Journal of Atmospheric and Terrestrial Physics最新文献

The time variations of the Schumann resonance peak frequencies for the first three modes are presented in the vertical electric component measured in the Nagycenk Observatory (47.6°N, 16.7°E) from May 1993 to August 1994. The average daily frequency patterns are different for the three modes, and each mode shows a distinct seasonal variation. The recurrence of this seasonal variation is also shown. The daily frequency range, in which the frequencies shift, is wider in winter than in summer in all three modes. The mean frequency level also shows a seasonal variation in the third mode. A spring-autumn asymmetry has been found in case of the first mode.

Whistler mode signals from the NAA transmitter (24 kHz) received at Faraday, Antarctica are processed to obtain the Doppler shift at a much higher time resolution than has previously been possible. This has allowed the observation of pulsations of about 13 mHz frequency which are believed to be associated with hydromagnetic waves in the magnetosphere. The pulsations are observed separately on signals with a number of discrete group delay features that can be interpreted as individual whistler ducts. Using the measured pulsation phase over the array of ducts the phase velocity and wave normal direction of the hydromagnetic wave in the equatorial plane are estimated. The direction of propagation is consistent with a source on the dayside magnetopause.

The association between whistler mode Doppler shifts and hydromagnetic waves has been reported before but not, as far as we are aware, using an experimental technique that allows measurements on individual ducts in order to determine the direction of propagation of the hydromagnetic wave.

The possibility of the generation of decameter scale ionospheric plasma density irregularities, that must be responsible for dusk scatter, by the plasma gradient drift instability (GDI) at F-region altitudes is considered. It is shown that the dusk scatter could be produced by the ion density perturbations which appear as a result of the development of the GDI produced by the maximum westward plasma drift in the region poleward of the trough minimum. Possible reasons for the appearance of growth of the GDI waves as a result of the development of the trough plasma GDI during or just after sunset in the F-region are discussed. It is shown that, if the GDI begins after sunset, then the influence of the drift velocity shear results in the action of the GDI during 1–2 hours after sunset, which is close to the duration of dusk scatter.

The HWM90 thermospheric wind model has been revised in the lower thermosphere and extended into the mesosphere, stratosphere and lower atmosphere to provide a single analytic model for calculating zonal and meridional wind profiles representative of the climatological average for various geophysical conditions. Gradient winds from CIRA-86 plus rocket soundings, incoherent scatter radar, MF radar, and meteor radar provide the data base and are supplemented by previous data driven model summaries. Low-order spherical harmonics and Fourier series are used to describe the major variations throughout the atmosphere including latitude, annual, semiannual, local time (tides), and longitude (stationary wave 1), with a cubic spline interpolation in altitude. The model represents a smoothed compromise between the original data sources. Although agreement between various data sources is generally good, some systematic differences are noted, particularly near the mesopause. Overall root mean square differences between dar.a and model values are on the order of 15 m/s in the mesosphere and 10 m/s in the stratosphere for zonal winds, and 10 m/s and 5 m/s respectively for meridional winds.

Diurnal variations of decay time of heater-induced small-scale irregularities in the mid-latitude ionospheric F-layer were measured by means of diagnostic stimulated electromagnetic emissions (DSEE). The abrupt (15–20 min) and very strong (10-fold or more) increase in DSEE decay times was observed simultaneously over a wide height range around a turbulence location. This increase was assumed to be dictated by a natural mechanism, supporting artificial irregularities by utilization of the diagnostic wave energy. Analysis of the experimental data, concerning features of both heater-induced and natural irregu larities, shows that such a natural mechanism was initiated by the S_q current system. To account for small-scale irregularity growth, the thermomagnetic instability realized for a downward directed field-aligned current was considered. This instability allows us to explain the natural generation of irregularities with scale lengths of 25 m or longer.

At the magnetopause, solar wind plasma interacts with the terrestrial magnetic field, with the consequent entry of solar wind energy into the magnetosphere and the ionosphere. Geomagnetic activity is one of the results. Planetary geomagnetic indices, e.g. Kp, Ap, Am, etc, have been designed to measure solar particle radiation by its magnetic effects. Long-term averages of these indices have established that solar wind energy input into the ionosphere maximizes around equinoctial months with minima around the solstices. Although considerable progress has been made to explain qualitatively the semiannual variation o1' geomagnetic activity, its component parts, representing the axial and equinoctial hypotheses, have not so far been put together with a high degree of quantitative precision. This paper demonstrates that the semiannual trend of geomagnetic activity can be reproduced quantitatively with good precision by using accurate astronomical data relating to the Sun-Earth geometry. The key factor is the combination of the varying solar declination and the heliographic latitude of the Earth during different months. Analysis shows that the seasonal trend of solar wind-magnetopause coupling is, in fact, controlled by a combination of the two competing theories, the axial and equinoctial, which have been advanced over the years to explain the semiannual variation in geomagnetic activity. Planetary ion density of the F2 layer of the ionosphere (F2pd) is another index of relatively higher accuracy which also shows marked maxima around the equinoxes. The observed seasonal trend of F2pd can be reproduced by using the semiannual trend of geomagnetic activity as derived from astronomical data with a correlation coefficient of 0.98. This analysis also brings out another important fact that the planetary indices, Kp, Ap, Am and AA, are somewhat deficient as they respond to solar declination only and do not bring out the contribution of the heliographic latitude of the Earth.

The spectral slope of the middle atmospheric wind is an important index of the gravity wave and turbulence processes. Gaps exist in MF radar spaced-antenna winds data because significance criteria are built into the analysis. These cause a smearing of the spectrum and seriously modify the slope, as well as affect the absolute power at high frequencies. A comparison between sites with different gap rates must account for this. Different methods of dealing with these gaps are tested in this paper. The periodogram (with linear interpolation across gaps), the correlogram, and the Lomb-Scargle analyses are compared on synthetic data with known slope, and also with some of the best measured data (less than 20% gaps), both with added gaps to a maximum of 50%. The periodogram is seen to be the best choice. Parallel calculations on real data and synthetic data with the real gaps inserted are used to compare 1992 summer and winter spectral slopes from the Saskatoon MF radar. The latter are also compared with those of winter spectra from the two CNSR (Canadian Network for Space Research) radars which, with Saskatoon, form a ∼ 500 km array. A similar process is used to compare the seasonal variation of absolute power (10–100 min) at the three sites.

Using synchronous measurements made by microbarograph and a seismograph with a vertical pendulum, the sign of the vertical direction of the wave energy flux in the atmosphere at the ground has been derived for oscillations in the ∼0.5–4 h period range and which occur simultaneously in the Earth and atmosphere. The seismic oscillations shown could generate atmospheric oscillations by the ‘piston’ mechanism. A change of sign of the flux direction is also observed.

The variations of the column ozone densities at Middle-Asian stations and several others in the former USSR were examined by auto- and cross-correlation analyses. Periodic processes with amplitude of tens of DU and periods of 15 to 25 days occur simultaneously at many stations. The movement of this wave disturbance is directed towards the south-east and can also be seen in the weather maps at 500 hPa and 100 hPa, and the driving force is most probably meteorological air motion.