[1] Spacecraft potential measurements by the EFW electric field experiment on the Cluster satellites can be used to obtain plasma density estimates in regions barely accessible to other type of plasma experiments. Direct calibrations of the plasma density as a function of the measured potential difference between the spacecraft and the probes can be carried out in the solar wind, the magnetosheath, and the plasmashere by the use of CIS ion density and WHISPER electron density measurements. The spacecraft photoelectron characteristic (photoelectrons escaping to the plasma in current balance with collected ambient electrons) can be calculated from knowledge of the electron current to the spacecraft based on plasma density and electron temperature data from the above mentioned experiments and can be extended to more positive spacecraft potentials by CIS ion and the PEACE electron experiments in the plasma sheet. This

[1] Magnetic flux transfer events (FTEs) are signatures of unsteady magnetic reconnection, often observed at planetary magnetopauses. Their generation mechanism, a key ingredient determining how they regulate the transfer of solar wind energy into magnetospheres, is still largely unknown. We report THEMIS spacecraft observations on 2007-06-14 of an FTE generated by multiple X-line reconnection at the dayside magnetopause. The evidence consists of (1) two oppositely-directed ion jets converging toward the FTE that was slowly moving southward, (2) the cross-section of the FTE core being elongated along the magnetopause normal, probably squeezed by the oppositely-directed jets, and (3) bidirectional field-aligned fluxes of energetic electrons in the magnetosheath, indicating reconnection on both sides of the FTE. The observations agree well with a global magnetohydrodynamic model of the FTE generation under large geomagnetic dipole tilt, which implies the efficiency of magnetic flux transport into the magnetotail being lower for larger dipole tilt. Citation: Hasegawa, H., et al. (2010), Evidence for a flux transfer event generated by multiple X-line reconnection at the magnetopause, Geophys. Res. Lett., 37, L16101,

The bright night-time aurorae that are visible to the unaided eye are caused by electrons accelerated towards Earth by an upward-pointing electric field. On adjacent geomagnetic field lines the reverse process occurs: a downward-pointing electric field accelerates electrons away from Earth. Such magnetic-field-aligned electric fields in the collisionless plasma above the auroral ionosphere have been predicted, but how they could be maintained is still a matter for debate. The spatial and temporal behaviour of the electric fields-a knowledge of which is crucial to an understanding of their nature-cannot be resolved uniquely by single satellite measurements. Here we report on the first observations by a formation of identically instrumented satellites crossing a beam of upward-accelerated electrons. The structure of the electric potential accelerating the beam grew in magnitude and width for about 200 s, accompanied by a widening of the downward-current sheet, with the total current remaining constant. The 200-s timescale suggests that the evacuation of the electrons from the ionosphere contributes to the formation of the downward-pointing magnetic-field-aligned electric fields. This evolution implies a growing load in the downward leg of the current circuit, which may affect the visible discrete aurorae.

We have studied in detail multi‐spacecraft observations of the exterior cusp on 04 February 2001, during a steady northward Interplanetary Magnetic Field (IMF) interval. At a radial distance of 11 Re, Cluster encountered a well‐bounded region where the magnetic field exhibited very low diamagnetic values and the ions displayed high levels of isotropisation. We refer to this region as the Stagnant Exterior Cusp (SEC). Its equatorward edge is magnetopause like, whereas on the poleward side of the SEC, high‐speed plasma jets were observed consistent with a reconnection site poleward of the cusp. The SEC/magnetosheath boundary is characterized by abrupt changes in the magnetic field and plasma parameters that satisfy the Walén test, and by an S‐shaped magnetic hodogram. The latter may suggest the presence of an intermediate/slow transition.

[1] We analyze Cluster data to explore the statistical properties of the magnetosheath electron boundary layer, observed outside the high-latitude dayside magnetopause, under northward interplanetary magnetic field (IMF). We investigate the dependence of the presence and directionality of heated magnetosheath electrons in this layer on the geomagnetic dipole tilt and IMF tilt angles. The statistical results illustrate that the dipole tilt angle primarily controls the directionality of heated electrons in the magnetosheath boundary layer outside of the magnetopause. By contrast, the effect of the IMF tilt angle appears marginal. If the presence of such heated electrons is taken to be the signature of magnetosheath field lines that have reconnected with the high-latitude magnetic field of the Earth, tailward of the cusp, these results indicate that the dipole tilt determines in which hemisphere high-latitude reconnection of a given magnetosheath field line occurs first. The marginal impact of the IMF tilt angle may indicate that its potential effect is partially removed by the IMF passage through the bow shock and subsequent magnetic field draping at the dayside magnetopause. The frequent detection of bidirectional heated electrons outside the magnetopause additionally suggests that magnetosheath field lines may frequently reconnect in both hemispheres. Such a finding would support double highlatitude reconnection as a potential mechanism for low-latitude boundary layer formation under northward IMF.

[1] A sunlit conductive spacecraft, immersed in tenuous plasma, will attain a positive potential relative to the ambient plasma. This potential is primarily governed by solar irradiation, which causes escape of photoelectrons from the surface of the spacecraft, and the electrons in the ambient plasma providing the return current. In this paper we combine potential measurements from the Cluster satellites with measurements of extreme ultraviolet radiation from the TIMED satellite to establish a relation between solar radiation and spacecraft charging from solar maximum to solar minimum. We then use this relation to derive an improved method for determination of the current balance of the spacecraft. By calibration with other instruments we thereafter derive the plasma density. The results show that this method can provide information about plasma densities in the polar cap and magnetotail lobe regions where other measurements have limitations.

The partition of energy flux in magnetic reconnection is examined experimentally using Cluster satellite observations of collisionless reconnection in Earth's magnetotail. In this plasma regime, the dominant component of the energy flux is ion enthalpy flux, with smaller contributions from the electron enthalpy and heat flux and the ion kinetic energy flux. However, the Poynting flux is not negligible, and in certain parts of the ion diffusion region the Poynting flux in fact dominates. Evidence for earthward-tailward asymmetry is ascribed to the presence of Earth's dipole fields. DOI: 10.1103/PhysRevLett.110.225001 PACS numbers: 94.30.cp, 52.35.Vd, 94.30.ct In plasmas, magnetic reconnection across current sheets releases magnetic energy, heating the plasma and creating jets [1][2][3]. Since magnetic reconnection lies at the heart of numerous space, solar, astrophysical, and laboratory plasma phenomena, understanding the pathways and mechanisms by which released energy is divided into different forms is an important problem. This is particularly relevant for situations where detailed in situ measurements of the plasma and the reconnection region cannot be made, and/or for remote observations which rely on only one component of the plasma (e.g., electron-generated synchrotron radiation).Scaling arguments and resistive MHD simulations predict that for antiparallel symmetric reconnection configurations in the limit of inflow ! 0, the outward energy flux is split equally between the kinetic energy flux and the enthalpy flux [4]. However, more generally the enthalpy flux is predicted to exceed the kinetic energy flux [5,6]. Further analysis using hybrid simulations has shown that the ion enthalpy flux may in fact account for $75% of the outward energy flow ( inflow ¼ 0:1) [7]. A common feature of these studies, in addition to neglecting heat fluxes, is that the outward Poynting flux is negligible, in part because scaling arguments lead to the conclusion that the magnetic field in the outflow is small. However, recent particle-incell simulations have shown the existence of kinetic Alfvén wave structures in the vicinity of the separatrices, related to ion diffusion region Hall fields that are associated with collisionless reconnection [8,9]. These structures are associated with significantly larger Poynting fluxes in the reconnection outflow than previously expected and extend very long distances from the X line [10].While the existence of this Poynting flux was detected using in situ magnetotail data from the Cluster satellites [10,11], this previous work did not establish the extent to which this Poynting flux was significant in the context of other energy fluxes, nor did it establish the partition of energy flux. Here we address this question by presenting new analysis of the energy flux associated with antiparallel symmetric reconnection in the Earth's magnetotail. We concentrate on the vicinity of the diffusion region, rather than where the jets interact with the dipole field region, finding that whilst the io...

[1] We present initial results from a statistical study of Cluster multispacecraft flux transfer event (FTE) observations at the high-latitude magnetopause and low-latitude flanks from February 2001 to June 2003. Cluster FTEs are observed at both the highlatitude magnetopause and low-latitude flanks for both southward and northward IMF. Among the 1222 FTEs, 36%, 20%, 14%, and 30% are seen by one, two, three, and four Cluster satellites, respectively. There are 73% (27%) of the FTEs observed outside (inside) the magnetopause, which might be caused by the motion of FTEs toward the magnetosheath when they propagate from subsolar magnetopause to the midlatitude and high-latitude magnetopause and low-latitude flanks. We obtain an average FTE separation time of 7.09 min, which is at the lower end of the previous results. The mean B N peak-peak magnitude of Cluster FTEs is significantly larger than that from low-latitude FTE studies. FTE B N peak-peak magnitude clearly increases with increasing absolute magnetic latitude (MLAT), it has a weaker dependence on magnetic local time (MLT) with a peak near the magnetic local noon, and it has a complex dependence on Earth dipole tilt with a peak at around zero. FTE periodic behavior is found to be controlled by MLT, with a general increase of FTE separation time with increasing MLT, and by Earth dipole tilt, with a peak FTE separation time at around zero Earth dipole tilt. There is no clear dependence of FTE separation time on MLAT. There is a weak increase of FTE B N peak-peak magnitude with increasing FTE separation time, and we see no clear dependence of it on FTE B N peak-peak time. When no FTE identification thresholds are used, more accurate calculations of some FTE statistical parameters, including the mean B N peak-peak time, can be obtained. Further, comparing results with different thresholds can help obtain useful information about FTEs. Citation: Wang, Y., et al. (2005), Initial results of high-latitude magnetopause and low-latitude flank flux transfer events from 3 years of Cluster observations,