Observation of a Non Thermal Continuum radio event during the CLUSTER Tilt campaign 17th CLUSTER...

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

1) Observations

2) Analysis

3) Results

OUTLINE


Y

X

Y

X

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

Magnetosheath

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


• 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.

Medium latitude plasmasphere

28 August 2008

X

Z

Y

C4 C3

C3

C4

A new form of NTC revealed by CLUSTER



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

Fuh

DawnDawn

DuskDusk

Z

Y

X GSM

Polar cap


Fp


• Magnetic activity: quiet period, preceding a moderate perturbation

• Expanded plasmasphere (outer boundary at L ~ 8 in dusk sector)

Case event presentation : geophysical context



• 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


•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

43

Iso-fce surface


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 (E0

2, 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)

k


azimuth


3) Results

Spin modulation on all SCE field analysis on C3-C4 pair: coherent with plane wave elliptic polarization (e = 0.85) kel within a cone of ~10° half angleAssumption of circular polarization: kcirc at ~ 10° from kel

Directivity analysis

azimuth ( [0, π[)

k orientation (GSE coordinates)

zenith angle ([0, π[ )

90 100 110 120 130 140 (deg)

400

40

20

20

0

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


Source position

Ray path in rotating meridian plane of SM coordinate system

Iso-fce

1 kHz2

34

B Tsyganenko

df

Wave source position: questions raised

3) Results


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?

?

C4 MLT

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.


Observation of a Non Thermal Continuum radio event during the CLUSTER Tilt campaign 17th CLUSTER...

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