J. M. Bosqued

The Cluster Ion Spectrometry (CIS) Experiment

The Cluster Ion Spectrometry (CIS) experiment is a comprehensive ionic plasma spectrometry package on-board the four Cluster spacecraft capable of obtaining full three-dimensional ion distributions with good time resolution (one spacecraft spin) with mass per charge composition determination. The requirements to cover the scientific objectives cannot be met with a single instrument. The CIS package therefore consists of two different instruments, a Hot Ion Analyser (HIA) and a time-of-flight ion COmposition and DIstribution Function analyser (CODIF), plus a sophisticated dual-processor-based instrument-control and Data-Processing System (DPS), which permits extensive on-board data-processing. Both analysers use symmetric optics resulting in continuous, uniform, and well-characterised phase space coverage. CODIF measures the distributions of the major ions (H+, He+, He++, and O+) with energies from ~0 to 40 keV/e with medium (22.5°) angular resolution and two different sensitivities. HIA does not offer mass resolution but, also having two different sensitivities, increases the dynamic range, and has an angular resolution capability (5.6° × 5.6°) adequate for ion-beam and solar-wind measurements.

Simultaneous observations of field‐aligned beams and gyrating ions in the terrestrial foreshock

[1] We examine an energetic (2–30 keV) upstream ion event presenting a clear doublepeak spectrum observed � 1 RE upstream from the bow shock. The lower-energy (E � 3.5 keV) peak is associated with an ion beam propagating along the magnetic field direction, while the higher-energy (E � 13 keV) peak is associated with gyrating ions having pitch angles � 30� . The latter population progressively extends to lower energies over the span of the event. During times when the field-aligned beams were observed, the interplanetary magnetic field was remarkably steady, while the appearance of the 30� pitch angle gyrating ions was accompanied by the onset of large-amplitude ultralow frequency fluctuations of the magnetic field. Our analysis indicates that the gyrating ions had guiding centers on field lines downstream of the field-aligned component but that both populations could be sampled simultaneously because of the orbits of the former. We find that the downstream limit of the field-aligned beams is populated with protons having a speed 1.68 times the solar wind velocity, which is inconsistent with any known shockrelated emission mechanisms. This boundary makes an angle of 77� with respect to the Sun-Earth line in agreement with theoretical predictions. Just downstream of this rapid transition, gyrating ions having a flow speed of 1.52 times the solar wind speed are observed in association with ULF waves. Like the field-aligned beams, the gyrating ions reported here have streaming speeds inconsistent with any known shock emission mechanisms. While the simultaneous observation of field-aligned and gyrating components is possible because of the large gyration orbits of the latter, the observational sequence is consistent with a very sharp (]1 gyroradius) boundary separating the guiding centers of each. Explicit observations of such a sharp demarcation between these populations have not been reported before, and they place a significant constraint on the production mechanisms of the two populations. Our interpretation of these observations provides a refinement of the usual framework for foreshock morphology. INDEX TERMS: 2116 Interplanetary Physics: Energetic particles, planetary; 2164 Interplanetary Physics: Solar wind plasma; 2134 Interplanetary Physics: Interplanetary magnetic fields; 2154 Interplanetary Physics: Planetary bow shocks; 7851 Space Plasma Physics: Shock waves; KEYWORDS: foreshock boundary, ultralow frequency waves, bow shock, field-aligned beam, magnetic moment, shock emission mechanism

Cluster four spacecraft measurements of small traveling compression regions in the near-tail

Cluster four spacecraft measurements of small travelling compression regions in the near-tail

Energetic magnetospheric oxygen in the magnetosheath and its response to IMF orientation: Cluster observations

[1] We present Cluster observations made during an outbound orbit on 10 December 2000. After exiting the magnetosphere at midlatitude, Cluster spent a long time skimming the magnetopause moving to lower latitude along an orbit approximately in the ZY GSM plane on the dusk flank of the magnetopause. During this time, magnetospheric oxygen with energy >10 keV was observed continuously both in the magnetosphere and in the magnetosheath by the Cluster Ion Spectrometry (CIS) plasma experiment. While the oxygen density is roughly constant in the magnetosheath throughout the event, its velocity shows a strong dependence on the magnetosheath magnetic field orientation: low speeds, corresponding to almost isotropic distribution functions, occur for northward magnetic field, and high speeds, corresponding to beam-like distribution function occur for southward magnetic field. Mainly, two different processes have been discussed to explain the energetic particles escaping from the magnetosphere: flow along reconnected magnetospheric and magnetosheath field lines or crossing of the magnetopause when the particle gyroradii are comparable with the magnetopause thickness. The presence of the oxygen population cannot be readily explained in the framework of the reconnection theory. Instead, the observations are successfully reproduced by a model based on magnetopause crossing by finite gyroradius, provided the magnetosheath convection is taken into account together with the magnetosheath magnetic field orientation. Moreover, the presence of quasi-periodic motion of the magnetopause surface with period of approximately 5 min are evidenced by the analysis.

Effect of a northward turning of the interplanetary magnetic field on cusp precipitation as observed by Cluster

The immediate effect of the rotation of the interplanetary magnetic field (IMF) from southward to northward on cusp precipitation has been rarely observed by a polar orbiting satellite in the past. The four Cluster spacecraft observed such an event on 23 September 2004 as they were crossing the polar cusp within 2–16 min from each other. Between the first three and the last spacecraft crossing the cusp, the IMF rotated from southward to northward with a dominant By (GSM) component. For the first time we can examine the changes in the particle precipitation immediately after such IMF change. The first two spacecraft observed typical IMF-southward ion dispersion, while the last one observed both an IMF-southward-like dispersion in the boundary layer and an IMF-northward dispersion in the cusp. After the IMF turning, the cusp is shown to have grown in size in both the poleward and equatorward directions. A three-dimensional magnetohydrodynamic simulation is used to determine the locations of the sources of the ions and the topology of the magnetic field during the event.

Correlation between suprathermal electron bursts, broadband extremely low frequency waves, and local ion heating in the midaltitude cleft/low-latitude boundary layer observed by Cluster

with localized extra low frequency (ELF) (1–10 Hz) magnetic field wave power with broadband spectra. Our study shows that strong ion heating was observed only in the region with electron field-aligned anisotropy more than 2. In addition, comparison of particle data from two spacecraft, which crossed the heating region with a time difference of 4 min, shows the correlation between ion outflow fluxes and fluxes of the injected electrons. Whereas ELF electromagnetic waves are localized inside the ion heating region, ELF electrostatic waves are detected throughout the cleft/cusp/mantle regions, where strong ion heating was not observed, suggesting that electromagnetic ELF waves heat ions in the cleft region. Owing to the absence of magnetosheath ions and strong field-aligned currents, we suppose that inside ‘‘electron-only’’ cleft region the suprathermal electron bursts are most likely an energy source for the wave destabilization. We suggest that the location of the heating region and the level of the outflow ion fluxes could be related to electron injection in the cleft in such events. INDEX TERMS: 2724 Magnetospheric Physics: Magnetopause, cusp, and boundary layers; 2736 Magnetospheric Physics: Magnetosphere/ionosphere interactions; 7807 Space Plasma Physics: Charged particle motion and acceleration; 7867 Space Plasma Physics: Wave/particle interactions; KEYWORDS: cleft electron observations, dayside ion outflow, ion heating in the cleft/cusp, wave/ particle interaction

Motion of auroral ion outflow structures observed with CLUSTER and IMAGE FUV

[1] During February 2001 the CLUSTER satellites recorded a number of perigee passes through the midnight auroral zone. We concentrate on one pass, on 23 February 2001, when structured outflow was observed. Simultaneous observations of the aurora were available from the FUV instrument on IMAGE. The features in the ion outflow observed by the Cluster Ion Spectrometry (CIS) experiment are compared with the auroral activity. Observations from the multiple CLUSTER spacecraft are used to determine the velocity of the outflow structures. We find a good correspondence between the observed ion outflow and the auroral arcs, with the highest energy outflow corresponding to the brightest arcs. The features at the equatorward edge, which are trapped precipitating ions, are stationary. In addition, the increased velocity structure at the poleward edge is also stationary. However, the bulk of the ion outflow structures, which are observed between these boundaries, are moving equatorward with a velocity of roughly 7 km/s, which corresponds to a velocity of 0.7 km/s at 100 km. One feature is observed moving poleward, at the same time that the auroral arc is expanding poleward. Comparisons with the motion of the auroral arcs and with the convection velocity measured by the EDI instrument on CLUSTER show that the motion of the structures in general agrees with the convective motion of the field lines.