Electron Flux Profiles Research Articles

On the time needed to reach an equilibrium structure of the radiation belts

AbstractIn this study, we complement the notion of equilibrium states of the radiation belts with a discussion on the dynamics and time needed to reach equilibrium. We solve for the equilibrium states obtained using 1‐D radial diffusion with recently developed hiss and chorus lifetimes at constant values of Kp = 1, 3, and 6. We find that the equilibrium states at moderately low Kp, when plotted versus L shell (L) and energy (E), display the same interesting S shape for the inner edge of the outer belt as recently observed by the Van Allen Probes. The S shape is also produced as the radiation belts dynamically evolve toward the equilibrium state when initialized to simulate the buildup after a massive dropout or to simulate loss due to outward diffusion from a saturated state. Physically, this shape, intimately linked with the slot structure, is due to the dependence of electron loss rate (originating from wave‐particle interactions) on both energy and L shell. Equilibrium electron flux profiles are governed by the Biot number (τDiffusion/τloss), with large Biot number corresponding to low fluxes and low Biot number to large fluxes. The time it takes for the flux at a specific (L, E) to reach the value associated with the equilibrium state, starting from these different initial states, is governed by the initial state of the belts, the property of the dynamics (diffusion coefficients), and the size of the domain of computation. Its structure shows a rather complex scissor form in the (L, E) plane. The equilibrium value (phase space density or flux) is practically reachable only for selected regions in (L, E) and geomagnetic activity. Convergence to equilibrium requires hundreds of days in the inner belt for E > 300 keV and moderate Kp (≤3). It takes less time to reach equilibrium during disturbed geomagnetic conditions (Kp ≥ 3), when the system evolves faster. Restricting our interest to the slot region, below L = 4, we find that only small regions in (L, E) space can reach the equilibrium value: E ~ [200, 300] keV for L = [3.7, 4] at Kp = 1, E~[0.6, 1] MeV for L = [3, 4] at Kp = 3, and E~300 keV for L = [3.5, 4] at Kp = 6 assuming no new incoming electrons.

Long-term relativistic radiation belt electron responses to GEM magnetic storms

We present a long-term radiation belt simulation for a 200-day period starting on 25 January 1991, which includes both six geomagnetic storms identified by the Geospace Environment Modeling (GEM) focus group and non-stormy periods of the Combined Release and Radiation Effects Satellite (CRRES) mission, using 3-D time-dependent Versatile Electron Radiation Belt (VERB) code, and compare the simulation results with a multisatellite phase space density (PSD) reanalysis obtained using Kalman filtering of observations from CRRES, GEO, GPS, and Akebono satellites, as well as with the CRRES MEA 1MeV electron observation. The processes accounted for in the model are radial diffusion-driven by ultra-low frequency (ULF) electromagnetic fluctuations and local (pitch-angle and energy) scattering by plasmaspheric hiss and chorus waves, respectively, inside and outside the plasmasphere. The observations show that a significant decrease in the relativistic electrons in the outer radiation belt is observed in association with the solar wind dynamic pressure enhancement during the main phase of each storm, while during the recovery phase, different types of relativistic electron flux profiles are identified: increased, decreased, and unchanged relative to the pre-storm flux level. First, for an increase of relativistic electrons relative to the pre-storm flux level, the comparison of simulation with reanalysis shows that inward radial diffusion and local acceleration coupled with each other result in a net acceleration. Second, for a decrease or lack of change in relativistic electrons, competing effects of pitch-angle scattering, outward diffusion, and acceleration produce the net decrease in electron PSD and fluxes. The results show that the overall time evolution of the radiation belt is in good agreement with our model simulations, indicating that modeling, including radial diffusion and pitch-angle scattering, is reasonable in predicting the general long-term structure of the outer radiation belt. In addition, with the assistance of local acceleration by chorus waves, the overall flux level in the outer radiation belt becomes comparable to the observation.

High resolution radially symmetric nanostructures from simultaneous electron beaminduced etching and deposition

Electron beam induced etching (EBIE) and deposition (EBID) are promising fabricationtechniques in which an electron beam is used to dissociate surface-adsorbed precursormolecules to achieve etching or deposition. Spatial resolution is normally limited by theelectron flux distribution at the substrate surface. Here we present simultaneous EBIE andEBID (EBIED) as a method for surpassing this resolution limit by using adsorbatedepletion to induce etching and deposition in adjacent regions within the electron fluxprofile. Our simulation results indicate the possibility of growth control of radiallysymmetric nanostructures at the sub-1 nm length scale on bulk substrates. Thetechnique is well suited to the fabrication of ring-shaped nanostructures such as thoseemployed in plasmonics, sensing devices, magneto-optics and magnetoelectronics.

Flux profile scanners for scattered high-energy electrons

The paper describes the design and performance of flux integrating Cherenkov scanners with air-core reflecting light guides used in a high-energy, high-flux electron scattering experiment at the Stanford Linear Accelerator Center. The scanners were highly radiation resistant and provided a good signal to background ratio leading to very good spatial resolution of the scattered electron flux profile scans.

Plasma Sheath Formation in a Groove Target.

Ion and electron flux profiles could be affected by the geometric of the target. In this study, a groove target with the groove width close to the ion gyro-radius was used to investigate this effect to the sheath formation. Several other types of target were also studied for comparison. It was found that the unbalance of ion and electron fluxes induced potential differences in the targets. Due to the potential difference electric currents are generated in the targets to compensated for the differences. When insulators are inserted in the targets, the sheath potentials change to balance ion and electron fluxes at the each position in the targets.

Nanometer-scale lithography on Si(001) using adsorbed H as an atomic layer resist

We describe nanometer-scale feature definition in adsorbed hydrogen layers on Si(001) surfaces by exposure to low energy electrons from a scanning tunneling microscope tip. Feature sizes range from <5 to ≳40 nm as a function of bias voltage (5–30 V) and exposure dose (1–104 μC/cm). We show that the cross section for electron stimulated desorption of hydrogen has a threshold at 6–8 eV and is nearly constant from 10 to 30 eV, so that above threshold the feature profiles are a direct reflection of the electron flux profile at the surface. Radial flux distributions are best fit by a simple exponential function, where the decay length is dependent primarily on the tip–sample separation. Low intensity tails at large radius are also observed for high bias emission. Comparison to field emission simulations shows that our tip has an ‘‘effective radius’’ of approximately 30 nm. Simulations demonstrate that tip geometry and tip–sample separation play the dominant role in defining the electron flux distribution, and that optimum beam diameter at the sample is obtained at small tip–sample separation (low bias) with sharp tips. We show that adsorbed hydrogen is a robust resist that can be used as a mask for selective area deposition of metals by chemical vapor deposition. Fe lines 10 nm wide are deposited by pyrolysis of Fe(CO)5 in areas where H has been desorbed, with minimal nucleation in the H-passivated areas.

Kilometer scale structures in auroral X ray arcs detected by combining satellite X ray images with in situ energetic electron flux profiles

X ray images of the Earth's auroral zone give a large‐scale view of the precipitation patterns of energetic electrons through the bremsstrahlung produced by the electrons stopping in the atmosphere. Here we present X ray maps based on observations by the XRIS imager on the S81‐1 polar orbiting satellite; discrete X ray arcs are the most prominent features in these data. The full width half maximum spatial resolution of XRIS was 40 km, but high time resolution measurements of electrons above 16 keV could detect 2 km spatial variations in one dimension along the track of the satellite. The technique was used to study 26 auroral arcs observed in 4‐40 keV X rays by the XRIS and showed that in 2/3 of the cases the full width of the precipitation region was smaller than 40 km. Furthermore, comparisons with electron data lead to the conclusion that significant spectral variations occur on kilometer size scales.

The effect of the poloidal divertor neutralizer geometry on the scrape-off plasma temperature and density

The poloidal magnetic divertor operating in the cool-high density plasma mode represents a good solution to the tokamak first wall erosion and impurity transport problems. It also, in general, improves the energy confinement time in the beam-heated discharges. In the existing experiments, the neutralizer plates are oriented close to normal to the flux surfaces in the poloidal plane, but the planned high-power experiments require that the plate be sharply angled to reduce the power flux density. We have studied the effect of changing the plate angle on the radial plasma temperature and density profiles at the plate. We find that some plate orientations shift the electron flux profile relative to the power flux profile, resulting in unfavorable temperature profiles. The question of existence of a steady-state solution was also addressed. For a steady state to exist, the radial neutral fluxes have to exactly cancel the radial diffusive ion fluxes. Depending on the diffusion coefficients, the plasma density may have to increase with radius to satisfy this condition. The implications of these conclusions for the INTOR, and CIT experiments is also discussed.

A model of impulsive acceleration and transport of energetic particles in Mercury's magnetosphere

A qualitative model of substorm processes in the Mercury magnetosphere is presented based on Mariner 10 observations obtained in 1974–1975. The model is predicated on close analogies observed with the terrestrial case. Particular emphasis is given to energetic particle phenomena as observed by Mariner on March 29, 1974. We support the suggestion that energetic particles up to ∼500 keV are produced by strong induced electric fields at 3 ∼ 6 RM in the Hermean tail in association with substorm neutral line formation. The bursts of energetic particles produced are, in this model, subsequently confined on closed field lines near Mercury and drift adiabatically on quasi‐trapped orbits for many tens of seconds. Such gradient and curvature drift of the particles can explain prominent periodicities of 5–10 s seen in the Mariner > 170‐keV electron flux profiles.

Undisturbed trapping boundary for energetic electrons at low altitudes

Dawn‐dusk traversals of the auroral zone by the German polar orbiting satellite Azur are used to reexamine the previously reported dawn‐dusk asymmetry of the boundary position of the 40‐keV electrons. This asymmetry, i.e., higher latitudes of the boundary positions in the dawn sector than in the dusk sector, disappears during prolonged periods of low electrojet activity. Symmetric boundary positions, i.e., significantly lower than average boundary positions in the dawn sector, are found whenever the electron flux decreases smoothly to the cosmic ray background over the polar cap and when there is no enhanced precipitation. Similar, undisturbed flux profiles observed by Ogo 6 at E > 30 keV are used to extend the study to all local times. The resulting average boundary contour has an almost circular shape, centered at 2° away from the magnetic pole on the midnight meridian and is compared with the average position of quiet auroral arcs and also with the average pattern of field‐aligned currents for AL < 100 nT. The average boundary contour of undisturbed 30‐ and 40‐keV electron flux profiles is interpreted as the high‐latitude limit of stable trapping in the undisturbed magnetosphere.

The inter-relationship of hard X-ray and EUV bursts during solar flares

A comparison is made between the flux-versus-time profile in the EUV band and the thick target electron flux profile as inferred from hard X-rays for a number of moderately large solar flares. This complements Kane and Donnelly's (1971) study of small flares. The hard X-ray data are from ESRO TD-1A and the EUV inferred from SFD observations. Use of a χ2 minimising method shows that the best overall fit between the profile fine structures obtains for synchronism to ≲5 s which is within the timing accuracy. This suggests that neither conduction nor convection is fast enough as the primary mechanism of energy transport into the EUV flare and rather favours heating by the electrons themselves or by some MHD wave process much faster than acoustic waves. The electron power deposited, for a thick target model, is however far greater than the EUV luminosity for any reasonable assumptions about the area and depth over which EUV is emitted. This means that either most of the power deposited is conducted away to the optical flare or that only a fraction ≲1–10% of the X-ray emitting electrons are injected downwards. Recent work on Hα flare heating strongly favours the latter alternative - i.e. that electrons are mostly confined in the corona.

Energetic Electrons in Jupiter's Magnetosphere

view Abstract Citations (71) References (58) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Energetic Electrons in Jupiter's Magnetosphere Coroniti, F. V. Abstract A theoretical model for the energetic electron fluxes in the Jovian magnetosphere is developed. Electrons are transported inward from the solar wind or Jovian magnetospheric tail by radial diffusion. The radial diffusion is driven by fluctuating ionospheric dynamo electric fields associated with a neutral- wind tidal eigenmode at ionospheric altitudes. The tidal mode is excited by the electromagnetic coupling of the solar wind to the polar ionosphere; the radial diffusion coefficient which results from disturbed solar-wind flows is 2 X 1 L3 Rj2 per day. Two injection models are considered: (1) electron penetration through the dayside magnetopause-low-energy model; and (2) injection of electrons from an assumed magnetospheric tail high-energy model. Both thermal solar-wind electrons and energetic solar-flare electrons are considered. In the outer zone (7 < L < 20) the electron fluxes are limited to a stably trapped level by whistler-mode precipitation loss. The Galilean satellites are assumed to sweep out all electrons which have access to the satellites. In the inner zone (1.2 < L < 4), a steady-state radial diffusion equation with synchrotron energy loss is solved to the relativistic electron-flux profile, the mean energy, and the total synchrotron power emitted by a flux tube. The synchrotron-emission bandwidth and the flux density at Earth are estimated; the tail-injection model yields the best agreement with the radiometric data. Subject headings: atmospheres, planetary - hydromagnetics - Jupiter - solar wind Publication: The Astrophysical Journal Supplement Series Pub Date: March 1974 DOI: 10.1086/190296 Bibcode: 1974ApJS...27..261C full text sources ADS |