1. Observation of a Non Thermal Continuum radio event during the CLUSTER Tilt campaign 17th CLUSTER Workshop Uppsala, Sweden, 12 – 15 May 2009 Uppsala May 2009 1/13 Pierrette Décréau (1), Séna Kougblenou (1), Jean –Louis Rauch (1), Jean – Gabriel Trotignon (1), Xavier Vallières (1), Sandrine Grimald (2) and Patrick Canu (3) (1)LPC2E, Orléans, France (2)MSSL, Holmbury St Mary, United Kingdom (3)LPP, Vélizy, France

2. 1) Observations 2) Analysis 3) Results OUTLINE Uppsala May 2009 2/13

3. Gurnett, 1975  Pioneer studies show that NTC radiation emerges from sources at PlasmaPause boundary layer, beaming predominantly near equatorial plane. The lower frequency band is trapped inside the magnetospheric cavity. 26/02/2001 Magnetosphere Trapped NTC Cusp Escaping NTC 20 40 60 Magneto sheath F (kHz) 2 00:30 03:50 07:10 UT Cluster observations largely confirm this view Introductory remarks 1) Observations: Non Thermal Continuum form analyzed Uppsala May 2009 3/13

4. Cluster revealed a new form of NTC: banded emissions (Grimald et al., 2008) The plasmapause radiates over a large latitudinal range. Waves emitted (at local plasma frequencies) are banded at local F ce. Medium latitude plasmasphere 28 August 2008 X Z Y C4C3 C4 A new form of NTC revealed by CLUSTER 1) Observations: Non Thermal Continuum form analyzed Uppsala May 2009 4/13

5. Case event presentation : spectral and orbit characteristics Date: 18/05/2008 (tilt campaign); 12:00 -12:30 UT Spectral features: bands in the 15 – 25 kHz range; df ~ 2.5 kHz (< local f ce = 3 kHz) Observer’s position: dawn lobes; Cluster heads toward dusk plasmasphere F uh Dawn Dusk Z Y X GSM Polar cap 1) Observations: Non Thermal Continuum form analyzed FpFp Uppsala May 2009 5/13

6. Magnetic activity: quiet period, preceding a moderate perturbation Expanded plasmasphere (outer boundary at L ~ 8 in dusk sector) Case event presentation : geophysical context 1) Observations: Non Thermal Continuum form analyzed Uppsala May 2009 6/13

7. NTC bands provide remote information of a plasmapause knee feature (at source) which we attempt to localize, in order to learn about plasmaspheric dynamics. In this event, geomagnetic conditions announce a possible drainage of the plasmaspheric body. The frequency range of NTC is associated to a PPause knee at low densities (< 10 cm-3). Such features could not be observed by IMAGE, which nevertheless revealed complex plasmasphere contours (plume, shoulder, fingers …). Motivation 2)Analysis After Carpenter, 1970 Shoulder ? Plume Shoulder Plume Uppsala May 2009 7/13

8. The frequency distance between bands gives access to magnetic field magnitude local to the source, i. e. an ‘iso-fce’ surface which can be modeled in 3D; 3D ray path orientation (from tilted SC pair) points to the source position Directivity in spin planes of the two other SC add supporting information 2) Analysis Strategy 1 2 4 3 Iso-f ce surface Uppsala May 2009 8/13

9. 2) Analysis Wave measurements from a tilted pair Assumption: E field combines two linear fields, au and bv, oscillating in quadrature. Same conditions at C3 and C4.  Processing: The signal intensity measured in spin plane, modulated with antenna orientation, is characterized by three parameters (E 0 2, α 2,  ), leading to possible orientations of meridian planes P 4a & P 4b. Combining C3 and C4 possible meridian planes lead to candidates au / bv pairs, tested regarding the main assumption. Either no solution is obtained, or an optimal solution validated. Results: test the assumption; estimate au and bv quantities; obtain k orientation (all vectors with sign ambiguity) k Uppsala May 2009 9/13

10. azimuth Uppsala May 2009 10/13 3) Results Spin modulation on all SC E field analysis on C3-C4 pair: coherent with plane wave elliptic polarization (e = 0.85) k el within a cone of ~10° half angle Assumption of circular polarization: k circ at ~ 10° from k el Directivity analysis azimuth  ( [0, π[) k orientation (GSE coordinates) zenith angle  ([0, π[ ) 90 100 110 120 130 140 (deg) 40 0 20 0  

11. k orientation is measured at (C3_C4) position at 4.6 MLT, 12:20 U :  = 126° ;  = 119° in the band 15 – 25 kHz. a rectilinear ray path (free space approximation) cuts successive meridian planes (SM coordinates) until the gyro- frequency of a modeled magnetic field meets the df band spacing the (unrealistic) dipole model indicates a source near the equator beaming in an unrealistic way towards the observatory Tsyganenko model – not including By IMF component - points to a source at L=11, MLT = 6.6, beaming in a realistic way directivity on other SC indicates similar  values –compatible with above all uncertainty factors cumulated lead to an unprecise source position – could be refined 3) Results Uppsala May 2009 11/13 Source position Ray path in rotating meridian plane of SM coordinate system Iso-f ce 1 kHz 2 3 4 B Tsyganenko df

12. Wave source position: questions raised 3) Results Uppsala May 2009 12/13 The source is found at L = 11, not far from a connection with the low latitude boundary layer. What is the actual topology? Are field lines at source close/open? (could be checked with Superdarn) The ray path orientation between source and Cluster indicates the presence of a shoulder, ~ MLT aligned.. What is the actual 3D shape of plasmasphere? ? C 4 MLT

13. Conclusion Search of the source of a banded NTC wave observed from the polar cap (4.6 MLT) by CLUSTER tilted pair Tilted pair allows to measure wave polarization characteristics, confirming propagation in an L_O mode Path direction is estimated in 3D (with a 180° ambiguity) Use of a realistic static magnetic field model points to a region source at high L value in the dawn (6.6) MLT sector the low average density (~ 5 cm -3 ) at the source, is observed at medium latitudes (Z = -4.5 R E ) along a (closed?) flux tube connected to the low latitude boundary layer (LLBL) Those results confirm that the plasmapause radiates NTC waves at medium latitudes, in this case in the MI coupling region. Uppsala May 2009 13/13