Search

We present a statistical analysis of >2,100 bipolar electrostatic solitary waves (ESWs) collected from 10 quasi‐perpendicular Earth's bow shock crossings by Magnetospheric Multiscale spacecraft. We developed and implemented a correction procedure for reconstruction of actual electric fields, velocities, and other properties of ESW, whose spatial scales are typically comparable with or smaller than spatial distance between voltage‐sensitive probes. We found that more than 95% of the ESW are of negative polarity with amplitudes typically below a few Volts and 0.1Te (5–30 V or 0.1–0.3Te for a few percent of ESW), spatial scales of 10–100 m or λD–10λD, and velocities from a few tens to a few hundred km/s that is on the order of local ion‐acoustic speed. The spatial scales of ESW are correlated with local Debye length λD. The ESW have electric fields generally oblique to magnetic field and they propagate highly oblique to shock normal N; more than 80% of ESW propagate within 30° of the shock plane LM. In the shock plane, ESW typically propagates within a few tens of degrees of local magnetic field projection BLM and preferentially opposite to N × BLM. We argue that the ESW of negative polarity are ion holes produced by ion‐ion streaming instabilities. We estimate ion hole lifetimes to be 10–100 ms, or 1–10 km in terms of traveling distance. The revealed statistical properties will be useful for quantitative studies of electron thermalization in the Earth's bow shock. Plain Language Summary: The Earth's bow shock is a natural laboratory for in‐situ analysis of plasma processes in supercritical collisionless shock waves. The current consensus is that quasi‐static magnetic and electric fields play a major role in electron heating, while scattering by waves results in thermalization of electron velocity distribution functions shaped by quasi‐static fields. Among various waves observed in the Earth's bow shock, electrostatic fluctuations deserve particular attention, because they are always present in the shock transition region. This study is focused on experimental analysis of electrostatic fluctuations in the Earth's bow shock using 3D electric field measurements by the Magnetospheric Multiscale (MMS) spacecraft. We presented a statistical analysis of electrostatic solitary waves (ESWs) using a data set of more than 2,000 solitary waves selected in 10 Earth's bow shock crossings. In contrast to previous interpretations, we showed that ESWs in the Earth's bow shock are predominantly ion phase‐space holes, which is a strong indication that electrostatic fluctuations in the Earth's bow shock are produced by ion‐ion streaming instabilities. We obtained statistical distributions of various properties of ion holes, which will be of value for quantifying electron thermalization by electrostatic fluctuations in collisionless shocks. Key Points: More than 95% of bipolar electrostatic structures in the Earth's bow shock are ion holesIon holes typically have electric fields oriented oblique to local magnetic fieldWe estimated lifetimes of ion holes to be 10–100 ms or 1–10 km in terms of traveling distance [ABSTRACT FROM AUTHOR]

Martian subsolar electron temperatures obtained below 250 km are examined using data obtained by instruments on the Mars Atmosphere Evolution Mission (MAVEN) during the three subsolar deep dip campaigns and a one‐dimensional fluid model. This analysis was done because of the uncertainty in MAVEN low electron temperature observations at low altitudes and the fact that the Level 2 temperatures reported from the MAVEN Langmuir Probe and Waves instrument are more than 400 K above the neutral temperatures at the lowest altitudes sampled (~120 km). These electron temperatures are well above those expected before MAVEN was launched. We find that an empirical normalization parameter, neutral pressure divided by local electron heating rate, organized the electron temperature data and identified a similar altitude (~160 km) and time scale (~2,000 s) for all three deep dips. We show that MAVEN data are not consistent with a plasma characterized by electrons in thermal equilibrium with the neutral population above 100 km. Because of the lack data below 120 km and the uncertainties of the data and the cross sections used in the one‐dimensional fluid model above 120 km, we cannot use MAVEN observations to prove that the electron temperature converges to the neutral temperature below 100 km. However, the uncertainty of the electron temperature altitude profile below 120 km does not impact our understanding of the role of electron temperature in determining ion escape rates because ion escape is determined by electron temperatures above 180 km. Key Points: Observations of Martian subsolar electron temperatures below 160 km from MAVEN deep campaigns are reported and analyzedThe observed electron temperatures are significantly higher than those expected before the MAVEN was obtainedElectron neutral temperature equilibrium occurs below 100 km [ABSTRACT FROM AUTHOR]

Exploring low-loss two-dimensional plasmon modes is considered central for achieving light manipulation at the nanoscale and applications in plasmonic science and technology. In this context, pump–probe spectroscopy is a powerful tool for investigating these collective modes and the corresponding energy transfer processes. Here, I present a first-principles study on non-equilibrium Dirac plasmon in graphene, wherein damping channels under ultrafast conditions are still not fully explored. The laser-induced blueshift of plasmon energy is explained in terms of thermal increase of the electron–hole pair concentration in the intraband channel. Interestingly, while damping pathways of the equilibrium graphene plasmon are entirely ruled by scatterings with acoustic phonons, the photoinduced plasmon predominantly transfers its energy to the strongly coupled hot optical phonons, which explains the experimentally-observed tenfold increase of the plasmon linewidth. The present study paves the way for an in-depth theoretical comprehension of plasmon temporal dynamics in novel two-dimensional systems and heterostructures.

The thermalization length and spatial distribution of electrons in liquid water were simulated for initial electron energies ranging from 0.1 eV to 100 keV using a dynamic Monte Carlo code. The results showed that electrons were decelerated for thermalization over a longer time period than was previously predicted. This long thermalization time significantly contributed to the series of processes from initial ionization to hydration. We further studied the particular deceleration process of electrons at an incident energy of 1 eV, focusing on the temporal evolution of total track length, mean traveling distance, and energy distributions of decelerating electrons. The initial prehydration time and thermalization periods were estimated to be approximately 50 and 220 fs, respectively, indicating that the initial prehydration began before or contemporaneously with the thermal equilibrium. Based on these results, the prehydrated electrons were suggested to play an important role during multiple DNA damage induction. [ABSTRACT FROM AUTHOR]

Abstract: The Metropolis method of calculating the electron–ion geminate escape probability in high-mobility systems has been applied to determine the initial thermalization distance distribution in several non-polar liquids for agreement with measured free-ion yield. Calculations have been performed for a series of hydrocarbons and other liquids, at room temperature, over the range of electron mobility μ from about 1cm2 V−1 s−1 to 100cm2 V−1 s−1, and for liquid methane over the temperature interval 122–192K (μ=50–800cm2 V−1 s−1). The results clearly confirm inapplicability of the standard Onsager theory of ion recombination to analyze experimental results for high-mobility liquids. The mean thermalization distances determined by the Metropolis method are significantly shorter than those obtained using the Onsager theory. Some regularities in the thermalization distance in various liquids are revealed, which were unobserved in earlier studies because of the use of the Onsager theory. The concepts proposed earlier that the mean thermalization distance in liquids of similar structure is scaled by either the density or the dielectric constant are only partially corroborated. [Copyright &y& Elsevier]

Abstract: A Monte Carlo model is utilized for studying the behavior of electrons in the afterglow of an analytical microsecond dc pulsed glow discharge. This model uses several quantities as input data, such as electric field and potential, ion flux at the cathode, the fast argon ion and atom impact ionization rates, slow electron density, the electrical characterization of the pulse (voltage and current profiles) and temperature profile. These quantities were obtained by earlier Monte Carlo — fluid calculations for a pulsed discharge. Our goal is to study the behavior of the so-called Monte Carlo electrons (i.e., those electrons created at the cathode or by ionization collisions in the plasma which are followed by using the Monte Carlo model) from their origin to the moment when they are absorbed at the cell walls or when they have lost their energy by collisions (being transferred to the group of slow electrons) in the afterglow of the pulsed discharge. The thermalization of the electrons is a phenomenon where the electron-electron Coulomb collisions acquire a special importance. Indeed, in the afterglow the cross sections of the other electron reactions taken into account in the model are very low, because of the very low electron energy. We study the electron energy distributions at several times during and after the pulse and at several positions in the plasma cell, focusing on the thermalization and on the behavior of the electrons in the afterglow. Also, the time evolution of the rates of the various collision processes, the average electron energy, the densities of Monte Carlo and slow electrons and the ionization degree are investigated. [Copyright &y& Elsevier]

Abstract: Some features of the stationary and dynamic states of the electron are critically examined. Outline of a quantum mechanical description of electron thermalization is attempted qualitatively. The effects of both the mean free path and the reaction inefficiency on electron–ion geminate escape probability are investigated by a recently developed Metropolis method. The trapped state is interpreted in terms of Anderson localization, yielding an approximate number of molecules interacting with the trapped electron. [Copyright &y& Elsevier]

The localized surface plasmon dynamics of laser ablated gold and gold-silver alloy nanoparticles (NPs) near interband and intraband excitation wavelengths were investigated using femtosecond (fs) transient absorption spectroscopy. Interband excitation with 70 fs laser pulses at a wavelength of 400 nm demonstrated plasmon photo-bleach and a transient absorption in the wings of the bleach spectrum. With intraband excitation, Ag 50 Au 50 alloy NPs depicted a fast electron thermalization time of ~180 fs. A slower decay of positive absorption from alloy NPs was observed when compared to pure Au NPs. These NPs demonstrated a rapid initial electron thermalization and exhibited similar electron-phonon relaxation times. The third-order nonlinear optical (NLO) properties were measured using the Z-scan technique with 1 kHz repetition rate fs pulses at 800 nm. It is observed that the alloy NPs possessed large value of the NLO susceptibility χ(3) (~10−13 esu) compared to pure gold NPs at 800 nm finding applications in different areas of photonics. • Au and Au 50 Ag 50 nanoparticles prepared using femtosecond laser ablation. • Z-scan technique used to study the NLO properties. • Femtosecond dynamics obtained using transient absorption measurements. • Strong NLO coefficients obtained with femtosecond pulses. [ABSTRACT FROM AUTHOR]