Search

Two time periods, each covering both quiet and disturbed conditions (growth phase, breakup, and post breakup phase), are studied. Electric and magnetic field measurements, carried out in the near-Earth plasma sheet (NEPS), are used to calculate the two components (radial and azimuthal) of the electric E × B/B2 drift. These calculations are compared with independent estimates of the ion flow direction deduced from ion flux measurements. During active periods, the two flow directions coincide to a large degree. Evidence is given for two regimes of transport: (1) During the growth phase, and after the active phase, the electric field (radial and azimuthal) and hence the azimuthal and radial flow velocities are small in the near-equatorial region. This is interpreted as the consequence of an electrostatic field that tends to shield the induced electric field associated with time-varying external conditions. (2) During active phases (breakup and pseudobreakup), however, large-amplitude bursts in E × B/B2 radial and azimuthal components (interpreted as flow bursts), with typical velocities of the order of 100 km s−1, are observed. The direction of these flow bursts is somewhat arbitrary, and in particular, for the two substorm events described here, sudden reversals in the flow direction are observed. These fast flow bursts coincide with intense low-frequency electromagnetic fluctuations: current-driven Alfven waves (CDA waves) with frequency ƒ ≃ ƒH+, the proton gyrofrequency. CDA waves produce “anomalous” collisions on timescales shorter than the electron bounce period, thus violating the second adiabatic invariant for electrons. As a consequence, the electrostatic shielding is destroyed, which leads to enhanced radial transport. Thus the transport in the NEPS seems to be controlled by a microscopic current-driven instability.

International audience; ULF waves at frequencies of the order of the proton gyrofrequency are systematically detected at the early development of substorm breakups. The observed characteristics of these ULF waves, namely their polarization and δE/δB ratio are consistent with being electromagnetic waves driven unstable by a parallel current. In order to take into account properly wave particle interactions, a kinetic approach is used. We show that a parallel drift between electrons and ions leads to a strong instability, resulting from a coupling between the shear Alfvén (SA) mode and the fast magnetosonic mode via this drift. We call it current‐driven Alfvén instability (CDA). We have carried out a parametric study of this current‐driven electromagnetic instability in a parameter range adapted to conditions prevailing at the geostationary orbit before and during breakup. We conclude that even a modest parallel drift between electrons and ions (Vd), caused by a parallel current, can destabilize CDA waves. When the ratio between Vd/VA (VA being the Alfvén velocity) increases, the CDA mode couples with SA mode. These two modes have a substantial parallel electric field that leads to a fast parallel diffusion of the electrons. We suggest that this parallel diffusion leads to an interruption of the parallel current.

International audience; A new approach is proposed to calculate the self-consistent parallel electric field associated with the response of a plasma to quasi-static electromagnetic perturbations (co < kllVA, where vn is the Alfv•n velocity and kll the parallel component of the wave vector). Calculations are carried out in the case of mirror geometry, for co < coo (coo being the particle bounce frequency). For the sake of simplification the 3 of the plasma is assumed to be small. Apart from this restriction, the full Vlasov-Maxwell system of equations has been solved within the constraints described above (co < kllVA and co < coo) by [Le Contel et al., this issue] (LC00, hereafter), who describe self-consistently the radial transport of particles during the substorm growth phase. LC00 used an expansion in the small parameter T•/T• (T•/T• is typically 0.1 to 0.2 in the plasma sheet) to solve the quasi-neutrality condition (QNC). To the lowest order in T•/T• < 1, they found that the QNC implies (1) the existence of a global electrostatic potential (k0 which strongly modifies the perpendicular transport of the plasma and (2) the parallel electric field vanishes. In the present study, we solve the QNC to the next order in T•/Ti and show that a field-aligned potential drop proportional to T•/Ti does develop. We compute explicitly this potential drop in the case of the substorm growth phase modeled as in LC00. This potential drop has been calculated analytically for two regimes of parameters, cod < co and w• > co (•-j being the bounce averaged magnetic drift frequency equal to ky•-•, where ky is the wave number in the y direction and •-• the bounce averaged magnetic drift velocity). The first regime (•-j < co) corresponds to small particle energy and/or small ky, while the second regime (•-j > co) is adapted to large energies and/or large ky. In particular, in the limit co• < co and Iv•l < where uv is the diamagnetic velocity proportional to the pressure gradient, we find a parallel electric field proportional to the pressure gradient and directed toward the ionosphere in the dusk sector and toward the equator in the dawn sector. This parallel electric field corresponds to a potential drop of a few hundred volts that can accelerate electrons and produce a differential drift between electrons and ions.

International audience; [1] From one year (2004) of thermosphere total density data inferred from CHAMP/ STAR accelerometer measurements, we calculate the global thermosphere response to auroral magnetic activity forcing at middle and low latitudes using a method based on a singular value decomposition of the satellite data. This method allows separating the large-scale spatial variations in the density, mostly related to altitude/latitude variations and captured by the first singular component, from the time variations, down to timescales on the order of the orbital period, which are captured by the associated projection coefficient. This projection coefficient is used to define a disturbance coefficient that characterizes the global thermospheric density response to auroral forcing. For quiet to moderate magnetic activity levels (Kp < 6), we show that the disturbance coefficient is better correlated with the magnetic am indices than with the magnetic ap indices. The latter index is used in all empirical thermosphere models to quantify the auroral forcing. It is found that the NRLMSISE-00 model correctly estimates the main features of the thermosphere density response to geomagnetic activity, i.e., the morphology of Universal Time variations and the larger relative increase during nighttime than during daytime. However, it statistically underestimates the amplitude of the thermosphere density response by about 50%. This underestimation reaches 200% for specific disturbed periods. It is also found that the difference between daytime and nighttime responses to auroral forcing can statistically be explained by local differences in magnetic activity as described by the longitude sector magnetic indices. Citation: Lathuillère, C., M. Menvielle, A. Marchaudon, and S. Bruinsma (2008), A statistical study of the observed and modeled global thermosphere response to magnetic activity at middle and low latitudes