Electron precipitation in the post midnight sector of the auroral zones

Comprehensive measurements of the angular distributions and energy spectra of electron intensities with electrostatic analyzer arrays on board the low-altitude satellite Injun 5 are reported. These are for the post-midnight sector of the auroral zones during the high-intensity events accompanying magnetic substorms. Precipitation features on closed terrestrial field lines well equatorward of the trapping boundary for energetic electrons with E greater than 45 keV were examined. No evidences of maxima in the differential energy spectra or of strongly field-aligned currents which are indicative of quasi-static electric fields aligned parallel to the geomagnetic field were found. Precipitation of low-energy electron intensities fluctuated on time scales greater than 2 seconds as viewed at the satellite position. This precipitation was characterized by isotropy for all pitch angles outside the atmospheric backscatter cone

Polar plasmas as observed by Dynamics Explorers 1 and 2

Plasma measurements from the Dynamics Explorer 1 and 2 satellites were used to characterize the polar cap environment. Analysis of numerous polar-cap passes indicate that, in general, three major regimes of plasma exist: (1) polar rain--electrons with magnetosheath-like energy spectra but much lower densities, most intense near the cusp and weakening toward the central polar cap; (2) polar wind--low energy upward flowing ions with both field-aligned and conical distributions; and (3) acceleration events--sporadic events consistent with the acceleration of electrons and positive ions by parallel electric fields. (1) to (3) were observed at high altitudes by Dynamics Explorer 1, while (1) and (3) were also observed at low altitudes by Dynamics Explorer 2. The plasma parameters associated with these plasma regimes are presented and discussed in terms of source and acceleration mechanisms

Kilometric radiation power flux dependence on area of discrete aurora

Kilometer wavelength radiation, measured from distant positions over the North Pole and over the Earth's equator, was compared to the area of discrete aurora imaged by several low-altitude spacecraft. Through correlative studies of auroral kilometric radiation (AKR) with about two thousand auroral images, a stereoscopic view of the average auroral acceleration region was obtained. A major result is that the total AKR power increases as the area of the discrete auroral oval increases. The implications are that the regions of parallel potentials or the auroral plasma cavities, in which AKR is generated, must possess the following attributes: (1) they are shallow in altitude and their radial position depends on wavelength, (2) they thread flux tubes of small cross section, (3) the generation mechanism in them reaches a saturation limit rapidly, and (4) their distribution over the discrete auroral oval is nearly uniform. The above statistical results are true for large samples collected over a long period of time (about six months). In the short term, AKR frequently exhibits temporal variations with scales as short as three minutes (the resolution of the averaged data used). These fluctuations are explainable by rapid quenchings as well as fast starts of the electron cyclotron maser mechanism. There were times when AKR was present at substantial power levels while optical emissions were below instrument thresholds. A recent theoretical result may account for this set of observations by predicting that suprathermal electrons, of energies as low as several hundred eV, can generate second harmonic AKR. The indirect observations of second harmonic AKR require that these electrons have mirror points high above the atmosphere so as to minimize auroral light emissions. The results provide evidence supporting the electron cyclotron maser mechanism

Region-based representation of video sequences with uniform background motion for a content-based image coding

International audienceContent-based image coders become the center of attention now for the currently emerging standard MPEG-4. A method based on the spatio-temporal segmentation for motion image coding is developed in this paper. The method is designed for the sequences characterized by homogeneous global motion (camera motion) and the presence of semantic objects having proper motions. The property of the method is the fitness of the moving border to spatial contours of regions which allows for a high quality of predicted images without any error encoding

Region-based coding scheme for the transmission of video sequences via channels of varying very low bit rate

International audienceRegion-based coding methods can provide a solution of maximal quality for the transmission of video sequences through channels of low bit-rate due to their 'scene- adaptive' nature. Here, the visual quality of frames predicted by motion compensation on nearly semantic homogeneous regions is better than for the conventional block-based and quad-tree based methods. If the error signal is not encoded, the most bit-consuming component is the description of the regions borders and of the topology of the segmentation map. A planar graph with polygonal arcs is used to represent the geometrical form and relations of regions in each frame. A method allowing to adapt the segmentation description to the variable available bit-rate is proposed based on rate-distortion theory and constrained optimization. The method uses the concept of description layers. The 'basic' layer contains only the triplet nodes of the graph and the vertices of the highest contrast and curvature. The 'maximal enhancement' layer contains all the nodes and polygonal vertices of the segmentation graph. The choice of 'optimal' layer for each polygonal arc is done independently, minimizing Lagrangian cost function. The last one combines rate and distortion measures. The entropy estimates for the encoding of all vertices of a given arc are taken as the rate measure. The distortion measure is the total sum of squared DFD in the area delimited by the basic layer and maximal enhancement layer for a given arc. The experiments on the 420-625 CCIR videoconference sequence showed a 30 percent decrease of the bit-rate for an unnoticeable loss in quality