H. Rème’s research while affiliated with Research Institute in Astrophysics and Planetology, French National Centre for Scientific Research and other places

Approaches regarding how to turn the instrument background counts into scientifically valuable data are presented in this Technical Report on Methods. The background counts due to penetrating energetic particles of radiation belts detected on Cluster CIS HIA and CODIF instruments and the Double Star HIA instrument are used in these approaches. In HIA spectrograms, the background counts are seen simultaneously in all energy channels marking the entry and exit of the radiation belts by the spacecraft, therefore, the locations of the boundaries of the outer and inner belts can be determined. In the case when HIA measurements are not readily available, a new method is proposed in which supplementary data streams within the CODIF telemetry is exploited. It employs separate counts that register “start,” “stop,” and “non‐valid” signals increasing in the presence of penetrating particles even when no corresponding increase are shown in the energy‐time spectrograms. The locations of the radiation belt boundaries are defined by following the changes in counts gradients with time and visual inspection of all the available measurements. The July–August 2007 and September–October 2012 time periods are analyzed for method demonstration on a presence of a third radiation belt, or storage ring.

We statistically investigate the influence of the solar wind dynamic pressure (SW Pdyn) on the field‐aligned currents (FACs) in the magnetotail with 1,492 FAC cases from July to October in 2001 and 2004, which covers 74 Cluster crossings of the plasma sheet boundary layer (PSBL) in both storm time and non‐storm time. The FAC density in the magnetotail is derived from the magnetic field data with the four‐point measurement of Cluster, and the SW Pdyn is taken from ACE data. The results indicate the FAC density becomes stronger with increasing SW Pdyn. The statistics show that the FAC occurrence increased monotonically with SW Pdyn in the three levels (Weak: SW Pdyn < 2 nPa; Medium: 2 nPa ≤ SW Pdyn ≤ 5 nPa; Strong: SW Pdyn > 5 nPa). The FAC density increased with increasing SW Pdyn, while its footprint (invariant latitude, ILAT) in the polar region decreased with increasing SW Pdyn. The response of the FAC to SW Pdyn in the magnetotail had a north‐south hemispheric asymmetry. The FAC density had a better correlation with SW Pdyn in the Northern hemisphere, while the footprint had a better correlation with SW Pdyn in the Southern hemisphere. Possible underlying mechanisms for our results are analyzed and discussed. However, it requires more observations and simulation studies to find out the mechanism of north‐south asymmetry.

In this study, we reported the large‐amplitude and fast‐damped flapping of the plasma sheet, which co‐occurred with magnetic reconnection. Data from the Double Star TC‐1 and Cluster satellites were used to analyze the features of the plasma sheet flapping 1.4 RE earthward of an ongoing magnetic reconnection event. The flapping was rapidly damped, and its amplitude decreased from the magnetohydrodynamics scale to the subion scale in 5 min. The variation in the flapping period (from 224 to 20 s) indicated that the source of the flapping had highly dynamic temporal characteristics. The plasma sheet flapping propagated duskward through a kink‐like wave with a velocity of 100 km/s, which was in agreement with the group velocity of the ballooning perturbation. A correlation analysis between the magnetic reconnection and plasma sheet flapping indicated that the magnetic reconnection likely facilitated the occurrence of ballooning instability by altering the state of plasma in the downstream plasma sheet. In this regard, the reconnection‐induced ballooning instability could be a potential mechanism to generate the flapping motion of the plasma sheet.

Conjunction observations of the magnetic field and plasma by Cluster and TC‐1 at the dayside magnetosphere are presented to investigate the sequential flux ropes transferred from the low latitude boundary layer to the high‐altitude cusp on 10 March 2004.Three sequential flux ropes originating from the dayside low latitude magnetopause are first detected by TC‐1. After ~5.3 min, three sequential flux ropes accumulated with energetic oxygen ions are also detected by Cluster in the high‐altitude cusp. The recurrence period of these flux ropes is ~3 min. The number density of energetic oxygen ions in the cusp flux rope is ~0.25 cm⁻³ detected from CIS/CODF instrument on Cluster. It is found that oxygen ions with energy lager than 10 keV have a narrow pitch angle (less than 90°) distribution in the southern high‐altitude cusp. While oxygen ions with energy less than 10 keV are distributed in a wide pitch angle from 0 to 180°. Counter‐streaming energetic oxygen ions are found in these flux ropes in the high‐altitude cusp.This result suggests that the oxygen ions with energy less than 10 keV in the high‐altitude cusp have two source regions. One is from the dayside magnetopause, and the other is from the low‐altitude cusp. Our investigations first provide evidence that flux ropes at dayside low‐latitude magnetopause can carry energetic oxygen ions into the high‐altitude cusp region.

The influence of the interplanetary magnetic field (IMF) cone angle θ (the angle between the IMF direction and the Sun-Earth line) on the invariant latitudes (ILATs) of the footprints of the field-aligned currents (FACs) in the magnetotail has been investigated. We performed a statistical study of 542 FAC cases observed by the four Cluster spacecraft in the northern hemisphere. The results show that there are almost no FACs when the IMF cone angle is less than 10°, and there are indications of the FACs in the PSBLs being weak under the radial IMF conditions. The footprints of the large FAC (>10 nA/m2) cases are within ILATs <71° and mainly within IMF cone angles θ>60°, which implies that the footprints of the large FACs mainly expand equatorward with large IMF cone angle. The equatorward boundary of the FAC footprints in the polar region decreases with increasing IMF cone angle (and has a better correlation for northward IMF), which shows that the IMF cone angle plays an important controlling role in FAC distributions in the magnetosphere-ionosphere coupling system. There is almost no correlation between the poleward boundary and the IMF cone angle for both northward and southward IMF. This is because the poleward boundary movement is limited by an enhanced lobe magnetic flux. This is the first time a correlation between FAC foot prints in the polar region and IMF cone angles has been determined.

Singly charged oxygen ions, O+, energized by kinetic Alfvén wave eigenmode (KAWE) in the plasma sheet boundary layer during dipolarizations of two intense substorms,10:07 UT on 31 August 2004 and 18:24 UT on 14 September 2004, are investigated by Cluster spacecraft in the magnetotail. It is found that after the beginning of the expansion phase of substorms, O+ ions are clearly energized in the direction perpendicular to the magnetic field with energy larger than 1 keV in the near-Earth plasma sheet (NEPS) during magnetic dipolarizations. The pitch angle distribution of these energetic O+ ions is significantly different from that of O+ ions with energy less than 1 keV before substorm onset which is in the quasi-parallel direction along the magnetic field. The KAWE with the large perpendicular unipolar electric field, Ez ~ -20 mV/m, significantly accelerate O+ ions in the direction perpendicular to the background magnetic field. We present good evidences that O+ ions origin from the ionosphere along the magnetic field line in the northward lobe can be accelerated in the perpendicular direction during substorm dipolarizations. The change of the move direction of O+ ions is useful for O+ transferring from the lobe into the central plasma sheet in the magnetotail. Thus KAWE can play an important role in O+ ions transfer process from the lobe into the plasma sheet during intense substorms.

Comets contributed to Earth's atmosphere Models of xenon's origin in Earth's atmosphere require an additional, unknown source that has been a mystery for several decades. Marty et al. measured isotopic ratios of xenon released from comet 67P/Churyumov-Gerasimenko and found that they match the heretofore unknown source. The xenon appears to have been trapped in ice within the comet since before the solar system formed. Comets contributed about a quarter of the xenon on Earth, which constrains the amount of other materials (such as water) delivered to our planet by comets. Science , this issue p. 1069

The coma and the comet-solar wind interaction of comet 67P/Churyumov-Gerasimenko changed dramatically from the initial Rosetta spacecraft encounter in August 2014 through perihelion in August 2015. Just before equinox (at 1.6 AU from the Sun), the solar wind signal disappeared and two regions of different cometary ion characteristics were observed. These “outer” and “inner” regions have cometary ion characteristics similar to outside and inside the ion pileup region observed during the Giotto approach to comet 1P/Halley. Rosetta/DFMS ion mass spectrometer observations are used here to investigate the H3O⁺/H2O⁺ ratio in the outer and inner regions at 67P/ Churyumov-Gerasimenko. The H3O⁺/H2O⁺ ratio and the H3O⁺ signal are observed to increase in the transition from the outer to the inner region and the H3O⁺ signal appears to be weakly correlated with cometary ion energy. These ion composition changes are similar to the ones observed during the 1P/Halley flyby. Modeling is used to determine the importance of neutral composition and transport of neutrals and ions away from the nucleus. This modeling demonstrates that changes in the H3O⁺/H2O⁺ ratio appear to be driven largely by transport properties and only weakly by neutral composition in the coma.

Utilizing conjunction observations of the Geotail and ACE satellites from 1998 to 2005, we investigated the temporal evolutions of the solar wind conditions prior to the formation of X-lines in the near-Earth magnetotail. We first show the statistical properties of Bz, By, density, and velocity of the solar wind related to the 374 tail X-line events. A superposed epoch analysis is performed to study the temporal evolutions of the solar wind conditions 5 hours prior to the tail X-lines. The solar wind conditions for tail X-lines during southward IMF (SW-IMF) and northward IMF (NW-IMF) are analyzed. The main results are as follows: 1) For events classified as SW-IMF, near-Earth X-line observations in the magnetosphere are preceded by ~2 hour intervals of southward IMF; 2) For events classified a NW-IMF, the northward IMF orientation preceding near-Earth X-line observations lasts ~ 40 minutes.