Abstract. Data from the four spacecraft Cluster mission during a high altitude cusp crossing on 13 February 2001 are presented. The spacecraft configuration has one leading spacecraft, with the three trailing spacecraft lying in a plane that corresponds roughly to the nominal magnetopause surface. The typical spacecraft separation is approximately 600 km. The encounter occurs under conditions of strong and steady southward Interplanetary Magnetic Field (IMF). The cusp is identified as a seven-minute long depression in the magnetic field, associated with ion heating and a high abundance of He + . Cusp entry involves passage through a magnetopause boundary that has undergone very significant distortion from its nominal shape, is moving rapidly, and exhibits structure on scales of the order of the spacecraft separation or less. This boundary is associated with a rotation of the magnetic field, a normal field component, and a plasma flow into the cusp of approximately 35 km/s. However, it cannot be identified positively as a rotational discontinuity. Exit from the cusp into the lobe is through a boundary that is initially sharp, but then retreats tailward at a few km/s. As the leading spacecraft passes through this boundary, there is a plasma flow out of the cusp of approximately 30 km/s, suggesting that this is not a tangential discontinuity. A few minutes after exit from the cusp, the three trailing the spacecraft see a single cusp-like signature in the magnetic field. There is an associated temperature increase at two of the three trailing spacecraft. Timing measurements indicate that this is due to cusp-like regions detaching from the rear of the cusp boundary, and moving tailward. The magnetic field in the cusp is highly disordered, with no obvious relation between the four spacecraft, indicative of structure on scales <<600 km. However, the plasma moments show only a gradual change over many minutes. A similar cusp crossing on 20 FebruCorrespondence to: P. J. Cargill (p.cargill@ic.ac.uk) ary 2001 also shows a field depression and highly dynamic boundaries.

The structure of the quiet‐time magnetotail is studied using a test particle simulation. Vlasov equilibria are obtained in the regime where υD = Ey c/Bz is much less than the ion thermal velocity and are self‐consistent in that the current and magnetic field satisfy Ampere’s law. Force balance between the plasma and magnetic field is satisfied everywhere. The global structure of the current sheet is found to be critically dependent on the source distribution function. The pressure tensor is nondiagonal in the current sheet with anisotropic temperature. A kinetic mechanism is proposed whereby changes in the source distribution results in a thinning of the current sheet.

The sheath formed between a magnetized plasma and a particle absorbing wall is examined for the case in which the magnetic field intercepts the wall at a small angle 0°&lt;ε≲9°, where sin ε=B⋅n̂/‖B‖, and n̂ is the unit normal to the wall. The model is time-independent and one-dimensional (1-D) with all functions varying only in the direction normal to the wall. The ions are modeled by a Maxwellian velocity distribution which is modified by the condition that ions, which would have hit the wall, are absent. For the electrons a fluid description is used, including the effects of electron–neutral collisions. The transport of particles due to turbulent electrostatic fluctuations is modeled by a constant electric field perpendicular to both B and n̂. It is found that in the range of angles under consideration, there are two distinct regimes of sheath formation. If ε≲ν̄=ν/Ωe (grazing incidence), where ν is the electron–neutral collision frequency and Ωe is the electron cyclotron frequency, then the properties of the sheath are determined by a parameter λ which is the ratio of the convective (E×B) and diffusive electron flows. If λ≲1, the wall potential is negative and the sheath scale length is on the order of an ion gyroradius. If λ≳1, the wall potential is positive and, for large λ, the sheath is characterized by two scales: a short length, which is a decreasing function of λ, adjacent to the wall, and the ion gyroradius farther from the wall. For ε≫ν̄, (oblique incidence) the potential at the wall is negative with a magnitude close to that of the unmagnetized plasma and is only weakly dependent on ε. In addition, for this case, the sheath scale length is on the order of an ion gyroradius and is weakly dependent on ε, larger values of ε resulting in a slightly shorter scale length.

Abstract.In this paper we present simulation results and observational data from the Geotail comprehensive plasma instrument of an ion distribution function signature which arises due to nonlinear particle dynamics in the quiet time magnetotail. The signature manifests itself as peaks and valleys in the ion distribution function whose separation scales as the fourth root of the particle energy. The Geotail observations represent the first independent corroboration of this signature since it was seen in ISEE i data by Chen et al. [1990]. The simulations demonstrate that the signature is present in the pitch-angle-resolved distribution even in the case of perfectly symmetric particle sources in the northern and southern hemispheres. When combined with magnetometer data, we show how the peaks and valleys may be used to determine the current sheet thickness using a single satellite. The current sheet thickness determined in this fashion is less than but consistent with other measurements of the current sheet.

The effects of collisional processes on the nonlinear particle dynamics in the magnetotail are considered. A simple collision operator is developed to model the effects of pitch‐angle and energy scattering. It is found that the phase space partition persists for up to moderate scattering amplitudes in pitch‐angle and energy, and that certain distribution function features are robust even in the presence of large amplitude collisions. It is shown that if the collisions are due to short scale length electrostatic fields, excessively large field amplitudes are required to significantly alter the phase space structures and the resulting distribution function features.

The effects of a constant cross‐tail magnetic field By on the phase space structures and the observational signatures of nonlinear particle dynamics in the quiet time magnetotail are examined. The separation of phase space into dynamically distinct regions (transient, stochastic and integrable) is found to persist albeit with considerable complications. In addition, it is shown that if |By| is less than the Bz component, the resonant coherent chaotic scattering of particles and the resulting distribution function features persist. If |By| ≳ Bz, on the other hand, the majority of the particles are forward scattered through the current sheet, and there is no discernible signature of the resonance in the distribution function. This is significant in that the presence or absence of the resonance effect in observed distribution functions can constrain the magnitude of By.

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