Observation of a Non Thermal Continuum radio event during the CLUSTER Tilt campaign

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

2. OUTLINE • Observations • Analysis • Results Uppsala May 2009 2/13

3. Observations: Non Thermal Continuum form analyzed F (kHz) Magnetosheath Magnetosphere 60 Escaping NTC 40 Trapped NTC Cusp 20 2 00:30 03:50 07:10 UT Gurnett, 1975 26/02/2001 Introductory remarks • 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. • Cluster observations largely confirm this view Uppsala May 2009 3/13

4. Observations: Non Thermal Continuum form analyzed C4 C3 Z X Y 28 August 2008 A new form of NTC revealed by CLUSTER • 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 Fce. C3 C4 Medium latitude plasmasphere Uppsala May 2009 4/13

5. Observations: Non Thermal Continuum form analyzed 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 fce = 3 kHz) • Observer’s position: dawn lobes; Cluster heads toward dusk plasmasphere Z X GSM Y Fuh Polar cap Fp Dawn Dusk Dusk Dawn Uppsala May 2009 5/13

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

7. Analysis Plume Shoulder ? Plume After Carpenter, 1970 Shoulder Motivation • 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 …). Uppsala May 2009 7/13

8. Analysis Strategy • 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 1 2 3 4 Iso-fce surface Uppsala May 2009 8/13

9. 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. k • Processing: The signal intensity measured in spin plane, modulated with antenna orientation, is characterized by three parameters (E02, α2, ), leading to possible orientations of meridian planes P4a & P4b. 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) Uppsala May 2009 9/13

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

11. Results BTsyganenko 4 3 2 1 kHz df Iso-fce Ray path in rotating meridian plane of SM coordinate system Source position • 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 Uppsala May 2009 11/13

12. 3) Results ? C4 MLT Wave source position: questions raised • 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? Uppsala May 2009 12/13

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 RE) 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