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

Abstract

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.

Wide-banded NTC radiation: local to remote observations by the four Cluster satellites

“…Such maps have been used in Grimald at al. (2008) and in Décréau et al (2013). We take the opportunity of this new analysis of NTC events previously published to correct an error which appears in the maps published in the two papers quoted above.…”

“…8 and 9). Note that the resulting orientations are only a crude indication of correct values, since the underlying assumption (circular polarization of electric field) is likely not fully satisfied (see Décréau et al (2013) for an estimation of possible errors), and that measurement uncertainties are large. Nevertheless, Fig.…”

“…The case event on 18 May 2008 has been presented in detail in Décréau et al (2013). It happens during the Cluster tilt campaign, when the pair of satellites C3-C4 is used as a single observatory, enabling to derive the geometric properties of a planar electromagnetic wave.…”

Recent highlights from Cluster, the first 3-D magnetospheric mission

“…On 3 April 1997, the SPC approved the Cluster II mission to be launched in 2000 on two Soyuz rockets (Credland and Schmidt, 1997). In a little more than 3 years, the fastest implementation time for an ESA space mission, the four Cluster spacecraft were rebuilt, tested and successfully launched on 16 July and 9 August 2000 (Escoubet et al, 2001). …”

“…2). Décréau et al (2013) showed that classical triangulation, in this case using three of the non-tilted spacecraft situated thousands of kilometres apart, can lead to a source location that is nowhere near the boundaries where NTC generation occurs. The erroneous source location, far from these boundaries, given by triangulation is attributed to small deviation from the assumed polarisation of the emission.…”