R. Rüster

Absolute reflectivities and aspect sensitivities of VHF radio wave scatterers measured with the SOUSY radar

Abstract By accurately calibrating the SOUSY radar in West Germany it has been made possible to measure absolute values of effective reflection coefficients and turbulence structure constants. Some typical values of these parameters as a function of altitude are presented. Such profiles are presented for both a vertically directed beam, and also for two beams directed 7° off-vertical. Comparisons of powers on the vertical and off-vertical beams show that scatter became more aspect sensitive at the tropopause and in the lower stratosphere, but, unexpectedly, scatter was observed to become considerably more isotropic in the higher regions of the stratosphere (above 15–18 km) on this occasion. An enhancement of signal from the tropopause occurred not only on the vertical beam, but also on the off-vertical beams. Comparisons of signal strengths scattered from the mesosphere and measured with the vertical and off-vertical beams showed that for the present observations mesospheric scatter was close to isotropic. The backscatter cross-sections at VHF have been compared with other measurements at medium and high frequencies at other locations, and these comparisons help set some limits on the scales of turbulent and specular scatterers in the mesosphere.

Complementary Code and Digital Filtering for Detection of Weak VHF Radar Signals from the Mesosphere

Measurements of the structure and the dynamics of the middle atmosphere with a fine height resolution have been carried out using the SOUSY-VHF-Radar (SOUSY = SOUnding SYstem), operated by the Max-Planck-Institute for Aeronomy in the Harz mountains in Germany. Since the echoes from the middle atmosphere are coherent within a time scale of the order of a second, the received signals are over-sampled and added coherently for that time period. This integration, which is equivalent to a digital combfiltering, improves the signal-to-noise ratio. It is carried out by a hardware adder. In order to detect echoes from the mesosphere, an increase in the effective radiated power is necessary. By applying a 32-element complementary code to long transmitted pulses, the average power is increased and radar returns from the height range between about 65 and 90 km have been recorded with a height resolution of 300 m. The special hardware and software used is described. Some observational results from mesospheric measurements are presented.

Remote sensing of the atmosphere by VHF radar experiments

Problems in atmospheric dynamics, in particular in micro- and mesoscale processes, which can be solved by radar investigations, are summarized. The method of atmospheric radar experiments is described. Some relevant results obtained with the new SOUSY-VHF-Radar are presented, including observations of a warm-front passage, of layered structures, and of vertical and horizontal velocities in the troposphere. A power profile of radar echoes from heights up to the stratosphere proves that the SOUSY-VHF-Radar in its final operating state will be able to investigate structures and dynamics in the mesosphere too.

VHF radar observations of nonlinear interactions in the summer polar mesosphere

Abstract VHF radar measurements of velocities and echo power in the summer polar mesosphere have been analysed using maximum entropy, bispectral and cross-spectral methods in order to study wave-wave interactions. The results show nonlinear interactions of second and even third order between the diurnal and semidiurnal tides and planetary waves with periods of 2 and 3 days in the velocity field. Similar analyses of time series of echo power suggest corresponding variations of the temperature field.

Mean state densities, temperatures and winds during the MAC/SINE and MAC/EPSILON campaigns

Abstract During 1987 two major field campaigns were conducted, mainly in northern Norway (in summer and late autumn), in which a total of 41 (26+15) in-situ temperature profiles were obtained by different techniques such as passive falling spheres, ionizalion gauges and mass spectrometers. Simultaneously, ground-based measurements of OH-temperatures and sodium lidar temperatures were performed for approximately 85 h and 104 h, respectively. In addition, a total of 67 (37 + 30) wind profiles were measured by in-situ techniques. Several radar systems measured winds almost continuously before, during and after the campaigns. The mean temperature profile for the summer campaign showed major deviations from a recently published reference atmosphere (CIRA 1986), whereas the differences between observations and model are smaller in autumn. In general, both the summer and autumn mean wind profiles agreed with CIRA 1986. Minor differences were attributed to tidal biases of the observations and ageostrophic components.

The effects of thermospheric winds on the ionosphere at low and middle latitudes during magnetic disturbances

Abstract Thermospheric temperature changes associated with the preferential heating of the atmosphere at high latitudes during magnetic disturbances are shown to produce an additional storm-time equatorward wind of about 50 m s −1 at lower-middle latitudes. This is sufficiently large to substantially alter the normal daytime thermospheric winds at these latitudes; during equinox and winter conditions, for example, the normal poleward daytime wind is reversed. This storm-time wind opposes the poleward transport of ionization responsible for the equatorial anomaly and it will thus influence the latitudinal variation of electron concentration at lower-middle latitudes. It is suggested that the effect of the storm-time wind will be to produce negative storms during the day in summer; in winter positive storms may be expected during minor disturbances and negative storms during major disturbances.

SOLUTION OF THE COUPLED IONOSPHERIC CONTINUITY EQUATIONS AND THE EQUATIONS OF MOTION FOR THE IONS, ELECTRONS, AND NEUTRAL PARTICLES.

Abstract The time-dependent continuity equations for four different ion species and the equations of motion for the ions, electrons and neutral particles are discussed and a method of solving them numerically is described. The calculations include the effects of neutral air winds, electric fields, viscosity, Coriolis force, geomagnetic declination, a time-dependent incoming flux and different electron, ion and neutral particle temperatures. The various assumptions which have to be made in order to solve the equations and some limitations of the present work, are also discussed.

VHF radar observation of wave instability and turbulence in the mesosphere

Abstract At mesospheric heights, VHF radar measurements reveal strong signal power bursts which have the same period as simultaneously observed short-period velocity oscillations. Both the power bursts and the velocity oscillations occur in layers of maximum vertical wind shear generated by tidal or long-period gravity waves with apparent vertical wavelengths of the order of 10 km. A comparison with similar power bursts measured in the troposphere during a jet stream passage leads to the conclusion that the short-period velocity oscillations are due to a Kelvin-Helmholtz instability. This instability in turn generates superadiabatic lapse rates so that strong turbulence can occur which produces the observed signal power bursts.

VHF radar observations in the summer polar mesosphere indicating nonlinear interaction

Abstract VHF radar observations at mesospheric heights have been made during the MAC/SINE campaign in summer 1987 in Northern Norway. For a period of 32 days one hour averages of mesospheric velocities within the height range from about 80 to 90 km are derived and analysed using various spectral methods. The mean zonal winds clearly reveal a strong summer westward circulation of up to 40 m/s. The meridional flow, being smaller (∼ 10 m/s), is directed equatowards. Tidal motions mainlyconsist of the dominant semidiurnal and the weaker diurnal tide with amplitudes of about 15 m/s and 10 m/s, respectively, but slow a relatively strong temporal variability. Using the maximum entropy method variations with periods of about 2 d and 16 h are also observed. Bispectral analysis indicates wave-wave interaction.

VHF radar observations of tides at polar latitudes in the summer mesosphere

Abstract Doppler measurements at mesospheric heights were carried out in June 1984 using the mobile SOUSY VHF radar at Andenes/Norway (69°N, 16°E). The temporal wind variations at heights between 80 and 95 km reveal wave motions with periods of about 12 h, 14.5 h, 24 h and 54 h. The propagating semi-diurnal tide has amplitudes up to 20 m s −1 and a vertical wavelength of about 30 km. The diurnal tide is much weaker and the phase shows a more complex variation with height. The 14.5 h period motion reveals a maximum amplitude of 15 m s −1 at about 84 km and a vertical wavelength of 15 km.