Dynamics Of Energetic Electrons Research Articles

Flare-accelerated Electrons in the Kappa Distribution from X-Ray Spectra with the Warm-target Model

Abstract X-ray observations provide important and valuable insights into the acceleration and propagation of nonthermal electrons during solar flares. Improved X-ray spectral analysis requires a deeper understanding of the dynamics of energetic electrons. Previous studies have demonstrated that the dynamics of accelerated electrons with a few thermal speeds are more complex than those with significantly higher speeds. To better describe the energetic electrons after injection, a model considering energy diffusion and thermalization effects in flare conditions (the warm-target model) has recently been developed for spectral analysis of hard X-rays. This model has demonstrated how the low-energy cutoff, which can hardly be constrained in cold-target modeling, can be determined. However, the power-law form may not be the most suitable representation of injected electrons. The kappa distribution, which is proposed as a physical consequence of electron acceleration, has been applied successfully in RHESSI spectral analysis. In this study, we employ the kappa-form injected electrons in the warm-target model to analyze two M-class flares, observed by RHESSI and the Spectrometer/Telescope for Imaging X-rays, respectively. The best-fit results show that the kappa-form energetic electron spectrum generates lower nonthermal energy than the power-law form when producing a similar photon spectrum in the fit range. We also demonstrated that the fit parameters associated with the kappa-form electron spectrum can be well determined with small uncertainty. Further, the kappa distribution, which covers the entire electron energy range, enables the determination of key electron properties such as total electron number density and average energy at the flare site, providing valuable information on electron acceleration processes.

Influences of Solar Wind Parameters on Energetic Electron Fluxes at Geosynchronous Orbit Revealed by the Deep SHAP Method

AbstractSolar wind is an intermediary in energy transfer from the Sun into the Earth's magnetosphere, and is considered as a decisive driver of energetic electron dynamics at the geosynchronous orbit (GEO). Based on machine learning technology, several models driven by solar wind parameters have been established to predict GEO electron fluxes. However, the relative contributions of different solar wind parameters on GEO electron fluxes are still unclear. Recently, a feature attribution method, Deep SHapley Additive exPlanations (Deep SHAP) is proposed to open black boxes of machine learning models. In this study, we use the Deep SHAP method to quantify contributions of different solar wind parameters with an artificial neural network (ANN) model. Backpropagating the prediction results of this ANN model from 2011 to 2020, SHAP values for four solar wind parameters (interplanetary magnetic field (IMF) BZ, solar wind speed, solar wind dynamic pressure, and proton density) are calculated and comprehensively analyzed. The results suggest that solar wind speed with a lag of 1 day is the most important driver. We further investigate relative roles of different parameters in three specific electron fluxes variation events (corresponding to electron fluxes reaching a local maximum, a local minimum, and unchanged, respectively). The results suggest that high solar wind speed and southward IMF BZ (high dynamic pressures) facilitate increases (decreases) of electron fluxes. These findings help reveal the underlying physical mechanisms of GEO electron dynamics and help develop more accurate prediction models for GEO electron fluxes.

Unveiling Energetic Particle Dynamics in the Near‐Earth Environment From CubeSat Missions

AbstractThe discovery of the Van Allen radiation belts marked a prominent milestone in space physics. Recent advances, through the measurements of two CubeSat missions, have shed new light on the dynamics of energetic particles in the near‐Earth environment. Measurements from CSSWE, a student‐led mission, revealed that the decay of low‐energy neutrons, associated with cosmic rays impacting the atmosphere, is the primary source of relativistic electrons at the inner edge of the inner belt (Li et al., Nature, 2017, https://doi.org/10.1038/nature2464). Recently CIRBE captured striking details of energetic electron dynamics (Li et al., GRL, 2024, https://doi.org/10.1029/2023gl107521), further demonstrating high‐quality science achievable with CubeSat missions.

Constraining the Influence of Callisto's Perturbed Electromagnetic Environment on Energetic Particle Observations

AbstractThis study focuses on constraining the role that Callisto's perturbed electromagnetic environment had on energetic charged particle signatures observed during the Galileo mission. To do so, we compare data from the Energetic Particle Detector (EPD) obtained during four close encounters of the moon with a model framework that combines hybrid simulations for low‐energy plasma and test‐particle tracing simulations for high‐energy particles. By comparing model results for energetic particle dynamics in both uniform and perturbed electromagnetic fields, we systematically disentangle the role that geometric effects (i.e., absorption of particles by Callisto's solid surface) have on observed energetic particle signatures compared to those associated with Callisto's perturbed electromagnetic environment (generated by the moon's induced magnetic field and plasma interaction currents). We show that observed flux drop‐outs in the energetic ion pitch angle distributions (PADs) are largely driven by their absorption by Callisto's surface: their large gyroradii exceed the size of the moon, facilitating their impact onto the icy surface and preventing their detection by EPD. However, features observed in the energetic electron PADs can only be explained with an accurate representation of the moon's perturbed environment, since electrons closely follow the orientation of the electromagnetic fields. Our findings therefore illustrate the key role that the moon's induced field and magnetospheric plasma interaction have on the dynamics of energetic electrons, emphasizing the importance of accurately modeling Callisto's locally perturbed electromagnetic environment when attempting to interpret data from past and future encounters, including those anticipated from the upcoming JUICE mission.

On the Two Approaches to Incorporate Wave‐Particle Resonant Effects Into Global Test Particle Simulations

AbstractEnergetic electron dynamics in the Earth's radiation belts and near‐Earth plasma sheet are controlled by multiple processes operating on very different time scales: from storm‐time magnetic field reconfiguration on a timescale of hours to individual resonant wave‐particle interactions on a timescale of milliseconds. The most advanced models for such dynamics either include test particle simulations in electromagnetic fields from global magnetospheric models, or those that solve the Fokker‐Plank equation for long‐term effects of wave‐particle resonant interactions. The most prospective method, however, would be to combine these two classes of models, to allow the inclusion of resonant electron scattering into simulations of electron motion in global magnetospheric fields. However, there are still significant outstanding challenges that remain regarding how to incorporate the long term effects of wave‐particle interactions in test‐particle simulations. In this paper, we describe in details two approaches that incorporate electron scattering in test particle simulations: stochastic differential equation (SDE) approach and the mapping technique. Both approaches assume that wave‐particle interactions can be described as a probabilistic process that changes electron energy, pitch‐angle, and thus modifies the test particle dynamics. To compare these approaches, we model electron resonant interactions with field‐aligned whistler‐mode waves in dipole magnetic fields. This comparison shows advantages of the mapping technique in simulating the nonlinear resonant effects, but also underlines that more significant computational resources are needed for this technique in comparison with the SDE approach. We further discuss applications of both approaches in improving existing models of energetic electron dynamics.

Mode Transition Induced by Gas Heating Along the Discharge Channel in Capacitively Coupled Atmospheric Pressure Micro Plasma Jets

The effects of neutral gas heating along the direction of the gas flow inside the discharge channel of a parallel plate micro atmospheric pressure plasma jet, the COST-jet, on the spatio-temporal dynamics of energetic electrons are investigated by experiments and simulations. The plasma source is driven by a single frequency sinusoidal voltage waveform at 13.56 MHz in helium with an admixture (0.05–0.2%) of nitrogen. Optical emission spectroscopy measurements are applied to determine the spatio-temporally resolved electron impact excitation dynamics from the ground state into the He I (3s)3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^3$$\\end{document}S1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_1$$\\end{document} state and the rotational temperature of nitrogen molecules at different positions along the direction of the gas flow inside the 30 mm long discharge channel. The gas temperature, which is assumed to be equal to the N2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document} rotational temperature, is found to increase along the discharge channel. This effect is attenuated as the nitrogen concentration is increased in the gas mixture, leading to an eventually constant temperature profile. The experimental data also reveal a plasma operating mode transition along the discharge channel from the Ω\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Omega$$\\end{document}- to the Penning-mode and show good agreement with the results of 1d3v kinetic simulations, which spatially resolve the inter-electrode space and use the gas temperature as an input value. The simulations demonstrate that the increase of the gas temperature leads to the observed mode transition. The results suggest the possibility of using the nitrogen admixture and the feed gas temperature as additional control parameters, (i) to tailor the plasma operating mode along the direction of the gas flow so that the production of specific radicals is optimized; and (ii) to control the final gas temperature of the effluent. The latter could be particularly interesting for biological applications, where the upper gas temperature limit is dictated by the rather low thermal damage threshold of the treated material.

Role of “positive phase bunching” effect for long-term electron flux dynamics due to resonances with whistler-mode waves

Resonant interactions with electromagnetic whistler-mode waves are a primary driver of energetic electron dynamics in the Earth's radiation belts. The most intense waves can resonate with electrons nonlinearly, and effects of such nonlinear resonant interactions significantly differ from the classical quasi-linear diffusion. There have been continuous efforts on the theoretical investigation and implementation of these effects into radiation belt models, but not all nonlinear effects have been revealed yet. The two most investigated effects are phase trapping and phase bunching, which are responsible for electron acceleration and precipitation into the Earth's atmosphere, respectively, i.e., for the first cyclotron resonance with waves generated at the equator and propagating to higher latitudes, phase trapping increases electrons' energy, whereas phase bunching decreases the electron pitch-angle (and magnetic moment). However, recent studies reported a new effect called positive phase bunching, which may increase the electron pitch-angle and move them away from the loss-cone. This paper aims to characterize possible contributions of this effect to long-term electron dynamics, including multiple resonant interactions. Using an iterated mapping technique, we show that although the positive phase bunching effect can modify electron trajectories, it does not change the average rate of electron mixing in phase space. Thus, this effect may be safely neglected in long-term simulations of radiation belt dynamics. We also discuss possible verification of the positive phase bunching effect using short (single resonance), bursty electron precipitation events.

Formation of Banded Chorus Waves by Propagation From Multiple Sources

AbstractChorus waves are electromagnetic emissions in the magnetosphere and play a crucial role in energetic electron dynamics. Observations show that these waves often exhibit a two‐band structure with a power gap near half the electron cyclotron frequency. However, the mechanism behind this gap remains an open question, despite the existence of multiple theories. Here we propose a model to explain the banded structure by the different propagation properties of the lower and upper band waves within a density crest. This requires independent sources for lower and upper band waves, and unducted propagation of upper band waves to the observing location. The model naturally generates a gap near half the electron cyclotron frequency and can be extended to explain lower‐band‐only, upper‐band‐only, and no‐gap whistler waves. The model calls for further research on density irregularities in the magnetosphere and their effects on shaping wave properties.

Deep learning model of hiss waves in the plasmasphere and plumes and their effects on radiation belt electrons

Hiss waves play an important role in removing energetic electrons from Earth’s radiation belts by precipitating them into the upper atmosphere. Compared to plasmaspheric hiss that has been studied extensively, the evolution and effects of plume hiss are less understood due to the challenge of obtaining their global observations at high cadence. In this study, we use a neural network approach to model the global evolution of both the total electron density and the hiss wave amplitudes in the plasmasphere and plume. After describing the model development, we apply the model to a storm event that occurred on 14 May 2019 and find that the hiss wave amplitude first increased at dawn and then shifted towards dusk, where it was further excited within a narrow region of high density, namely, a plasmaspheric plume. During the recovery phase of the storm, the plume rotated and wrapped around Earth, while the hiss wave amplitude decayed quickly over the nightside. Moreover, we simulated the overall energetic electron evolution during this storm event, and the simulated flux decay rate agrees well with the observations. By separating the modeled plasmaspheric and plume hiss waves, we quantified the effect of plume hiss on energetic electron dynamics. Our simulation demonstrates that, under relatively quiet geomagnetic conditions, the region with plume hiss can vary from L = 4 to 6 and can account for up to an 80% decrease in electron fluxes at hundreds of keV at L > 4 over 3 days. This study highlights the importance of including the dynamic hiss distribution in future simulations of radiation belt electron dynamics.

The electrical asymmetry effect in electronegative CF4 capacitive RF plasmas operated in the striation mode

The Electrical Asymmetry Effect (EAE) provides control of the mean ion energy at the electrodes of multi-frequency capacitively coupled radio frequency plasmas (CCP) by tuning the DC self-bias via adjusting the relative phase(s) between the consecutive driving harmonics. Depending on the electron power absorption mode, this phase control affects the ion flux in different ways. While it provides separate control of the mean ion energy and flux in the α-mode, limitations were found in the γ- and Drift-Ambipolar modes. In this work, based on experiments as well as kinetic simulations, the EAE is investigated in the striation-mode, which is present in electronegative CCPs driven by low frequencies. The discharge is operated in CF4 and is driven by two consecutive harmonics (4/8 MHz). The simulation results are validated against measurements of the DC self-bias and the spatio-temporally resolved dynamics of energetic electrons. To include heavy particle induced secondary electron emission realistically, a new computationally assisted diagnostic is developed to determine the corresponding secondary electron emission coefficient from a comparison of the DC self-bias obtained experimentally and from the simulations. Based on the validated simulation results, the EAE is found to provide separate control of the mean ion energy and flux in the striation mode, while the axial charged particle density profiles and the number of striations change as a function of the relative phase. This is understood based on an analysis of the ionization dynamics.

Local enhancement of electron heating and neutral species generation in radio-frequency micro-atmospheric pressure plasma jets: the effects of structured electrode topologies

The effects of structured electrode topologies on He/O2 radio frequency micro-atmospheric pressure plasma jets driven at 13.56 MHz are investigated by a combination of 2D fluid simulations and experiments. Good qualitative agreement is found between the computational and experimental results for the 2D spatio-temporally resolved dynamics of energetic electrons measured by phase resolved optical emission spectroscopy, 2D spatially resolved helium metastable densities measured by tunable diode laser absorption spectroscopy and 2D spatially resolved atomic oxygen densities measured by two photon absorption laser induced fluorescence. The presence of rectangular trenches of specific dimensions inside the electrodes is found to cause a local increase of the electron power absorption inside and above/below these surface structures. This method of controlling the electron energy distribution function via tailored surface topologies leads to a local increase of the metastable and atomic oxygen densities. A linear combination of trenches along the direction of the gas flow is found to result in an increase of the atomic oxygen density in the effluent, depending linearly on the number of trenches. These findings are explained by an enhanced Ohmic electric field inside each trench, originating from (a) the low electron density, and, consequently, the low plasma conductivity inside the trenches, and (b) the presence of a current focusing effect as a result of the electrode topology.

Energetic Magnetospheric Particle Fluxes Onto Callisto's Atmosphere

AbstractThis study investigates how Callisto's perturbed electromagnetic environment—generated by the moon's interaction with the low‐energy Jovian magnetospheric plasma—affects the dynamics of high‐energy ions and electrons. We constrain how these perturbed fields influence the energetic particle fluxes deposited onto the top of Callisto's atmosphere between energies of 4.5 keV ≤ E ≤ 11.8 MeV. We use a hybrid simulation to model the variability in Callisto's perturbed electromagnetic environment over a synodic period by considering three representative scenarios of the moon's plasma interaction, corresponding to various distances of the moon to the Jovian magnetospheric current sheet. The local field perturbations are maximized near the center of the sheet (forming, e.g., signatures of field‐line pileup, draping, and Alfvén wings) whereas far from the sheet, a mere superposition of the moon's induced dipole with the background field largely explains the perturbations. We then apply a test‐particle approach to investigate the dynamics of energetic electrons and ions (protons, oxygen, and sulfur) while exposed to these fields. Since electron gyroradii are smaller than Callisto, the field perturbations generate small‐scale non‐uniformities in their flux patterns onto the moon, while the ion flux patterns are more homogeneous. Energetic electrons dominate the number flux onto the atmosphere, whereas ions dominate the energy flux. Over a synodic period, the flux patterns onto Callisto's exobase closely resemble those when the moon is near the current sheet center, since the differential energetic particle fluxes in the ambient plasma decrease by an order of magnitude when the moon travels far outside of the sheet.

An on-orbit cross-calibration between the relativistic electron observations from BeiDou M04 and GPS ns63

Understanding the radiation belts comprehensively has prominent significance both for aerospace engineering and space weather. For both the development of radiation belts models and the research into the dynamics of energetic electrons in the inner magnetosphere, the availability of global, well cross-calibrated data is crucial. In this report, we carry out an on-orbit cross-calibration between the observations from the High Energy Electron Detector (HEED) on Bei Dou navigation satellite system (BD) of China and Combined X-ray and Dosimeter (CXD) on Global Positioning System (GPS) constellation using the relativistic electron flux data from 2012 to 2014 and obtain the systematic offsets between the two systems. The fitting results show that there is a good consistency with the electron observations from M04 and ns63 with linear fitting slopes between relativistic electron fluxes from HEED and CXD all near 0.9. Using the cross-calibration results, the observations of greater than 1 MeV electrons of M04-HEED and GPS-CXD are assimilated well by removing the system deviation. Taking advantage of assimilated electron observations from M04 and ns63, the evolution of electron energy spectrums during the two sequential magnetic storms in March 2013 is investigated. Different dynamic properties presented by assimilated observations suggest different physical process dominant during these storms. Furthermore, this case study demonstrates the significance of the cross-calibration between M04 and ns64 observations and the usefulness of assimilated observations. This work is the first try to show that the relativistic electron observation from BD is compared with the same type operational satellite GPS. It could be expected that huge assimilated observations using the cross-calibration presented in this work would be extremely useful for statistical radiation belt modeling research, space weather nowcasting and forecasting, and comprehensive scientific understanding of energetic electron dynamic processes in the radiation belts.

Harmonic Electron Cyclotron Maser Emission along the Coronal Loop

Efficient radiation at second and/or higher harmonics of Ωce has been suggested to circumvent the escaping difficulty of the electron cyclotron maser emission mechanism when it is applied to solar radio bursts, such as spikes. In our earlier study, we developed a three-step numerical scheme to connect the dynamics of energetic electrons within a large-scale coronal loop structure with the microscale kinetic instability energized by the obtained nonthermal velocity distribution and found that direct and efficient harmonic X-mode (X2 for short) emission can be achieved due to the strip-like features of the distribution. That study only considered the radiation from the loop top at a specific time. Here we present the emission properties along the loop at different locations and timings. We found that, in accordance with our earlier results, few to several strip-like features can appear in all cases, and the first two strips play the major role in exciting X2 and Z (i.e., the slow extraordinary mode) that propagate quasi-perpendicularly. For the four sections along the loop, significant excitation of X2 is observed from the upper two sections, and the strongest emission is from the top section. In addition, significant excitation of Z is observed for all loop sections, while there is no significant emission of the fundamental X mode. The study provides new insight into coherent maser emission along the coronal loop structure during solar flares.

Simultaneous occurrence of four magnetospheric wave modes and the resultant combined scattering effect on radiation belt electrons

We report a representative concurrent event of four wave modes at <italic>L</italic> ≈ 5.0, including electrostatic electron cyclotron harmonic (ECH) waves, exohiss, magnetosonic (MS) waves, and electromagnetic ion cyclotron (EMIC) waves, based on the observations from Van Allen Probe A on October 15, 2015. The diffusion coefficients induced by these waves are calculated by using both the Full Diffusion Code and test particle simulations. Moreover, the scattering effects of these waves on energetic electrons are simulated by using a two-dimensional Fokker–Planck diffusion model. The results show that ECH waves mainly scatter low-pitch-angle (&lt; 20°) electrons at 0.1–10 keV; exohiss can significantly scatter hundreds of kiloelectron volt electrons to form a reversed energy spectrum; MS waves mainly affect high-pitch-angle electrons (&gt; 60°); and EMIC waves scatter only &gt; 5 MeV electrons. The combined scattering effects of exohiss and MS waves are stronger than those of exohiss alone. The top-hat pitch angle distributions produced by exohiss are relaxed after adding the effect of MS waves. Because the energies of electrons scattered by ECH waves and EMIC waves are much lower and higher than those scattered by exohiss and MS waves, respectively, the combined scattering effects with the addition of ECH and EMIC waves show little difference from the results for the combination of MS waves and exohiss. These results suggest that distinct wave modes can occur simultaneously and scatter electrons in combination or individually, which requires careful consideration in future global simulations of the complex dynamics of radiation belt energetic electrons.

Bounce Resonance Scattering of Radiation Belt Energetic Electrons by Extremely Low‐Frequency Chorus Waves

AbstractBounce‐resonant diffusion coefficients of radiation belt energetic electrons induced by extremely low‐frequency (ELF) chorus waves are comprehensively evaluated using Van Allen Probes observations on December 22, 2014. ELF chorus waves can efficiently scatter 85° &lt; eq &lt; 89° electrons with bounce resonance pitch angle scattering rates above 10−4/s and energy scattering rates above 10−7/s. By comparing diffusion coefficients due to bounce resonance with those due to cyclotron and Landau resonances, we found that bounce resonance diffusion rates have slight energy dependence for &gt;100 keV electrons while Landau‐resonant scattering rates decrease significantly in the MeV energy range. These findings suggest that bounce resonances by ELF chorus waves have potentially significant contributions to the dynamics of energetic electrons and should be considered in the further modeling of Earth's radiation belts.

Transitional regime of electron resonant interaction with whistler-mode waves in inhomogeneous space plasma.

Resonances with electromagnetic whistler-mode waves are the primary driver for the formation and dynamics of energetic electron fluxes in various space plasma systems, including shock waves and planetary radiation belts. The basic and most elaborated theoretical framework for the description of the integral effect of multiple resonant interactions is the quasilinear theory, which operates through electron diffusion in velocity space. The quasilinear diffusion rate scales linearly with the wave intensity, D_{QL}∼B_{w}^{2}, which should be small enough to satisfy the applicability criteria of this theory. Spacecraft measurements, however, often detect whistle-mode waves sufficiently intense to resonate with electrons nonlinearly. Such nonlinear resonant interactions imply effects of phase trapping and phase bunching, which may quickly change the electron fluxes in a nondiffusive manner. Both regimes of electron resonant interactions (diffusive and nonlinear) are well studied, but there is no theory quantifying the transition between these two regimes. In this paper we describe the integral effect of nonlinear electron interactions with whistler-mode waves in terms of the timescale of electron distribution relaxation, ∼1/D_{NL}. We determine the scaling of D_{NL} with wave intensity B_{w}^{2} and other main wave characteristics, such as wave-packet size. The comparison of D_{QL} and D_{NL} provides the range of wave intensity and wave-packet sizes where the electron distribution evolves at the same rates for the diffusive and nonlinear resonant regimes. The obtained results are discussed in the context of energetic electron dynamics in the Earth's radiation belt.

Direct Observational Evidence of the Simultaneous Excitation of Electromagnetic Ion Cyclotron Waves and Magnetosonic Waves by an Anisotropic Proton Ring Distribution

AbstractMagnetosonic (MS) waves and electromagnetic ion cyclotron (EMIC) waves are two important plasma wave modes in the magnetosphere. Previous simulations have shown that both waves could be generated by a ring‐like proton distribution, while direct observational evidence has yet to be reported. Here, we present simultaneous observations of MS and EMIC waves and a detailed case analysis. The linear growth rates estimated for both waves are in good agreement with the observed wave frequency spectra. The measured proton distribution evolution is also compared with the simulation results, providing direct observational evidence for the previous theoretical prediction that anisotropic ring‐like proton distributions could excite MS and EMIC waves simultaneously. Our findings are crucial for understanding the generation mechanisms of and relation between MS and EMIC waves and for evaluating their combined effects on energetic electron and ion dynamics.

Energetic Electron Enhancement and Dropout Echoes Induced by Solar Wind Dynamic Pressure Decrease: The Effect of Phase Space Density Profile

AbstractThe interplanetary (IP) shock with the sudden increase of solar wind dynamic pressure can cause the acceleration and transportation of energetic electrons in the inner magnetosphere. In this study, we further examine the influence of the sudden drop of solar wind dynamic pressure on magnetospheric energetic particles, through case study of energetic electron drift echoes and statistics with 25 cases from 2012 to 2017. We found that there were both dropout and enhancement drift echoes of electrons in some dynamic pressure decrease events, while other events only show electron dropout drift echoes. In addition, we analyzed the electron phase space density (PSD) and found that the radial profile of electron PSD were different for the two types of events, suggesting that the electron PSD plays an important role in the variations of energy electron fluxes induced by the dynamics pressure decrease. The results of this study, combined with previous studies of energetic electron dynamics caused by IP shocks, can provide a more general picture of both solar wind dynamic pressure decrease and increase effects on the energetic electrons.

Towards the optimisation of direct laser acceleration

Experimental measurements using the OMEGA EP laser facility demonstrated direct laser acceleration (DLA) of electron beams to (505 ± 75) MeV with (140 ± 30) nC of charge from a low-density plasma target using a 400 J, picosecond duration pulse. Similar trends of electron energy with target density are also observed in self-consistent two-dimensional particle-in-cell simulations. The intensity of the laser pulse is sufficiently large that the electrons are rapidly expelled along the laser pulse propagation axis to form a channel. The dominant acceleration mechanism is confirmed to be DLA and the effect of quasi-static channel fields on energetic electron dynamics is examined. A strong channel magnetic field, self-generated by the accelerated electrons, is found to play a comparable role to the transverse electric channel field in defining the boundary of electron motion.