Journal of Atmospheric and Terrestrial Physics最新文献

A model of solar light scattering in the terrestrial spherical atmosphere is developed on the basis of a numerical solution of the radiation transfer equation. It allows us to model the twilight sky brightness. This brightness for the range of solar zenith angles of 92–106 degrees and wavelength 530 tun has been compared with the experimental data and with other calculated results. Modelling has been carried out for standard altitude distributions of the molecular number density and ozone absorption coefficient and for the aerosol model of Toon and Pollack (1976). The different layers' contribution and the contribution of different orders of scattering from'some altitudes to the zenith intensity have been obtained. The role of multiple scattering in the twilight sky brightness and a hypothesis on the high turbidity of the upper atmosphere are discussed.

Observations of the OH (8-3) band airglow emission, using a multichannel tilting filter type photometer, have been carried out at Calgary (51°N, 114°W), Canada, since 1981. In this paper recent measurements of the nocturnal, seasonal and solar flux variations of the mesopause temperature, obtained from the rotational temperature of the OH (8-3) band observations, are presented. The data presented span the ascending phase of the present solar cycle viz. 1987–1988 (low solar activity) and 1990 (high solar activity). Good correlations (r = 0.73) between the OH (8-3) band rotational temperature and the 10.7 cm solar flux were observed. The mean temperature for the period investigated was about 210 K. The seasonally averaged nocturnal variations show only small irregular excursions, possibly associated with solar tides and the passage of gravity waves in the mesopause region. However, the observed rotational temperatures show considerable night-to-night changes.

The characteristics of extremely high-latitude dayside auroras are examined by using auroral TV data obtained at Godhavn, Greenland, and simultaneous DMSP particle data. Two different kinds of aurora are found near the pre-noon sector, namely (1) the polar arc: this aurora is observed during quiet periods and originates from the dayside region. It is related to about 100 eV electron precipitation or less, and (2) the polar corona: this aurora is observed during disturbed periods and the appearence latitute of this aurora is confined within a certain region about 70–80° MLAT. It is related to a few hundred eV electrons. These results suggest that the origin of the polar arc seems to be the plasma mantle or low-latitude boundary layer, and the origin of the polar corona seems to be the low-latitude boundary layer or Boundary Plasma Sheet.

The Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) facility is a new and major facility for atmospheric research. It is located at Andøya in Northern Norway. One of the important facilities of ALOMAR is the Doppler Wind and Temperature System (DWTS). The DWTS will determine atmospheric wind and temperature profiles between about 8 and 90 km altitude from the Doppler shift and broadening of the lidar signal Rayleigh back-scattered from the atmosphere. The DWTS uses a double-etalon Fabry-Perot interferometer to perform the high-resolution spectral analysis of the back-scattered lidar signal, and to reject the bright background light from the daytime sky. After spectral analysis, the Fabry-Perot fringes are imaged onto a multi-ring anode imaging photon detector which provides, pulse-by-pulse, time-resolved detection of the spectrum of the laser light back-scattered from the atmosphere. The double-etalon Fabry-Perot interferometer has been designed to detect the returned signal during daytime, and thus summer-time conditions at ALOMAR, as well as during night-time. The entire optical system has been designed to maximise the transmission and detection of light, to make maximum use of the faint signals available from high-altitude regions, up to around 80–90 km. This paper reports on the objectives and design of the ALOMAR DWTS, and presents some initial results obtained during commissioning periods in October 1994 and January 1995.

During 1992 and 1993, record low total ozone values were observed over the middle and high northern latitudes. The ozone data from the long-operating station at Belsk, Poland, have been used to examine their departures from climatological behaviour in 1992 and 1993. It seems that not only do the exceptionally low ozone amounts recorded over the northern mid-latitudes need an explanation but also their occurrence for two years in a row. One of the possible mechanisms which may be responsible for this event is suggested to be connected with the occurrence of stratospheric minor warmings. They occur without a breakdown of the polar vortex but only with the displacement of very cold air towards lower latitudes (as in January 1992 and February 1993). It is known that air masses in the polar vortex have been chemically disturbed and, when they arrive over the sunlit middle latitudes, chemical destruction of ozone is likely to occur. During the periods under study, the strongest negative total ozone deviations correspond to strong negative temperature deviations at 30 hPa and to large potential vorticity values; this points to the presence over Europe of air masses of polar vortex origin. It has been shown that the characteristics of mid-winter stratospheric warmings and the interannual variability of winter-spring total ozone averages at Belsk are associated with each other.

An energetic auroral proton entering the atmosphere will, by charge exchange in collisions with atmospheric constituents, alternate between being a proton H⁺ and a neutral hydrogen atom H. This study provides a procedure to evaluate the auroral Doppler shifted and broadened hydrogen Balmer profile as a function of initial energy, flux, pitch angle and view angle relative to the geomagnetic field. The differential proton energy flux entering the atmosphere is deduced using ground-based measurements of H_α and H_β from Nordlysstasjonen in Adventdalen, Longyearbyen. The main assumptions are that the geomagnetic field lines are: parallel and vertical, and that the pitch angle of the H/H⁺-particle is preserved in collisions with atmospheric constituents before being thermalized. This numerical method estimates the fate of the auroral H/H⁺-particle in the atmosphere, and from measured Doppler profiles the corresponding incoming particle flux can be deduced. Optimization of the method will continue through extensive use of observational data.

A new model of the lowest part of the D-region is obtained by a trial-and-error inversion method. Its basic feature is a step-like transition between 55 and 70 km which is not depicted in most ionospheric models. The seasonal differences of this are considered to be quite important: the bottom of the ionsphere is found to be lower in summer and spring, the gradient of the profile below the CR-layer is stronger in winter, and a well defined ‘valley’ exists around 70 km in spring. By simulating the ionospheric response to a, solar flare (SID-effect) in summer and in winter, an attempt was made to verify the obtained seasonal peculiarities of the quiet ionosphere.

We present a numerical model which describes the global distribution and the thunderstorm-driven electromagnetic excitation in the extremely low frequency-(ELF) range. The model, in its present stage, builds on the parameterised world-wide distribution and temporal occurrence of thunderstorms and on electromagnetic wave propagation in the Earth-ionosphere cavity. The ionospheric D layer and the surface of the Earth are treated as very good conductors, and the ELF propagation in the Earth-ionosphere waveguide is considered to be isotropic but damped. The return strokes are regarded as the only transmitters and for the purpose of a qualitative simulation these strokes are approximated by vertical Hertz dipoles with normalized dipole moments.

Our model has been checked against data from two full days of ELF observations at a remote field station (located at 50.4° N, 9.3° E). The comparison, while qualitatively satisfactory, has revealed some quantitative limitations of the model which lead to suggestions for its improvement. These include a more accurate description of the world-wide lightning occurrence, with refined spatial and temporal resolution and possibly an account of systematic variations in the lighting dipole moment with respect to geographic latitude and frequency.

A simplified full-wave method adapted to the propagation of very obliquely incident LF radio waves is developed. For a selected ionosphere model the wave-field structure is calculated inside a horizontally stratified ionosphere and the peculiarities of the reflected field are clearly described. The penetration of the investigated radio waves in the lower ionosphere at noon-time is found to be restricted to a layer several wavelengths thick. The reflected wave is created entirely by the mechanism of partial reflections and the region responsible for its formation is usually below 70 km. The influence of some typical parameters of the electron density profile, as well as the atmospheric pressure and temperature, on the attenuation of the investigated radio waves is demonstrated. It is also found that the reflection at very oblique incidence depends mainly on the height of the bottom of the ionosphere.