Nightside Plasma Sheet Research Articles

THEMIS observations of electron acceleration associated with the evolution of substorm dipolarization in the near‐Earth tail

AbstractWe present the evolution of dipolarizations in the near‐Earth tail during a substorm on 15 March 2009, based on the two‐point measurements in the nightside plasma sheet at X ~ −8.0 RE. The earthward‐moving dipolarization, the magnetic flux pileup, and the tailward‐moving dipolarization were observed. For the 30–200 keV electrons, betatron acceleration was the dominant process, which was caused by the much larger gradient of the magnetic field there during the earthward‐moving dipolarization or by a local compression of the magnetic field during the magnetic flux pileup and tailward‐moving dipolarization. These near‐perpendicular distributions for the 30–200 keV electrons are interpreted as produced by a two‐step acceleration: Electrons were first accelerated in the dipolarization fronts in the midtail or the near‐Earth tail and then were further accelerated near the tail current disruption region. For the more than 200 keV electrons, Fermi acceleration was the dominant process, which was caused by the shrinking length of magnetic field line during the tailward‐moving dipolarization. The source region of the more than 200 keV electrons may be near the tail current disruption region, but these electrons were accelerated locally. These field‐aligned electrons can precipitate into the ionosphere and form the discrete auroral arcs. Two parallel arcs were clearly observed around the substorm onset: one propagated equatorward, another propagated poleward. We suggest that the earthward‐moving dipolarization, the magnetic flux pileup, and the tailward‐moving dipolarization near the tail current disruption region can well explain the auroral evolution around the substorm onset.

Reduction of viscous potential for northward interplanetary magnetic field as seen in the LFM simulation

The two primary methods responsible for solar wind magnetosphere coupling are magnetic reconnection and the viscous interaction. The viscous interaction is generated due to the antisunward dragging of plasma inside the magnetopause by the plasma flowing in the magnetosheath, creating a return flow deeper inside the magnetosphere and producing a circulation pattern. This viscous circulation pattern is mapped into the ionosphere via magnetic field lines, which results in ionospheric electric field in the nonrotating Earth's frame. We measure this interaction in terms of an electric potential, the viscous potential. In this paper, we use the results obtained from the Lyon‐Fedder‐Mobarry (LFM) simulation model during periods of purely northward interplanetary magnetic field (IMF) for different solar wind velocity and ionospheric conductivity, showing a reduction of the viscous potential with increasing magnitude of northward IMF. The viscous potential is found to settle around 5–10 kV for large +Bz values. The decrease in viscous potential was found to be associated with a weak or nonexistent sunward plasma flow in the nightside plasmasheet. Instead, the return flow to the dayside occurs at high latitudes and is associated with the reconnection topology and dynamics that occur during northward IMF periods. We also show that the magnetosphere remains closed during purely northward IMF, except for two small regions—one on each hemisphere, where the magnetic reconnection occur. We argue that the reduction of the viscous potential is due to a reduction of the velocity shear across the magnetopause and the lack of sunward convection in the equatorial tail.

Magnetospheric convection and magnetopause shadowing effects in ULF wave‐driven energetic electron transport

Magnetospheric radiation belt electron transport in the presence of ULF waves and a convection electric field is investigated using a model that includes a nightside plasma sheet source and electron losses by magnetopause shadowing. Narrow‐band ULF waves launched from a prescribed dayside magnetopause source are shown to interact with trapped and untrapped equatorially mirroring electrons within the magnetosphere. For magnetic moments less than 8 keV/nT and a strong convection electric field (in the order of 5 mV/m), we find that a limb of untrapped plasma sheet electrons extending across the dayside magnetosphere into the afternoon sector provides a phase space density (PSD) source for injection to lower L‐shells by ULF waves, causing a rapid enhancement in PSD. However, the same ULF wave activity gives rise to a rapid dropout in PSD for electrons with a higher magnetic moment or in the presence of a weaker convection electric field, since in this case the plasma sheet electrons escape through the magnetopause before they reach the afternoon sector. In their place, a lack of PSD, or PSD “holes” can be periodically injected from the magnetopause to lower L‐shells by ULF waves, leading to the rapid depletion in average PSD. In each case, the magnitude and extent in L‐shell of the PSD enhancement or depletion is strongly dependent on the amplitude of ULF waves in the afternoon sector, and is significantly augmented by the overlap of drift‐resonant islands and an associated stochastic transport layer.

THEMIS observations of ULF wave excitation in the nightside plasma sheet during sudden impulse events

Sudden impulses (SIs) are an important source of ultra low frequency (ULF) wave activity throughout the Earth's magnetosphere. Most SI‐induced ULF wave events have been reported in the dayside magnetosphere; it is not clear when and how SIs drive ULF wave activity in the nightside plasma sheet. We examined the ULF response of the nightside plasma sheet to SIs using an ensemble of 13 SI events observed by THEMIS (Timed History of Events and Macroscale Interactions during Substorms) satellites (probes). Only three of these events resulted in ULF wave activity. The periods of the waves are found to be 3.3, 6.0, and 7.6 min. East‐west magnetic and radial electric field perturbations, which typically indicate the toroidal mode, are found to be stronger and can have phase relationships consistent with standing waves. Our results suggest that the two largest‐amplitude ULF responses to SIs in the nightside plasma sheet are tailward‐moving vortices, which have previously been reported, and the dynamic response of cross‐tail currents in the magnetotail to maintain force balance with the solar wind, which has not previously been reported as a ULF wave driver. Both mechanisms could potentially drive standing Alfvén waves (toroidal modes) observed via the field‐line resonance mechanism. Furthermore, both involve frequency selection and a preference for certain driving conditions that can explain the small number of ULF wave events associated with SIs in the nightside plasma sheet.

Characteristics of the plasma distribution in Mercury's equatorial magnetosphere derived from MESSENGER Magnetometer observations

Localized reductions in the magnetic field associated with plasma pressure in Mercury's magnetospheric cusp and nightside plasma sheet have been routinely observed by the Magnetometer on the MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) spacecraft. We present a statistical analysis of near‐equatorial magnetic depressions to derive the structure of Mercury's plasma sheet pressure. Because the plasma pressure in the magnetosphere correlates with solar wind density, the pressures were normalized to a Mercury heliocentric distance of 0.39 AU. A model magnetic field was used to map observations obtained on the ascending and descending orbit nodes to the magnetic equator and revealed the presence of plasma in a toroidal section extending on the nightside from dusk to dawn. Mapping the data to invariant magnetic latitude shows that the pressure is symmetric about the magnetic equator. The average pressure normalized for heliocentric distance is 1.45 nPa and exhibits a weak, 0.05 nPa/h, dusk‐to‐dawn gradient with local time. The plasma sheet pressure can vary between successive orbits by an order of magnitude. Unlike the predictions of some global simulations of Mercury's magnetosphere, the plasma enhancements do not form a closed distribution around the planet. This difference may arise from the idealized solar wind and interplanetary magnetic field conditions used in the simulations, which maximize the size and stability of the magnetosphere, thus promoting the formation of drift paths that close around the planet. For typical plasma sheet energies, 5 keV, the first adiabatic invariant for protons fails to be conserved even within 500 km altitude at midnight, implying that stochastic processes must be considered in plasma sheet transport.

Can flow bursts penetrate into the inner magnetosphere?

[1] Recent studies have shown that only a small fraction of fast-flow bursts observed at mid-tail penetrate into the inner magnetosphere, raising questions regarding their role in particle injections. Motivated by these findings, we compared observations at two radially-aligned Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft in the high-beta nightside plasma sheet to determine which physical parameter controls the penetration efficiency of flow bursts observed at the outer spacecraft. We showed that the inferred plasma tube entropy PV5/3 demonstrates better prediction efficiency than other parameters (e.g., Vx or Bz). Comparing its minimal value at the outer spacecraft during the flow burst to its preflow value at the inner spacecraft allows us to distinguish between penetrating and non-penetrating events. Our results explain the relatively small number of deeply penetrating BBFs and provide a strong argument in favor of the bubble model of fast-flow bursts and plasma injections.

Dynamics and seasonal variations in Saturn's magnetospheric plasma sheet, as measured by Cassini

We analyze electron plasma, energetic ion, and magnetic field data from four almost vertical Cassini passes through the nightside plasma sheet of Saturn (segments of the high-latitude orbits of the spacecraft) separated in two subsets: two passes of identical geometry from January 2007 with Cassini crossing the equatorial plane in the postmidnight sector at a distance of similar to 21 Saturn radii (R-S) and two passes from April 2009, also of identical geometry, with Cassini crossing the equatorial plane in the premidnight sector again at a distance of similar to 21 R-S. The vertical structure and variability of the plasma sheet is described for each individual pass, and its basic properties (scale height, vertical displacement, tilt angle, hinging distance) are computed. The plasma sheet presents an energy-dependent vertical structure, being thicker by a factor of similar to 2 in the energetic particle range than in the electron plasma. It further exhibits intense dynamical behavior, evident in the energetic neutral atom emission. In two of the four passes, we observe a clear north-south asymmetry, presumably a combined result of vertical plasma sheet motion and short time scale dynamics. Comparison between the 2007 and 2009 passes reveals a clear change in the tilt and vertical offset of the planetary nightside plasma sheet, which progressively becomes aligned to the solar wind direction as we approach Saturnian equinox (August 2009). Temperature, pressure, and number density in the center of the sheet remain relatively stable and essentially unaffected by the seasonal change.

The effect of smoothed solar wind inputs on global modeling results

This study investigates the role of fluctuations in the solar wind parameters in triggering a magnetic storm and assesses the storm simulation ability of the Space Weather Modeling Framework (SWMF) through a model–data comparison. The event of 22 September 1999 is examined through global magnetosphere simulations, using as input Advanced Composition Explorer (ACE) observations (4 min temporal resolution) along with running averages of this data with windows of 60, 120, and 180 min. It is noted that for this storm the model produces a two phase, fast then slow recovery phase due to a sudden drop in plasma sheet density during the interval of southward interplanetary magnetic field (IMF). Also, smoothing the input with a window larger than 60 min changes the entire magnetosphere and reduces the plasma sheet density and pressure, therefore a less intense storm develops. It is worth mentioning that the main phase (measured from Storm Sudden Commencement to minimum Dst) for this magnetic storm lasted about 3 h. This explains the change in the Dst profile for the 120 and 180 min averaged input. Averaging only IMF Bz or solar wind density reveals that all input parameters are important for the development of the storm, but Bz is the most significant. Also, comparison with Dst predictions (using the formula of O'Brien and McPherron (2000)) are presented and discussed. For all cases studied, there are no significant differences for Cross Polar Cap Potential (CPCP) in both hemispheres, while the nightside plasma sheet density shows a sharp drop when the input is averaged over 60 min or more. Our results indicate that the magnetosphere responds nonlinearly to the changes in the energy input, suggesting the need for a threshold in the amount of energy transferred to the system in order for the ring current to develop. Further increase of the energy input leads to a saturation limit where the inner magnetosphere response is no longer affected by any additional amount of energy contained within high‐frequency oscillations, because the magnetospheric system acts as a low‐pass filter on the interplanetary magnetic field.

Evidence that solar wind fluctuations substantially affect global convection and substorm occurrence

We have used examples of Poker Flat and Sondrestrom incoherent‐scatter radar observations of flows within the ionospheric mapping of the nightside plasma sheet and of Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft observations within the nightside plasma sheet to investigate whether the features found in the companion paper by Kim et al. (2009) within the dayside polar cap are also seen within the nightside plasma sheet. We find evidence that this is indeed the case: intensified interplanetary ULF fluctuations substantially enhance nightside convection flows and the fluctuations are reflected in the fluctuation power of the nightside flows. Additionally, our observations show evidence for an enhancement of earthward convection within the inner plasma sheet and an increase in plasma pressure within the plasma sheet in association with enhanced interplanetary ULF fluctuations. We have also found evidence that the enhancement in convection and plasma sheet pressure associated with strong interplanetary fluctuations may lead to a dramatic increase in substorm occurrence under northward interplanetary magnetic field conditions. More detailed testing of the above results is needed. However, if corroborated, it would indicate that interplanetary ULF fluctuations have a substantial effect on global convection and are an important contributor to the large‐scale transfer of solar wind energy to the magnetosphere‐ionosphere system, to plasma sheet structure and dynamics, and to the occurrence of disturbances such as substorms.

Plasma sheet P5/3 and n and associated plasma and energy transport for different convection strengths and AE levels

To understand the large‐scale plasma sheet thermodynamics, we have used Geotail data and a formula for estimating flux tube volume to investigate statistically the equatorial distributions of P5/3 and n for slow flowing plasma in the nightside plasma sheet and compare them with the physical bases of ideal MHD and the Rice Convection Model (RCM). We have examined the distributions under three conditions: (1) weak convection and low AE, (2) enhanced convection and low AE, and (3) enhanced convection and high AE. The overall n decreases significantly with increasing convection or AE, while the overall P5/3 remains similar. We found that P5/3 drops significantly earthward along the estimated electric drift paths near midnight, inconsistent with ideal MHD. Examination of Pk5/3 and the electric and magnetic drift paths of different energy invariants, where Pk is the partial pressure of a specific energy invariant, shows that the strong duskward drift of the above thermal‐energy particles due to magnetic drift, together with there being significantly fewer higher‐energy particles from the dawn flank than from the tail, results in the strong earthward decrease of P5/3. We also found that d(Pk5/3)/dt = 0 and d(nk)/dt = 0 along the electric and magnetic drift paths used in the RCM are good approximations for slow flowing pressure‐bearing plasma sheet ions. The distributions of n indicate that low‐energy plasma becomes dominant when convection and AE are low, and their transport from the flanks may be affected by nonadiabatic processes not included in ideal MHD or the RCM.

Evolution of dipolarization in the near-Earth current sheet induced by Earthward rapid flux transport

Abstract. We report on the evolution of dipolarization and associated disturbances of the near-Earth current sheet during a substorm on 27 October 2007, based upon Cluster multi-point, multi-scale observations of the night-side plasma sheet at X~−10 RE. Three dipolarization events were observed accompanied by activations on ground magnetograms at 09:07, 09:14, and 09:22 UT. We found that all these events consist of two types of dipolarization signatures: (1) Earthward moving dipolarization pulse, which is accompanied by enhanced rapid Earthward flux transport and is followed by current sheet disturbances with decrease in BZ and enhanced local current density, and subsequent (2) increase in BZ toward a stable level, which is more prominent at Earthward side and evolving tailward. During the 09:07 event, when Cluster was located in a thin current sheet, the dipolarization and fast Earthward flows were also accompanied by further thinning of the current sheet down to a half-thickness of about 1000 km and oscillation in a kink-like mode with a period of ~15 s and propagating duskward. Probable cause of this "flapping current sheet" is shown to be the Earthward high-speed flow. The oscillation ceased as the flow decreased and the field configuration became more dipolar. The later rapid flux transport events at 09:14 and 09:22 UT took place when the field configuration was initially more dipolar and were also associated with BZ disturbance and local current density enhancement, but to a lesser degree. Hence, current sheet disturbances induced by initial dipolarization pulses could differ, depending on the configuration of the current sheet.

Imaging cold ions in the plasma sheet from the Equator‐S satellite

Energy‐dispersed H+ structures observed by Equator‐S in the sub‐keV range in the dawn sector of the ring current region were used to perform the remote sensing of cold ions in the near‐earth plasma sheet. A time history of the source distribution function in the nightside plasma sheet was reconstructed by using the phase space mapping method under a time‐dependent convection electric field model. We obtained the following major conclusions: (1) A cold H+ component (∼10 eV) exists in the near‐earth plasma sheet. (2) The cold component is readily distinguished from the main component of the plasma sheet (>∼a few keV). (3) The cold component is distributed over a relatively wide area in MLT. (4) Energy‐dispersed H+ structures in the inner magnetosphere results from temporal variation of the cold component. Our scheme will be applicable for investigating long‐term plasma processes acting on the cold component whose behavior is largely unknown.

The storm‐time access of solar wind ions to the nightside ring current and plasma sheet

We investigated the storm‐time entry of solar wind ions into the magnetosphere by carrying out a large‐scale kinetic particle tracing calculation for the 24–25 September 1998 and the 17 April 2002 geomagnetic storms. For each storm, we simulated the magnetosphere by using a global magnetohydrodynamic (MHD) code that was driven by solar wind and interplanetary magnetic field (IMF) data from spacecraft upstream of Earth. We then launched ions in the solar wind in the global time‐dependent fields obtained from the MHD simulation, beginning prior to storm onset and extending into the main phase. We collected those ions that successfully reached the nightside plasma sheet and ring current, and then determined the mechanisms of entry and transport responsible for the injection of these ions into the magnetotail. We found that, in agreement with quiet time entry results, the IMF By component strongly influenced whether ions entered through the dawn or dusk flanks, and changes in IMF By were immediately reflected on the ion entry pattern. Also, in addition to the usual dayside merging source, ion entry into the magnetosphere was facilitated by dynamic pressure enhancements, during which entry occurred over a wide swath of latitudes and extended from the dayside to locations far downtail. More significantly, because of the warmer storm‐time ion population in the magnetosheath, ions were more readily affected by inhomogeneities and rapid changes in the IMF, which caused solar wind ions to gradient drift onto open field lines and reach the inner magnetosphere.

Comment on “Jupiter: A fundamentally different magnetospheric interaction with the solar wind” by D. J. McComas and F. Bagenal

Comment on “Jupiter: A fundamentally different magnetospheric interaction with the solar wind” by D. J. McComas and F. Bagenal

Solar wind dependence of ion parameters in the Earth's magnetospheric region calculated from CLUSTER observations

Abstract. Moments calculated from the ion distributions (~0–40 keV) measured by the Cluster Ion Spectrometry (CIS) instrument are combined with data from the Cluster Flux Gate Magnetometer (FGM) instrument and used to characterise the bulk properties of the plasma in the near-Earth magnetosphere over five years (2001–2005). Results are presented in the form of 2-D xy, xz and yz GSM cuts through the magnetosphere using data obtained from the Cluster Science Data System (CSDS) and the Cluster Active Archive (CAA). Analysis reveals the distribution of ~0–40 keV ions in the inner magnetosphere is highly ordered and highly responsive to changes in solar wind velocity. Specifically, elevations in temperature are found to occur across the entire nightside plasma sheet region during times of fast solar wind. We demonstrate that the nightside plasma sheet ion temperature at a downtail distance of ~12 to 19 Earth radii increases by a factor of ~2 during periods of fast solar wind (500–1000 km s−1) compared to periods of slow solar wind (100–400 km s−1). The spatial extent of these increases are shown in the xy, xz and yz GSM planes. The results from the study have implications for modelling studies and simulations of solar-wind/magnetosphere coupling, which ultimately rely on in situ observations of the plasma sheet properties for input/boundary conditions.

An auroral westward flow channel (AWFC) and its relationship to field-aligned current, ring current, and plasmapause location determined using multiple spacecraft observations

Abstract. An auroral westward flow channel (AWFC) is a latitudinally narrow channel of unstable F-region plasma with intense westward drift in the dusk-to-midnight sector ionosphere. AWFCs tend to overlap the equatorward edge of the auroral oval, and their life cycle is often synchronised to that of substorms: they commence close to substorm expansion phase onset, intensify during the expansion phase, and then decay during the recovery phase. Here we define for the first time the relationship between an AWFC, large-scale field-aligned current (FAC), the ring current, and plasmapause location. The Tasman International Geospace Environment Radar (TIGER), a Southern Hemisphere HF SuperDARN radar, observed a jet-like AWFC during ~08:35 to 13:28 UT on 7 April 2001. The initiation of the AWFC was preceded by a band of equatorward expanding ionospheric scatter (BEES) which conveyed an intense poleward electric field through the inner plasma sheet. Unlike previous AWFCs, this event was not associated with a distinct substorm surge; rather it occurred during an interval of persistent, moderate magnetic activity characterised by AL~−200 nT. The four Cluster spacecraft had perigees within the dusk sector plasmasphere, and their trajectories were magnetically conjugate to the radar observations. The Waves of High frequency and Sounder for Probing Electron density by Relaxation (WHISPER) instruments on board Cluster were used to identify the plasmapause location. The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) EUV experiment also provided global-scale observations of the plasmapause. The Cluster fluxgate magnetometers (FGM) provided successive measurements specifying the relative location of the ring current and filamentary plasma sheet current. An analysis of Iridium spacecraft magnetometer measurements provided estimates of large-scale ionospheric FAC in relation to the AWFC evolution. Peak flows in the AWFC were located close to the peak of a Region 2 downward FAC, located just poleward of the plasmapause. DMSP satellite observations confirmed the AWFC was located equatorward of the nightside plasmasheet, sometimes associated with ~10 keV ion precipitation.

Theory of energetic particle transport in the magnetosphere: A noncanonical approach

This paper presents a new transport theory suitable for the study of energetic particles in the magnetospheric environment. A Fokker‐Planck diffusion equation governing the variation of the particle distribution function, along with its isotropic and bounce average approximations, is established in the phase space of location and momentum, which is similar to how cosmic rays are treated by the heliospheric community. The equation includes essentially all the particle transport mechanisms: streaming, convection, drift, adiabatic energy change, acceleration by parallel electric field, focusing, and diffusions in location, momentum, and pitch angle. All the coefficients of the equation are directly linked to plasma and magnetic configurations including electromagnetic fluctuations. The theory can form a base for developing full‐scale numerical models of energetic particle transport including acceleration in the magnetosphere. Unlike the conventional theory using canonical variables which is only applicable to the radiation belt of the inner magnetosphere, this theory includes the effects of nonuniform magnetospheric convection and thus is valid for the entire magnetosphere. The derivation of the transport equation has identified some important particle transport mechanisms that have not received careful study by the magnetospheric community. It is found that the compression of magnetospheric plasma can play a significant role in particle acceleration and trapping. The compressional acceleration is the first‐order particle acceleration mechanism, very similar to particle acceleration at shock waves in other space plasma environments. This acceleration mechanism is operable to both electrons and ions. A model map of magnetospheric convection flow shows that the compressional particle acceleration is a large‐scale phenomenon and it is the strongest in the near‐Earth nightside plasma sheet. Magnetospheric compression can also drive particles toward the 90° equatorial pitch angle, which is ideal for trapping them in the geomagnetic field.

Equatorial distributions of the plasma sheet ions, their electric and magnetic drifts, and magnetic fields under different interplanetary magnetic field Bz conditions

To understand the nightside plasma sheet structure under different interplanetary magnetic field (IMF) Bz conditions, we have investigated statistically the equatorial distributions of ions and magnetic fields from Geotail when the IMF has been continuously northward or southward for shorter or longer than 1 hour. A dawn‐dusk density (temperature) asymmetry with higher density (temperature) on the dawn (dusk) side is seen in the near‐Earth plasma sheet during northward IMF, resulting in roughly dawn‐dusk symmetric pressure. As southward IMF proceeds, the density asymmetry weakens while the temperature asymmetry maintains, resulting in higher pressure on the dusk side. The plasma sheet is relatively colder and denser near the flanks than around midnight. The flux distributions show that the density asymmetry is due to ions <∼3 keV, and the temperature asymmetry is due to ions above thermal energy. The perpendicular flow shows that ions divert around the Earth mainly through the dusk side in the inner plasma sheet because of westward diamagnetic drift. The magnetic fields indicate that field lines are more stretched during southward IMF. Ions' electric and magnetic drift paths evaluated from the observations show that for thermal energy ions, magnetic drift is as important as electric drift. Comparison of the distributions of the observed phase space density with the evaluated drift paths at different energies indicates that the electric and magnetic drift transport is responsible for the observed dawn‐dusk asymmetries in the plasma sheet structure.

Energetic electron signatures in an active magnetotail plasma sheet

Particles with energies of tens to hundreds of keV provide a powerful diagnostic of the acceleration processes that characterise the Earth’s magnetosphere, in particular the highly dynamic nightside plasma sheet. Such energetic particles can be detected by the RAPID experiment, onboard the quartet of Cluster spacecraft. We present results from the study of a series of quasi-periodic, intense energetic electron signatures in the magnetotail revealed by RAPID Imaging Electron Spectrometer (IES) observations some 19 Earth radii ( R E) downtail, associated with the passage of a highly geoeffective, high-speed solar wind stream. The RAPID-IES signatures – interpreted in combination with magnetic field and lower-energy electron measurements from the FGM and PEACE experiments on Cluster, respectively, and with reference to energetic electron observations from the CEPPAD-IES instrument on Polar – are understood in terms of repeated encounters of the Cluster spacecraft with the tail plasma sheet in response to the resultant tail reconfiguration in each of a series of substorms. We consider the Cluster response for two of these substorms (identified according to the conventional expansion phase onset indicators of particle injection at geosynchronous orbit and Pi2 pulsations at Earth) in terms of two possible tail configurations in which a Near-Earth Neutral Line forms either antisunward or sunward of the Cluster spacecraft. The latter scenario, in which the reconnection X-line is assumed to form sunward of Cluster and subsequently migrate downtail such that the spacecraft become engulfed in a tailward expanding plasma sheet, is shown to be more consistent with the observations.

The HIA instrument on board the Tan Ce 1 Double Star near-equatorial spacecraft and its first results

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 RE 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 ion beams close to the magnetopause. The huge interest in the orbit of TC-1 is strongly demonstrated.