Journal of Atmospheric and Terrestrial Physics最新文献

We recorded shadow bands just before and just after the total phase of the solar eclipse of 3 November 1994. The recordings were made using two broad-band silicon photodiodes separated by 100 mm. They were mounted on a plate that faced the eclipsed Sun, which was at an altitude of 32.4° as seen from our observing site 4500 m above sea level between Putre and Lake Chungara in northern Chile. The irradiance fluctuations associated with the shadow bands were around 0.008 W m⁻² rms on a background of about 2–8 W m⁻². The cross-correlation function indicates that the shadow bands were moving at a speed of about 1.8 m s⁻¹ perpendicular to their extent. The power spectral density functions are in accord with the shadow band theory of Codona (1986). We carried out a similar experiment in Baja California during the eclipse of 11 July 1991. In spite of teh considerable differences between the two circumstances, our results on the two occasions are broadly similar.

Diurnal variations in the propagation direction of atmospheric gravity waves, and the travelling ionospheric disturbances to which they give rise, have been observed in many experimental observations and several modelling studies have demonstrated that this is primarily due to the corresponding diurnal rotation in the direction of the thermospheric wind. Other variations have been attributed to seasonal or other effects, but the effects of variations in the thermospheric temperature have not previously been analysed in detail. We present results from a study of the propagation of gravity waves through a layered atmosphere in which the thermospheric wind and temperature are derived from a three-dimensional time-dependent model. The analysis has been carried out for a range of wave speeds and periods, and for a range of times, seasons and propagation azimuths. Results suggest that a significant diurnal variation in the transmission coefficient for waves propagating through the thermosphere exists with seasonally dependent maxima. Transmission increases for increasing wave period up to about 50 min, after which it remains approximately constant. Maximum transmission occurs for wave phase speeds around 200–250 m/s and falls to zero for speeds less than about 100 m/s. An exception to this rule occurs for waves with periods less than 40 min and speeds less than 50 m/s for which significant transmission appears to be theoretically possible.

Two techniques of mesospheric wind velocity measurement are compared. One is the spaced antenna technique and the other is a technique described in this paper which deconvolves radar data in order to determine the positions and Doppler shifts of D-region scatterers. Both techniques were employed in the height interval 70–96 km using the Bribie Island radar (152°E, 27°S) operating at 1.98 MHz. A median discrepancy of 15 m/s between the two sets of wind measurements was observed. This was reduced to about 7 m/s when both techniques were forced to use the same echo information when performing a wind velocity measurement. In performing a measurement, the deconvolution technique has the advantage of being able to use the echo information within a height interval whereas the spaced antenna technique uses echo information within a range interval. However the deconvolution technique has disadvantages including much greater computer time requirements.

This paper discusses how key characteristics of temperate zone sporadic-E can be determined by making use of the combined experiences of amateur radio observations and ionosonde data. There are advantages to using this unified approach in the study of the phenomenon, and to understand and draw conclusions from the data. A brief history of amateur radio involvement in the phenomenon is provided along with actual radio observations in the 50 MHz band. Long term ionosonde data collected at Boulder, Colorado were examined using the benefit of radio experience in order to find what correlations may exist with other phenomena. Some significant conclusions regarding characteristics of temperate zone sporadic-E can be drawn through this approach.

By the full-wave algorithm with Fourier synthesis, 3-D propagation of a whistler beam incident on the pre-dawn lower ionosphere at very low latitude is numerically investigated. Processes of transmission, reflection, and coupling with the Earth-ionosphere waveguide are discussed via the wave energy and polarisation distributions and their dependence on the wave parameters and the ionospheric profile (such as the E_s-layer). It is shown that the dominant wave above 90 km altitude has the propagation characteristics of the magneto-ionic whistler mode, and absorption, spreading, reflection and mode conversion mainly occur at, and are greatly affected by, the bottom of the ionosphere. It is found that the transmitted energy density along the Earth's surface is reduced by 20 dB or more. Beam transmission loss varies asymmetrically with the incident angle, but changes little with the frequency. In the region 150 km (for 5 kHz) away from the ‘exit area’ where whistlers emerge, the bearing measurements using ground-based VLF direction-finders may be in error because direction-finding algorithms assume plane wave propagation. Only a small portion (about −25 dB at 5 kHz) of the incident energy is reflected up to an altitude of 150 km, and major reflection takes place in a small range of altitude at the bottom of the ionosphere with little spreading and lateral shift with respect to the incident beam. Reflection is enhanced considerably at lower frequency. Our results also suggest that an E_s-layer or an ionospheric gradient refracting waves to higher latitudes would be favorable factors for multi-hop echoes to be received on the ground.

A ray-tracing method for the precise calculation of “range errors” and “group delay errors” of VHF transionospheric radio waves has been developed. The method allows quantitative estimation of the influence of the geomagnetic field and the ionosphere at large zenith angles.

The problem of electromagnetic field disturbances produced by the interaction between winds of acoustic gravity waves (AGW) origin and the ionospheric plasma has been considered. It is shown that, when not allowing the electrostatic approach, electromagnetic field disturbances represent shear Alfvén and compressional modes modified by ionospheric Pedersen and Hall conductivities. It is further shown that the quasielectrostatic Alfvén type disturbances give the main contribution to electric field perturbations. Magnetic field perturbations due to Alfvén and compressional modes have the same order of magnitude. Two numerical models for simulation of the problem under consideration have been developed. The first model is intended for the simulation of Alfvén type disturbance production and transmission into the magnetosphere, taking into account the dipole geometry of the geomagnetic field, but a mutual transformation of Alfvén and compressional modes is ignored. The second model is constructed for the simulation of both electromagnetic field disturbance production and their mutual transformation in the ionosphere. The results of numerical simulations with these models show that there is an opportunity for AGW activity monitoring in the lower thermosphere by ground-and satellite-based recordings of magnetic and electric field variations.

The penetration of the quasi-DC electric fields, E due to electrified clouds, into the middle- and high-latitude ionosphere is theoretically studied during the initial stage of cloud charge separation. The electrification process is characterized by a source function S(t), whose variations are on the time scale of the relaxation process in the cloud. A first-order approximation solution for the time variation of E is obtained as an explicit function of the electric field in the steady-state case. Some features of the time variations of E at different altitudes (in the ionosphere and also in the atmosphere) are investigated, depending on the source function and the relaxation time constants. This result can also be applied for slowly electrifying clouds which do not produce lightning.

The definition of geomagnetic storms dates back to the turn of the century when researchers recognized the unique shape of the H-component field change upon averaging storms recorded at low latitude observatories. A generally accepted modeling of the storm field sources as a magnetospheric ring current was settled about 30 years ago at the start of space exploration and the discovery of the Van Allen belt of particles encircling the Earth. The Dst global ‘ring-current’ index of geomagnetic disturbances, formulated in that period, is still taken to be the definitive representation for geomagnetic storms. Dst indices, or data from many world observatories processed in a fashion paralleling the index, are used widely by researchers relying on the assumption of such a magnetospheric current-ring depiction. Recent in situ measurements by satellites passing through the ring-current region and computations with disturbed magnetosphere models show that the Dst storm is not solely a main-phase, growth to disintegration, of a massive current encircling the Earth. Although a ring current certainly exists during a storm, there are many other field contributions at the middle-and low-latitude observatories that are summed to show the ‘storm’ characteristic behavior in Dst at these observatories. One characteristic of the storm field form at middle and low latitudes is that Dst exhibits a lognormal distribution shape when plotted as the hourly value amplitude in each time range. Such distributions, common in nature, arise when there are many contributors to a measurement or when the measurement is a result of a connected series of statistical processes. The amplitude-time displays of Dst are thought to occur because the many time-series processes that are added to form Dst all have their own characteristic distribution in time. By transforming the Dst time display into the equivalent normal distribution, it is shown that a storm recovery can be predicted with remarkable accuracy from measurements made during the Dst growth phase. In the lognormal formulation, the mean, standard deviation and field count within standard deviation limits become definitive Dst storm parameters.