Alain Roux

Solitary kinetic Alfvén waves: A study of the Poynting flux

Using a particular mode of the FREJA wave experiment (three magnetic and one electric simultaneous measurements), we have investigated the electromagnetic structure of the solitary kinetic Alfven waves observed in the topside ionosphere. It is shown that these strong electromagnetic spikes (ΔE ≈ 100 mV/m and ΔB ≈ 10 nT) have mainly a rotational character with, nevertheless, a tiny compressional component (ΔB‖/ΔB ≈ 10 %). They seem to be associated with small-scale (100 m) tubular current structures. On the assumption that their electric component is perpendicular to the magnetic one their Poynting flux is estimated. Values of the order of 10−3 W/m2 are measured.

Characterization of low frequency oscillations at substorm breakup

Field and particle data recorded on the geostationary satellite GEOS 2 are used to investigate the electric and magnetic signatures of a substorm characterized by a dispersionless injection of energetic electrons and ions. Three types of field variations are observed: (1) Long-period oscillations with period of ∼ 300 s, interpreted as oscillations of entire field lines. These oscillations develop as second harmonic standing waves and correspond to coupled shear Alfven-slow magnetosonic modes. They grow after the most active period of the breakup. (2) Short-period transient oscillations with periods of ∼ 45–65 s, interpreted as wave modes trapped in a current layer which develops prior to the substorm breakup and is disrupted at breakup. These oscillations also correspond to a coupled shear Alfven-slow magnetosonic mode (coupled via magnetic field curvature effects in a high-β plasma). The short-period transient oscillations are only observed during the most active period of the breakup. (3) A nonoscillatory sharp increase observed on both the parallel magnetic component and the energetic ion flux, averaged over one satellite rotation, interpreted as evidence for the fast magnetosonic mode which in view of the simultaneous large impulsive increase in the azimuthal electric field, appears to propagate radially outwards, transporting the substorm breakup downtail.

Electric fields with a large parallel component observed by the Freja spacecraft: Artifacts or real signals?

Using plasma wave data sampled by the Freja spacecraft from the topside ionosphere during auroral conditions, the possible existence of electric fields with an intense parallel component (a few tens of millivolts per meter) with respect to the Earths magnetic field is discussed. When Freja crosses large-amplitude solitary electromagnetic structures (ΔE ≈ 100 mV/m and ΔB ≈ 10 nT, identified as being solitary kinetic Alfven waves), strong electric spikes are sometimes detected along a direction almost parallel to the static magnetic field. The possible sources of errors due to the plasma inhomogeneities and/or to the magnetic connection between the electric probe and the spacecraft body are reviewed and discussed. In particular, using an indirect technique based on the reconstruction of the electric field hodograms, it is shown that these sources of errors have no influence on our conclusions. Unless unknown mechanisms strongly affect the validity of double-probe measurements in some circumstances, it is then concluded that an electric field with a parallel component 2–3 orders of magnitude larger than expected from the theory of kinetic Alfven waves can develop in the topside ionosphere.

Plasma sheet dynamics in the Jovian magnetotail: Signatures For substorm‐like processes ?

During Galileos orbit G2 in 1996 the Energetic Particles Detector (EPD) onboard the spacecraft detected a number of particle bursts with large radial/antisunward anisotropies in the distant Jovian magnetotail [Krupp et al., 1998]. In this letter we focus on a detailed analysis of one of the bursts. Prior to the onset of the burst, particle intensities at low energies increase over several hours. This phase can be interpreted as a plasma loading phase. It ends after the onset of strong distortions in the magnetic field with a bipolar excursion of the north-south component being the most prominent feature. The subsequent plasma sheet encounters show that the plasma sheet has thinned considerably. Accelerated/heated ion beams first from the Jovian direction and then later from the tail direction are seen at the plasma sheet and lobe interfaces and intense radio and plasma wave emissions are detected. The event is tentatively interpreted as a dynamical process, where the Jovian magnetotail is internally driven unstable by mass loading of magnetic flux tubes.

A study of the Jovian “energetic magnetospheric events” observed by Galileo: role in the radial plasma transport

Using the Galileo Plasma Wave Subsystem (PWS) experiment, we analyze the large-scale energetic events that recurrently occur in the Jovian magnetosphere. As described by Louarn et al. [1998], these sporadic phenomena are associated with enhancements in the flux of the various auroral radio emissions, with the creation of new sources of radiation in the Io torus, and correspond to large fluctuations in the magnetodisc density. These events have been interpreted as sudden releases of energy in the Jovian magnetosphere. In order to better characterize them we study an extended PWS data set (orbits G2, G7, and G8) corresponding to 130 days of observations during which 32 energetic events have been unambiguously identified. We conclude the following: (1) The periods of enhanced energy releases in the magnetosphere (as indicated by increases in the auroral radio flux) are almost systematically initiated by an energetic event. (2) The periodicity of the events changes from one orbit to the other and it is shown that the more frequent they are, the denser is the plasma sheet. (3) In a large majority of cases the events occur as the disc is thin and relatively depleted in plasma. A few hours after the events, a thick and heavily populated magnetodisc is observed. It then thins, and its density progressively decreases over a timescale of a few tens of hours. Our interpretation is that the events correspond to sequences of rapid plasma loading of the magnetodisc that are followed by much more progressive evacuations of the magnetodisc plasma. The global plasma content of the magnetodisc would thus increase with their frequency. This study suggests that the energetic events are related to an instability developing in the external part of the Io torus or in the close magnetodisc that sporadically injects new plasma populations in the more distant magnetodisc. The associated transport process appears to be efficient enough to explain the outward evacuation of a significant fraction of the plasma created at the Io orbit (a few 1028 ions/s).

Disruption of parallel current at substorm breakup

We study the development of substorm breakups characterized by dispersionless injections of energetic particles at the geostationary orbit. The corresponding magnetic signature is a fast change from tail-like to dipole-like configuration with transient superimposed low frequency oscillations (T∼1 mn). We show that intense waves (δB ≈ 1 nT) with shorter periods (1 s) systematically develop at breakup, and that their intensification is strongly related to the dipolarization and to the fast increase of energetic electrons. These “higher frequency” (F ∼1 Hz) waves appear as short lasting bursts, strongly confined across the magnetic field. Hence they look like kinetic Alfven waves and are likely to have finite parallel electric fields, thereby resonating with electrons. We compute the diffusion coefficient and show that electrons are heated along the parallel direction and can gain up to 5 keV in a few tens of seconds. This fast parallel diffusion of electrons leads to cancellation of the parallel current and therefore to a complete modification of the current system.

Heating of proton conics by resonant absorption in a multicomponent plasma: 1. Experimental evidence

ELF emissions observed on the low-altitude AUREOL 3 satellite are seen in association with H+ ions at large pitch angle. The flux in the upward direction for ∼120° pitch angle is found to be equal to or larger than the flux at ∼60° pitch angle, which provides evidence for transverse acceleration at or below the spacecraft. These emissions have a sharp lower-frequency cutoff of the transverse components of the electric field and a narrow peak at, or, more precisely, just below the proton gyrofrequency fH+. This narrow peak is more easily seen on the parallel component and appears as a narrow line on the spectrogram of this component. A statistical study of the occurrence of this line at f ∼ fH+ is presented. It is shown that this line is observed at relatively high invariant latitude within the light ion trough where a strong depletion of thermal H+ ions occurs. Detailed analysis of ELF waves observed just below fH+ demonstrates that they propagate in the left-hand mode. These observations are interpreted as a signature of mode conversion from a fast magnetosonic mode into a slow proton cyclotron mode. It is suggested that this slow proton cyclotron wave can accelerate protons up to a few hundreds electron volts in the transverse direction. This mode conversion process can operate over a much broader of large altitude range than covered by AUREOL 3; it is a likely candidate for explaining the formation of H+ conics. Theoretical calculations that support the above conclusions are given in a companion paper by Le Queau et al. (this issue).

Heating of protons by resonant absorption in a multicomponent plasma: 2. Theoretical model

A theory is proposed to explain the selective heating of protons in a direction transverse to the geomagnetic field lines, which has been observed on board the AUREOL 3 satellite, as shown in a companion paper. This process, which occurs only in multicomponent plasma, ultimately leads to the formation of a conical distribution, inside and above the heating region. It is shown that magnetosonic Alfven waves generated at higher altitudes can undergo a resonant mode conversion process as they propagate downward. The efficiency of the subsequent resonant absorption depends rather strongly on the propagation angle of the waves and on the relative abundance of H+ ions: up to 25% of the incident energy flux can be transferred to protons. The present theory moreover foresees that a strong parallel electric field component has to occur at the altitude at which the wave frequency matches the proton gyrofrequency. The corresponding spectral signature has been unambiguously identified in the AUREOL 3 data, together with an intensification of the upward flux of suprathermal protons, which indicates the presence of proton conics. We have verified that within the uncertainties of the measurements the energy balance between the incoming electromagnetic power and the outgoing flux of kinetic energy is well explained by the theory.

Substorm expansion phase: Observations from Geotail, Polar and IMAGE network

[1]xa0We describe the signature of a substorm detected in the midtail while Geotail was located close to the midnight meridian. At the same time, the UVI imager on board Polar identifies a bulge which develops at low latitude and rapidly expands towards the north, east, and west, corresponding to the expansion phase. The magnetograms of the IMAGE network are consistent with these observations; during the expansion phase they give evidence for a northward expansion of the magnetic perturbation. Mapping indicates that the Geotail footprint is located north of the initial bulge and south of the high-latitude oval. During the expansion phase, Geotail is located in the center of the neutral sheet and detects an ion flow velocity, perpendicular to Bo and directed tailward while Bz changes from positive to negative. During the recovery phase, Geotail, which is not at the center of the sheet anymore, detects an ion velocity directed earthward but essentially field aligned, while Bz is positive and the high-latitude auroral structure is located north of Geotail footprint. The radial component of the velocity is always dominant. We interpret these observations as evidence for a tailward moving dipolarization front that first destroys the inner part of the thin current sheet (TCS) formed during the growth phase. This dipolarization/current disruption starts in the near-Earth plasma sheet and expands tailward. This “erosion” produces a negative Bz component at the earthward edge of the TCS. For large enough distances this contribution can eventually be dominant, thereby producing a negative Bz. In this interpretation the formation of an X-point/X-line is the consequence of the erosion of the currents in near-Earth tail. Present observations give evidence for an association between increases in the ion velocity and small-scale Alfvenic fluctuations. The plasma sheet electrons are heated via interaction with the waves. This heating is preferentially along the direction of the magnetic field. Large ion flow velocities coincide with wave observations.

Filamentation of plasma in the auroral region by an ion-ion instability: A process for the formation of bidimensional potential structures

A two-dimensional, explicit, electrostatic particle code is used to investigate the nonlinear behavior of electrostatic ion waves generated by an ion beam flowing through a thermal ion and electron background in a strongly magnetized plasma ({omega}{sub ce} {much gt} {omega}{sub pe} where {omega}{sub ce} and {omega}{sub pe} are the electron gyrofrequency and the plasma frequency). To follow the nonlinear evolution of these ions waves, a long-lasting simulation is run with a large simulation grid: 128 {times} 512{lambda}{sub d}. Beam ions are shown to generate oblique waves. The nonlinear beatings between these oblique waves produce purely transverse waves, which leads to a strong modulation of the density and of the electric potential in a direction transverse to the magnetic field. The transverse scale of these essentially field-aligned filaments is L{sub {perpendicular}} = 10 {rho}{sub i} where {rho}{sub i} is the ion Larmor radius of beam ions. Within these filaments, relatively stable field-aligned density and potential structures develop. The typical size, along the magnetic field, of these structures is L{sub {parallel}} = 10 {lambda}{sub d}, the density is modulated by 30%, and the electric potential is as large as T{sub e} within these structures. Unlike the potential structures that develop in amorexa0» two-component plasma with downgoing electrons, these structures move upward. These characteristics are in good agreement with the weak double layers recently detected by Viking.«xa0less