Hot electron energy relaxation in lattice-matched InAlN/AlN/GaN heterostructures: The sum rules for electron-phonon interactions and hot-phonon effect

Using the dielectric continuum (DC) and three-dimensional phonon (3DP) models, energy relaxation of the hot electrons in the quasi-two-dimensional channel of lattice-matched InAlN/AlN/GaN heterostructures is studied theoretically, taking into account non-equilibrium polar optical phonons, electron degeneracy, and screening from the mobile electrons. The electron power dissipation and energy relaxation time due to both half-space and interface phonons are calculated as functions of the electron temperature Te using a variety of phonon lifetime values from experiment, and then compared with those evaluated by the 3DP model. Thereby particular attention is paid to examination of the 3DP model to use for the hot-electron relaxation study. The 3DP model yields very close results to the DC model: with no hot phonons or screening the power loss calculated from the 3DP model is 5% smaller than the DC power dissipation, whereas slightly larger 3DP power loss (by less than 4% with a phonon lifetime from 0.1 to 1 ps) is obtained throughout the electron temperature range from room temperature to 2500 K after including both the hot-phonon effect (HPE) and screening. Very close results are obtained also for energy relaxation time with the two phonon models (within a 5% of deviation). However the 3DP model is found to underestimate the HPE by 9%. The Mori-Ando sum rule is restored by which it is proved that the power dissipation values obtained from the DC and 3DP models are in general different in the pure phonon emission process, except when scattering with interface phonons is sufficiently weak, or when the degenerate modes condition is imposed, which is also consistent with Register’s scattering rate sum rule. The discrepancy between the DC and 3DP results is found to be caused by how much the high-energy interface phonons contribute to the energy relaxation: their contribution is enhanced in the pure emission process but is dramatically reduced after including the HPE. Our calculation with both phonon models has obtained a great fall in energy relaxation time at low electron temperatures (Te < 750 K) and slow decrease at the high temperatures with the use of decreasing phonon lifetime with Te. The calculated temperature dependence of the relaxation time and the high-temperature relaxation time ∼0.09 ps are in good agreement with experimental results

Momentum relaxation due to polar optical phonons in AlGaN/GaN heterostructures

Using the dielectric continuum (DC) model, momentum relaxation rates are calculated for electrons confined in quasi-two-dimensional (quasi-2D) channels of AlGaN/GaN heterostructures. Particular attention is paid to the effects of half-space and interface modes on the momentum relaxation. The total momentum relaxation rates are compared with those evaluated by the three-dimensional phonon (3DP) model, and also with the Callen results for bulk GaN. In heterostructures with a wide channel (effective channel width >100 Å), the DC and 3DP models yield very close momentum relaxation rates. Only for narrow-channel heterostructures do interface phonons become important in momentum relaxation processes, and an abrupt threshold occurs for emission of interface as well as half-space phonons. For a 30-Å GaN channel, for instance, the 3DP model is found to underestimate rates just below the bulk phonon energy by 70% and overestimate rates just above the bulk phonon energy by 40% compared to the DC model. Owing to the rapid decrease in the electron-phonon interaction with the phonon wave vector, negative momentum relaxation rates are predicted for interface phonon absorption in usual GaN channels. The total rates remain positive due to the dominant half-space phonon scattering. The quasi-2D rates can have substantially higher peak values than the three-dimensional rates near the phonon emission threshold. Analytical expressions for momentum relaxation rates are obtained in the extreme quantum limits (i.e., the threshold emission and the near subband-bottom absorption). All the results are well explained in terms of electron and phonon densities of states

The response of the magnetosphere-ionosphere system to a sudden dynamic pressure enhancement under southward IMF conditions

The magnetospheric response to step-like solar wind dynamic pressure increases under southward IMF conditions is studied using the University of Michigan MHD code. A two phased response in the ionosphere is observed, similar to what is observed when the IMF is northward by looking into the residual potential and field-aligned current (FAC) patterns in the ionosphere. The first phase response right after the high pressure enhancement hits the magnetopause is associated with a pair of FACs downward in the postnoon and upward in the prenoon region. These FACs are caused by dusk-to-dawn electric fields inside the dayside magnetopause launched by a fast mode compressional wave. The second phase response shows another pair of potential cells as well as FACs in opposite polarity, which originates from magnetospheric vortices on the equatorial plane. The vortices appear to be formed by the recovery of the system from the fast mode wave

Simulation of non-hydrostatic gravity wave propagation in the upper atmosphere

The high-frequency and small horizontal scale gravity waves may be reflected and ducted in non-hydrostatic simulations, but usually propagate vertically in hydrostatic models. To examine gravity wave propagation, a preliminary study has been conducted with a global ionosphere–thermosphere model (GITM), which is a non-hydrostatic general circulation model for the upper atmosphere. GITM has been run regionally with a horizontal resolution of 0.2° long × 0.2° lat to resolve the gravity wave with wavelength of 250 km. A cosine wave oscillation with amplitude of 30 m s⁻¹ has been applied to the zonal wind at the low boundary, and both high-frequency and low-frequency waves have been tested. In the high-frequency case, the gravity wave stays below 200 km, which indicates that the wave is reflected or ducted in propagation. The results are consistent with the theoretical analysis from the dispersion relationship when the wavelength is larger than the cutoff wavelength for the non-hydrostatic situation. However, the low-frequency wave propagates to the high altitudes during the whole simulation period, and the amplitude increases with height. This study shows that the non-hydrostatic model successfully reproduces the high-frequency gravity wave dissipation

Role of vertical ion convection in the high‐latitude ionospheric plasma distribution

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94649/1/jgra18345.pd

Validation of SWMF magnetic field and plasma

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94788/1/swe345.pd

Establishing Social Work Practices in England: The Early Evidence

Social Work Practices (SWPs) were established in England in 2009 to deliver social work services to looked after children and care leavers. The introduction of independent social work-led organisations generated controversy focused on issues such as the privatisation of children's services and social workers' conditions of employment. This paper reports early findings from the evaluation of four of these pilots, drawing on interviews with children and young people, staff, and local authority and national stakeholders. The SWPs assumed a variety of organisational forms. The procurement process was demanding, with protracted negotiations over matters such as budgetary control and providing a round-the-clock service. Start-up was facilitated by an established relationship between the SWP provider and the local authority. Once operational, SWPs continued to rely on local authorities for various functions; in most cases, local authorities retained control of placement budgets. Levels of consultation and choice offered to children and young people regarding the move to an SWP varied considerably. Children's understanding about SWPs was generally low except in the pilot where most children retained their original social worker. These early findings show some dilution of the original SWP model, while the pilots' diversity allows the benefits of particular models to emerge

Effects of seasonal changes in the ionospheric conductances on magnetospheric field‐aligned currents

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95211/1/grl22574.pd

How reversible is sea ice loss?

It is well accepted that increasing atmospheric CO<sub>2</sub> results in global warming, leading to a decline in polar sea ice area. Here, the specific question of whether there is a tipping point in the sea ice cover is investigated. The global climate model HadCM3 is used to map the trajectory of sea ice area under idealised scenarios. The atmospheric CO<sub>2</sub> is first ramped up to four times pre-industrial levels (4 × CO<sub>2</sub>), then ramped down to pre-industrial levels. We also examine the impact of stabilising climate at 4 × CO<sub>2</sub> prior to ramping CO<sub>2</sub> down to pre-industrial levels. Against global mean temperature, Arctic sea ice area is reversible, while the Antarctic sea ice shows some asymmetric behaviour – its rate of change slower, with falling temperatures, than its rate of change with rising temperatures. However, we show that the asymmetric behaviour is driven by hemispherical differences in temperature change between transient and stabilisation periods. We find no irreversible behaviour in the sea ice cover

Exploring the efficacy of different electric field models in driving a model of the plasmasphere

The dynamics of the plasmasphere are strongly controlled by the inner magnetospheric electric field. In order to capture realistically the erosion of the nightside plasmapause and the formation of the drainage plume in a model of the plasmasphere, the electric field must be accurate. This study investigates how well five different electric field models drive the Dynamic Global Core Plasma Model during eight storm periods. The five electric field models are the Volland‐Stern analytic formula with Maynard‐Chen Kp dependence, two versions of the Weimer statistical models (96 and 05), and two versions of the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) technique using magnetometer and DMSP satellite data. Manually extracted plasmapause locations from images taken by the EUV instrument on the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) satellite, as described by Goldstein et al. (2005), were compared to the simulation results throughout the main phase of the eight events. Three methods of calculating the plasmapause were employed to determine the best fit to EUV data, using the maximum gradient, a constant density contour (fit method), and the location in which the modeled density fell significantly below the specified saturation density for the given radial position (saturation method). It was found that the simulations driven by the Weimer (1996) model produced the best fit overall and that the fit and saturation methods worked best for matching the model results to the observations. Key Points The Weimer [1996] model works quite well for driving the plasmasphere A saturation technique for determining the plasmapause location in introduced Plasmapause determined by IMAGE may not be the steepest gradient in densityPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/108010/1/jgra51094.pd