Dynamic processes and kinetic structure of collisionless reconnection at the dayside magnetopause: comparison between GEOTAIL observations and computer simulations

Abstract

Abstract. In this paper we report new kinetic features of ions and electrons observed in the vicinity of the reconnection layer on 10 January 1997. This event has a threedimensional magnetic field topological structure, which is much more complex than the previously suggested twodimensional magnetic configuration. The ion distributions are non-gyrotropic and electrons show non-Maxwellian distribution functions. Acceleration of multiple ion beams, both parallel and perpendicular to the local magnetic field, have been observed. The perpendicular acceleration of the multiple ion beams can be explained by plasma mixing between the meandering ions accelerated around the ion diffusion region and the cold ions convected directly from the magnetosheath without passing through the X-line region. The parallel acceleration of the multiple ion beams can be understood by the fact that high-velocity ions ejected from the vicinity of the X-line mix with the plasma flowing directly across the boundary. We observed the kinetic effect of the separation of the electron and ion edges due to the time-offlight effect. It is stressed that kinetic processes are the key to understanding these new observations that cannot be adequately explained by magnetohydrodynamic (MHD) models.

Cylindrical lower‐hybrid electron holes at the Earth's dayside magnetopause

“…The fastest growing mode is generated when the unperturbed electron beam speed matches the phase velocity of the corresponding Langmuir wave, k z v z ,0 ≈ ω p , e . These data are consistent with the satellite observations of large amplitude Langmuir waves following the fast reconnection and the emergence of electron holes [ Deng et al , 2003; Cattell et al , 2002, 2005], whose duration is 5–20 Langmuir wave periods. Conversely, for the oblique Buneman instability of lower‐hybrid waves with tan θ = k z / k ⊥ = 1/15, we have γ ∼ ω R ∼ 0.7 ω LH , for a mode that is driven by an electron beam whose velocity is somewhat larger than the parallel phase velocity of lower‐hybrid waves, namely k z v z ,0 ∼ 3 ω LH .…”

“…2. The electron temperature is usually anisotropic in the magnetopause, T e ∥ ≫ T e ⊥ (the subscripts ∥ and ⊥ denote the directions parallel and perpendicular to the ambient magnetic field, respectively), both in the strong shear dayside region and in the low latitude boundary layer at its dawn/dusk flanks [ Phan and Paschmann , 1996; Deng et al , 2003; Nishida , 2000]. This is true also in other regions where the magnetic reconnection takes place, e.g., in the magnetotail [ Mozer et al , 2004; Wygant et al , 2002] or which are traversed by parallel electron beams, such as the auroral region [ Janhunen et al , 2004; Ergun et al , 1999].…”

“…These two linearly unstable modes are likely to saturate by the trapping of resonant electrons, yielding parallel (or Langmuir‐type) and oblique (or lower‐hybrid) electron holes, respectively. The Langmuir‐type electron holes have been known from satellite observations [ Deng et al , 2003; Cattell et al , 2002, 2005], and they have been extensively investigated both analytically and numerically. In the following we will study only the oblique structures in order to address the observations of perpendicular electric fields by Mozer et al [2004].…”

“… Mozer et al [2004] speculate that these coherent structures result neither from the spatial gradients of the plasma properties, which are known to produce bipolar electric fields, nor from solitary Langmuir‐wave‐type electron phase‐space structures (holes), whose electric fields are mostly parallel to the ambient magnetic field. The latter, very rapid nonlinear structures whose duration is in the range (typically 5–20 times longer) of the Langmuir wave period T p , e ( T p , e = 2π/ω p , e , where ω p , e = ( n 0 e 2 /ε 0 m e ) 1/2 is the electron plasma frequency, n 0 is the unperturbed plasma number density, and e , m e are the magnitude of the electron charge and the mass, respectively), have been often observed to accompany the magnetic reconnection events, both at the Earth's dayside magnetopause with southward IMF (observed by Polar [ Cattell et al , 2002] and Geotail [ Deng et al , 2003] spacecrafts) and in the Earth's magnetotail by Cluster ([ Cattell et al , 2005]). Noticeably, Cattell et al [2005] commonly recorded also very large amplitude lower‐hybrid waves, having electric fields up to 250 mV/m, perpendicular to the ambient magnetic field, as well as the holes in the lower‐hybrid time range, accompanied by the bursts of the upper‐hybrid waves.…”

“…Noticeably, Cattell et al [2005] commonly recorded also very large amplitude lower‐hybrid waves, having electric fields up to 250 mV/m, perpendicular to the ambient magnetic field, as well as the holes in the lower‐hybrid time range, accompanied by the bursts of the upper‐hybrid waves. Computer simulations by Deng et al [2003] and Drake et al [2003] revealed that large‐amplitude Langmuir and lower‐hybrid waves, respectively, are produced by two types of Buneman instability (parallel and oblique) of the magnetic field‐aligned electron beams that come from electron acceleration by the reconnection electric fields. The development of thin current sheets parallel to the guide magnetic field, whose thickness is of the order of the electron skin depth ∼ c /ω p , e , is well known from the simulations of collisionless reconnection [see, e.g., Del Sarto et al , 2003, and references therein].…”