A. Roux

Fast flow during current sheet thinning

[1]xa0In this paper we report Cluster observation of a fast flow event in the plasma sheet associated with a small auroral substorm intensification at 1838 UT on August 12, 2001. Cluster, located in the plasma sheet, experienced significant thinning of the current sheet associated with a high-speed Earthward flow of 900 km/s. By using the four spacecraft magnetic field data and a Harris-type current sheet model, it was estimated that the thickness of the current sheet changes from about 1 RE before the flow observation down to 400 km, i.e., close to the ion inertia length. In the vicinity of this thin current sheet there were also signatures of enhanced current density off the center of the neutral sheet, consistent with recent Geotail results.

Magnetic island formation between large-scale flow vortices at an undulating postnoon magnetopause for northward interplanetary magnetic field

[1]xa0Time History of Events and Macroscale Interactions during Substorms multispacecraft observations are presented for a ∼2-h-long postnoon magnetopause event on 8 June 2007 that for the first time indicate that the trailing (sunward) edges of Kelvin-Helmholtz (KH) waves are commonly related to small-scale <0.56 RE magnetic islands or flux transfer events (FTE) during the growth phase of these surface waves. The FTEs typically show a characteristic bipolar BN structure with enhanced total pressure at their center. Most of the small-scale FTEs are not related to any major plasma acceleration. TH-A observations of one small FTE at a transition from the low-latitude boundary layer (LLBL) into a magnetosheath plasma depletion layer were reconstructed using separate techniques that together confirm the presence of a magnetic island within the LLBL adjacent to the magnetopause. The island was associated with a small plasma vortex and both features appeared between two large-scale (∼1 RE long and 2000 km wide) plasma vortices. We propose that the observed magnetic islands may have been generated from a time-varying reconnection process in a low ion plasma beta (βi < 0.2) and low 8.3° field shear environment at the sunward edge of the growing KH waves where the local magnetopause current sheet may be compressed by the converging flow of the large-scale plasma vortices as suggested by numerical simulations of the KH instability.

Characterization of small-scale structures at the magnetopause from ISEE measurements

High-resolution data (covering up to 8 Hz) from the flux gate magnetometers on the two ISEE spacecraft are used to analyze ultralow-frequency (ULF) fluctuations observed at the magnetopause and in the adjacent layers. Intersatellite correlations are computed to show that the same structure can be identified in the fluctuations observed on both spacecraft when the interspacecraft distance is small. Then the possibility of deducing the velocity of the structure from two-point measurements is discussed; it is shown that it can be estimated only in certain cases.

Global distribution of electrostatic electron cyclotron harmonic waves observed on THEMIS

[1]xa0A global, statistical analysis of electrostatic electron cyclotron harmonic (ECH) waves is performed using THEMIS wave data. Our results confirm the high occurrence of 5). The strongest (≥1 mV/m) ECH waves are enhanced during geomagnetically disturbed periods, and are mainly confined close to the magnetic equator (∣λ∣ 8) magnetosphere where chorus emissions are statistically weak.

A model of electromagnetic electron phase-space holes and its application

[1]xa0Electron phase-space holes (EHs) are indicators of nonlinear activities in space plasmas. Most often they are observed as electrostatic signals, but recently Andersson et al. [2009] reported electromagnetic EHs observed by the THEMIS mission in the Earths plasma sheet. As a follow-up to Andersson et al. [2009], this paper presents a model of electromagnetic EHs where the δE × B0 drift of electrons creates a net current. The model is examined with test-particle simulations and compared to the electromagnetic EHs reported by Andersson et al. [2009]. As an application of the model, we introduce a more accurate method than the simplified Lorentz transformation of Andersson et al. [2009] to derive EH velocity (vEH). The sizes and potentials of EHs are derived from vEH, so an accurate derivation of vEH is important in analyzing EHs. In general, our results are qualitatively consistent with those of Andersson et al. [2009] but generally with smaller velocities and sizes.

What is the nature of magnetosheath FTEs

Cluster multipoint measurements are used to study two successive magnetosheath flux transfer events (FTEs). Magnetic field lines in the leading region are found to be closed magnetospheric field lines. For event 1 these field lines are wounded up by a large current structure oriented eastward and moving poleward. Conversely, the trailing region corresponds to opened magnetic field lines. For both events the leading edge of the FTEs is a tangential discontinuity separating the magnetosheath from closed field lines. In the case of event 1 magnetosheath ions are accelerated through the FTE trailing edge via a rotational discontinuity and penetrate on closed field lines through a second discontinuity. Thus, the ion jet is accelerated equatorward of the spacecraft but the backtracking of the discontinuities and the lack of dispersion show that ion acceleration occurs at less than 2u2009RE from Cluster. On the other hand the extrapolation forward indicates that the FTE bulge steepens as in simulations of Dorelli and Bhattacharjee (2009). Evidence is given for the penetration of magnetosheath ions inside the core of the FTE, on closed field lines. Magnetosheath electrons are accelerated in parallel and antiparallel directions on open and on closed field lines, thus breaking the frozen-in condition. Event 2 is also split in two distinct regions but no evidence is found for accelerated bidirectional magnetosheath electrons. For event 2 the two discontinuities at the trailing region are stacked together when they are crossed by the spacecraft, suggesting that the current splitting is a reconnection signature.

Io's interaction with the Jovian magnetosphere

When the Galileo orbiter passed within 1000 km of Io on December 7, 1995, the data collected were every bit as exciting as the Galileo team had expected, given the unique interaction between Io and the Jovian magnetosphere. The spacecraft detected intense beams of electrons propagating parallel to the magnetic field and a variety of unexpectedly strong wave phenomena surrounding Io that are associated with Ios production of new ions. There was also evidence for an intrinsic magnetic field, possibly generated by a dynamo within Io, and a cool “ionospheric”plasma flowing away from Io, removing tens of kilograms of material per second, not nearly a ton per second, as previously thought.