Medium Energy Electron Research Articles

A downward revision of a recently reported proton auroral LBH emission efficiency

[1] A significantly higher N2 Lyman-Birge-Hopfield (LBH) emission efficiency for auroral proton precipitation compared to model calculations was reported by Knight et al. (2008) based on a statistical study utilizing coincident far ultraviolet and particle data from the sensors Special Sensor Ultraviolet Spectrographic Imager (SSUSI) and Special Sensor J/5 (SSJ/5) on board the DMSP satellite F16. Here, the quantity of interest from that study is the median ratio of LBH column emission rates (CERs) from SSUSI and derived from SSJ/5 spectra using monoenergetic emission yields. The median ratio was found to be 2.83 for proton aurora, suggesting the need for significant increases in currently used LBH proton/H-atom impact cross sections. A key step in their analysis was extrapolation of SSJ/5 spectra above 30 keV. Limited testing of this algorithm using NOAA Polar Orbiting Environmental Satellites Total Energy Detector and Medium Energy Proton and Electron Detector (TM) data found no significant bias. This work reports on a more detailed investigation of the algorithm's performance, also using TM data, and has uncovered a bias that reduces the median column emission rates (CER) ratio to 1.75. Within expected uncertainties, including calibration, this still calls for cross section increases but to a lesser extent. The discovered bias becomes apparent with CER thresholding that was overlooked during testing by Knight et al. (2008). Thresholding at 400 Rayleighs (R) is necessary since Knight et al. excluded CERs < 400 R in deriving their median ratio. We show that the algorithm's performance degrades with increasing energy flux of the precipitation. A method is reported for eliminating most of the bias which utilizes auroral Ly α, whose emission strength is closely coupled to spectral hardness.

Middle atmosphere response to the solar cycle in irradiance and ionizing particle precipitation

Abstract. The impact of NOx and HOx production by three types of energetic particle precipitation (EPP), auroral zone medium and high energy electrons, solar proton events and galactic cosmic rays on the middle atmosphere is examined using a chemistry climate model. This process study uses ensemble simulations forced by transient EPP derived from observations with one-year repeating sea surface temperatures and fixed chemical boundary conditions for cases with and without solar cycle in irradiance. Our model results show a wintertime polar stratosphere ozone reduction of between 3 and 10 % in agreement with previous studies. EPP is found to modulate the radiative solar cycle effect in the middle atmosphere in a significant way, bringing temperature and ozone variations closer to observed patterns. The Southern Hemisphere polar vortex undergoes an intensification from solar minimum to solar maximum instead of a weakening. This changes the solar cycle variation of the Brewer-Dobson circulation, with a weakening during solar maxima compared to solar minima. In response, the tropical tropopause temperature manifests a statistically significant solar cycle variation resulting in about 4 % more water vapour transported into the lower tropical stratosphere during solar maxima compared to solar minima. This has implications for surface temperature variation due to the associated change in radiative forcing.

First evidence of mesospheric hydroxyl response to electron precipitation from the radiation belts

[1] Utilizing observations from the Medium Energy Proton and Electron Detector (MEPED) on board the Polar Orbiting Environmental Satellite (POES) and the Microwave Limb Sounder (MLS) on board the Aura satellite, we demonstrate that there is a strong link between 100–300 keV loss cone electron count rates observed in the outer radiation belt and nighttime OH concentrations in the middle mesosphere at 71–78 km altitude. In theory, this can be expected because the ionization caused by energetic electron precipitation (EEP) leads to odd hydrogen (HOx) production through ionic reactions. However, this is the first time that OH production due to EEP has been observed. We consider daily mean data from 2 months, March 2005 and April 2006, which were selected because of (1) relatively high count rates of radiation belt electrons observed and (2) the absence of solar proton events that could mask the EEP effects. The results show that at 55–65° magnetic latitude (equivalent to McIlwain L shells 3.0–5.6) increases in electron count rates by 2 orders of magnitude are accompanied by increases in nighttime OH concentration of 100%. There is a high correlation between MEPED and MLS data such that 56–87% of the OH variation can be explained by changes in EEP. Because the relation between MEPED count rate observations and the flux of electrons actually entering the atmosphere is not trivial, we discuss the possibility of using OH observations to obtain an estimate of EEP forcing that could be used in atmospheric modeling.

Experimental evidence for direct penetration of ULF waves from the solar wind and their possible effect on acceleration of radiation belt electrons

The event of March 12–19, 2009, when a moderately high-speed solar wind stream flew around the Earth’s magnetosphere and carried millihertz ultralow-frequency (ULF) waves, has been analyzed. The stream caused a weak magnetic storm (D st min = −28 nT). Since March 13, fluxes of energetic (up to relativistic) electrons started increasing in the magnetosphere. Comparison of the spectra of ULF oscillations observed in the solar wind and magnetosphere and on the Earth’s surface indicated that a stable common spectral peak was present at frequencies of 3–4 mHz. This fact is interpreted as evidence that waves penetrated directly from the solar wind into the magnetosphere. Possible scenarios describing the participation of oscillations in the acceleration of medium-energy (E > 0.6 MeV) and high-energy (E > 2.0 MeV) electrons in the radiation belt are discussed. Based on comparing the event with the moderate magnetic storm of January 21–22, 2005, we concluded that favorable conditions for analyzing the interaction between the solar wind and the magnetosphere are formed during a deep minimum of solar activity.

Modeling of hydrogen ground state rotational and vibrational temperatures in kinetic plasmas

A dipole-quadrupole electron-impact excitation model, consistent with molecular symmetry rules, is presented to fit ro-vibronic spectra of the hydrogen Fulcher-α Q-branch line emissions for passively measuring the rotational temperature of hydrogen neutral molecules in kinetic plasmas with the coronal equilibrium approximation. A quasi-rotational temperature and quadrupole contribution factor are adjustable parameters in the model. Quadrupole excitation is possible due to a violation of the 1st Born approximation for low to medium energy electrons (up to several hundred eV). The Born–Oppenheimer and Franck–Condon approximations are implicitly shown to hold. A quadrupole contribution of 10% is shown to fit experimental data at several temperatures from different experiments with electron energies from several to 100 eV. A convenient chart is produced to graphically determine the vibrational temperature of the hydrogen molecules from diagonal band intensities, if the ground state distribution is Boltzmann. Hydrogen vibrational modes are long-lived, surviving up to thousands of wall collisions, consistent with multiple other molecular dynamics computational results. The importance of inter-molecular collisions during a plasma pulse is also discussed.

Changes in upper mesospheric and lower thermospheric temperatures caused by energetic particle precipitation

A statistical evaluation on the upper mesospheric and lower thermospheric temperature effects caused by energetic particle precipitation is performed on the basis of data from the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) and NOAA 15, 16, and 17 satellites. By combining particle measurement from the medium energy proton and electron detectors (MEPED) on board the NOAA satellites, maps of the global particle precipitation can be obtained close in time to the SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) temperature retrieval. Using large data sets, sorted by season, local time, and geomagnetic latitude, we investigated whether there are significant temperature effects in the upper mesosphere and lower thermosphere associated with the energetic particle precipitation. During both May/June and October/November 2003, we found a temperature increase related to particle precipitation at all heights above 100 km. In general, we did not find a consistent immediate temperature modification below 100 km associated with increased particle flux. Considering the temperatures retrieved during the extraordinary large geomagnetic storms in late October 2003, we found a cooling effect associated with energetic particle precipitation.

Changes of structure in potassium depleted binary silicate glass studied by MD

Ab-initio (DFT) molecular dynamics was used to simulate the structural relaxation of 15K 2O·85SiO 2 glass after the removal of potassium ions. The simulations mimic the experimental findings for potassium-silicate glass irradiated with medium-energy electrons (tens of keV) during which glass becomes fully depleted of alkali ions after some time of the continuous irradiation. Simulations include both volume compression and expansion, found experimentally at different doses. Non-bridging oxygen atoms were created by potassium removal from the prepared glass structure. Changes after alkali atoms removal and the following structural relaxation were characterized in terms of PP RDFs and PP CNFs. The preferential formation of O–O bonds after alkali atom removal was confirmed. The peroxide/ozonide bonds were pretty stable; they do not decay until 2000 K. The relaxation is driven in way Continuous Random Network modified by allowed O–O bonds is formed. Important role of structural defects, namely 5-coordinated Si and sharing edge polyhedra, was confirmed by the direct view of network evolution.

On the fine structure of medium energy electron fluxes in the auroral zone and related effects in the ionospheric D-region

Abstract. This study is based on measurements of trapped and precipitated electrons of energy &gt;30 keV and &gt;100 keV observed by polar orbiting environmental satellites during overpasses of the imaging riometer at Kilpisjärvi, Finland. The satellites are in sun-synchronous orbits of about 850 km altitude, recording the electron fluxes at 2-s time resolution. The riometer measures the radiowave absorption at 38.2 MHz, showing the spatial pattern within a 240 km field of view. The analysis has focussed on two areas. Having found a close correlation between the radiowave absorption and the medium-energy electron fluxes during satellite overpasses, empirical relationships are derived, enabling one quantity to be predicted from the other for three sectors of local time. It is shown that small-scale variations observed during a pass are essentially spatial rather than temporal. Other properties, such as the spectra and the relation between precipitated and trapped components, are also considered in the light of the theory of pitch angle scattering by VLF waves. It is found that the properties and behaviour depend strongly on the time of day. In the noon sector, the precipitated and trapped fluxes are highly correlated through a square law relationship.

Use of POES SEM‐2 observations to examine radiation belt dynamics and energetic electron precipitation into the atmosphere

The coupling of the Van Allen radiation belts to the Earth's atmosphere through precipitating particles is an area of intense scientific interest. Currently, there are significant uncertainties surrounding the precipitating characteristics of medium energy electrons (&gt;20 keV), and even more uncertainties for relativistic electrons. In this paper we examine roughly 10 years of measurements of trapped and precipitating electrons available from the Polar Orbiting Environmental Satellites (POES)/Space Environment Monitor (SEM‐2), which has provided long‐term global data in this energy range. We show that the POES SEM‐2 detectors suffer from some contamination issues that complicate the understanding of the measurements, but that the observations provide insight into the precipitation of energetic electrons from the radiation belts, and may be developed into a useful climatology for medium energy electrons. Electron contamination also allows POES/SEM‐2 to provide unintended observations of &gt;700 keV relativistic electrons. Finally, there is an energy‐dependent time delay observed in the POES/SEM‐2 observations, with the relativistic electron enhancement (electrons &gt;800 keV) delayed by approximately one week relative to the &gt;30 keV electron enhancement, probably due to the timescales of the acceleration processes. Observations of trapped relativistic electron fluxes near the geomagnetic equator by GOES show similar delays, indicating a “coherency” to the radiation belts at high and low orbits, and also a strong link between trapped and precipitating particle fluxes. Such large delays should have consequences for the timing of the atmospheric impact of geomagnetic storms.

Origin of energetic electron precipitation &gt;30 keV into the atmosphere

Energetic electrons are deposited into the atmosphere from Earth's inner magnetosphere, resulting in the production of odd nitrogen (NOx). During polar night, NOx can be transported to low altitudes, where it can destroy ozone, affecting the atmospheric radiation balance. Since the flux of energetic electrons trapped in the magnetosphere is related to solar activity, the precipitation of these electrons into Earth's atmosphere provides a link between solar variability and changes in atmospheric chemistry which may affect Earth's climate. To determine the global distribution of the precipitating flux, we have built a statistical model binned by auroral electrojet (AE) index, magnetic local time (MLT), and L shell of E &gt; 30 keV precipitating electrons from the Medium Energy Proton and Electron Detector (MEPED) on board the NOAA Polar Orbiting Environmental Satellites (POES) low‐altitude satellites NOAA‐15, NOAA‐16, NOAA‐17, and NOAA‐18. We show that the precipitating flux increases with geomagnetic activity, suggesting that the flux is related to substorm activity. The precipitating fluxes maximize during active conditions where they are primarily seen outside of the plasmapause on the dawnside. The global distribution of the precipitating flux of E &gt; 30 keV electrons is well‐correlated with the global distribution of lower‐band chorus waves as observed by the plasma wave experiment onboard the Combined Release and Radiation Effects Satellite (CRRES) satellite. In addition, the electron precipitation occurs where the pitch angle diffusion coefficient due to resonant interaction between electrons and whistler mode chorus waves is high, as calculated using the pitch angle and energy diffusion of ions and electrons (PADIE) code. Our results suggest that lower‐band chorus is very important for scattering &gt;30 keV electrons from Earth's inner magnetosphere into the atmosphere.

Electron-photon emision of as&lt;sub&gt;2&lt;/sub&gt;S&lt;sub&gt;3&lt;/sub&gt;

By the method of electron-photon spectroscopy was investigated interaction medium energy electrons with surface As 2 S 3 films. The revealed features in the received spectra are explained. The absolute output of photons from a surface of As 2 S 3 films, deposited on surface of silicon and K-8 glass, at bombardment by electrons with energy 450 eV in a range of wave-length 200-800 nanometers which makes N 1 = 7,57*10 -4 phot./nm·el. and N 2 =1,75*10 -3 phot./nm·el. is calculated.

Avalanche Photodiode Arrays Enable Large-Area Measurements of Medium-Energy Electrons

We demonstrate the performance of a large-area avalanche photodiode array as a medium-energy electron detector. This avalanche photodiode array is composed of eight pixels with 1.2 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of total active area. The energy resolution (Delta <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i> ) is as low as 4.5% in full width at half maximum for 25-keV electrons, and the linearity is excellent. Monte Carlo modeling by Geant4 shows that the detection efficiency of this device is appropriate for the detection of electrons ranging from < 5 to > 200 keV. This new device can be coupled with magnetic deflectors, electrostatic analyzers, and other physics instruments requiring position-sensitive or large-area solutions for the detection of electrons in this energy range.

Auger Intensity from Si(001)2×2–Al Surface Excited by Wave-Field in Medium-Energy Electron Diffraction

From dynamical analysis of rocking curves from both reflection high-energy electron diffraction (RHEED) and medium-energy electron diffraction (MEED), the surface of Si(001)2×2–Al was confirmed to have a parallel-dimer structure. Furthermore, dynamical calculation, usually used to determine RHEED intensity, was confirmed to also be effective for the calculation of MEED intensity. The incident electron density distribution, or “wave-field”, was calculated for MEED conditions on the basis of the surface structure model. The wave-field at the surface is very sensitive to changes in diffraction conditions, such as the incident glancing angle. The Al(LMM) Auger electron intensity emitted from the Si(001)2×2–Al surface during MEED correlated to the wave-field intensity on the Al atomic rows relatively well.

Correlation between Auger Intensity and Wave-Field Intensity for Si(001)2*2-Al Surface

According to dynamical theory of electron diffraction, the incident electron density distribution, or “wave-field”, was calculated for a parallel-dimer structure of Si(001)2×2-Al surface on the condition of medium-energy electron diffraction (MEED). The surface structure and the calculated method were confirmed to be effective by the rocking-curve analysis of diffracted beam intensity. The wave-field at the surface is very sensitive to changes in diffraction conditions, such as the incident glancing angle. The Al(LMM) Auger electron intensity emitted from the Si(001)2×2-Al surface during MEED incident beam rocking, that is named beam rocking Auger electron spectroscopy (BRAES), has been found to be correlated to the calculated wave-field intensity on the Al atomic rows. The BRAES profile of adsorbate Al(LMM) differs from that of substrate Si(LVV). [DOI: 10.1380/ejssnt.2009.97]

A noise attenuation method for medium-energy electron measurements in the radiation belt

From the viewpoint of plasma particle measurements in the radiation belt, background noise is a serious problem. High-energy particles penetrating the sensor shielding generate spurious signals, and their count rate often can be comparable to the true signals. In order to attenuate such background noise during medium-energy (5–83 keV) electron measurements, we propose the double energy analyses (DEA) method. DEA is conducted by a combination of an electrostatic analyser (ESA) and avalanche photo-diodes (APDs); ESA and APD independently determine the energy of each incoming particle. By using the DEA method, therefore, the penetrating particles can be rejected when the two energy determinations are inconsistent; spurious noise are caused only when the deposited energy at an APD is by chance consistent with the measured energy by ESA. We formulate the noise count rate and show the advantage of DEA method quantitatively.

High-resolution detection of 100 keV electrons using avalanche photodiodes

With two electron beam sources, we have tested two new Hamamatsu [Hamamatsu Photonics K.K., Shizuoka, Japan 〈 http://www.hamamatsu.com/ 〉 ] avalanche photodiodes (APDs) of spl 3988 and spl 6098 to detect electron beams up to 100 keV. Though our previous results showed the effectiveness and the advantage of an APD to measure 2–40 keV electrons, its upper limit was not high enough to detect so-called medium-energy electrons. In addition to the limitation of its detectable range, the response at different energies was also not linear. These newly developed APDs, which have thicker depletion-layers, provide full coverage of this missing range along with a good linearity. The depletion-layer thickness was increased to 140 μ m for both APDs, the dead-layer of spl 3988 became 10 times thicker than that of spl 6098. The thin-surface dead-layer and thick depletion-layer of spl 6098 allows the detection of electrons from 3 keV up to 100 keV with a good linearity and with an excellent energy resolution of 4 keV at 100-keV electrons. The wide dynamic range from 3 keV to 100 keV of those APDs will increase their appeal in detecting electrons for space plasma research.

Mechanisms of charging of insulators under irradiation with medium-energy electron beams

The electron emission and charge characteristics of a large number of massive insulators are investigated qualitatively and quantitatively. It is demonstrated that irradiation of insulators with a medium-energy continuous electron beam leads to a decrease in the equilibrium energy of incident electrons (the second critical energy) as compared to the theoretical value. The equilibrium state of charging to saturation is attained for times from several seconds to several hundred seconds depending on the irradiation current density, the electron energy, and the insulator material. The mechanisms of charging are explained in the framework of the model according to which irradiation is accompanied by the formation of a charged double layer that consists of a positively charged layer (with the thickness equal to the escape depth of secondary electrons) and a negatively charged layer (with the thickness equal to the penetration depth of primary electrons).

Inelastic Interaction of Medium-Energy Electrons with Ni Surface Studied by Absolute Reflection Electron Energy Loss Spectrum Analysis and Monte Carlo Simulation

The results of the investigation of the inelastic interaction of medium-energy electrons with the Ni surface were described. The inelastic mean free path (IMFP), the surface excitation parameter (SEP) and the differential SEP (DSEP) were deduced simultaneously from an absolute reflection electron energy loss spectroscopy (REELS) spectrum. The present IMFPs show a good agreement with those calculated using the TPP-2M predictive equation. The dependence of the SEPs on the electron energy is similar to that calculated using Werner’s and Gertgely’s predictive equations. The DSEPs show a reasonable agreement with the theoretical DSEPs calculated using Tung’s theory with optical data. The derived IMFP, SEP and DSEP were applied to Monte Carlo simulation of REELS spectra, in which energy loss processes of signal electrons due to surface excitations were taken into account. The simulated REELS spectra reproduce the experimental absolute REELS spectra well without any fitting parameters, indicating that surface excitations play an important role in the inelastic interaction of medium-energy electrons with the solid surface.

Molecular-beam epitaxy of [formula omitted] Heusler alloy thin films epitaxially grown on Si(0 0 1)

We report on the investigation of structural and magnetic properties of the ternary Heusler alloy Co 2 MnSi grown on Si(0 0 1) by molecular-beam epitaxy. Low-energy electron diffraction (LEED), inelastic medium-energy electron diffraction (IMEED) and X-ray photoelectron diffraction (XPD) measurements clearly show the growth of crystalline Co 2 MnSi . The best crystallographic and magnetic quality of the Co 2 MnSi films have been achieved after codeposition of the three Co, Mn and Si elements on the Si(0 0 1) substrate held at 587 K. Quantitative determinations of magnetic anisotropies were performed using transverse bias initial inverse susceptibility and torque measurements (TBIIST). Co 2 MnSi reveals to have an in-plane fourfold magnetocrystalline anisotropy with easy axis along 〈 0 1 0 〉 directions for evaporation fluxes perpendicular to the substrate surface. On the other hand, grazing-incidence fluxes invariably generate a dominant uniaxial in-plane magnetic anisotropy contribution with easy axis perpendicular to the incidence plane.

Electron emission and charging of natural diamond under irradiation with medium-energy electrons

The processes of charging of a natural diamond crystal irradiated by an electron beam with primary electron energies in the range 1–30 keV have been experimentally investigated. The charging kinetics has been studied as a function of the irradiation dose and the electron-emission properties of the crystal. The following fundamental characteristics have been determined: the second crossover equilibrium energy of the beam electrons and the values of high-voltage surface potentials and accumulated charges.