B. Lefebvre

Evidence for newly closed magnetosheath field lines at the dayside magnetopause under northward IMF

[1]xa0We analyze the structure of the high-latitude magnetopause under steady interplanetary magnetic field (IMF). We use 56 magnetopause encounters of Cluster spacecraft from 2001 to 2003 to explore the statistical properties of the magnetosheath electron boundary layer, observed outside the high-latitude dayside magnetopause. We focus on the occurrence of low absolute parallel heat flux in this layer and its dependence on the magnetic field clock angle simultaneously measured by Cluster. The low absolute parallel heat fluxes result from the presence of bidirectional heated electrons in the magnetosheath electron boundary layer and are primarily observed when the local magnetic field is northward. The bidirectional heated electrons are interpreted as the signature of newly closed magnetosheath field lines that have reconnected at the high-latitude magnetopause, tailward of the cusp, in both hemispheres. This study strongly suggests that double high-latitude reconnection is a tenable mechanism for the formation of the low-latitude boundary layer and potentially of the cold, dense plasma sheet under northward IMF. Although the efficiency (in terms of mass and energy transfer) of this mechanism is still to be investigated, it is an obvious way of capturing solar wind plasma under northward IMF.

Electron anisotropy constraint in the magnetosheath: Cluster observations

[1] The whistler anisotropy instability is driven by the condition T⊥ e /T∥ e > 1, where the subscript e denotes electrons and the other subscripts denote directions relative to the background magnetic field B o . Instability growth leads to enhanced field fluctuations which scatter the electrons; theory and simulations show that this scattering imposes an upper bound on the electron anisotropy in the form T⊥ e /T∥ e -1 = S e /β α e∥ e with fitting parameters 0.1? S e ? 1 and 0.5? α e < 0.7 over 0.10 ≤ β∥ e < 1.0 where β∥ e = 8πn e T∥ e /B 2 o . Here measurements from the PEACE instrument on the Cluster 1 spacecraft show that electron anisotropies in two crossings of the dayside terrestrial magnetosheath are constrained statistically by this equation with S e ≃ 0.2 and α e ≃ 0.6. This is the first reported observation of this constraint in a space plasma. Citation: Gary, S. P., B. Lavraud, M. F. Thomsen, B. Lefebvre, and S. J. Schwartz (2005), Electron anisotropy constraint in the magnetosheath: Cluster observations.

Electron dynamics and cross-shock potential at the quasi-perpendicular Earth's bow shock

[1] The evolution of the electron distribution function through quasi-perpendicular collisionless shocks is believed to be dominated by the electron dynamics in the large-scale coherent and quasi-stationary magnetic and electric fields. We investigate the electron distributions measured on board Cluster by the Plasma Electron and Current Experiment (PEACE) instrument during three quasi-perpendicular bow shock crossings. Observed distributions are compared with those predicted by electron dynamics resulting from conservation of the first adiabatic invariant and energy in the de Hoffmann-Teller frame, for all pitch angles and all types of trajectories (passing and, for the first time, reflected or trapped). The predicted downstream velocity distributions are mapped from upstream measurements using an improved Liouville mapping technique taking into account the overshoots. Furthermore, for one of these crossings we could take advantage of the configuration of the Cluster quartet to compare mapped upstream velocity distributions with those simultaneously measured at a relatively well magnetically connected downstream location. Consequences of energy and adiabatic invariant conservation are found to be compatible with the observed electron distributions, confirming the validity of electron heating theories based on DC fields as zeroth-order approximations, but some systematic deviations are found between the dynamics of low-and high-adiabatic invariant electrons. Our approach also provides a way to estimate the cross-shock electric potential profile making full use of the electron measurements, and the results are compared to other estimates relying on the steady state dissipationless electron fluid equations. At the temporal resolution of the instruments, the scales associated to the change of the potential generally appear to be comparable to those of the magnetic field, but some differences between the methods appear within the shock transition. It is argued that potentials evaluated from Liouville mapping rely on less restrictive hypotheses and are therefore more reliable. Finally, we show how, in contrast to methods using electron velocity moments, the technique can be used to produce high-time-resolution electric potentials and discuss the electric potential profiles through the shock. Citation: Lefebvre, B., S. J. Schwartz, A. F. Fazakerley, and P. Decreau (2007), Electron dynamics and cross-shock potential at the quasi-perpendicular Earths bow shock,

Modeling of Cluster's electric antennas in space: Application to plasma diagnostics

[1]xa0The main characteristics of the long-boom electric antennas installed on board the Cluster satellites are derived from finite element modeling in a kinetic and isotropic space plasma, in the frequency range of about 1–100 kHz. The model is based on the surface charge distribution method in quasi-static conditions. The impedances of both types of antenna, i.e., the double-wire and the double-probe, are computed versus the frequency normalized with respect to the local plasma frequency and for several different Debye lengths. Most of the code outputs are checked using analytic estimations for better understanding of the involved physical mechanisms. As a by-product, the effective length of the double-probe antenna and the mutual impedance between the two antennas are computed by the code. It is shown that if it had been possible to implement such measurements on board, one would have been able not only to determine accurately the electric characteristics of the antennas but also to estimate the local plasma parameters. Nevertheless, an interesting feature predicted by the model has been checked recently in orbit by running a special mode of operation for testing the mutual impedance measurement. The preliminary results are globally consistent with the predictions, except that they suggest that our Maxwellian model for the electron distribution should be revised in order to explain the unexpected low-frequency response. After analysis of the electron flux measurements obtained simultaneously, it appears that a rough adjustment of the electron distribution with a two-component distribution allows us to account for the observations.

Generation of downshifted oscillations in the electron foreshock: A loss‐cone instability

[1]xa0Measurements performed aboard Cluster spacecraft near Earths bow shock on 24 January 2001 provide convincing evidence of a loss-cone feature within the electron foreshock region. This feature is formed by suprathermal electrons with energies 15–45 eV and pitch angles 130°–150° and is always accompanied by electrostatic waves with frequencies well below the local plasma frequency. An instability analysis shows that these downshifted oscillations can result from a loss-cone instability of electron cyclotron modes rather than from the beam instability as previously suggested.