Incident Electron Spectrum Research Articles

Internal Dielectric Charging Simulation of a Complex Structure With Different Shielding Thicknesses

To find the real risk points of internal dielectric charging (IDC), 3-D IDC simulation is performed to a typical dielectric structure considering GEO severe radiation environment (FLUMIC3). Charge transportation simulation is carried out by Geant4 and electric field is numerically computed according to the current conservation law. The aluminum shielding thickness is varied to investigate the shielding dependence. By adjusting the lower energy limit of the incident electron spectrum according to the shielding thickness, a technique is proposed to improve the simulation efficiency. The statistic condition in Geant4 is checked and attention is paid to the influence of mesh on the peak electric field. The simulation data show that the peak electric field occurs on the grounding edge and local mesh refinement on the critical charging point is of significance to make sure the real charging level. Increasing the shielding thickness could mitigate IDC. However, this mitigation effect is weak in the case where radiation-induced conductivity dominates the total conductivity (at low temperature). In this case, even using a 3-mm aluminum shielding, the peak electric field can reach the level of $10^{7}$ V/m.

Application of AF-NUMIT2 to the Modeling of Deep-Dielectric Spacecraft Charging in the Space Environment

A 1-D numerical iteration software tool (NUMIT) designed for laboratory modeling has been refined, developed, and greatly enhanced, so that it can be used to model and predict the deep charging of dielectrics on spacecraft. The new tool, called Air Force NUMIT version 2 (AF-NUMIT2), can now model isotropically incident electron fluxes from realistic energy spectra that change with time. The model has been extended to incident electron fluxes an order of magnitude lower than before (≥10 keV), and aspects of the model have been checked against Monte Carlo simulations. Electron energy flux spectra, as measured by Combined Release and Radiation Effects Satellite, have been used in dozens of simulations of Kapton charging over multiple days. Kapton has been modeled with both dark conductivities and radiation-induced conductivities extending over two orders-of-magnitude. The result has been a surprisingly consistent relationship between maximum internal electric-field strengths and conductivity, regardless of the particular incident electron spectra.

Radiation Induced Conductivity in Teflon FEP Irradiated With Multienergetic Electron Beam

Teflon Fluorinated Ethylene Propylene (FEP), used as thermal blankets on satellites, has a specific electrical behavior under space ionizing environment. Charging behaviors of this dielectric material, under electron beam irradiation, is of special interest for future spacecraft needs. Hence, ground experimental parameters have been adjusted in order to distinguish between the different physical mechanisms that steers FEP charging potential. The effect of incident electron spectrum, electric field, and dose rate has been investigated through surface potential measurements during and after the irradiation process. The experimental results especially reveal that charging potential evolution as a function of time is not strongly dependent upon the electron spectrum and the electric field but varies noticeably with dose rate. These results have been analyzed in the light of a physical model that takes into account ionization, trapping/detrapping, and recombination mechanisms for negative and positive charges.

Benchmarking of Monte Carlo simulation of bremsstrahlung from thick targets at radiotherapy energies

Several Monte Carlo systems were benchmarked against published measurements of bremsstrahlung yield from thick targets for 10-30 MV beams. The quantity measured was photon fluence at 1 m per unit energy per incident electron (spectra), and total photon fluence, integrated over energy, per incident electron (photon yield). Results were reported at 10-30 MV on the beam axis for Al and Pb targets and at 15 MV at angles out to 90 degrees for Be, Al, and Pb targets. Beam energy was revised with improved accuracy of 0.5% using an improved energy calibration of the accelerator. Recently released versions of the Monte Carlo systems EGSNRC, GEANT4, and PENELOPE were benchmarked against the published measurements using the revised beam energies. Monte Carlo simulation was capable of calculation of photon yield in the experimental geometry to 5% out to 30 degrees, 10% at wider angles, and photon spectra to 10% at intermediate photon energies, 15% at lower energies. Accuracy of measured photon yield from 0 to 30 degrees was 5%, 1 s.d., increasing to 7% for the larger angles. EGSNRC and PENELOPE results were within 2 s.d. of the measured photon yield at all beam energies and angles, GEANT4 within 3 s.d. Photon yield at nonzero angles for angles covering conventional field sizes used in radiotherapy (out to 10 degrees), measured with an accuracy of 3%, was calculated within 1 s.d. of measurement for EGSNRC, 2 s.d. for PENELOPE and GEANT4. Calculated spectra closely matched measurement at photon energies over 5 MeV. Photon spectra near 5 MeV were underestimated by as much as 10% by all three codes. The photon spectra below 2-3 MeV for the Be and Al targets and small angles were overestimated by up to 15% when using EGSNRC and PENELOPE, 20% with GEANT4. EGSNRC results with the NIST option for the bremsstrahlung cross section were preferred over the alternative cross section available in EGSNRC and over EGS4. GEANT4 results calculated with the "low energy" physics list were more accurate than those calculated with the "standard" physics list.

Large negative lunar surface potentials in sunlight and shadow

We survey Lunar Prospector Electron Reflectometer measurements from times when the Moon was located in the geomagnetic tail, in order to characterize the occurrence of large (>500 V) negative lunar surface potentials, as identified by upward‐going electron beams. We find that charging to these levels is rare, but that such charging events do occasionally occur in both sunlight and shadow. Large surface potentials are found primarily in the plasma sheet, and their occurrence depends mainly on the incident electron spectrum rather than surface properties. Most examples occur in shadow, where the current from energetic electrons dominates. However, some occur in sunlight, suggesting the occasional presence of either lower photoemission than expected or non‐monotonic potentials. Depending on the electric field scale height, significant electric fields of up to ∼100 V/m may sometimes exist at the lunar surface.

Analysis of the Martian atmosphere for riometry

The riometer (relative ionospheric opacity meter) technique for measuring changes in the electron density of the atmosphere is useful for investigating the effects of precipitating electron fluxes, as well as those of solar photoionization and solar and galactic cosmic rays. The efficacy of extending the riometer method to the Martian ionosphere is examined, and it is found that characteristics of the Martian atmosphere make it more sensitive than the terrestrial atmosphere to riometer absorption by several orders of magnitude for the same incident electron spectrum. The increased sensitivity is due both to the lower altitudes that energetic electrons can attain in the thinner Martian atmosphere, as well as to the enhanced effects of electron-neutral collisions with Martian carbon dioxide molecules versus terrestrial nitrogen molecules (these gases are the major atmospheric constituents at altitudes sensitive to riometer measurements in the respective atmospheres). Even though the Martian environment does not have magnetospheric regions capable of trapping and accelerating solar wind electrons, this enhanced sensitivity may make it possible to examine effects of the solar wind on the atmosphere that can only be examined on Earth within the narrow boundaries of the auroral oval and geomagnetic cusp/cleft regions.

Quasiperiodic ∼5–60 s fluctuations of VLF signals propagating in the Earth‐ionosphere waveguide: A result of pulsating auroral particle precipitation?

Subionospheric very low frequency and low‐frequency (VLF/LF) transmitter signals received at middle‐latitude ground stations at nighttime were found to exhibit pulsating behavior with periods that were typically in the ∼5–60 s range but sometimes reached ∼100 s. The amplitude versus time shape of the pulsations was often triangular or zigzag‐like, hence the term “zigzag effect.” Variations in the envelope shape were usually in the direction of faster development than recovery. Episodes of zigzag activity at Siple, Antarctica (L ∼4.3), and Saskatoon, Canada (L ∼4.2), were found to occur widely during the predawn hours and were not observed during geomagnetically quiet periods. The fluctuations appeared to be caused by ionospheric perturbations at the ∼ 85 km nighttime VLF reflection height in regions poleward of the plasmapause. We infer that in the case of the Saskatoon and Siple data, the perturbations were centered within ∼500 km of the stations and within ∼ 100–200 km of the affected signal paths. Their horizontal extent is inferred to have been in the range ∼50–200 km. The assembled evidence, supported by Corcuffs [1996] recent research at Kerguelen (L ∼3.7), suggests that the underlying cause of the effect was pulsating auroral precipitation. The means by which that precipitation produces ionospheric perturbations at 85 km is not yet clear. Candidate mechanisms include (1) acoustic waves that propagate downward from precipitation regions above the ∼ 85 km VLF reflection level; (2) quasi‐static perturbation electric fields that give rise to E×B drifts of the bottomside ionosphere; (3) secondary ionization production and subsequent decay at or below 85 km. Those zigzag fluctuations exhibiting notably faster development than recovery probably originated in secondary ionization produced near 85 km by the more energetic (E >40 keV) electrons in the incident electron spectrum.

Relationship between energy flux Q and mean energy 〈E〉 of auroral electron spectra based on radar data from the 1987 CEDAR Campaign at Sondre Stromfjord, Greenland

The incoherent scatter radar at Sondre Stromfjord, Greenland, measured electron density profiles from 90 to 500 km during four auroral events over a 3‐hour period on February 28, 1987. The profiles were obtained with the radar pointed along the magnetic field near zenith at 15‐s intervals. Under the assumption that proton/H atom precipitation was unimportant during these events a representation of the incident electron flux was obtained by fitting calculated profiles with measured profiles in the vicinity of their peaks (lower E region). Maxwellian and Gaussian electron distributions with high‐ and low‐energy tails were used to generate the calculated profiles. The distributions were specified in terms of average energy 〈E〉 and energy flux Q. We find that we can clearly distinguish between profiles that result from a Maxwellian incident electron spectrum and those that result from a Gaussian spectrum. Interpreting Gaussian and Maxwellian spectra as representative of discrete and diffuse aurora, respectively, the measurements indicated good correlation between 〈E〉 and Q for discrete aurora, while essentially no correlation was observed for diffuse aurora. This is consistent with current understanding that discrete auroras are produced by electrons accelerated by magnetic field‐aligned potential drops whereas diffuse auroras are produced by pitch angle diffusion of plasma sheet electrons into the loss cone.

Deducing composition and incident electron spectra from ground‐based auroral optical measurements: A study of auroral red line processes

We conclude from a study of the production and loss of O(¹D) in auroras that the “traditional” sources, direct electron inpact excitation of atomic oxygen and dissociative recombination of molecular oxygen ions, can account for most of the O I 6300‐Å emission rate. In a specific application of the model to the comprehensive observation of an auroral event by Sharp et al. (1979), we show that there is no compelling need for the reaction N(²D) + O2 → NO + O(¹D). We also present a study of the sensitivity of the red line emission rate to a wide variety of input conditions.

Deducing composition and incident electron spectra from ground‐based auroral optical measurements: Theory and model results

We examine the problem of monitoring composition and the behavior of precipitating electron spectra in auroras using N2+ 4278 Å (blue), O I 6300 Å (red), O I 7774 Å (narrow; e+O), and O I 7774 Å (broad; e+O2) as observed from the ground. Calculated column emission rates for these features as well as those of narrow O I 8446 Å and broad O I 8446 Å are presented as functions of the hardness of the incident electron spectrum and the concentrations of O and O2. Selected ratios of these rates such as narrow/broad (for 7774 Å) and red/blue are also presented. Incident spectra are characterized by Maxwellian energy distributions with characteristic energies ranging from 0.1 to 8 keV. Composition is modeled using a Jacchia (1977) model where scaling factors are applied to the O and O2 number densities. Scaling factors for O range from 0.1 to 1, and factors of 1 and 1.5 are considered for O2. As a group, the above rates and ratios show considerable variation over the just described parameter ranges, making them attractive for monitoring incident electron energy flux, its mean energy, and the concentrations of O and O2 relative to N2. The red line is a key feature of the above group which is sensitive to the low‐energy portion (subkilovolt) of the incident electron spectrum. Since this can vary considerably from one aurora to another having the same approximate mean energy, it becomes an added parameter to be considered within any algorithm using the red line. Paper 2 (Meier et al., this issue) discusses this problem as part of a detailed investigation of the red line. In the current paper, one particular representation of a low‐energy component is considered for making the red line calculations. A final subject of this paper is the use of temperatures deduced from measurements of rotational line distributions and atomic line Doppler widths to infer mean energies of precipitating electrons. Calculated effective temperatures versus hardness of the incident electron spectrum are presented and discussed in the context of more precise techniques for relating measurements of rotational line distributions and Doppler widths to electron spectral hardness.

Deducing composition and incident electron spectra from ground‐based auroral optical measurements: Variations in oxygen density

Ground‐based optical observations have been made of the N2+ (4278 Å), O I (6300 Å), O I (7774 Å), and O I (8446 Å) auroral emissions. Data were collected on 12 nights from 1984 to 1986 at observatories in the auroral zone and were analyzed using the Strickland et al. (this issue) auroral electron impact excitation model results. It was found that during quiet geomagnetic conditions these data were consistent with O densities, at altitudes from 110 to 200 km, less than one‐half the densities given in the Jacchia 1977 or MSIS 1983 models. During highly disturbed periods such as during the great February 1986 storm, where Ap reached 202, the O density decreased an additional factor of 8. During the same storm the O2 density increased by less than a factor of 2 from its quiet value. These data also imply that the decrease in the O density is larger at higher altitudes during disturbed periods, in qualitative agreement with the heating model of Hays et al. (1973).

Optical and radar characterization of a short‐lived auroral event at high latitude

Observations of optical emission intensities and incoherent scatter radar returns in the magnetic zenith were compared in a study carried out at Sondre Stromfjord (Λ = 76.1°) in Greenland. The results were used to test the consistency of a theoretical model of ion chemistry and optical emissions in aurora and to explore the accuracy of relations between optical measurements and the average energy of the incident electrons. The incident primary electron spectrum and its temporal variation were inferred from zenith electron density profiles from the radar. The inferred primary energy spectrum at the peak intensity of the event approximated a Maxwellian distribution of characteristic energy 1.3 keV accelerated by an energy increment between 2 and 5 keV. Average energies inferred from the radar electron density profiles, from the N2+ rotational temperature and the I(6300)/I(4278) ratio were in good agreement. The variation of the I(8446)/I(4278) ratio was studied and was found to be promising as an index of average incident electron energy. An empirical relation between this ratio and average energy was derived from the data. The observed values of I(4278) exceeded the theoretical values derived from the ionization rate profiles deduced from the radar data by a factor near 2.0. Observed electron density profiles and theoretical profiles calculated from optical data were in good agreement provided that the optically inferred ion production rates were reduced by the same factor of 2. This discrepancy is probably the cumulative result of small errors in instrument calibrations, viewing geometry, recombination coefficients and the excitation and ionization cross sections used in the model.

Dependence of auroral middle UV emissions on the incident electron spectrum and neutral atmosphere

In this paper we examine the relationship between three important middle UV auroral emissions, the electron energy spectrum, and the neutral atmosphere. We display calculated emission rates for the Vegard‐Kaplan and second positive band systems of N2 and the O I (¹S → ³P) 2972 Å line. All three features are proportional to the incoming energy flux, but they have different dependences on the characteristic energy of the incident electron spectrum and on the concentration of atomic oxygen. Therefore, in principle, observations of the intensities of these three emission features could be used to characterize the incident electron spectrum. Unfortunately, uncertainties in the chemistry of the ¹S state of atomic oxygen and the A³Σu+ state of nitrogen make it difficult to use such observations quantitatively. We discuss these uncertainties and possible alternative far ultraviolet features such as O I 1356 Å and 1304 Å and the N2 Lyman‐Birge‐Hopfield bands.

The effect of electric field-induced vertical convection on the precipitation E-layer

The effects of perpendicular electric fields on the high-latitude nocturnal precipitation E-layer are studied in terms of model calculations. The deformations of the electron density profiles caused by vertical plasma movements are described assuming various incident electron energies and flux densities, as well as different field intensities and directions. Manifestations of the profile variations in vertical sounding observations and changes in the conductivity profiles are examined. The results show that field-induced vertical convection is important in plasma densities even higher than 10 11 m −3. Erroneous results may be obtained if the effect of the electric field is neglected when determining the incident electron spectrum from a measured ionospheric density profile.

Dependence of auroral FUV emissions on the incident electron spectrum and neutral atmosphere

In this paper we examine the relationship among certain prominent auroral FUV emission features, the incident electron spectrum, and the model neutral atmosphere. Given the neutral atmosphere, we show that for simple models of the incident electron spectrum (Maxwellian and Gaussian in energy), satellite measurements of FUV emission features, in principle, determine the incident electron spectrum. We also discuss the relationship between the incident electron spectrum and the E region plasma density profile for the continuous (diffuse) aurora and for a stable arc.

A model of energy deposition of energetic electrons and EUV emission in the Jovian and Saturnian atmospheres and implications

A model of the interaction between incident electron precipitation and H2 atmospheres is described. The local degraded primary and secondary electron energy distributions are calculated by using the continuous slowing down approximation. The altitude distribution of the ionization rate and various H and EUV H2 emissions are calculated for four different incident electron spectra. A total EUV H2 emission efficiency of 10.6 kR/incident erg cm−2 s−1 is obtained for a pure H2 atmosphere. Comparison with the Voyager Jupiter observations indicates that an incident energy flux of about 8 ergs cm−2 s−1 was present at the time of the encounter if the emission is located in an H2‐dominated region. The local thermospheric heating rate was about 4 ergs cm−2 s−1 for Jupiter and of the order of 0.1 erg cm−2 s−1 for Saturn. A globally averaged atomic hydrogen production rate of ∼1×1010 atoms/cm² s−1 is induced by the Jovian auroral electron precipitation, largely exceeding the solar EUV dissociation rate.

Energy spectra of incident electrons from auroral ionisation profiles

The production of ionospheric ionisation at night by auroral electrons and its height dependence are discussed. It is found that the altitude dependence of the effective recombination coefficient α e of the ions involved is of great importance in determining the ionisation profile produced by a given incident electron spectrum. Electron density profiles obtained in the auroral zone at night are examined for several nights during conditions of diffuse aurora. The incident electron spectra are computed from these measurements.

Observations of the optical spectrum of the dayside magnetospheric cleft aurora

Optical spectra of the cleft aurora in the region 5000–8500 Å were measured in December, 1977 at Cape Parry, N.W.T. A Michelson interferometer was used at a resolution of 10 cm −1. The auroral features observed were OI (5577, 6300-64, 7774, 8446 Å), OII (7319-30 Å), NI (5200 Å), Hα, O 2 atm (1,1), some weak N 21P bands and possibly some Meinel bands of N 2 +. In addition, nightglow emissions of Na and OH were observed. Theoretical predictions of the OI and NI emission rates using the model of Link et al. (1980) fit the observed rates reasonably well if a 40 eV Maxwellian incident electron spectrum is assumed. The predicted rates for OII exceed the observed value by a factor of 4. It is suggested that the ionization cross-section may be over-estimated.

Calculations of soft auroral bremsstrahlung and Kα line emission at satellite altitude

Satellite altitude (835km) fluxes of 0.1–10 keV auroral X-rays are calculated for some typical forms of incident electron spectra. Both bremsstrahlung and K ga line emission are included. These calculations indicate that K α emission can increase the observable intensity of satellite altitude auroral X-rays by up to 100 times, thereby making soft X-rays an ideal medium for producing images of the auroral morphology.

Auroral bremsstrahlung spectra in the atmosphere

A procedure for computing the bremsstrahlung X-ray flux generated by auroral electrons in a plane, parallel atmosphere is described in detail. An integral expression which gives the X-ray flux at a specified photon energy and atmospheric depth for an arbitrary incident electron spectrum is set up. Some examples of X-ray spectra obtained from the numerical solution of this multidimensional integral are illustrated and in several cases are compared with previous Monte Carlo results. The energy deposition by bremsstrahlung X-rays can be determined from their local flux spectra.