An empirical model of the Earth's horizontal wind fields: HWM07

The new Horizontal Wind Model (HWM07) provides a statistical representation of the horizontal wind fields of the Earth's atmosphere from the ground to the exosphere (0-500 km). It represents over 50 years of satellite, rocket, and ground-based wind measurements via a compact Fortran 90 subroutine. The computer model is a function of geographic location, altitude, day of the year, solar local time, and geomagnetic activity. It includes representations of the zonal mean circulation, stationary planetary waves, migrating tides, and the seasonal modulation thereof. HWM07 is composed of two components, a quiet time component for the background state described in this paper and a geomagnetic storm time component (DWM07) described in a companion paper

Power spectrum analysis of ionospheric fluctuations with the Murchison Widefield Array

Low-frequency, wide field-of-view (FOV) radio telescopes such as the Murchison Widefield Array (MWA) enable the ionosphere to be sampled at high spatial completeness. We present the results of the first power spectrum analysis of ionospheric fluctuations in MWA data, where we examined the position offsets of radio sources appearing in two data sets. The refractive shifts in the positions of celestial sources are proportional to spatial gradients in the electron column density transverse to the line of sight. These can be used to probe plasma structures and waves in the ionosphere. The regional (10–100 km) scales probed by the MWA, determined by the size of its FOV and the spatial density of radio sources (typically thousands in a single FOV), complement the global (100–1000 km) scales of GPS studies and local (0.01–1 km) scales of radar scattering measurements. Our data exhibit a range of complex structures and waves. Some fluctuations have the characteristics of traveling ionospheric disturbances, while others take the form of narrow, slowly drifting bands aligned along the Earth's magnetic field

Mesure de la pluie avec un radar spatial : étude de la contamination par l'écho de surface dans les lobes secondaires d'antenne

On considère le cas d'un radar (14 à 35 GHz) de mesure des précipitations opérant depuis une plate-forme satellitaire, tel celui défini pour le projet de mission spatiale B.E.S.T., en utilisant une antenne à ouverture réelle. Dans certaines configurations, le signal pluie provenant des échos reçus par le lobe principal dans les diverses portes de distance peut être contaminé, voire occulté, par les échos rétrodiffusés sur la surface (terre ou océan), via les lobes secondaires de l'antenne si le niveau de ces derniers n'est pas assez faible. On présente une étude analytique générale de ce problème reliant les spécifications sur le diagramme de gain d'antenne aux taux de contamination possibles. Les résultats sont analysés en fonction des paramètres géométriques de la visée, des caractéristiques de définition de l'instrument, de la structure des précipitations interceptées et des propriétés de la surface

Study of an airborne doppler radar Project Asterix

Translated from French (Cent. Rech. Phys. Environ. Terr. Planet., Tech. Note CRPE--159, 1987)SIGLEAvailable from British Library Document Supply Centre- DSC:9022.552(MO-Trans--1667)T / BLDSC - British Library Document Supply CentreGBUnited Kingdo

The RONSARD radars : a versatile C band dual Doppler facility

In this paper the authors describe a System of two Doppler radars (the RONSARD System) operated for the study of precipitating clouds (fronts and convective storms). In a first part, emphasis is placed on the System requirements concerning both transportability and flexibility, due to the wide range of atmosphere phenomena to be observed. These requirements lead to an original implementation of the aerial, to a special design of the receiving chain (dual polarization capability, wide dynamic range, absolute self calibration capability) and to the implementation of real time digital subsystems (Amplitude and Phase Memory, Ground clutter cancellor, Fourier Transform processor, Ambiguous Velocity corrector and a Spectrum Moment Computer). The second part is devoted to a brief review of the scientific programs involving the RONSARD System and to the présentation of some preliminary results

F-Region Electric Fields from the St-Santin Multistatic Incoherent Scatter Facility

The French Incoherent Scatter facility was extended to a tristatic system in December 1972 and to a quadristatic one in October 1973. As each new receiver could supply data for one more component of the ion drift velocity, it became possible to measure first one component of the electric field and later the full electric field vector, in a plane perpendicular to the magnetic field. The conjugate sunrise behaviour of the electric field appears to be of particular interest. Indeed, the location of the French observatory is such that during the winter solstice the time difference between conjugate and local sunrise reaches a maximum of 4 hours thus allowing for an easy separation of the two effects. Under such conditions the occurrence of conjugate sunrise is characterized by a rapid change of the electric field correlated with the appearance of the phenomenon of spread F on the ionograms. These observations are discussed in the light of the theoretical work of H. C. Carlson and J. C. G. Walker on the electrodynamic drift in the nocturnal F region caused by a rapid change in the electrical conductivity of the conjugate ionosphere at the time of sunrise. The day time electric fields have a magnitude which is compatible with what is expected from either the dynamo or the magnetospheric electric field current theoretical calculations