ESA Science & Technology - Publication Archive

The relationship between the average structure of the inner magnetospheric large-scale electric field and geomagnetic activity levels has been investigated by Double Star TC-1 data for radial distances between 4.5 R_e and 12.5 R_e and MLT between 18:00 h and 06:00 h from July to October in 2004 and 2005. The sunward component of the electric field decreases monotonically as radial distance increases and approaches zero as the distance off the Earth is greater than 10 R_e. The dawn-dusk component is always duskward. It decreases at about 6 R_e where the ring current is typically observed to be the strongest and shows strong asymmetry with respect to the magnetic local time. Surprisingly, the average electric field obtained from TC-1 for low activity is almost comparable to that observed during moderate activity, which is always duskward at the magnetotail (8 R_e~12 R_e).

Since early 2004 the Chinese spacecraft Tan Ce 1 (TC-1), first component of the Double Star (DSP) mission, has been on an equatorial elliptical orbit (13.4 R_E apogee), allowing the study of the dynamics of the Earth's magnetosphere in conjunction with the four European Cluster spacecraft (19.6 R_E apogee). The Cluster and Double Star spacecraft orbits are such that the spacecraft are almost in the same meridian, allowing conjugate studies. The four Cluster spacecraft highly eccentric polar orbit at 4 R_E perigee permits them to sample the ring current, the radiation belts, and the outer plasmasphere from south to north, almost following the same magnetic flux tube (latitudinal profile), whereas TC-1, with its very low-perigee equatorial orbit, gives the plasma profile across L shells. Coordinated ion measurements provided by the Cluster Ion Spectrometry and Hot Ion Analyzer instruments onboard Cluster and TC-1, respectively, obtained during quiet conditions, disturbed geomagnetic conditions, and an intense storm, are used to analyze crossings of the plasmasphere and the ring current region. Multiple narrow ion energy bands ("nose-like" structures) are simultaneously observed by both Cluster and TC-1. These observations reveal the large-scale character of these structures and pose a challenge for the simulation and modeling of the inner magnetosphere populations.

The Double Star TC-1 magnetosheath pass on 26 February 2004 is used to investigate magnetic field fluctuations. Strong compressional signatures which last for more than an hour have been found near the magnetopause behind a quasi-perpendicular bow shock. These compressional structures are most likely mirror mode waves. There is a clear wave transition in the magnetosheath which probably results from the change of the interplanetary magnetic field (IMF) cone angle. The wave characteristics in the magnetosheath are strongly controlled by the type of the upstream bow shock.

The equatorial and polar satellites of the Double Star Project (DSP) were launched successfully on December 29, 2003 and July 25, 2004, respectively, and both of them are operating smoothly. The DSP provides a good opportunity for investigating the structure of the magnetosphere. Based on the DSP data collected during 2004, we have surveyed the distribution of the magnetic fields and plasmas in the magnetosphere. It is found that: (1) Near the Earth's equatorial plane within geocentric distances of less than 7 R_E, the Earth's magnetic field is dipolar. In the vicinity of the magnetopause, the magnetic field is enhanced by a factor of about 1.5, and on the nightside, the magnetic field can vary significantly from the Earth's dipole field, likely caused by the presence of the near-Earth tail current sheet. (2) In the day-side magnetosheath, the electron and ion densities are usually both in the range of 10-30 cm^-3; the ion and electron temperatures are usually about 200 and 50 eV, respectively. The flow pattern is usually smooth, with a low velocity in the subsolar region and with significantly higher velocities in the dawn and dusk flanks. (3) In the region between the magnetopause and plasmasphere the density is low, approximately 0.5-5 cm^-3, and the temperature is high, about 1-10 keV for ions and 0.1-5 keV for electrons. The ion temperature has an apparent anisotropy, with the ratio of the perpendicular and parallel temperatures being about 1.0-1.3 for the night-and dusk-side magnetosphere and about 1.3-2.0 for the day-and dawn-side magnetosphere. -- Remainder of abstract is truncated --

A method is presented for retrieving the magnetospheric ion distribution from Energetic Neutral Atom (ENA) measurements made by the NUADU instrument on the TC-2 spacecraft. Based on the already well-established method of constrained linear inversion, an iterance technique suitable for the low count ENA measurements has been developed which is tolerant of the noise background. By the iterance technique, it is possible for the first time to simultaneously retrieve the magnetospheric ion distribution and the exospheric neutral density, and further to recover global ENA emissions in three dimensions. The technique is applied to a representative ENA image recorded in energy channel 2 (protons: 50-81 keV) of the NUADU instrument during a major geomagnetic storm and it is, thereby, shown that the retrieval method developed provides a useful tool for extracting ion distribution information from ENA data.

The characteristic and properties of ULF waves in the plasmasphere boundary layer during two very quiet periods are present. The ULF waves were detected by Double Star TC-1 when the spacecraft passed through the plasmasphere in an outbound and inbound trajectories, respectively. A clear association between the ULF waves and periodic variations of energetic ions fluxes was observed. The observations showed that the wave frequency was higher inside the plasmasphere than outside. The mechanism generating these ULF waves and possible diagnosing of the "classical plasmapause" location with the ULF wave were discussed.

An event of Cluster-Double Star conjunction observations of magnetic reconnection at high latitude magnetopause nightside of both cusps and solar wind transport into magnetosphere caused by such reconnection process has been investigated. During northward IMF, Cluster/SC1 observed accelerated flows and ion heating associated with magnetic reconnection at high latitude magnetopause nightside of southern cusp. And Double Star observed cold dense solar wind plasma transported into dayside magnetosphere. The analysis on such conjunction observations shows that: (1) during northward IMF, magnetic reconnection occurs at high latitude nightside of southern cusp, accompanied by accelerated flows that are observed by Cluster/SC1; (2) the direction of the accelerated flows, with its sunward component V_x, dawnward component V_y, northward component V_z, is quite consistent with the theoretical anticipation under the condition of northward IMF with dawnward component B_y; (3) reconnection can heat plasma more in parallel direction than in perpendicular direction, to a level of about 4 keV; (4) with reconnection taking place at high latitude magnetopause nightside of the southern cusp, TC-1 observed cold and dense plasma transported into magnetosphere; (5) by reconnection at high latitude magnetopause nightside of both cusps, solar wind flux tube can be captured by magnetosphere and pulled into dayside magnetosphere. This event presents further observational evidence for magnetic reconnection at high latitude magnetopause nightside of both cusps as an important mechanism of solar wind transport into magnetosphere.

The magnetic field variations are analyzed in the range of time periods from 4 s to 240 s in the magnetosheath observed by the Double Star TC-1 and Cluster in 2004. The characteristics of the magnetic field fluctuations are strongly controlled by the angle between the upstream interplanetary magnetic field (IMF) and the normal of the bow shock. Generally speaking, the magnetic field fluctuations in the quasi-parallel magnetosheath are more intense than those in the quasi-perpendicular ones. Almost purely compressional waves are found in the quasi-perpendicular magnetosheath. With the increase of the local plasma beta, both the magnitude and direction of the magnetic field fluctuate more intensely. There exists an inverse correlation between the local temperature anisotropy T_perp./T_parallel and the plasma beta.

From June 1, 2004 to October 31, 2006, a total 465 high-speed flow events are observed by the TC-1 satellite in the near-Earth region (-13 R_E < x < -9 R_E, |Y|<10 R_E, |Z|<5 R_E). Based on the angle between the flow and the magnetic field, the high-speed flow events are further divided into two types, that is, field-aligned high-speed flow (FAHF) in the plasma sheet boundary and convective bursty bulk flow (BBF) in the center plasma sheet. Among the total 465 high-speed flow events, there are 371 FAHFs, and 94 BBFs. The CHF are mainly concentrated in the plasma sheet, the intersection angle between the flow and the magnetic field is larger, the magnetic field intensity is relatively weak. The FHF are mainly distributed near the boundary layer of the plasma sheet, the intersection angle between the flow and magnetic field is smaller, and the magnetic field intensity is relatively strong. The convective BBFs have an important effect on the substorm.

We report on the THEMIS and Double Star TC-1 observations at the geocentric distances of 3 < R < 8 R_E during substorms on March 23, 2007. THEMIS crossed the inner boundary of the equatorial dusk-side plasma in the string-of-pearls configuration, allowing the dynamics of particle populations to be traced within time ranges from hours to 10 minutes. Observations show co-existence of the plasma sheet ion population (5 - 30 keV) with the ring current ion population (100 - 1000 keV) at 4 < R < 6 R_E . The plasma sheet population was characterized by pronounced "nose"-like dispersion with the spectral density maximum at ~10 keV. The plasma sheet boundary, defined by a sharp decrease of the ~1 keV electron flux, moved inward to R = 4 and outward back to ~8 RE within about 1 hour. Local enhancements of the plasma sheet (1 - 5 keV) electron flux with the characteristic time scale of 2 - 10 min were detected at 5 < R < 6 R_E during the substorm.

We present a statistical study on reconnection occurrence at the dayside magnetopause performed using the Double Star TC1 plasma and magnetic field data. We examined the magnetopause crossings that occurred during the first year of the mission in the 0600-1800 LT interval and we identified plasma flows, at the magnetopause or in the boundary layer, with a different velocity with respect to the adjacent magnetosheath. We used the Walén relation to test which of these flows could be generated by magnetic reconnection. For some event we observed opposite-directed reconnection jets, which could be associated with the passage of the X-line near the satellite. We analyzed the occurrence of the reconnection jets and reconnection jet reversals in relation to the magnetosheath parameters, in particular the local Alfvèn Mach number, the plasma beta, and the magnetic shear angle. We also studied the positions and velocities of the reconnection jets and jet reversals in relation to the magnetosheath magnetic field clock angle. We found that the observations indicate the presence of a reconnection line hinged near the subsolar point and tilted according to the observed magnetosheath clock angle, consistently with the component merging model.

We report Double Star spacecraft observations of the dusk-flank magnetopause and its boundary layer under predominantly northward interplanetary magnetic field (IMF). Under such conditions the flank low-latitude boundary layers (LLBL) of the magnetosphere are known to broaden. The primary candidate processes associated with the transport of solar wind plasma into the LLBL are: (1) local diffusive plasma transport associated with the Kelvin-Helmholtz instability (KHI), (2) local plasma penetration owing to magnetic reconnection in the vicinity of the KHI-driven vortices, and (3) via a pre-existing boundary layer formed through double high-latitude reconnection on the dayside. Previous studies have shown that a cold population of solar wind origin is typically mixed with a hot population of magnetospheric origin in the LLBL. The present observations show the coexistence of three distinct ion populations in the dusk LLBL, during an interval when the magnetopause is unstable to the KHI: (1) a typical hot magnetospheric population, (2) a cold population that shows parallel temperature anisotropy, and (3) a distinct third cold population that shows perpendicular temperature anisotropy. Although no unambiguous conclusion may be drawn from this single event, we discuss the possible mechanisms at work and the origin of each population by envisaging three likely sources: hot magnetospheric plasma sheet, cold magnetosheath of solar wind origin, and cold plasma of ionospheric origin.

A method has been developed for extracting magnetospheric ion distributions from Energetic Neutral Atom (ENA) measurements made by the NUADU instrument on the TC-2 spacecraft. Based on a constrained linear inversion, this iterative technique is suitable for use in the case of an ENA image measurement, featuring a sharply peaked spatial distribution. The method allows for magnetospheric ion distributions to be extracted from a low-count ENA image recorded over a short integration time (5 min). The technique is demonstrated through its application to a set of representative ENA images recorded in energy Channel~2 (hydrogen: 50-81 keV, oxygen: 138-185 keV) of the NUADU instrument during a geomagnetic storm. It is demonstrated that this inversion method provides a useful tool for extracting ion distribution information from ENA data that are characterized by high temporal and spatial resolution. The recovered ENA images obtained from inverted ion fluxes match most effectively the measurements made at maximum ENA intensity.

We present energetic neutral atom (ENA) images in the energy range 45 to 50 keV for H and 92 to 138 keV for O measured by the Neutral Atom Detector Unit (NUADU) onboard Double Star TC-2 during a geomagnetic storm on 8 May 2005. We compare the ion fluxes deduced from inversion of the NUADU image with those calculated using the Comprehensive Ring Current Model (CRCM). This comparison shows that the two approaches are consistent when used to derive the configuration of the corresponding global ion distribution and the peak ion fluxes. The deduced peak ion flux is located in the premidnight sector at 1540 UT, while the deduced ion peak flux is located in the midnight sector at 1610 UT. There are strong ion fluxes in the region between L = 2 and L = 4 which form a closed loop configuration. The ion peak flux is about 2.2×10⁶/cm²/sr/keV/s. The deduced ion distribution agrees well with the NUADU measurement. The agreement between the inverted ion distributions and the CRCM results give us confidence in applying our ENA imaging and modeling techniques to the study of the evolution of the inner magnetosphere plasma distribution and the global dynamics of the ring current during magnetic storms.

This study uses two conjunctions between Cluster and Double Star TC-1 spacecraft together with global magnetohydrodynamic (MHD) simulations to investigate the large-scale configuration of magnetic reconnection at the dayside magnetopause. Both events involve southward interplanetary magnetic fields with significant B_y components. The first event occurred on 8 May 2004, while both spacecraft were exploring the dawn flank of the magnetosphere; TC-1 was skimming the magnetopause whereas Cluster was exploring higher latitudes. Results from a global MHD simulation show the formation of an equatorial merging line in the morning sector and suggest that the three-dimensional geometry of the merging region is mostly a radial juxtaposition of planes displaying X-type reconnection geometries. The second conjunction was on 6 April 2004. During this event, Cluster was located at high latitudes and close to the noon-midnight meridian, while TC-1 was exploring the dawnside at low latitudes. Analysis of the simulation reveals that both antiparallel and component merging occurred simultaneously. Three-dimensional rendering of the parallel electric field indicates that component merging initiated in the subsolar magnetopause. Simulation runs carried out using different parameters in the model suggest that the spread of the merging region depends on the local resistivity. The subsolar-merging region grows with increasing resistivity values and becomes patchy when a resistivity threshold is used. However, the region of component merging appears to remain spatially constrained to the subsolar region where stronger parallel electric fields occur and no clear connection with the antiparallel-merging regions is found for the range of parameters surveyed.

The objective of the paper is to asses the specific spectral scaling properties of magnetic reconnection associated fluctuations/turbulence at the earthward and tailward outflow regions observed simultaneously by the Cluster and Double Star (TC-2) spacecraft on 26 September 2005. Systematic comparisons of spectral characteristics, including variance anisotropy and scale-dependent spectral anisotropy features in wave vector space were possible due to the well-documented reconnection events, occurring between the positions of Cluster (X = - 14-16 R_e) and TC-2 (X = - 6.6 R_e). Another factor of key importance is that the magnetometers on the spacecraft are similar. The comparisons provide further evidence for asymmetry of physical processes in earthward/tailward reconnection outflow regions. Variance anisotropy and spectral anisotropy angles estimated from the multiscale magnetic fluctuations in the tailward outflow region show features which are characteristic for magnetohydrodynamic cascading turbulence in the presence of a local mean magnetic field. The multiscale magnetic fluctuations in the earthward outflow region are not only exhibiting more power, lack of variance, and scale-dependent anisotropies but also are having larger anisotropy angles. In this region the magnetic field is more dipolar and the main processes driving turbulence are flow breaking/mixing, perhaps combined with turbulence ageing and noncascade-related multiscale energy sources.

On October 8, 2004, the Cluster and Double Star spacecraft crossed the near-Earth (12-19 R_E) magnetotail neutral sheet during the recovery phase of a small, isolated substorm. Although they were separated in distance by ~7 R_E and in time by ~30 min, both Cluster and Double Star observed steady, but highly structured Earthward moving >1000 km/s high speed H⁺ beams in the PSBL. This paper utilizes a global magnetohydrodynamic (MHD) simulation driven by Wind spacecraft solar wind input to model the large-scale structure of the PSBL and large-scale kinetic (LSK) particle tracing calculations to investigate the similarities and differences in the properties of the observed beams. This study finds that the large-scale shape of the PSBL is determined by the MHD configuration. On smaller scales, the LSK calculations, in good qualitative agreement with both Cluster and Double Star observations, demonstrated that the PSBL is highly structured in both time and space, on time intervals of less than 2 min, and spatial distances of the order of 0.2-0.5 R_E. This picture of the PSBL is different from the ordered and structured region previously reported in observations.

The study of the neutral sheet is of fundamental importance in understanding the dynamics of the Earth's magnetosphere. From the earliest observation of the magnetotail, it has been found that the neutral sheet frequently appears to be in motion due to changing solar wind conditions and geomagnetic activity. Multiple crossings of the neutral sheet by spacecraft have been attributed to a flapping motion of the neutral sheet in the north-south direction, a wavy profile either along the magnetotail or the dawn-dusk direction. Cluster observations have revealed that the flapping motions of the Earth's magnetotail are of internal origin and that kink-like waves are emitted from the central part of the tail and propagate toward the tail flanks. This flapping motion is shown here to propagate at an angle of ~45° with x_GSM. A possible assumption that the flapping could be created by a wake travelling away from a fast flow in the current sheet is rejected. Other waves in the magnetotail are found in the ULF range. One conjunction event between Cluster and DoubleStar TC1 is presented where all spacecraft show ULF wave activity at a period of approximately 5 min during fast Earthward flow. These waves are shown to be Kelvin-Helmholtz waves on the boundaries of the flow channel. Calculations show that the conversion of flow energy into magnetic energy through the Kelvin-Helmholtz instability can contribute to a significant part of flow breaking between Cluster and DoubleStar TC1.

Accepted for publication. Unedited accepted manuscript available online 7 February 2008 as PDF