The WHISPER Relaxation Sounder and the CLUSTER Active Archive

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Abstract. The Cluster mission operated a "tilt campaign" during the month of May 2008. Two of the four identical Cluster spacecraft were placed at a close distance (∼ 50 km) from each other and the spin axis of one of the spacecraft pair was tilted by an angle of ∼ 46 • . This gave the opportunity, for the first time in space, to measure global characteristics of AC electric field, at the sensitivity available with long boom (88 m) antennas, simultaneously from the specific configuration of the tilted pair of satellites and from the available base of three satellites placed at a large characteristic separation (∼ 1 R E ). This paper describes how global characteristics of radio waves, in this case the configuration of the electric field polarization ellipse in 3-D-space, are identified from in situ measurements of spin modulation features by the tilted pair, validating a novel experimental concept. In the event selected for analysis, non-thermal continuum (NTC) waves in the 15-25 kHz frequency range are observed from the Cluster constellation placed above the polar cap. The observed intensity variations with spin angle are those of plane waves, with an electric field polarization close to circular, at an ellipticity ratio e = 0.87. We derive the source position in 3-D by two different methods. The first one uses ray path orientation (measured by the tilted pair) combined with spectral signature of magnetic field magnitude at source. The second one is obtained via triangulation from the three spacecraft baseline, using estimation of directivity angles under assumption of circular polarization. The two results are not compatible, placing sources widely apart. We present a general study of the level of systematic errors due to the assumption of circular polarization, linked to the second approach, and show how this approach can lead to poor triangulation and wrong source positioning. The estimation derived from the first method places the NTC source region in the dawn sector, at a large L value (L ∼ 10) and a medium geomagnetic latitude (35 • S). We discuss these untypical results within the frame of the geophysical conditions prevailing that day, i.e. a particularly quiet long time interval, followed by a short increase of magnetic activity.

Section: Output Of the Dsm Tool: Discussion Of A Selected Casementioning

confidence: 99%

Section: Measured Quantitiesmentioning

confidence: 99%

Remote sensing of a NTC radio source from a Cluster tilted spacecraft pair

Décréau¹,

Kougblénou²,

Lointier³

et al. 2013

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“…This study makes use of data from a number of experiments on board the Cluster spacecraft, including PEACE (Johnstone et al, 1997;Fazakerley et al, 2010), the Fluxgate Magnetometer (FGM) (Balogh et al, 1997;Gloag et al, 2010), the Electric Field and Waves (EFW) experiment (Gustafsson et al, 1997;Khotyaintsev et al, 2010), the Waves of High frequency and Sounder for Probing of Electron density by Relaxation (WHISPER) (Décréau et al, 1997;Trotignon et al, 2010), and the Spatio-Temporal Analysis of Field Fluctuations (STAFF) experiment (Cornilleau-Wehrlin et al, 1997.…”

Section: Datamentioning

confidence: 99%

Absence of the strahl during times of slow wind

Gurgiolo¹,

Goldstein

2017

Abstract. It is not uncommon during periods when the solar wind speed is less than 425 km s −1 to observe near 1 AU no evidence of a strahl population in either the electron solar wind or within the foreshock. Estimating the fluid flow within each energy step returned from the Plasma Electron And Current Experiment (PEACE) on board Cluster-2 often finds that in slow wind the GSE spherical flow angles in energies above where there is a clear core/halo signature are often close to radial with no evidence of a field-aligned flow. This signifies the lack of a strahl presence in the electron velocity distribution function (eVDF). When there is no obvious strahl signature in the data, the electrons above the core/halo in energy appear to be unstructured and smeared in angle. This can either be interpreted as due to statistical noise in low counting rate situations or the result of intense scattering. Regions where the strahl is seen and not seen are often separated by a very thin boundary layer. These transitions in the spacecraft frame of reference can be quite rapid, generally occurring within one to two spins (4-8 s).

“…The estimated spatial resolution of electron density observed is of the order of 1 km, i.e. a few times the electric antenna tip-to-tip length d, with d = 88 m. The electron density determination has been carried out in two steps: (1) a semi-automatic tool dedicated to the density production for the Cluster Active Archive (Trotignon et al, 2010) has been first applied; (2) the obtained electron density profiles are then checked manually in order to correct or filter spurious points. In the plasmasphere, resonances arise at the electron gyrofrequency F ce (and its harmonics nF ce ), the total electron plasma frequency F pe , the upper-hybrid frequency F UH and the Bernstein's mode frequencies F qn (Bernstein, 1958).…”

Section: Electron Density Determinationmentioning

confidence: 99%

Refilling process in the plasmasphere: a 3-D statistical characterization based on Cluster density observations

Lointier¹,

Darrouzet

Décréau³

et al. 2013

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Abstract. The Cluster mission offers an excellent opportunity to investigate the evolution of the plasma population in a large part of the inner magnetosphere, explored near its orbit's perigee, over a complete solar cycle. The WHISPER sounder, on board each satellite of the mission, is particularly suitable to study the electron density in this region, between 0.2 and 80 cm−3. Compiling WHISPER observations during 1339 perigee passes distributed over more than three years of the Cluster mission, we present first results of a statistical analysis dedicated to the study of the electron density morphology and dynamics along and across magnetic field lines between L = 2 and L = 10. In this study, we examine a specific topic: the refilling of the plasmasphere and trough regions during extended periods of quiet magnetic conditions. To do so, we survey the evolution of the ap index during the days preceding each perigee crossing and sort out electron density profiles along the orbit according to three classes, namely after respectively less than 2 days, between 2 and 4 days, and more than 4 days of quiet magnetic conditions (ap ≤ 15 nT) following an active episode (ap > 15 nT). This leads to three independent data subsets. Comparisons between density distributions in the 3-D plasmasphere and trough regions at the three stages of quiet magnetosphere provide novel views about the distribution of matter inside the inner magnetosphere during several days of low activity. Clear signatures of a refilling process inside an expended plasmasphere in formation are noted. A plasmapause-like boundary, at L ~ 6 for all MLT sectors, is formed after 3 to 4 days and expends somewhat further after that. In the outer part of the plasmasphere (L ~ 8), latitudinal profiles of median density values vary essentially according to the MLT sector considered rather than according to the refilling duration. The shape of these density profiles indicates that magnetic flux tubes are not fully replenished after 6 days of quiet conditions. In addition, the outer plasmasphere in the night and dawn sectors (22:00 to 10:00 MLT range) maintains an overall clear deficit of ionospheric population, when compared to the situation in the noon and dusk sectors (10:00 to 22:00 MLT range).