Philippe Louarn

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.

Multispacecraft observation of magnetic cloud erosion by magnetic reconnection during propagation

During propagation, Magnetic Clouds (MC) interact with their environment and, in particular, may reconnect with the solar wind around it, eroding away part of its initial magnetic flux. Here we quantitatively analyze such an interaction using combined, multipoint observations of the same MC flux rope by STEREO A, B, ACE, WIND and THEMIS on November 19-20, 2007. Observation of azimuthal magnetic flux imbalance inside a MC flux rope has been argued to stem from erosion due to magnetic reconnection at its front boundary. The present study adds to such analysis a large set of signatures expected from this erosion process. (1) Comparison of azimuthal flux imbalance for the same MC at widely separated points precludes the crossing of the MC leg as a source of bias in flux imbalance estimates. (2) The use of different methods, associated errors and parametric analyses show that only an unexpectedly large error in MC axis orientation could explain the azimuthal flux imbalance. (3) Reconnection signatures are observed at the MC front at all spacecraft, consistent with an ongoing erosion process. (4) Signatures in suprathermal electrons suggest that the trailing part of the MC has a different large-scale magnetic topology, as expected. The azimuthal magnetic flux erosion estimated at ACE and STEREO A corresponds respectively to 44% and 49% of the inferred initial azimuthal magnetic flux before MC erosion upon propagation. The corresponding average reconnection rate during transit is estimated to be in the range 0.12-0.22 mV/m, suggesting most of the erosion occurs in the inner parts of the heliosphere. Future studies ought to quantify the influence of such an erosion process on geo-effectiveness. ©2012. American Geophysical Union. All Rights Reserved.

A study of the large‐scale dynamics of the Jovian magnetosphere using the Galileo Plasma Wave Experiment

Using observations of the Galileo PWS experiment, we show that energetic phenomena recurrently occur in the jovian magnetosphere. They are characterized by intensifications of the auroral radio emissions and the creation of new sources of radiations in the outer regions of the Io torus. Simultaneously, modifications of the structure of the plasmasheet are observed at large distance (more than 60 Rj) from Jupiter. These large-scale processes, presenting a periodicity of 50 to 80 hours, could be linked to global instabilities of the jovian magnetosphere.

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.

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).

Observation of similar radio signatures at Saturn and Jupiter: Implications for the magnetospheric dynamics

We report on radio signatures observed at Saturn by the Cassini RPWS experiment which are strikingly similar to the Jovian “energetic events” observed by Galileo. They consist of sudden intensifications of the auroral radio emission (SKR) followed by the detection of a periodic narrowband radiation which most likely originates from Saturns plasma disk. About ten “events” have been observed in 2006, showing on average temporal scales ∼3 times longer than their Jovian counterparts. We analyze the conditions of generation and the visibility of the narrowband radiation and conclude that the Kronian “events” are most likely associated with plasma evacuation from the disk. These observations provide new insights on the role of internal energy releases in Saturns magnetosphere, known from other observations to be mainly driven by the solar wind.

Global magnetodisk disturbances and energetic particle injections at Jupiter

Measurements made by the Galileo Energetic Particles Detector and the plasma wave/radio instrument are analyzed to establish relationships between dynamic processes observed independently in the distant and the inner Jovian disk (at 80–120 Jovian radii (RJ) and 10–25 RJ, respectively). It is first shown that global magnetospheric disturbances identified from the radio emissions (the “energetic events” are well correlated with reconnection/reconfiguration events observed at the outer edge of the disk. Then, considering all Galileo perijoves, it is also demonstrated that the energetic events occurring as Galileo was at less than ~25 RJ are systematically associated with new injections of energetic particles seen from ~10 to 25 RJ. This demonstrates that major disturbances commonly affect the whole magnetodisk, from 10 to 80–120 RJ. Overall, their phenomenology involves simultaneous auroral activation, formation of new sources of radio emission (narrow-band kilometric radiation) and particle injections in the Io torus, magnetic reconfigurations, and radial flow bursts in the distant disk, over time scale of a few hours.

A multi‐instrument study of a Jovian magnetospheric disturbance

Using observations from different Galileo experiments (plasma wave system, magnetometer and energetic particle detector), we analyze a strong magnetospheric disturbance that occurs on day 311 of 1996 as Galileo was close to Jupiter (less than 15 Jovian radii). This perturbation is characterized by multiple injections of energetic particles in the inner magnetosphere and has been described as a possible analog of the terrestrial magnetic storm by Mauk et al. [1999]. We show here that it also corresponds to a large-scale magnetospheric perturbation similar to the “energetic events” described by Louarn et al., [1998, 2000]. It is associated with the development of a particular magnetic activity in the outermost part of the Io torus, over periods of 2–4 hours and in sectors of longitude with a typical 30°–80° longitudinal extension. At distances ranging from 10 to 13 Rj, the activity itself is characterized by the generation of low-frequency magnetic oscillations (18 min periodicity in the present case) that correlate with dispersionless energetic electron injections and modulations of the auroral radio flux. When they are observed a few hours after their formation, these injections present a weak energy-time dispersion and are still periodic. They then progressively mix and finally define a region of limited longitudinal extension where the density of energetic particles is particularly large. We show that this region corresponds to the source of the narrowband kilometric radiation (n-KOM). By combining remote sensing radio observations, in situ particle, and magnetic field measurements, we show that the active zone where the large scale disturbance initially develops most probably does not corotate and would even be almost fixed in local time. In the present case, the magnetospheric event is the consequence of two activations separated by a few hours. They occur in two separated longitude sectors and give rise to two different n-KOM sources. During the event, some 1012 W are transferred to the electron population. It is proposed that this set of phenomena is the manifestation of a sporadic dissipation of a part of the Io torus rotational energy and would be thus associated with the development of a large-scale instability in the external part of the Io torus.

Particle acceleration linked to Alfvén wave propagation on small scale density gradients

Abstract We study how Alfven waves propagate in the presence of sharp density gradients in the direction perpendicular to the ambient magnetic field. A fully electromagnetic electron guiding centre code is used for the simulation. During the propagation, initially parallel ( k ∥ = 0), transverse scales of the order of c/ω pe are quickly reached which contributes to the creation of a significant parallel component of the electric field in the region of density inhomogeneity. The effects of this field on the velocity distribution functions are then discussed. In particular, we show that they can present a strong deviation from their initial Gaussian shape (global shift in energy) due to the action of the parallel electric field. Evidences are then given for a net energy gain of the electrons, to the expense of the wave, during this process. This energy transfer mechanism may be relevant in order to explain the particle acceleration in the auroral plasma cavities.

Simulation Study of Spacecraft Electrostatic Sheath Changes With the Heliocentric Distances From 0.044 to 1 AU

In this paper, the electrostatic sheath of a simplified spacecraft is investigated for heliocentric distances varying from 0.044 to 1 AU, using the 3-D Particle in Cell software Satellite-Plasma Interaction System. The baseline context is the prediction of sheath effects on solar wind measurements for various missions, including the Solar Probe Plus mission (perihelion at 0.044 AU from the sun) and Solar Orbiter (SO) (perihelion at 0.28 AU). The electrostatic sheath and the spacecraft potential could interfere with the low-energy (a few tens of eV) plasma measurements, by biasing the particle distribution functions measured by the detectors. If the spacecraft charges to large negative potentials, the problem will be more severe as low-energy electrons will not be seen at all. The Solar Probe Plus and SO cases will be presented in details and extended to other distances through a parametric study, to investigate the influence of the heliocentric distance to spacecraft. Our main result is that, for our spacecraft model, the floating potential is a few volts positive from 1 AU to about 0.3 AU, while below 0.3 AU, the space charge of the photoelectrons and secondary electrons create a potential barrier that drives the spacecraft potential negative.