Patrick Robert

Plasma sheet instability related to the westward traveling surge

The detailed analysis of an isolated dispersionless substorm is performed on the basis of field and particle data collected in situ by the geostationary satellite GEOS 2 and of data from ground-based instruments installed close to the GEOS 2 magnetic footprint. These data give evidence for (1) quasi-periodic variations of the magnetic field configuration, which is alternatively taillike and dipolelike, (2) in-phase oscillations of the flux of energetic electrons, which is high when the configuration is dipolelike and vice versa, (3) a gradient in the flux of energetic ions, which is, on the average, earthward but undergoes large fluctuations around this average direction, and (4) large transient fluctuations of the quasi-dc electric field, which reverses its direction from eastward to westward. It is shown that these results are consistent with the development of an instability which leads to a westward propagating “wave”. The source of the instability is the differential drift of energetic electrons and ions in a highly stressed magnetic field configuration (in a high β plasma). Evidence is given for a system of localized field-aligned currents flowing alternately earthward and equatorward at the leading and trailing edges of the westward propagating wave. This current system resulting from the temporal development of the instability produces the so-called Pi 2 pulsations, at the ionospheric level. The closure of this current system in the equatorial region leads to a current antiparallel to the tail current, and therefore to its reduction or cancellation. This reduction/cancellation of the tail current restores the dipole magnetic field (dipolarization) and generates a large westward directed induced electric field (injection). Hence, dipolarization and injection are the consequences of the instability. Finally, it is suggested that the westward traveling surge observed simultaneously by all-sky cameras, close to the magnetic field of GEOS 2, is the image of the instability in the equatorial region transmitted to the upper atmosphere by precipitating electrons.

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 2 RE 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.

STAFF Instrument Products Distributed Through the Cluster Active Archive

Cluster Spatio-Temporal Analysis of Field Fluctuations (STAFF) high resolution data that are available at Cluster Active Archive (CAA) comprise two main parts, corresponding to data issued from the two onboard data analysers. The STAFF waveform analyser (STAFF-SC) provides data in the frequency range 0.1–10 Hz or 0.1–180 Hz, depending on the spacecraft telemetry rate. The Spectrum Analyser (STAFF-SA) calculates the complete spectral matrix elements for five wave components, the three magnetic components from the STAFF experiment and the two electric components from the Electric Field and Wave (EFW) experiment, in the frequency range 8–4,000 Hz. From STAFF-SC the CAA data comprise waveform data in telemetry units, complex spectra in physical units, dynamic spectra plots, and possibly in the future waveform data in physical units. The CAA products from STAFF-SA are the spectral matrix data in physical units. In the future, STAFF-SA value added products containing wave polarisation and propagation characteristics will be delivered.

Ion heating by fast magnetosonic waves and ring current-electron radiation belt coupling

During the main phase of geomagnetic storms ions are transported across the geomagnetic field towards the Earth from the outer magnetosphere. Convective (and inductive) electric fields transport low energy ions of a few tens of keV through dawn towards the dayside while higher energy ions are transported through dusk by the curvature and gradient drift. The higher energy ions can be trapped by the magnetic field to form the partial, and total, ring current. Recent observations, and simulations, show that when the ion distribution function is observed on the dayside magnetosphere it has a ring type distribution perpendicular to the magnetic field as a result of the different drift paths of the ions, and as a result of loss processes. It has been shown that these ring distribution can excite plasma instabilities resulting in fast magnetosonic waves that propagate across the magnetic field at frequencies between the proton cyclotron frequency and the lower hybrid frequency. These waves can accelerate radiation belt electrons, and heat thermal ions, but their effects on the ion ring distribution and the ring current have yet to be assessed. Here we study the effects of the waves on the ion distribution. We present wave data from the CLUSTER spacecraft with very high frequency resolution which shows magnetosonic waves above half the lower hybrid frequency with a line structure that has frequency spacing comparable to the local proton cyclotron frequency. We use the Ring current Atmosphere Model (RAM) to simulate the development of the ring current during a storm and find that ion ring distributions are predicted by the model and that they are unstable to the generation of fast magnetosonic waves with multiple spectral peaks. By modelling the multiple peaked spectra, we show that these waves are very effective in diffusing the ions at energies between 10 and 100 keV, and that energy diffusion rates are two orders of magnitude higher than pitch angle diffusion. Since diffusion does not extend into the loss cone, the effect of the waves is to diffuse ions to lower energies where they help remove the ion ring and to higher energies where they form a high energy tail with a large anisotropy. We solve the Fokker Planck equation to study the evolution of the ion distribution function, and compare the timescale for ion heating with that for ion transport. We conclude that fast magnetosonic waves are very effective process for both heating the ring current ions, and discuss whether the resulting ion distribution could affect the generation of electromagnetic ion cyclotron waves on the dayside magnetosphere. Since EMIC waves can cause rapid loss of radiation belt electrons our results show that fast magnetosonic waves may play an important role in the coupling the ring current to the electron radiation belts.