Journal of Atmospheric and Terrestrial Physics最新文献

The observed discrepancies between A1 absorption meaurements and numerical estimation of the same using IRI electron density profiles are attributed to the assumption made in the Sen-Wyleer generalized magneto-ionic theory that the momentum transfer collision frequency of electrons with neutrals is proportional to the square of the electron thermal speed. Based on Budden's (1985) suggestion that, in the lower thermosphere and mesosphere, the momentum transfer collision frequency is proportional to the electron thermal speed, a generalized magneto-ionic theory has been outlined. The new theory brings experimental measurements of A1 absorption closer to the theoretical deductions based on IRI-90 electron density profiles.

Low-latitude plasma drifts (zonal and meridional) in the F-region are inferred from the observed night-time thermospheric neutral wind velocities and temperature gradients, together with models for the neutral density (MSIS-86 model) and the electron density (IRI model). The thermospheric neutral winds and temperatures are derived from measurements of Doppler shifts and widths of the Oi 630.0 nm airglow emission line, respectively, using a Fabry-Perot interferometer at Cachoeira Paulista (23°S, 45°W), Brazil. The equations considered are the ideal gas law and the momentum equation for the thermosphere, which includes the time variation of the neutral wind, the pressure gradient which is related to the temperature and density gradients and the ion drag force. The present method to infer the night-time plasma drift using observed neutral parameters (time variation of neutral wind velocities and temperature gradients) showed results that are in reasonable agreement with our calculated plasma drifts and those observed in other low-latitude locations. On the other hand, it is surprising that sometimes the winds flow from the observed coldest sector to the hottest part of the thermosphere during many hours, suggesting that plasma drift can drive the neutral winds at low latitudes for a period of time.

A procedure is established to evaluate the Balmer excitation rates of Hα and Hβ to produce the corresponding volume emission rates versus height, using semi-empirical range relations of protons in air. The calculations are carried out with identified ion-energy particle spectra of the dayside aurora obtained by low altitude satellites. The calculated emission intensities of Hα and Hβ indicate that they are indeed observable by ground-based optical detection. Measurements of the dayside aurora from Nordlysstasjonen in Adventdalen, Svalbard, are discussed in relation to these calculations. Periodic bursts of auroral Hα and Hβ emissions are observed in the dayside aurora by ground-based photometers and spectrometers. The mean period between proton events is found to be 10 min on average. Furthermore, it is found that when the time between successive bursts decreases, the emission ratio of Hα and Hβ fluctuates indicating a step-like behaviour in the primary initial proton energy.

Knowledge of the quiet and disturbed conditions in the propagation medium is essential for quality control of transatmospheric radio signals. This holds equally for the troposphere and the ionosphere. This paper describes a climatology of ionospheric irregularities obtained from observations of celestial radio sources by radio interferometry, i.e. by the Westerbork Synthesis Radio Telescope (WSRT) in The Netherlands. This instrument is located at geomagnetic mid-latitude. All WSRT calibrator observations in the 22-year period 26 June 1970–31 December 1991 have been checked for manifestations of ionosopheric effects. Although seasonal effects are clear, the occurrence and ‘strength’ of ionospheric irregularities show no dependence on solar activity. Assuming that the frequency of occurrence of ionospheric disturbances in spring and autumn are similar, it is found that ‘ionospheric’ winter starts on day 348 ± 3 and all seasons last for 3 months. Medium-scale travelling ionospheric disturbances (TIDs) occur most frequently during the daytime in winter periods. The occurrence of non-periodic irregularities is, however, not a function of time in the day. The daily variation in the amplitude and frequency of the occurrence of the TIDs suggests that the solar terminator and Joule heating near the electrojets do not contribute substantially to their generation. Generation of gravity waves may be caused by winds and tides in the lower thermosphere-mesosphere. This has to be investigated further.

On the basis of the available data, a ‘disturbance measure’, indicating to what extent the ionosphere is ‘quiet’, is proposed. The output of this project may be of immediate use for different ionospheric investigations, such as ionospheric modelling and the study of excitation mechanisms for ionospheric irregularities.

Using numerical simulation of a non-stationary problem of thermodiffusion and diffusive spreading of the electron component of the dense cold ionospheric plasma, the processes of formation and relaxation of strong disturbances of the electron temperature and concentration in the E- and F-regions of the middle-latitude ionosphere are examined, taking into account the altitudinal distribution of the electron transport coefficients. The cases of local heating and heating at separated altitudes of the ionospheric plasma by powerful radio waves generated from ground-based HF-facilities are numerically investigated. The numerical simulations of the non-stationary problem are compared with the analytical evaluations carried out for the stationary and quasi-stationary heating models. Results obtained from numerical experiments give good explanations of the experimentally observed deformation of the altitudinal ionospheric plasma density profile and the creation of negative cavities in the upper ionosphere and positive cavities in the lower ionosphere during the process of plasma heating.

A nonlinear wave equation for a solitary one-dimensional inhomogeneity is studied. It is similar to the diffusion equation with the diffusion coefficient depending on the phase velocity. The phase velocity depends, in turn, on the electron density. A weak inhomogeneity moves with the velocity close to that of the electric drift. If this velocity exceeds the ion acoustic speed the effective diffusion is negative, and the inhomogeneity grows and contracts. The velocity of the growing inhomogeneity becomes smaller. It absorbs weaker and faster moving inhomogeneities from the back side. In a stationary regime, the ionosphere will be filled with rare but strong inhomogeneities with sharp back sides.

A new deconvolution technique similar to that used by Jones and Thiele (1991) was used to determine the positions and Doppler shifts of D-region scatteres at Bribie Island (152°E, 27°S). The new technique performs analysis in the frequency domain. It is introduced and measurements of the aspect sensitivity and angular spread of D-region backscatter at 1.98 MHz are shown. These results are similar to those obtained by others.

In the period March–November 1991, which was characterized by strong magnetic activity, two kinds of complementary VLF data were recorded at Kerguelen: (1) the amplitude of the subionospheric 22.3 kHz signal from the North-west Cape (NWC) transmitter in Australia, and (2) the integrated intensity in various VLF bands, and broadband waveforms (0.4–10 kHz). Comprehensive analysis of the data allowed one to identify and characterize four types of well-structured perturbations in the NWC signal amplitude, i.e.: classical Trimpi events associated with whistlers; hiss-induced electron precipitation events; inverted dome-shaped events identifiable with dome-shaped events recently observed at Durban by Friedel et al. (1993); and quasi-monochromatic oscillations, with a period of ∼7–15 s observed when Kerguelen is poleward of the plasmapause projection in the pre-dawn sector, similar to the zigzag effect described by Carpenter et al. (1985a). The purpose of this paper is to report on the results of investigations devoted to these various types of event; each of them is illustrated by examples.

The main new results relative to classical Trimpi events concern: the dependence on severe magnetic storms (out of 2300 events identified in June and July 1991—period of maximum occurrence—48% were observed on 6 days only in the aftermath of three large magnetic storms); the correlation between amplitude perturbation sign and NWC signal amplitude connected with sunset and sunrise on the east-to-west NWC-KER path; the nocturnal variation of recovery times for plasmaspheric whistler-associated Trimpi events (the mean value increases from 30 to 42 s at night); the detection in the midnight-dawn sector of short-duration Trimpi events (recovery times in the range ∼4–15 s) probably triggered by whistler waves propagating beyond the plasmapause when the latter is equatorward of, or close to, Kerguelen, 24–48 h following the onset of intense magnetic storms.

Whereas it is possible to interpret most of the characteristics and properties of classical Trimpi events in terms of electron precipitation induced by magnetospheric VLF waves—a process being favoured by the Occurrence of severe magnetic storms with probable injection of energetic particles—the dome and zigzag effects remain unexplained as yet.

Tide-like variations observed in data from an OH spectrometer and MF radar, co-located at the Buckland Park field station (34°38′S, 138°29′E) near Adelaide, South Australia, over six nights in September 1993 are presented. A prominent semi-diurnal tide-like variation in temperature with a magnitude of up to 40 K peak-to-peak is observed in the OH data. Temperature and wind amplitudes and phases are compared with a semi-diurnal model, as are the relationships between the quantities. In addition, the observed temperature variations are compared with those of previous researchers. With regard to the absolute values of the amplitudes and phases, the model and observations differ. However, the observed relationships between the quantities, in terms of phase, are found to agree well with the model for four of the six individual nights, as well as for data that are the average of the six nights.

Magnetic data from a meridional chain of stations in Greenland and AL-indices of magnetic activity have been used to study the relationship between magnetic perturbations in the dayside cleft region and substorm activity in the night-time auroral zone. The analysis of 14 substorms, isolated and prolonged, has shown that intensification of westward currents in the postnoon sector of the cleft precedes or accompanies substorm development in the night-time auroral zone. Westward currents appear in the northern cleft as substorm precursors even under the adverse influence of the IMF positive B_y component. These currents trend to extend in the prenoon sector. To explain the relationship between the cleft currents and auroral electrojet the connection between neutral layer currents and noon Birkeland currents is proposed. This connection can be realized by means of the source region acting just inside the daytime magnetopause owing to stationary reconnection of geomagnetic field and IMF, the source region flowing downstream to the tail magnetopause.