1. WHISPER Measurement of DC Magnetic field amplitude Cross Calibration Workshop 2-3 February 2006 AUTHORS: SURAUD Xavier, DECREAU Pierrette, MAZOUZ Farida, RAUCH Jean-Louis, Patrick CANU, ROCHEL Alban, TROTIGNON Jean-Gabriel, VALLIERE Xavier

2. OUTLINE 1)WHISPER Sounding Technique 2)Estimation of B field amplitude 3) Comparison withy FGM: Case events

3. WHISPER Sounding experiment Emission (time t, frequency f) Reception (time t + Δt, frequency f) PLASMA Emitted signal : sine wave train of central frequency Fc Frequency sweep: covered by 80 Fc values, 4 – 80 kHz range Time table tc (Fc): not a linear function (there and back goes) Time Table Frequency sweep f0 f1 t1t0t2 f2

4.  FFT range : [3, 80] KHz  FFT Resolution: 163Hz A slice in frequency, centred on emitted Fc, is selected after each sounding It represents the response of the plasma in this band. Reception and Wave Analysis The wave analyser: Time seriesFrequency spectrum Selected Frequency slice

5. WHISPER Sounding experiment An example of active spectrum SALSA 09/09/ 2000 00:09:18 Central triggered frequency The concatenation of triggered frequency slices forms the active spectrum This spectrum is dated from the central time of the sweep (duration 1.5s) Exact times of each reception (one per freq slice) are known

6. Fce harmonics Electron gyro-frequency estimation Resonance frequencies are known within +/- 163 Hz The harmonic n, yields Fce within +/- (163/n) Hz If the medium is stable during the sweep, a better precision can be achieved (correlation with a comb function ) Fce ( Hz) = 28 B (nT)

7. Compared WHISPER and FGM Fce estimations FGM data at 4sHigh res FGM data

8. ( 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 ) harmonic order 20020805 case study: Fce harmonics well separated from Fqn

9. (fce) (1)pre-treatment: (a) morphology (b) dynamics in a spectrum: position of resonances known at 163 Hz resolution (2) Hough transform: uses time continuity over a few spectra search for max correlation including position and slope of each ‘harmonic’ segment

10. WHISPER estimation of magnetic field modulus, compared to FGM measurement 20020805 case study: Position of each harmonic after pre-treatment Whisper resolution +/-0.5 nT

11. 20020725 case study: Whisper resolution Whisper and FGM compared time variations Conclusion: Whisper and FGM estimations are comparable

12. Perspectives: Could Whisper provide a useful reference of magnetic field value? Estimation from Fce harmonic of order n : ~1 nT at best (Higher orders perturbed by Fqn) Usage of full spectral information (when FGM measure is stationary over sweep): ~ 0.3 nT Image processing: Hough transform: to be determined Usage of statistical information (over few hours): ~ 0.1 nT should be achievable Confidence interval?