon dose. Correcting for this is essential to meet the science goals of the Roman Space Telescope. The CRNL effect is distinct from signal-level-dependent (but photon-rate-independent) nonlinearity. We describe a potential method for measuring CRNL, by illuminating the sensor with two modulated light sources running at different frequencies, f 1 and f 2. The CRNL-modified generation of photocharge in the material produces a variation in the accumulated charge versus time with Fourier components at the beat frequencies (f 1 − f 2) and (f 1 + f 2). Successive non-destructive read-up-the-ramp measurements could perhaps be used to quantify CRNL, and distinguish it from nonlinearities in the downstream electronics. A pair of LEDs driven with square waves at different frequencies are appropriate sources.

Near- and far scrape-off layer transport studies in detached, small-ELM ASDEX Upgrade discharges by means of EMC3-EIRENE

We compare various diagnostics characterizing an ASDEX Upgrade upper single-null discharge to EMC3-EIRENE simulations now including volumetric recombination and main chamber plasma-wall interaction but not yet drifts. The discharge is in a small-ELM regime and is approaching detachment due to a density ramp and nitrogen seeding. Time/ELM averaged upstream density, electron and ion temperature measurements as well as downstream ion saturation current, electron temperature and power flux measurements match the simulation within the error bars before and after reaching detachment, qualifying the code for this regime. Although the Ion Cyclotron Resonance Heating (ICRH) antenna limiters are 35 mm away from the separatrix, i.e. ten times the typical near-SOL power fall-off length 3.8 mm, they receive power fluxes of several hundreds of kW exceeding the ones to the outer target in the detached phase and strongly contribute to the particle fueling. This is explained by a substantially larger far-SOL power fall-off length 25 mm possibly caused by the small ELMs in the detached phase and the occurrence of a density shoulder.

Suprathermal Electron Acceleration by a Quasi-perpendicular Shock: Simulations and Observations

Abstract The acceleration of suprathermal electrons in the solar wind is mainly associated with shocks driven by interplanetary coronal mass ejections (ICMEs). It is well known that the acceleration of electrons is much more efficient at quasi-perpendicular shocks than at quasi-parallel ones. Yang et al. studied the acceleration of suprathermal electrons with observations at a quasi-perpendicular ICME-driven shock event to claim the important role of shock-drift acceleration (SDA). Here, we perform test-particle simulations to study the acceleration of electrons in this event, by calculating the downstream electron intensity distribution for all energy channels assuming an initial distribution based on the average upstream intensities. Using simulations, we obtain the results similar to the observations from Yang et al. as follows. It is shown that the ratio of downstream to upstream intensities peaks at about 90° pitch angle. In addition, in each pitch angle direction the downstream electron energy spectral index is much larger than the theoretical index of diffusive shock acceleration. Furthermore, the estimated drift length is proportional to the electron energy but the drift time is almost energy independent. Finally, we use a theoretical model based on SDA to describe the drift length and drift time especially, to explain their energy dependence. These results indicate the importance of SDA in the acceleration of electrons by quasi-perpendicular shocks.