Uprising whistlers, traversed the ionosphere and recorded on LEO satellites were widely thought to propagate almost vertically in the plasma. This picture involves, that the signal might propagate at a very large angle to magnetic field lines - at low and middle latitudes. Our real UWB, full-wave solution for oblique e.m. wave propagation in an anisotro- pic plasma [2] makes it possible properly to model the effect of the angle between the direction of the propagation and DEMETER launched on June Orbit parameters: micro-satellite (130 kg) low-altitude: 710 km inclination: 98.3º Scientific Payload: ICE, 3 electric sensors from DC up to 3.5 MHz, IMSC, 3 magnetic sensors from a few Hz up to 18 kHz, IAP, an ion analyzer, IDP, an energetic particle detector, ISL, a Langmuir probe ABSTRACT: The dispersion of fractional-hop whistlers crossing the ionosphere is determined by the propagation character- istics, physical conditions along their paths. Low-Earth- orbit (LEO) satellites provide a unique opportunity to monitor the ionosphere by recording and analyzing whistlers with different dispersions. Although this app- roach is an effective way to derive ionospheric paramet- res and their spatial and temporal distribution, despite the numerous satellite experiments fractional-hop whist- lers have been poorly analyzed and applied as remote sensing technique in the field of ionospheric research. The dispersion values of the vast majority of whistlers recorded on board of LEO satellites appear to fall into distinct clusters. The largest of these dispersion values are thought to correspond to one-hop whistlers excited in the magnetic conjugate region and reached the satel- lite position after long plasmaspheric propagation along geomagnetic field lines. Whistlers exhibiting the smal- lest dispersion are widely accepted as fractional-hop ones reaching the satellite almost vertically. As an unex- plained phenomenon a large number of whistlers, form- ing a clear cluster in the dispersion distribution may not be classified into any of these categories. Fractional-hop whistlers recorded on board the DEMETER satellite were possible to model and interpret applying our full-wave impulse propagation model describing obliquely propa- gating signals on a basis of ionospheric and magnetic field standard model. The results yielded a basically new picture of ionospheric whistler propagation. Dispersions of fractional-hop whistlers themselves exhibit bimodal distribution, varying with magnetic latitude of the satel- lite. The diverse dispersion values correspond to diffe- rent propagation directions from the lower ionosphere to the satellite and thus different angles between the direction of the propagation and the geomagnetic field along the path. Realistic, UWB modeling of a great number of whistlers appeared on the DEMETER ICE VLF recordings, acqui- red at large latitudinal range proved that the angle of oblique propagation is responsible for diverse disper- sions of fractional hop whistlers. The most probable 3D propagation directions of the fractional-hop whistlers in the ionosphere in the vicinity of the satellite can be determined at a given magnetic latitude. Acknowledgements: This work has been carried out by the R&D (OTKA) funds No. T and F and by the support of the Hungarian Space Office. As guest investigators of the DEMETER mission special thanks to the CNES for the supplied satellite data. References: [1] Hughes, A.R.W., Rice, W.K.M., (1997) A satellite study of low latitude electron and proton whistlers, J. Atmos. Sol- Terr. Phys., 59, 10, [2] Ferencz, Cs., O.E. Ferencz, D. Hamar and J.Lichtenberger (2001), Whistler Phenomena. Short impulse propagation, 260pp., Kluwer Academic Publishers, Dordrecht [3] Lichtenberger, J., Cs. Ferencz, D. Hamar, P. Steinbach, L. Bodnár (2004) Automatic whistler detection and analyzing system, Geophys. Res. Abs., Vol 6, Oblique whistler propagation in the ionosphere - results of the first application of oblique impulse propagation model on DEMETER burst recordings Steinbach 1, P., Ferencz 2, O.E., Ferencz 2, Cs., Lichtenberger 2, J., Hamar 2, D., Berthelier 3, J.J., Lefeuvre 4, F., Parrot 4, M. 1) Research Group for Geoinformatics and Space Sciences, Hungarian Academy of Sciences, Budapest 2) Space Research Group, Institute of Geography and Earth Sciences, Eötvös University, Budapest URL: 3) CETP/CNRS St. Maur, France 4) LPCE/CNRS Orléans, France Abstract No.: EGU06-A The difference of whistler dispersion values, occurring often overlapped on satellite wide- band recordings were analyzed and interpreted as up- rising and short-path one-hop signals, based only on low latitude data [1]. By extending the range of the satellite orbits to higher latitudes, and thus the analyzed whistler cases, our study clearly showed, that the dispersion values are far less than expected in the case of longitudinally propagated one-hop whistlers. The almost constant LEO altitude of the DEMETER satellite supports to follow and to describe the magnetic latitude dependence of the whistler dispersions. Dispersion values of analyzed whistlers exhibit clear bimodality at a given latitude with symmetry to the geomagnetic equator. (□ symbols represent mirrored points) The variation of distinct whistler dispersion values with the magnetic latitudes, along an equatorial orbit of the DEMETER satellite can be traced on the three recording spectra below. the magnetic field. (Simulated fractional-hop whistler spectra can be seen on the right. The red trace represents the longitudinal, while the others the increasingly oblique propagation cases up to 60º.) Large number of whistlers, detected [3] on DEMETER burst VLF recordings were analyzed in order to find the most probable paths of propagation up to the satellite and to explain the source of different dispersions. For this reason the full-wave solution has been applied and the crossed medium characterized by the IRI and IGRF standard models along the calculated paths in the vicinity of the satellite. The best fit results (see example bottom right) have drawn the most likely directions of the propagation, yielding a new description of ionospheric VLF propagation. Our results assure, that in a magnetoionic medium the propagation direction with respect to the magnetic field is a fundamental factor of the real waveform. Signals, supposed to propagate almost perpendicular to the magnetic field would have strongly dispersed shape and also small amplitude. Distinct regions can be drawn in the lower ionosphere, where from the propagation is probable or unlikely, they are determined by the satellite position and the local geomagnetic inclination – as seen below on the spatial distribution of the simulated oblique propagation angles around a satellite taken at 30ºN.