-
Possible effects of signal reception from different electrojet heights in a skewness of auroral coherent spectra are studied assuming that the "inherent" spectral line due to plasma turbulence is of type-2 and symmetrical. For reasonable ionospheric parameters, the altitude integrated spectra are expected to be skewed negatively for positive mean Doppler shift, in agreement with radar observations at small aspect angles. However, the spectra could be skewed positively if the turbulent layer or the electron density profile is shifted to high altitudes of ~120 km. This change of spectral shape will not be observed experimentally if, at the same time, either the electron collision frequency is enhanced or the "inherent" spectral width is increased. Observational results are discussed in view of the predictions given.
-
The continuum oscillation of a latitudinal range of closed geomagnetic field lines or shells appears to be a basic feature of the magnetosphere. Such oscillations are observed at the ground, and have been termed transient ULF pulsations. Earlier modelling showed that the apparent mean damping rate at the ground should be much greater than that in the magnetosphere. This modelling is extended to examine the time dependence of the magnetic field of transient pulsations as seen by a latitudinal chain of magnetometers. It is found that there should be significant temporal variation of both period and damping decrement observed at a given latitude, which could help to identify transient events even when the period variation with latitude is not obvious. Time-frequency analysis and analytical signal analysis do not seem to be effective in determining temporal parameter variation for the short, highly damped data segments typical of transient events. Least squares fitting of two decaying sinusoids gives surprisingly good results, but seems to have no physical basis, is difficult to interpret, and may be misleading. Least squares fitting of a single sinusoid with time-varying period and damping rate gives reasonably good fits. The resulting parameter variations with latitude may help to determine the structures of ionospheric current systems associated with transient ULF events. In particular, the time change of the period at a single station can determine where that station is relative to the ionospheric current maximum.
-
The Halley PACE HF radar has been operated in a new mode to provide very high time (10 s) and space (15 km) resolution measurements of the iono-spheric signatures of the cusp and the low-latitude boundary layer. The first data show that the iono-spheric signature of flux transfer events occur up to 300 km equatorward of regions showing the HF characteristics of the ionospheric cusp. Whilst larger flux transfer events are seen, on average, every 7 min, many much smaller and short-duration events have been identified. On one occasion DMSP data have been used to show that at least four flux transfer events are occurring simultaneously at the edge of the cusp over 2 h of MLT. There is strong, but not conclusive evidence, that reconnection at the magnetopause is both intermittent and patchy. These data also suggest that flux transfer events can be a significant contributor to the cross-polar cap potential.
-
The response of the magnetopause surface to time-varying solar wind dynamic pressure is examined. We argue that to a first approximation the magnetopause surface may be considered as analogous to an elastic membrane. Upon displacement from equilibrium resulting from a change in applied external pressure, it moves to a new equilibrium under the equation of motion of a forced, damped, simple harmonic oscillator. We derive this equation of motion by linearising for small perturbations the momentum equation for flow past a nonrigid ellipsoidal body in the Newton-Busemann limit. Though our approach is only an approximation to the real dynamics of the magnetopause boundary, it serves to demonstrate the importance of inertia in the system response. It allows us to estimate the natural eigenperiod of magnetopause oscillation as typically around 7 min, the precise value depending on solar wind conditions. However, the magnetopause eigenoscillation is furthermore found to be strongly damped, regardless of solar wind conditions. One consequence of these properties is that short-period fluctuations in the solar wind dynamic pressure elicit a suppressed magnetospheric response. We outline other theoretical expectations by which our model may be tested against observation, and discuss the implications of our findings for current interpretations of spacecraft observations made in the dynamic magnetopause environment.
-
A comparative study is made of the stepped structure of atmospherics during sunrise and sunset hours at three harmonically related frequencies in the VLF band, observed at the coast of the Bay of Bengal. The occurrences of the steps, their duration, time sequence and amplitudes are critically examined. Our analysis reveals that the start times of the third and second steps of the sunrise effect are always before ground sunrise, while the start of the main step is distributed on both sides of it. During sunset hours, the onset time of the main step is before the local ground sunset, but for the second and third steps, the onset times are after that. The results are interpreted by considering the multi-hop propagation as well as the distributed noise sources surrounding the observing site.
-
It is shown that the equations governing low-frequency acoustic-gravity waves in a stable stratified atmosphere can have localized dipole-vortex solutions (modons). They propagate in the horizontal direction with a speed that is larger than that of all possible linear internal waves.
-
Modelling and observational evidence indicate that interannual variabilities of dynamic height and sea surface temperature (SST) in the eastern part of the tropical Atlantic Ocean (Gulf of Guinea) are largely induced by preceding fluctuations in wind stress, mainly in the western equatorial basin. A wind-driven linear ocean model is used here to test the possibility of forecasting the abnormal dynamic heights. A control run of the model, forced by 1964–1993 wind stress monthly means, is first conducted. Yearly test runs (1964–1994) are subsequently performed from January to August by forcing the model with observed winds from January to May, and then by forcing with the May wind assumed to persist from June to August. During the last three decades the largest deviations of dynamic height simulated by the control run in the Gulf of Guinea in boreal summer would have been correctly forecast from wind data related only to conditions in May of each year. However, for weak climatic anomalies, the model may forecast overestimated values. For the most part (about 20 times during the last 30 years), the sign of the observed SST anomaly in the centre of the Gulf of Guinea during the boreal summer is identical to the sign of simulated anomalies of dynamic height deduced from both control and test runs. Along the eastern equatorial waveguide, the sea level forecasting skill slowly decreases from the first 2 weeks of June until the second 2 weeks of August, but remains high on both sides of the equator throughout boreal summer, as is expected from the adjustment in a linear ocean model. It is established that throughout the year in the Gulf of Guinea the accuracy of the 1-month forecast dynamic height anomaly provided by the simple linear method is greater than that of the 1-month forecast assuming persistence.
-
A numerical study of the development and structure of the sea breeze in Mallorca is presented using a meso-<beta> numerical model. The model includes a detailed representation of the soil and vegetation processes. The study covers a diurnal cycle. The results show that the model reproduces the main known features of the circulation and new ones appear, which seem to have an appreciable effect on the circulation during the decay of the sea breeze. The orography and soil dryness have been identified as the main factors determining the structure of the breeze. Three more experiments have been performed in order to isolate the effect of each factor.
-
Heat flux estimates obtained using the inertial dissipation method, and the profile method applied to radiosonde soundings, are assessed with emphasis on the parameterization of the roughness lengths for temperature and specific humidity. Results from the inertial dissipation method show a decrease of the temperature and humidity roughness lengths for increasing neutral wind speed, in agreement with previous studies. The sensible heat flux estimates were obtained using the temperature estimated from the speed of sound determined by a sonic anemometer. This method seems very attractive for estimating heat fluxes over the ocean. However allowance must be made in the inertial dissipation method for non-neutral stratification. The SOFIA/ASTEX and SEMAPHORE results show that, in unstable stratification, a term due to the transport terms in the turbulent kinetic energy budget, has to be included in order to determine the friction velocity with better accuracy. Using the profile method with radiosonde data, the roughness length values showed large scatter. A reliable estimate of the temperature roughness length could not be obtained. The humidity roughness length values were compatible with those found using the inertial dissipation method.
-
The two commonly used statistical measures of the air-sea heat flux, the sampling and classical means, have been compared using hourly reports over a 7-year-period from a weather ship stationed in the NE Atlantic. The sampling mean is the average over all flux estimates in a given period, where individual flux estimates are determined from ship reports of meteorological variables using the well-known bulk formulae. The classical mean is the flux derived by substituting period-averaged values for each of the meteorological variables into the bulk formula (where the averaging period employed is the same as that over which the fluxes are to be determined). Monthly sampling and classical means are calculated for the latent and sensible heat fluxes. The monthly classical mean latent heat flux is found to overestimate the sampling mean by an amount which increases from 1–2 W m<sup>–2</sup> in summer to 7 W m<sup>–2</sup> in winter, on average, over the 7-year-period. In a given winter month, the excess may be as great as 15 W m<sup>–2</sup>, which represents about 10% of the latent heat flux. For the sensible heat flux, any seasonal variation between the two means is of the order of 1 W m<sup>–2</sup> and is not significant compared to the interannual variation. The discrepancy between the two means for the latent heat flux is shown to arise primarily from a negative correlation between the wind speed and sea-air humidity difference, the effects of which are implicitly included in the sampling method but not in the classical. The influence of the dominant weather conditions on the sign and magnitude of this correlation are explored, and the large negative values that it takes in winter are found to depend on the typical track of the mid-latitude depressions with respect to the position sampled. In conclusion, it is suggested that sampling means should be employed where possible in future climatological studies.
