1. 1 Two questions still of the day: the Gendrin angle, the R.Gendrin view on the positioning of URSI F. Lefeuvre LPCE / CNRS – Univ Orléans

2. 2 The Gendrin angle

3. 3 θ 2 is the θ g Gendrin angle R. Gendrin, Planet Space Sci., 1961 From an Heliwell (1965) figure Notations θ = (k, B 0 ) f ce electron gyrofrequency f pe electron plasma frequency f H + proton gyrofrequency f LHR ~{f ce f H + } 1/2 lower hybrid resonance v g group velocity Condition for a propagation along B 0 (without duct) at f LHR < f < f ce /2 (W mode) (1) sin θ = 0 v g = ξ (f,f ce, f pe ), an isotropic emission is easily observable (2) cos θ = 2f/fce (θ = θg) all frequencies are propagated at the same v g [v g = 0.5 c (fce/fpe)], due to energetic considerations, isotropic emissions are not easily observable but emissions at cos θ = 2f/fce (e.g. Cerenkov emissions) may be observed

4. 4 R.A. Helliwell, Geophys. Res. Lett., 1995 The Gendrin mode of propagation is capable of supporting a new kind of wave particle interaction involving longitudinal resonance and called the Impulsive Wave Instability (IWI) The efficiency of the IWI instability is maximum when v g decreases, i.e. when the waves approach the equator.

5. 5 Observations of the Gendrin mode on satellite data - few identifications of propagation at the Gendrin angle for wave analysis performed under the plane wave hypothesis - numerous identifications for wave analysis performed from the Storey and Lefeuvre (1979, 1980) Wave Distribution Function (WDF) concept (distribution of the wave energy density as regards to the polar angle θ and the azimuthal angle Φ) Parrot and Lefeuvre (1986) In the case of a two-peaked WDF, the plane wave hypothesis provides an estimate ~ at the barycentre of the two peaks. X

6. 6. Examples of Gendrin mode observations. Analysis of plasmaspheric hiss events during a ISEE-1 pass in the equatorial region ( 3 < L < 5.6, -4° < MLAT < 4°). Solid circles are for main peaks, open circles for secondary peaks. (Storey et al., 1991). θgθgθgθg θres Analysis of GEOS 1 chorus events Recorded at L ~7.6, MLAT ~ 17° ( Hayakawa et al., 1990). (X) refers to impulsive chorus, and (∆) to normal rising tones.

7. 7 The R. Gendrin view on the positioning of URSI

8. 8 The objective of a Scientific Union is to stimulate and to coordinate, on an international basis, studies in specific domains ( for URSI, studies in the fields of radio, telecommunication, and electronic sciences) Roger Gendrin stimulated and coordinated studies: ● at URSI - Commission H Chair (1975 – 1977) - member of a preparatory group (Geophysics and Probing group) for the 1987 Corsendonk conference - very involved referee for the URSI WP on SPS (2007) ● at IAGA / IUGG (International Association of Geomagnetism and Aeronomy / International Union f Geodesy and Geophysics) - President (1987 – 191)

9. 9 Important Actions conducted by R. Gendrin (at URSI and at IAGA) concerned the distinction to be made between studies of wave phenomena in natural plasmas associated with : - geophysics - radiophysics ● 1975 URSI GA - creation of Commission H (former Commission IV on the magnetosphere) - modifications in the scientific objectives of the Inter-Unions URSI-IAGA WG - creation of 5 WGs: Plasma Instabilities (with G), Antennae in Plasmas (with B), Wave Analysis (with C), Radiofrequency Plasma Devices (with D), Active Wave Experiments

10. 10 ● Preparation of the (1987) Corsendonk conference Fields in which IAGA needs URSI - plasma physics in general (B,H) - physics of the interaction between waves and plasmas (G,H) - waves and the Atmosphere (F); - equipments to study this physics or to diagnose the environment characteristics (antennas, detectors, radar systems, signal analysis, active wave experiments) Other environmental problems of use to URSI - consequences of the environment on problems posed by the Telecommunication Sciences: auroral propagation, ionospheric scintillations (G, H); boundary layer physics, rain effects (F) Fields which are more relevant to IAGA - geophysical processes per se: variation of electron content, interaction between ionosphere and thermosphere, ionospheric convection, auroral processes, magnetospheric structures, etc.

11. 11 ● Other contributions - with Prof. Maha Abdalla (UCLA) and Hiroshi Matsumoto (Kyoto Univ.), development of numerical simulations through international schools - very concerned referee for the URSI WP on SPS satellites In 1987, Roger Gendrin received the John Howard Dillinger medal for his contribution to the “Study of waves of natural origin propagating in the surrounding of the Earth, and their influence on the behavior of the magnetosphere”.