CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE Laboratoire de
CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE Laboratoire de Physique et Chimie de l'Environnement et de l’Espace 3 A, avenue de la Recherche Scientifique F-45071 Orléans cedex 02, France WHISPER Instrument and Data Products in CAA Jean Gabriel Trotignon, Xavier Vallières, and Gabor Facskó HIA and WHISPER Data Comparison Meeting Bucharest, 21 -22 January 2009 Phone: (33 2) 38 25 52 63; Fax: (33 2) 38 63 12 34; E-mail: Jean-Gabriel. Trotignon@cnrs-orleans. fr
CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE Laboratoire de Physique et Chimie de l'Environnement et de l’Espace 3 A, avenue de la Recherche Scientifique F-45071 Orléans cedex 02, France WHISPER Instrument and Data Products in CAA Jean Gabriel Trotignon, Xavier Vallières, and Gabor Facskó Presentation outline 1. About WHISPER Experiment 2. WHISPER CAA products 3. Density Determination Methods 4. WHISPER Data Examples Conclusion Phone: (33 2) 38 25 52 63; Fax: (33 2) 38 63 12 34; E-mail: Jean-Gabriel. Trotignon@cnrs-orleans. fr
1. About WHISPER Experiment Waves of HIgh frequency and Sounder for Probing of Electron density by Relaxation It aims at total electron density evaluation and natural wave monitoring in the 2 -80 k. Hz range; It consists of a pulse transmitter, a sensitive radio receiver, a digital spectrum analyser and a controller unit; It uses the two EFW double sphere antennae to transmit and receive electric signals. EFW E-Field Antennae
The two long double sphere electric antennae of EFW are needed to transmit and receive Efield signals used by WHISPER. They consist of 4 orthogonal cable booms, carrying spherical sensors and deployed in the S/C spin plane. These Ey and Ez antennae have sphere-to-sphere separations of 88 m. The 8 cm in diameter spheres are made of aluminium and are coated by a conductive paint. The spheres each contain a high-impedance preamplifier that provides signals to the electronics boards and in particular to the WHISPER receiver one. Boom cables are composed of several wires, a coaxial, a kevlar braid, and a conductive outer braid. WHISPER transmitter is connected to the outer braids of Ey pair of booms.
1. About WHISPER Experiment (cont’d) Sounding Technique Frequency sweep fn fn+2 fn+1 fn+3 Reception at time t + Δt ü Sine wave-train sent through outer braids of Ey pair of booms during a short time (for example 1 ms) at a given central frequency fn (from a given Table); ü A few ms later, Whisper listens to the plasma response; ü The process is repeated, step by step, until the whole frequency range is covered (from 4 k. Hz to 81 k. Hz). tn tn+2 tn+1 tn+3 From a given time Table
1. About WHISPER Experiment (cont’d)
1. About WHISPER Experiment (cont’d) Current Time Resolution Pattern Ø Active Mode: – Max frequency resolution: 163 Hz – Maximum frequency range: [4, 82] k. Hz Ø Natural Mode: – Max frequency resolution: 163 Hz – Maximum frequency range: [2, 80] k. Hz
2. WHISPER CAA products 2. 1 Natural Waves Related Archives 2. 2 Energy Related Archives 2. 3 Sounding Related Archives 2. 4 Electron Density Related Archives
2. 1 Natural Waves Related Archives Most of the time Whisper listens to natural waves Ø [2 k. Hz, 80 k. Hz] band Ø On one of the two EFW antenna pairs: Ey or Ez Ø Spectrum obtained by on-board FFT • 512 or 256 bins generally, 64 and 128 are also possible • 163 Hz resolution • Only amplitudes, phases are lost Ø Three gains available: 12 d. B, 24 d. B, 36 d. B • 24/12, 36/24 & 36/12 modes switch automatically from high gain to low gain whenever too many overflows are encountered
2. 1 Natural Waves Related Archives (cont’d) Ø Passive spectra cannot all be transferred due to TM limitation Ø Compression and selection required: § Accumulation of spectra Number of accumulated spectra called “Average Number” Some accumulated spectra are not transmitted § Compression Spectrum values: from 32 bits to 8 bits or 6 bits Ø Two types of natural waves archives: § CP_WHI_NATURAL Natural (accumulated) spectra § CT_WHI_NATURAL_EVENTS Processing mode changes
2. 1 Natural Waves Related Archives (cont’d) CP_WHI_NATURAL § Spectrum (central) time § Time uncertainty in s [due to accumulation] § § § FFT size Reception antenna Spectrum values (in V 2 m-2 Hz-1) Frequencies (in k. Hz) Overflow saturation flag Average number (Number of accumulated spectra) § Number of gain changes during acquisition § Sizes of mantissa and exponent of compressed data
2. 2 Energy Related Archives v In each natural spectrum acquisition/accumulation interval, total power energies are accumulated and transmitted § On-board compression from 32 to 8 bits v Two archives: § CP_WHI_WAVE_FORM_ENERGY § PP_WHI (raw estimate of density, energy in 3 freq. bandwidths)
2. 2 Energy Related Archives (cont’d) CP_WHI_WAVE_FORM_ENERGY Ø Spectrum (central) time Ø Time uncertainty (in s) [due to accumulation] Ø Energy (in V 2 m-2 Hz-1) Ø Spectrum code § 1 if natural, 0 if active (0 unused yet, only for some specific modes) Ø Average number § Number of accumulated energies Ø Overflow code § Indicates the number of overflows
2. 3 Sounding Related Archives How passive and active spectra are reconstructed (frequency sweep, step by step, from 4 k. Hz to 81 k. Hz and back and forth moves).
2. 3 Sounding Related Archives (cont’d) Ø All the active spectra cannot again be transferred due to TM limitation Ø Compression and selection required: § Values compression (6/8 bits) § Reduced frequency sweep range § Strategy A • Active bins are grouped by pairs, only the greater in the pair is kept. Spectra are then filled by linear interpolation on ground. • Active/Passive ratios are coded on 2 bits and transmitted rather than sending passive data § Strategy B • All active bins are transmitted and Active/Passive ratios sent § Strategy C • All Active and Passive bins sent § Strategy D • Only Active data are transmitted
2. 3 Sounding Related Archives (cont’d) Six types of archives: q CP_WHI_ACTIVE • Sounding spectra q CP_WHI_PASSIVE_ACTIVE • Passive spectra q CP_WHI_ACTIVE_TO_PASSIVE_RATIO • Active to passive ratio (4 possible values for each bin) q CT_WHI_ACTIVE_EVENT • Sounding mode changes q CP_WHI_HK • Elementary sounding events, extracted from WEC housekeeping files q CP_WHI_SOUNDING_TIMES • simplified version of CP_WHI_ACTIVE files, only keeping sweep central sounding times
2. 3 Sounding Related Archives (cont’d) CP_WHI_ACTIVE § Spectrum (central) time § Time uncertainty (in s) [due to acquisition and sweep] § Receiving antenna § Active spectrum values (in V 2 m-2 Hz-1) § Frequencies (in k. Hz) § Scanned frequency table gives swept frequencies and sweep order § Sounding interval (in ms) Time between 2 successive emissions in a sweep § Emission/Reception delay Time between emission and reception § Number of gain changes § Compression parameters: mantissa and exponent sizes
2. 4 Electron Density Related Archives CP_WHI_ELECTRON_DENSITY § Central time of the spectrum from which the density is derived § Time uncertainty § Spectrum type (Active or Natural) § Computational method Used algorithm code § External data used E for EFW § Electron density (in cm-3) § Electron density uncertainty (in cm-3) If available (depends on the algorithm) § Quality factor (between 0 and 1) If available, based on the local contrast of the peak/cutoff
3. Density Determination Methods Solar Wind & Magnetosheath Ø Only one strong resonance is usually excited at, or closed to, the plasma frequency from which the electron density is directly derived. Ø Natural noise (for example in foreshock regions) and possible interferences should be removed. Ø In a first approach, the most intense frequency bin may be retained (like in PPDB).
3. Density Determination Methods (cont’d) Outbound Shock crossings MGN Resonance at Fpe Electromagnetic emissions at 2 Fpe Solar Wind
More Sophisticated Electron Density Computation in Magnetosheath Density from EFW S/C potential measurement Active spectra: spectra Selection of the most reliable active resonances Interpolation with EFW recalibrated signal Natural: Low-cutoff determination in a band centered on the interpolation
3. Density Determination Methods (cont’d) Magnetosphere Ø Several resonance series are observed. Ø n. Fce are easily identified due to their harmonicity, the initial Fce value is provided by MGM (Balogh et al. , 1997). Ø Bernstein’s resonances, Fuh & Fqn, can be computed for a given Fpe, then compared with the observations. The frequency at which the spectral energy reaches its maximum can also be taken as a rather good estimate of Fuh (method actually used for CAA density archive).
3. Density Determination Methods (cont’d) Outbound Crossing of the Earth’s Plasmapause 4 Fce 3 Fce Fq 4 Fq 3 Fq 2 2 Fce Fuh observed Fuh computed Fpe
Why Also Using Natural Emissions to Derive Ne? SOUNDING MODE NATURAL EMISSIONS WHISPER 3, March 18, 2005 Ø Time resolution is most often much better in Natural mode ~ 2 s compared to 52 or 104 s in current Sounding mode Ø Various signatures can be used to determine Ne between resonances
Natural Once identified from the sounder, characteristic frequencies can be unambiguously identified in passive emissions. The density is determined from active and passive observations: Ne (cm-3) = Fp 2 (k. Hz) / 81 Best time resolution: dt = 326 ms in BM No frequency calibration difficulty, ensuring good differential measurements Successive active and natural spectra measured in the Earth’s Plasmasphere by the WHISPER relaxation sounder onboard CLUSTER.
4. WHISPER Data Examples -- Plasma frequency in magnetosheath -- Plumes in plasmasphere -- Type III solar burst -- Thermal plasma in far magnetotail -- Overall magnetospheric path
Earth’s magnetosheath as seen by WHISPER-1: Ø when passive (top), and Ø active (middle). At the bottom, plasma density inferred from: Ø plasma resonances (crosses), and Ø lower cut-off frequency of thermal noise recorded when passive (red curve)
Electron Plasma Density (cm-3) Inbound Plume Outbound Plume 102 101 Inbound Plasmapause 04: 30 05: 00 Outbound Plasmapause 05: 30 06: 00 --- Rumba --- Salsa --- Samba --- Tango 06: 30 UT CLUSTER 1 -4, 11 April 2002 Electron density profiles measured In the Earth’s Plasmasphere by the relaxation sounder WHISPER (from plasma resonances) and the E-field and wave experiment EFW (from S/C potential), on 11 April 2002.
Lower-frequency cutoff at Fpe in the type III generation region Local Fpe Type III solar burst observed by WHISPER onboard CLUSTER 2.
Plasma frequency E-field resonances (top) and natural waves (bottom) observed by WHISPER-1 in the far magnetotail, on 30 Sept. 2002, at a geocentric distance of 18. 4 -18. 8 RE in the dusk sector (22 LT).
magnetosphere magnetosheath vs bow shock magnetopause E-field spectrograms measured by WHISPER onboard CLUSTER 1: natural waves (top), triggered waves (bottom). Magnetosphere is probed until 0610 UT, then the S/C is travelling into magnetosheath before entering solar wind at 0945 UT.
Top: resonances triggered by WHISPER from the plasmasphere to the solar wind. CLUSTER 1 crossed the magnetopause and bow shock at respectively 0610 UT and 0945 UT, so that the plasmasphere, the cusp regions, the magnetosheath and the solar wind were successively visited. Bottom: electron density derived from these observations. Cusp regions SW Plasmasphere Magnetosheath
CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE Laboratoire de Physique et Chimie de l'Environnement et de l’Espace 3 A, avenue de la Recherche Scientifique F-45071 Orléans cedex 02, France WHISPER Instrument and Data Products in CAA Jean Gabriel Trotignon, Xavier Vallières, and Gabor Facskó Conclusion The WHISPER relaxation sounder, aboard the 4 CLUSTER S/C, measures the E-Field from 2 to 80 k. Hz, but it is primarily an active experiment aimed at exciting radio waves at characteristic plasma frequencies. It delivers accurate and reliable values of the total plasma density (also B-field strength close to the Earth). Careful & accurate determinations of characteristic plasma frequencies are fundamental to interpret observed natural waves. Propagation characteristics of natural waves may then be used for valuable plasma density determination. Phone: (33 2) 38 25 52 63; Fax: (33 2) 38 63 12 34; E-mail: Jean-Gabriel. Trotignon@cnrs-orleans. fr
- Slides: 33