Current Sheet Region Research Articles

The effect of a normal magnetic field component on current sheet stability

A 2 2 5 - dimensional particle-mesh computer model for the simulation of the current-sheet region of the geomagnetic tail is described. Important features are (a) the use of Fast Fourier techniques for the efficient solution of Ampere's equation, (b) the incorporation of sources and sinks of particles, (c) facilities for simulating finite width effects and (d) the option of including a normal magnetic field component linking through the sheet. Simulations carried out using this model indicate that current sheets with a non-zero normal magnetic field component and an infinite width are stable. The particles trace out Speiser-like orbits in such a case. Sheets with B normal = 0 and a finite width are unstable with respect to the ion tearing-mode instability. However the presence of a normal magnetic field stabilises the system provided ρ 0<2 L y where ρ 0 is the characteristic length associated with the normal field and where L y is the width of the sheet. On the basis of these results it is suggested that a geomagnetic substorm occurs when the normal magnetic field drops below the critical value needed for stability.

Energetic protons in the Jovian magnetosphere

The time histories, angular distributions and energy spectra of energetic protons have been measured over an energy range extending from 0.2 to 20 MeV for the four passes of Pioneers 10 and 11 through the Jovian magnetosphere. The energetic particle data from these four passes are remarkably different. Azimuthal asymmetries appear to dominate with time variations also contributing to the very complex topology. On the inbound P‐10 pass the expected corotation anisotropy was not observed in the outer magnetosphere. The simplest explanation is that the particle reference frame (i.e., the magnetospheric plasma in this region) is moving nearly radially, suggesting the existence of a planetary wind at that time. Near the dawn meridian, particle streaming away from the planet begins at ∼15 RJ. On both the P‐10 inbound and P‐11 outbound passes, there are regions where only partial corotation is achieved. In the mid‐magnetosphere, field‐aligned streaming away from the near‐equatorial current sheet region is the most prominent and puzzling feature. At mid‐latitudes in the subsolar regime, the streaming pattern is more chaotic and its magnitude is smaller. Qualitative discussions are presented for a number of possible mechanisms which could produce this streaming. In the context of our present understanding of the Jovian magnetosphere, each of three mechanisms show promise: perpendicular electric fields, a strong azimuthal intensity gradient at the equator, and strong diffusion processes. In the Jovian wind regions, proton energy spectra are generally of the form E−γ, where E is the kinetic energy of the particle with γ generally ranging between values of 3.0 and 4.2. For the remaining times, the spectra are most frequently of the form exp −P/P0, where P is the proton momentum with P0 varying mainly between 8.0 and 12 MV.