Paolo Canu

Kelvin‐Helmholtz waves at the Earth's magnetopause: Multiscale development and associated reconnection

[1]xa0We examine traversals on 20 November 2001 of the equatorial magnetopause boundary layer simultaneously at ∼1500 magnetic local time (MLT) by the Geotail spacecraft and at ∼1900 MLT by the Cluster spacecraft, which detected rolled-up MHD-scale vortices generated by the Kelvin-Helmholtz instability (KHI) under prolonged northward interplanetary magnetic field conditions. Our purpose is to address the excitation process of the KHI, MHD-scale and ion-scale structures of the vortices, and the formation mechanism of the low-latitude boundary layer (LLBL). The observed KH wavelength (>4 × 104 km) is considerably longer than predicted by the linear theory from the thickness (∼1000 km) of the dayside velocity shear layer. Our analyses suggest that the KHI excitation is facilitated by combined effects of the formation of the LLBL presumably through high-latitude magnetopause reconnection and compressional magnetosheath fluctuations on the dayside, and that breakup and/or coalescence of the vortices are beginning around 1900 MLT. Current layers of thickness a few times ion inertia length ∼100 km and of magnetic shear ∼60° existed at the trailing edges of the vortices. Identified in one such current sheet were signatures of local reconnection: Alfvenic outflow jet within a bifurcated current sheet, nonzero magnetic field component normal to the sheet, and field-aligned beam of accelerated electrons. Because of its incipient nature, however, this reconnection process is unlikely to lead to the observed dusk-flank LLBL. It is thus inferred that the flank LLBL resulted from other mechanisms, namely, diffusion and/or remote reconnection unidentified by Cluster.

ULF wave identification in the magnetosheath: The k-filtering technique applied to Cluster II data

The magnetic fluctuations in the magnetosheath are studied, thanks to Cluster II data. The k-filtering technique is applied to explore ULF magnetic fluctuations using STAFF (Spatio-Temporal Analysis of a Field Fluctuations) data. Based on multipoint measurements, the k-filtering technique allows, for the first time, to estimate the Magnetic Field Energy Distribution (MFED) in both the angular frequency and wave vector space. We show how the localisation of the magnetic energy in the (ω, k) domain can be used to identify the linear modes that can propagate in the magnetosheath. A comparison between k-filtering results and prediction of the linear theory is performed. For the frequencies examined the magnetic energy seems to be distributed over the low frequency modes: mirror, Alfven, and slow modes. Estimation of Doppler shift shows that each frequency observed is the superposition of different frequencies in the plasma frame. This ``mixture of modes at a given observed frequency explains why the fluctuations are generally not observed to be polarized, as shown in previous studies. Some other implications on a weak turbulence approach of the magnetic fluctuations in the magnetosheath are discussed.

Equatorial electron density measurements in Saturn's inner magnetosphere

[1]xa0Upper hybrid resonance emissions detected by the Radio and Plasma Wave Science (RPWS) instrument on the Cassini spacecraft are used to obtain electron densities on five equatorial orbits of Saturn at radial distances ranging from 3 to 9 saturnian radii (RS). The electron density profiles for these orbits show a highly repeatable radial dependence beyond 5 RS, decreasing with increasing radial distance approximately as (1/R)3.63. Inside 5 RS, the electron density profiles are highly variable. We show that these radial variations are consistent with a centrifugally-driven outward transport of plasma from a source inside 5 RS.

Source of whistler emissions at the dayside magnetopause

Observations of whistler emissions are common in the magnetosphere near the dayside magnetopause. We show that one of the major source regions for these emissions is magnetic field minima that form along magnetic flux tubes at high latitudes. Using multispacecraft Cluster observations we experimentally confirm for the first time the existence of the magnetic field minima at high latitudes and we show that whistler emissions propagate away from the magnetic field minima. The strongest whistler emissions are observed on the magnetospheric flux tubes that are newly opened due to the magnetic reconnection. These flux tubes still have a density of magnetospheric plasma, but part of the high energy magnetospheric electrons have already been lost from the flux tubes. The partial loss of high energy electrons most probably causes anisotropy in electron distributions at high energies which should be the source of whistler emissions. Whistler emissions on opened flux tubes disappear as soon as the plasma density of flux tubes increases due to the entering of magnetosheath ions. We speculate that whistler emissions can most probably be used to trace the dynamics of the first opened field lines and thus the dynamics of magnetic reconnection sites.

Cluster observations of the high-altitude cusp for northward interplanetary magnetic field: A case study

Since January 2001, the multisatellite and multiinstrument CLUSTER mission gives a unique opportunity to study the structure and dynamics of the high-altitude polar cusp. On 17 March 2001, CLUSTER sampled the northern high-altitude cusp (J-9RE) around noon for more than 1 hour during very quiet interplanetary and magnetospheric conditions (P 0 driven patterns, including one or two lobe reconnection-cells in the dayside polar cap, according to the amplitude of the By component. The overall convection pattern responds in ∼3-5 min to abrupt changes in the IMF orientation. Successive electron and ion patches are interpreted as signatures of pulsed, enhanced reconnection in the high-latitude magnetopause, poleward of CLUSTER, at a distance estimated to be 8-12RE and, at times, less. A four-point boundary analysis demonstrates that reconnected flux tubes (Flux Transfer Events) convect with drift directions and velocities (6-15 km/s) in close agreement with the inferred convective patterns. Furthermore, CLUSTER demonstrates that boundary cusp motions, with a velocity up to ∼20 km/s, are immediately and directly induced by abrupt changes in the IMF orientation. Copyright 2003 by the American Geophysical Union.

Rapidly moving sources of upper band ELF/VLF chorus near the magnetic equator

[1]xa0Multiple simultaneous wideband (Gurnett et al., 2001) measurements on the Cluster spacecraft of upper band chorus emissions near the magnetic equator (at magnetic latitudes between −20° and 10° and L shells ranging between L = 4 and L = 5) are used to deduce the properties of the compact source regions of ELF/VLF chorus emissions. The frequency differences exhibited by the same chorus emissions observed on different spacecraft are interpreted (Inan et al., 2004) in terms of a differential Doppler shift, using a simple model involving rapidly moving sources traveling at speeds comparable to the parallel resonant velocity of counter-streaming gyroresonant electrons. Cluster observations are used to determine the location and extent along the Earths magnetic field lines of the source of chorus. Frequency and time differences between spacecraft are interpreted as a direct consequence of the rapid motion of highly localized source regions of chorus. In this paper, we examine the previously presented model of rapid motion of sources of chorus, extending the calculations to a three-dimensional space, using measurements of the four Cluster spacecraft. These calculations of source location and velocity as a function of frequency indicate that chorus sources move a distance of ∼6000 km along the field lines. The emitted chorus waves at the source are assumed to have a wide range of wave normal angles, but the rays reaching the spacecraft seem to be the ones with lower angles (with some exceptions). The ranges of velocity obtained vary with frequency around values ranging from ∼0.01c to ∼0.04c.

Experimental determination of the dispersion relation of magnetosonic waves

Magnetosonic waves are commonly observed in the vicinity of the terrestrial magnetic equator. It has been proposed that within this region they may interact with radiation belt electrons, accelerating some to high energies. These wave-particle interactions depend upon the characteristic properties of the wave mode. Hence, determination of the wave properties is a fundamental part of understanding these interaction processes. Using data collected during the Cluster Inner Magnetosphere Campaign, this paper identifies an occurrence of magnetosonic waves, discusses their generation and propagation properties from a theoretical perspective, and utilizes multispacecraft measurements to experimentally determine their dispersion relation. Their experimental dispersion is found to be in accordance with that based on cold plasma theory.

Whistlers observed by the Cluster spacecraft outside the plasmasphere.

[1]xa0Lightning generated whistlers are ubiquitous within the plasmasphere at both high and low altitudes, and these waves propagate efficiently in both ducted and nonducted modes. On the other hand, in the magnetospheric region outside the plasmasphere, lightning-generated whistlers are commonly observed at low altitudes (up to ∼6000 km) but only rarely at higher altitudes near the magnetic equatorial plane. The reasons for the lack of these waves at higher altitudes are not well understood. In the present paper we use data from the Wide Band Plasma (WBD) instruments on the four Cluster spacecraft to study the characteristics of lightning-generated whistlers observed on 4 separate days in 2001 at L shells ranging from L = 4 to L = 5, magnetic latitudes ranging from −20° to 10°, and Kp indices ranging from 3 to 6. The propagation paths of the lightning-generated whistlers are determined using a two-dimensional ray-tracing model to calculate the ray paths and group delays from the lower ionosphere to each of the four Cluster spacecraft over a range of frequencies (1 kHz < f < 8 kHz). The electron density distributions used for the ray-tracing calculations are derived from measurements with the Whisper relaxation sounder instrument. Our new results indicate that whistlers are observed outside the plasmasphere in the low-density regions only in the presence of large-scale irregularities within which the waves are “ducted.” This conclusion is sustained by an exhaustive search of whistlers outside the plasmasphere using all the Cluster passes during 2001 and 2002. In all the cases we found that dispersion characteristics are matched by ray-tracing simulations only if the whistlers are ducted. In some cases, whistler wave energy injected by an individual lightning discharge appears with significant smearing in time. The new results presented in this paper support a possible explanation of why whistlers outside the plasmasphere are rarely observed, based on wave conversion electromagnetic whistler mode to quasi-electrostatic lower hybrid mode (Bell et al., 2004).