摘要:

A comprehensive one‐dimensional model of the polar ionosphere has been used in conjunction with incoherent‐scatter radar data from Sondrestrom, Greenland, to determine downward heat fluxes and thermal ion outflows at very high latitudes. For periods of very quiet geomagnetic activity the model closely simulates the observed time‐dependent behavior of the electron density, ion and electron temperatures. To obtain this similarity between model and data, the upper boundary conditions of the model, namely downward heat flux, and magnetic field‐aligned ion flows, are continually adjusted with time to provide a best fit with data. The heat fluxes and ion flows are determined indirectly from this fitting procedure. The technique has been applied to a 10‐hour daytime data set for February 12, 1990, to search for enhanced downward heat fluxes and outward thermal ion fluxes associated with dayside auroral oval. Variations of heat flux ranged from about 2×109to 2×1010eV cm−2s−1, and vertical outward fluxes of ionization ranged from about zero to 8×108cm−2s−1. For both quantities the peak values occurred when the radar site was located under the dayside auroral oval. It is suggested that these marked upward thermal ion flows in the dayside auroral ionosphere may be associated with energetic O+ion outflows that have been observed at high altit

ISSN:0148-0227    

DOI:10.1029/94JA02071

年代:1995

数据来源: WILEY

摘要:

In this study, Atmosphere Explorer data and model results for the ion and electron temperature and the density of N+, O+, H+, and He+between 120 and 1400 km altitude are compared for two midlatitude ranges (L=2 and L=4), noon and midnight local time, winter and summer, at solar minimum. The data for the heavy atomic ions (O+and N+) show that their densities are greater at noon than at midnight for a given season and greater in summer than winter for a given local time. There is only a weak latitudinal variation in the density of these ions. The data show that the light ion (H+and He+) densities are greater at midnight than at noon and are generally greater in winter than summer. There is a strong latitudinal variation of the light ion densities, with the densities decreasing with increasing latitude. The model densities are in good agreement with the AE densities for N+, O+, and H+. Model He+densities are lower, by a factor of 2 or more, than the measured densities. Model ion and electron temperatures agree well with the measured temperatures with only a modest increase in plasmaspheric heating.

ISSN:0148-0227    

DOI:10.1029/94JA02306

年代:1995

数据来源: WILEY

摘要:

The early time evolution of an ionospheric electron depletion produced by a radially expanding electron attachment chemical release is studied with a two‐dimensional simulation model. The model includes electron attachment chemistry, incorporates fluid electrons, particle ions and neutrals, and considers the evolution in a plane perpendicular to the geomagnetic field for a low beta plasma. Timescales considered are of the order of or less than the cyclotron period of the negative ions that result as a by‐product of the electron attachment reaction. This corresponds to time periods of tenths of seconds during recent experiments. Simulation results show that a highly sheared azimuthal electron flow velocity develops in the radially expanding depletion boundary. This sheared electron flow velocity and the steep density gradients in the boundary give rise to small‐scale irregularities in the form of electron density cavities and spikes. The nonlinear evolution of these irregularities results in trapping and ultimately turbulent heating of the negative

ISSN:0148-0227    

DOI:10.1029/94JA02490

年代:1995

数据来源: WILEY

摘要:

This paper examines an isolated magnetospheric VLF/radio noise event that is highly suggestive of the triggering of terrestrial auroral kilometric radiation (AKR) by solar type III radio emission and of a close relation between AKR and broadband hiss. The solar type III burst was measured on polar HF riometers and was coincident with local dayside VLF/LF noise emission bursts at South Pole station. It was also coincident with AKR bursts detected on the AMPTE/IRM satellite, at the same magnetic local time as South Pole. On the basis of the close association of AKR and VLF bursts, and from geometrical considerations relating to wave propagation, it is likely that the AKR source was on the dayside and on field lines near South Pole station. The general level of geomagnetic activity was very low. However, an isolated magnetic impulse event (MIE) accompanied by a riometer absorption pulse was in progress when all of the VLF/radio noise bursts occurred. The very close association of the type III burst at HF with the AKR is consistent with external stimulation of the AKR, if a different, more immediate, triggering process than that implied by Calvert (1981) is invoked. It is suggested here that some of the HF solar radiant energy may decay into waves with frequencies comparable to those of the AKR by parametric excitation or some other process, thus providing the few background photons required for the generation of AKR by the Wu and Lee (1979) cyclotron maser instability. The AKR, perhaps by modifying the magnetospheric electron velocity distribution, might have produced the observed VLF emissions. Alternatively, the VLF emissions may have arisen from the same anisotropic and unstable electron distribution function responsible for the AKR. While the relationship of these emission features to the occurrence of the magnetic/absorption impulse may have been coincidental, the MIE could be a preconditioning agent for the ionosphere/magnetosphere plasma, making it susceptible to the external stimulation of the AKR.

ISSN:0148-0227    

DOI:10.1029/94JA02757

年代:1995

数据来源: WILEY

摘要:

Results of detailed numerical calculations of some parametric decay effects, arising in a magnetoplasma under the influence of a HF, sufficiently strong electric field , are given in this paper. The resonance branches and the VLF parametric resonances are calculated in the ionosphere at altitudesZ= 200, 300, and 400 km. Calculations in the resonance regions ωE=sω0(ω0is the electron Langmuir frequency,s= 1,2,…) were done in the cold palsma and also in the kinetic approximations. It is shown that the angle dependence ω1(Θ,Ep≠ 0) of the ELF (0<ω ≤ ΩB) resonance branch is close to the cosine law. This is in contrast with the earlier published results and with the angle dependence ω1(Θ,E= 0). This important effect and the other dependencies given in the paper may be used for the search of the parametric instabilities and of the electric field in the ionosphere and magnetosphere, especially by experiments in situ

ISSN:0148-0227    

DOI:10.1029/94JA01616

年代:1995

数据来源: WILEY