Abstract. On board the four Cluster spacecraft, the Cluster Ion Spectrometry (CIS) experiment measures the full, threedimensional ion distribution of the major magnetospheric ions (H + , He + , He ++ , and O + ) from the thermal energies to about 40 keV/e. The experiment consists of two different instruments: a COmposition and DIstribution Function analyser (CIS1/CODIF), giving the mass per charge composition with medium (22.5 • ) angular resolution, and a Hot Ion AnalCorrespondence to: H. Rème (Henri.Reme@cesr.fr) yser (CIS2/HIA), which does not offer mass resolution but has a better angular resolution (5.6 • ) that is adequate for ion beam and solar wind measurements. Each analyser has two different sensitivities in order to increase the dynamic range.

Abstract:The Rosetta mission has been designed to rendezvous with and escort comet 67P/Churyumov-Gerasimenko from a heliocentric distance of >3.6 AU, when the comet still has a low activity level, until perihelion passage at 1.25 AU where the comet reaches the maximum

Abstract. The inner magnetosphere's current mapping is one of the key elements for current loop closure inside the entire magnetosphere. A method for directly computing the current is the multi-spacecraft curlometer technique, which is based on the application of Maxwell-Ampère's law. This requires the use of four-point magnetic field high resolution measurements. The FGM experiment on board the four Cluster spacecraft allows, for the first time, an instantaneous calculation of the magnetic field gradients and thus a measurement of the local current density. This technique requires, however, a careful study concerning all the factors that can affect the accuracy of the J estimate, such as the tetrahedral geometry of the four spacecraft, or the size and orientation of the current structure sampled. The first part of this paper is thus providing a detailed analysis of the method accuracy, and points out the limitations of this technique in the region of interest. The second part is an analysis of the ring current region, which reveals, for the first time, the large latitudinal extent of the ring current, for all magnetic activity levels, as well as the latitudinal evolution of the perpendicular (and parallel) components of the current along the diffuse auroral zone. Our analysis also points out the sharp transition between two distinct plasma regions, with the existence of high diamagnetic currents at the interface, as well as the filamentation of the current inside the inner plasma sheet. A statistical study over multiple perigee passes of Cluster (at about 4 R E from the Earth) reveals the azimuthal extent of the partial ring current. It also reveals that, at these distances and all along the evening sector, there isn't necessarily a strong dependence of the local current density value on the magnetic activity level. This is a direct consequence of the ring current morphology evolution, as well as the relative positioning ofCorrespondence to: C. Vallat (claire.vallat@cesr.fr) the spacecraft with respect to the bulk of the ring current. It also proves the existence of a substantial ring current at these distances, all over the evening and the post-midnight sector.

Context. The Rosetta spacecraft is escorting comet 67P/Churyumov-Gerasimenko from a heliocentric distance of >3.6 AU, where the comet activity was low, until perihelion at 1.24 AU. Initially, the solar wind permeates the thin comet atmosphere formed from sublimation. Aims. Using the Rosetta Plasma Consortium Ion Composition Analyzer (RPC-ICA), we study the gradual evolution of the comet ion environment, from the first detectable traces of water ions to the stage where cometary water ions accelerated to about 1 keV energy are abundant. We compare ion fluxes of solar wind and cometary origin. Methods. RPC-ICA is an ion mass spectrometer measuring ions of solar wind and cometary origins in the 10 eV-40 keV energy range. Results. We show how the flux of accelerated water ions with energies above 120 eV increases between 3.6 and 2.0 AU. The 24 h average increases by 4 orders of magnitude, mainly because high-flux periods become more common. The water ion energy spectra also become broader with time. This may indicate a larger and more uniform source region. At 2.0 AU the accelerated water ion flux is frequently of the same order as the solar wind proton flux. Water ions of 120 eV-few keV energy may thus constitute a significant part of the ions sputtering the nucleus surface. The ion density and mass in the comet vicinity is dominated by ions of cometary origin. The solar wind is deflected and the energy spectra broadened compared to an undisturbed solar wind. Conclusions. The flux of accelerated water ions moving from the upstream direction back toward the nucleus is a strongly nonlinear function of the heliocentric distance.

[1] Using multipoint measurements from the Cluster ion spectrometry instruments and the research with adaptive particle imaging detectors, we identified new properties of multiple energy-dispersed ion structures in the plasma sheet boundary layer (PSBL). On 14 February 2001 at about 4.5 R E midnight local time, the PSBL was highly structured, showing several large-scale dispersed ion structures, which were substructured into several (up to four) beamlets with a quasiperiodicity of 1-3 min in the spacecraft frames. The different spacecraft (SC) recorded the first dispersed ion structures at different times and on different L shells at the outer edge of the PSBL within 2 min. Three different energy dispersions were associated with the dispersed ion structures. (1) The energy dispersion of the larger-scale structures was due to the decreasing energy of individual beamlets, covering energies from 2 to >40 keV. (2) Individual beamlets of each large-scale structure showed themselves energy dispersion along the peak flux line with varying slopes, but in all cases these slopes were steeper compared to the dispersion associated with the large-scale structure. (3) A third steep energy dispersion occurred at the beginning of each beamlet and covered an energy range from a few keV to >100 keV. This dispersion was associated with recurrent impulsive acceleration processes at 11-27 R E radial distance with a quasiperiodicity of 1-3 min. Moreover, most beamlets showed pitch angle dispersion. Superimposed on the dispersed ion structures were two transient ion injections, which had the same energy dispersion slope as described in item 3 (above), suggesting an association with the beamlets. The beamlets and one of the transient ion injections were recorded for different ion species: hydrogen, helium, and oxygen. Furthermore, echoes of beamlets were recorded, which makes this the first observation of bouncing ions in the PSBL. The echoes showed higher energy fluxes than the initial beamlets, indicating additional acceleration during subsequent current sheet crossings. Gradual thermalization of the initial beamlets after multiple current sheet crossings possibly led to the formation of the central plasma sheet. SC 1 and SC 3, longitudinally separated by only 100 km, recorded very different beamlet structures, which we interpret as a spatial effect; the two beamlet structures mapped into different magnetotail regions and underwent different spatiotemporal histories. Two possible scenarios are discussed to understand the spatiotemporal history of this highly structured PSBL.

Abstract. During the last 30 years, several magnetospheric missions have recorded the presence of narrow proton structures in the ring current region. These structures have been referred as "nose-like" structures, due to their appearance when represented in energy-time spectrograms, characterized by a flux value increase for a narrow energy range.Cluster's polar orbit, with a 4 R E perigee, samples the ring current region. The ion distribution functions obtained in-situ by the CIS experiment (for energies of ∼ 5 eV/q to 40 keV/q) reveal the simultaneous presence of several (up to 3) narrow nose-like structures. A statistical study (over one year and a half of CIS data) reveals that double nose structures are preferentially observed in the post-midnight sector. Also, the characteristic energy of the nose (the one observed at the lower energy range when several noses occur simultaneously) reveals a clear MLT dependence during quiet events (K p <2): a sharp transition in the energy range occurs in the pre-noon sector. Moreover, the multi-nose structures (up to 3 simultaneous noses) appear regardless of the magnetospheric activity level and/or the MLT sector crossed by the spacecraft.Numerical simulations of particles trajectories, using large-scale electric and magnetic field models are also presented. Most of the features have been accurately reproduced (namely the single and double noses), but the triple noses cannot be produced under these conditions and require to consider a more complex electric field model.

[1] This paper presents case studies of the dynamics of the terrestrial ions in the midplasma sheet and inside the lobes ($17-19 R E ). The high time resolution measurements of H + , He + , and O + ions made on board Cluster show that these ions are massively injected into the tail during substorms/storms from the nightside ionosphere and mainly dispersed by time of flight effects; a single oxygen injection being able to account for over 80% of the oxygen population of the midtail plasma sheet during storm time. Inside the lobes, during disturbed times, ionospheric oxygen ions appear as nearly monoenergetic beams, quasi field-aligned, propagating antisunward from the ionosphere, as expected from the ''cleft ion fountain.'' Most of the time, the ionospheric protons are very cold and their distribution function is not fully measurable by a charged spacecraft. However, we show that protons can, at times, acquire a large drift motion, reach energy larger than 30 eV, and be directly measurable by the CIS spectrometers. This occurs when large disturbances propagate in the plasma sheet boundary layer during both quiet and disturbed periods. We show that the large, variable, Alfvén waves generated by these plasma sheet disturbances modulate the energy of lobe protons and oxygen ions and inject electromagnetic energy, in excess of 15 Â 10 À3 W/m 2 , down to the polar cap. Deeper inside the plasma sheet boundary layer, large electric fields associated with the Earthward stream of plasma sheet particles give enough drift energy to a very cold ionospheric proton population to become fully detectable by the Cluster ion spectrometers.

Abstract. On 29 December 2003, the Chinese spacecraft Tan Ce 1 (TC-1), the first component of the Double Star mission, was successfully launched within a low-latitude eccentric orbit. In the framework of the scientific cooperation between the Academy of Sciences of China and ESA, several European instruments, identical to those developed for the Cluster spacecraft, were installed on board this spacecraft.The HIA (Hot Ion Analyzer) instrument on board the TC-1 spacecraft is an ion spectrometer nearly identical to the HIA sensor of the CIS instrument on board the 4 Cluster spacecraft. This instrument has been specially adapted for TC-1. It measures the 3-D distribution functions of the ions between 5 eV/q and 32 keV/q without mass discrimination.TC-1 is like a fifth Cluster spacecraft to study the interaction of the solar wind with the magnetosphere and to study geomagnetic storms and magnetospheric substorms in the near equatorial plane.HIA was commissioned in February 2004. Due to the 2 R E higher apogee than expected, some in-flight improvements were needed in order to use HIA in the solar wind in the initial phase of the mission. Since this period HIA has obtained very good measurements in the solar wind, the magnetosheath, the dayside and nightside plasma sheet, the ring current and the radiation belts. We present here the first results in the different regions of the magnetosphere and in the solar wind. Some of them are very new and include, for example, ion dispersion structures in the bow shock and ionCorrespondence to: H. Rème (henri.reme@cesr.fr) beams close to the magnetopause. The huge interest in the orbit of TC-1 is strongly demonstrated.