Kiloelectronvolt Electron Research Articles

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.

Whistler Waves in the Plasmasphere Boundary Layer: Nonlinear Parametric Excitation

AbstractWe present the initial results of numerical simulations of nonlinear coupling between quasi‐electrostatic lower hybrid oblique resonance and fast magnetosonic waves, which was suggested to explain the observations of Very Low Frequency (VLF) electromagnetic emissions at frequencies well above the lower hybrid resonance frequency in the turbulent plasmasphere boundary layer. These emissions represent a distinctive subset of the substorm/storm‐related VLF whistler activity contributing to the alteration of the outer radiation belt boundary. As the turbulent plasmasphere boundary layer is interior to the plasma sheet inner boundary and thus devoid of substorm‐injected kiloelectron volt electrons, the “standard” whistler generation mechanism must be excluded. The simulations show that the parametric coupling of lower hybrid oblique resonance and magnetosonic waves creates VLF electromagnetic emissions with the spectral characteristics consistent with the observations.

Statistical Properties of Hiss in Plasmaspheric Plumes and Associated Scattering Losses of Radiation Belt Electrons

AbstractWhistler mode hiss acts as an important loss mechanism contributing to the radiation belt electron dynamics inside the plasmasphere and plasmaspheric plumes. Based on Van Allen Probes observations from September 2012 to December 2015, we conduct a detailed analysis of hiss properties in plasmaspheric plumes and illustrate that corresponding to the highest occurrence probability of plumes at L = 5.0–6.0 and MLT = 18–21, hiss emissions occur concurrently with a rate of &gt;~80%. Plume hiss can efficiently scatter ~10‐ to 100‐keV electrons at rates up to ~10−4 s−1 near the loss cone, and the resultant electron loss timescales vary largely with energy, that is, from less than an hour for tens of kiloelectron volt electrons to several days for hundreds of kiloelectron volt electrons and to &gt;100 days for &gt;5‐MeV electrons. These newly obtained statistical properties of plume hiss and associated electron scattering effects are useful to future modeling efforts of radiation belt electron dynamics.

Particle Beams in the Vicinity of Magnetic Separatrix According to Near‐Lunar ARTEMIS Observations

AbstractWe study characteristics of ion and electron beams observed during 101 crossings of the near‐separatrix region by Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) spacecraft in the magnetotail. We found that accelerated ion beams are observed under any level of geomagnetic activity. A duration of earthward moving ion beams is statistically longer (≤ 10 min) than a duration of tailward ion beams (≤ 4 min), which can be due to the transient character of ion acceleration in the vicinity of the near‐Earth neutral line (NENL). Energetic characteristics of earthward and tailward ion beams are similar indicating similar acceleration conditions at ion kinetic scales at both sides of an X line independently of its location. Conversely, electron velocity distributions observed near magnetic separatrix earthward of the distant neutral line (DNL) differ from those observed tailward of the NENL. Earthward of the DNL a scattered and thermalized electron population without energetic field‐aligned beams is observed near the separatrix. On the contrary, tailward of the NENL field‐aligned electron beams accelerated to a few kiloelectron volts are detected. These observations show that near DNL the electron scattering and thermalization dominate over the direct acceleration, whereas stronger electric fields in the NENL produce substantial population of field‐aligned kiloelectron volt electrons.

Local Generation of High‐Frequency Plasmaspheric Hiss Observed by Van Allen Probes

AbstractThe generation of a high‐frequency plasmaspheric hiss (HFPH) wave observed by Van Allen Probes is studied in this letter for the first time. The wave has a moderate power spectral density (∼10−6 nT2/Hz), with a frequency range extended from 2 to 10 kHz. The correlated observations of waves and particles indicate that HFPH is associated with the enhancement of electron flux during the substorm on 6 January 2014. Calculations of the wave linear growth rate driven by the fitted electron phase space density show that the electron distribution after the substorm onset is efficient for the HFPH generation. The energy of the contributing electrons is about 1–2 keV, which is consistent with the observation. These results support that the observed HFPH is likely to be generated locally inside the plasmasphere due to the instability of injected kiloelectron volt electrons.

On the Initial Enhancement of Energetic Electrons and the Innermost Plasmapause Locations: Coronal Mass Ejection‐Driven Storm Periods

AbstractUsing Van Allen Probes' observations and established plasmapause location (Lpp) models, we investigate the relationship between the location of the initial enhancement (IE) of energetic electrons and the innermost (among all magnetic local time sectors) Lpp over five intense storm periods. Our study reveals that the IE events for ~30‐keV to ~2‐MeV electrons always occurred outside of the innermost Lpp. On average, the inner extent of the IE events (LIE) for &lt;800‐keV electrons was closer to the innermost Lpp when compared to the LIE for &gt;800‐keV electrons that was found consistently at ~1.5 RE outside of the innermost Lpp. The IE of tens of kiloelectron volts electrons was observed before the IE of hundreds of kiloelectron volt electrons, and the IE of &gt;800‐keV electrons was observed on average 12.6 ± 2.3 hr after the occurrence of the earliest IE event. In addition, we report an overall electron (~30 keV to ~2 MeV) flux increase outside the plasmasphere during the selected storm periods, in contrast to the little change of energy spectrum evolution inside the plasmasphere; this demonstrates the important role of the plasmasphere in shaping energetic electron dynamics. Our investigation of the LIE‐Lpp relationship also provides insights into the underlying physical processes responsible for the dynamics of ~30‐keV to ~2‐MeV electrons.

An Empirical Model of Radiation Belt Electron Pitch Angle Distributions Based On Van Allen Probes Measurements

AbstractBased on over 4 years of Van Allen Probes measurements, an empirical model of radiation belt electron equatorial pitch angle distribution (PAD) is constructed. The model, developed by fitting electron PADs with Legendre polynomials, provides the statistical PADs as a function of L‐shell (L = 1–6), magnetic local time, electron energy (~30 keV to 5.2 MeV), and geomagnetic activity (represented by the Dst index) and is also the first empirical PAD model in the inner belt and slot region. For megaelectron volt electrons, model results show more significant day‐night PAD asymmetry of electrons with higher energies and during disturbed times, which is caused by geomagnetic field configuration and flux radial gradient changes. Steeper PADs with higher fluxes around 90° pitch angle and lower fluxes at lower pitch angles for higher‐energy electrons and during active times are also present, which could be due to electromagnetic ion cyclotron wave scattering. For hundreds of kiloelectron volt electrons, cap PADs are generally present in the slot region during quiet times and their energy‐dependent features are consistent with hiss wave scattering, while during active times, cap PADs are less significant especially at outer part of slot region, which could be due to the complex energizing and transport processes. The 90°‐minimum PADs are persistently present in the inner belt and appear in the slot region during active times, and minima at 90° pitch angle are more significant for electrons with higher energies, which could be a critical evidence in identifying the underlying physical processes responsible for the formation of 90°‐minimum PADs.

Radiation-Induced Damage and Recovery of Ultra-Nanocrystalline Diamond: Toward Applications in Harsh Environments.

Ultra-nanocrystalline diamond (UNCD) is increasingly being used in the fabrication of devices and coatings due to its excellent tribological properties, corrosion resistance, and biocompatibility. Here, we study its response to irradiation with kiloelectronvolt electrons as a controlled model for extreme ionizing environments. Real time Raman spectroscopy reveals that the radiation-damage mechanism entails dehydrogenation of UNCD grain boundaries, and we show that the damage can be recovered by annealing at 883 K. Our results have significant practical implications for the implementation of UNCD in extreme environment applications, and indicate that the films can be used as radiation sensors.

Evaluation of computational models and cross sections used by MCNP6 for simulation of characteristic X-ray emission from thick targets bombarded by kiloelectronvolt electrons

This paper evaluates the accuracy of the single-event (SE) and condensed-history (CH) models of electron transport in MCNP6.1 when simulating characteristic Kα, total K (=Kα+Kβ) and Lα X-ray emission from thick targets bombarded by electrons with energies from 5keV to 30keV. It is shown that the MCNP6.1 implementation of the CH model for the K-shell impact ionization leads to underestimation of the K yield by 40% or more for the elements with atomic numbers Z<15 and overestimation of the Kα yield by more than 40% for the elements with Z>25. The Lα yields are underestimated by more than an order of magnitude in CH mode, because MCNP6.1 neglects X-ray emission caused by electron-impact ionization of L, M and higher shells in CH mode (the Lα yields calculated in CH mode reflect only X-ray fluorescence, which is mainly caused by photoelectric absorption of bremsstrahlung photons). The X-ray yields calculated by MCNP6.1 in SE mode (using ENDF/B-VII.1 library data) are more accurate: the differences of the calculated and experimental K yields are within the experimental uncertainties for the elements C, Al and Si, and the calculated Kα yields are typically underestimated by (20–30)% for the elements with Z>25, whereas the Lα yields are underestimated by (60–70)% for the elements with Z>49. It is also shown that agreement of the experimental X-ray yields with those calculated in SE mode is additionally improved by replacing the ENDF/B inner-shell electron-impact ionization cross sections with the set of cross sections obtained from the distorted-wave Born approximation (DWBA), which are also used in the PENELOPE code system. The latter replacement causes a decrease of the average relative difference of the experimental X-ray yields and the simulation results obtained in SE mode to approximately 10%, which is similar to accuracy achieved with PENELOPE. This confirms that the DWBA inner-shell impact ionization cross sections are significantly more accurate than the corresponding ENDF/B cross sections when energy of incident electrons is of the order of the binding energy.

Monte Carlo simulation of characteristic x-ray emission from thick samples bombarded by kiloelectronvolt electrons

We describe systematic Monte Carlo (MC) calculations of characteristic K and L x-ray emission from thick samples bombarded by kiloelectronvolt electrons. The simulations were performed using the general-purpose MC code PENELOPE, which was modified by introducing a new database of electron-impact ionization cross sections calculated from the distorted-wave Born approximation (DWBA). The calculated yields, defined as the number of photons emerging from the target per unit solid angle and per incident electron, are compared with experimental measurements available from the literature, which pertain to single-element materials with atomic numbers ranging from Z = 6 up to Z = 82 and electron beam energies from a few kiloelectronvolts up to 40 keV. To reveal the dependence of the characteristic x-ray yields on the adopted ionization cross sections, simulations were also performed using cross sections based on the plane-wave Born approximation (PWBA). Our calculations confirm that, in the considered energy range, the DWBA is considerably more accurate than the PWBA.

Possible mechanism for radial ion acceleration in a beam-plasma discharge

A mechanism is proposed that can lead to radial ion acceleration in a plasma discharge excited by an electron beam in a relatively weak longitudinal magnetic field. The mechanism operates as follows. The beam generates an azimuthally asymmetric slow potential wave, which traps electrons. Trapped magnetized electrons drift radially with a fairly high velocity under the combined action of the azimuthal wave field (which is constant for them) and a relatively weak external longitudinal magnetic field. The radial electron flux generates a radial charge-separation electric field, which accelerates unmagnetized plasma ions in the radial direction. The ion flux densities and energies achievable in experiments with kiloelectronvolt electron beams in magnetic fields of up to 100 G are estimated.

Measurement of conductivity and charge storage in insulators related to spacecraft charging

Improved experimental methods are discussed for laboratory measurement of conductivity and electric field in insulating spacecraft material intended for space radiation and plasma environments. These measurement techniques investigate the following features: 1) measurements of conductivity are up to four orders of magnitude smaller than those determined by existing standard methods. 2) Conductivity is altered as radiation accumulates and trapping states fill with electrons. 3) With intense kiloelectronvolt electron irradiation, electrons are continually emitted for hours from the irradiated surface after the irradiation ceases. 4) Charging induced by electron irradiation is strongly modified by the electron-hole pairs that the irradiation generates in the insulator. 5) High field effects at 10/sup 6/ V/cm act strongly on the electron-hole pairs and on electrons in shallow traps to provide extended conductivity. 6) The capacitance of the sample can be measured in the same apparatus along with the other testing. 7) Visible light can be used to investigate conduction by electrons (or holes) emitted from shallow trapping levels. The qualitative physics of such processes in solid dielectrics has long been known, and instrumentation is developed here for measuring the effects in practical spacecraft charging applications.

Xe laser pumped by fast electrons generated in a barrier discharge

A compact pulsed generator of a kiloelectronvolt electron beam based on an open barrier discharge in a dense gas is proposed and developed. A stable discharge was observed in the experimental setup in a broad range of pressures up to the atmospheric pressure. At moderate pressures of the gas (~ 5 — 20 Torr), the efficient generation of an electron beam was observed with a current density of ~ 1 A cm-2 and the discharge area of ~ 100 cm2. The electron energy was ~ 2 — 10 keV. The pulse repetition rate could be varied from 10 Hz to 10 kHz without changing the electron-beam parameters. By injecting an electron beam into a neutral gas, lasing was obtained at the transitions of a Xe atom in the 2-μm region with an output power of ~ 0.22 mW kHz-1.

The charging behaviour and internal electric field of PMMA irradiated by a kiloelectronvolt electron beam

A scanning electron microscope (SEM) is employed in the investigation of the charging behaviour of polymethylmethacrylate (PMMA) specimens. Charges trapped in the PMMA specimens are measured using the SEM mirror image method under different conditions of radiation time, accelerating voltages and beam currents. The influence of the surface potential established and the internal electric field arising in the PMMA specimen is studied, and the latter is found to play a more important role. There exists a saturation value for the charge that can be trapped in the specimen under a fixed charging condition. Beyond the saturation value, the trapped charge destabilizes and surface discharge is observed. A Gaussian model of charge distribution is used for the calculation of the electric field. The strength of the electric field initiating surface discharge is estimated.

A two-satellite study of nightside flux transfer events in the plasma sheet

High time resolution measurements of magnetic and electric fields and of the kiloelectronvolt electron flux at X ∼ −21 R e in the central magnetotail were used in the study of short-duration (time scale 1 min) magnetic field events during two consecutive substorms on 23 March 1979. These events were observed at ISEE-1 and 2 (having 0.3 R e separation along the Z-coordinate), both inside and outside of the thinned plasma sheet, close (in time) to the polarity reversal of the plasma flow from tailward to earthward direction in association with the transient plasma sheet expansions. The enhanced magnetic flux closure (southward B z during tailward streaming events, northward during earthward streaming events), plus enhanced plasma flow during these events, suggest that they are nightside magnetic flux transfer events (NFTEs). The typical behaviour of the magnetic field during a NFTE (compression, rotation then field decrease), the signatures of overpressure in such structures and their close correlation with ground-based observed auroral activations and high energy particle bursts signify that NFTE structures may result from the impulsive reconnection in the plasmasheet. Systematic differences in the magnetic variations found at ISEE-1 and 2 indicate current concentration at the outer plasmasheet boundary during the passage of a NFTE. Two NFTEs appeared to have structures similar to magnetic flux ropes. The NFTE structure and associated current system, as well as the interpretation of NFTE signatures in terms of the impulsive reconnection, are discussed. During the “poleward leap” stage of one substorm, the spacecraft within the central part of the plasma sheet observed the intense earthward magnetic flux transport associated with superimposed NFTE structures. The estimated amount of this flux transfer explained well the observed amplitude of the poleward expansion of the westward electrojet in the conjugate auroral zone.

Mapping of the auroral horn into the magnetotail

The auroral horn is a frequently observed brightening of a pre-existing auroral arc immediately after a substorm onset has taken place. The tip of the horn propagates westward (and sometimes eastward) with a speed of 10–30 km s −1 in the ionosphere, which is considerably faster than the westward expansion speed of the main auroral bulge. The field-aligned mapping of the horn into the magnetotail is studied using the 1989 version of the Tsyganenko magnetic field model. It is argued that the source of the horn propagates from the location of the substorm onset in the tail current sheet toward the plasma sheet boundary layer. The propagation speed in the tail is larger than a reasonable expansion speed of a current disruption region in the current sheet. A natural fast propagation mode across the tail is a magnetosonic wave, which may interact with kiloelectron volt electrons resulting in enhanced pitch angle scattering into the loss cone and brightening of the auroral luminosity along the horn. Thus the auroral horn may be an expression of communication between the current sheet and the plasma sheet boundary layer shortly after the substorm onset.

Soft X-rays arising from kiloelectronvolt electron motion in a solid

On the basis of a combined study of kinetic transport equations formulated both for quantum and for classical systems the characteristics of the movement of fast electron streams in solids are examined.It is found that in process of this movement the primary electrons must exhibit periodic cross-sectional oscillations, from which soft X-rays arise.

A technique for interpretation of auroral bremsstrahlung X-ray spectra

The precipitating kiloelectronvolt electrons in the auroral zone that produce enhanced E-region ionization also produce bremsstrahlung X-rays. The spectral distribution of the emitted X-rays contains information from which the energy spectrum of the precipitating electrons may be inferred. Modeling of the interaction of auroral electrons with the atmosphere shows that the flux F of vertically-emitted X-rays as a function of energy K is given approximately by F(K) = g 1(K) ∫ K ∞ (E − K)J(E) dE where J( E) is the differential flux of electrons in the energy range E − E + dE and g 1( K) is a modified X-ray production function. With this functional form for the production function the quantity F(K) g 1(K) is an exponential if the electron distribution is exponential. Thus, by dividing the observed X-ray fluxes at energy K by g 1( K) and fitting the result to an exponential function, the average energy and energy flux of the electron distribution can be determined directly. The technique is especially useful for determining ionospheric properties that depend on the integral of the electron fluxes, for example, the total energy flux, the E-region peak density and the height-integrated electrical conductances. The best-fit exponential electron spectrum yields values for these parameters that are very nearly the same as the values computed from the actual spectrum, even though the actual spectrum may deviate substantially from an exponential. Accurate knowledge of the function g 1 ( K) is important in applying the technique to observations. For X-ray fluxes emitted at an escape angle of 180° from the nadir direction and assuming an isotroplc distribution of incident electrons g 1( K) is approximately equal to 1.96 × 10 −5 K −1.9 when energies are expressed in kiloelectronvolts. The values derived here for g 1( K) agree well with those derived from simultaneous measurements of electron fluxes and resulting X-ray bremsstrahlung emission.

High‐latitude F region irregularities observed simultaneously with ISIS 1 and the Chatanika radar

Simultaneous measurements of the high‐latitude ionosphere were made by the Chatanika radar and the ISIS 1 topside sounder and cylindrical electrostatic probe (CEP) when the satellite was at a height of about 700 km. The variation of enhancements and valleys of electron density with invariant latitude at the satellite, on the scale of a few hundred kilometers, agrees reasonably well with the radar results, even though the satellite displacement changed from almost coincident to about 500 km magnetic west of the radar scan path. These density enhancements comprise a collection of large‐scale (tens of kilometers) irregularities or ‘blobs.’ Evidence for the formation of these blobs was obtained from the sounder. Natural, intense radio noise (hiss) was observed on the sounder ionograms recorded on the poleward side of the auroral zone, indicating the presence of precipitating kiloelectron volt electrons. A few minutes after these ionograms were recorded the radar scanned the same invariant latitude region and a large‐scale ionization enhancement, or ‘blob,’ not present 13 minutes earlier, had developed in the bottomside F layer. ISIS 1 telemetry dropouts resulting from medium‐scale irregularities, and enhanced irregularities down to about 200 m, observed with the CEP, are observed in association with blobs. Scatter signatures observed on the ISIS ionograms may be associated with large‐scale irregularities that can be identified in the radar data. The scattering detected by the sounder is caused by 30‐ to 60‐m wavelength irregularities (small‐scale) that presumably are created by instabilities operating on the edges of the large‐scale irregularities. The small‐scale irregularities, however, are observed on both poleward and equatorward directed gradients. If the gradient‐drift instability is responsible for these irregularities, then they either convect to the point of observation, are due to zonal gradients, or are field‐aligned extensions of irregularities generated at lower heights.