Engineering topics
Publications and source records attributed to Borg, H..
The density and velocity distribution of cometary water group ions was measured by the Giotto spacecraft in the regions upstream and downstream of the 'bow shock' at Comet Grigg-Skjellerup. The results show that the distributions of ions are ring-like until quite close to the shock, the timescales for pitch angle and energy diffusion appear similar and the ion density follows a r exp -2 dependence.
The existence of hydromagnetic waves in the mass-loaded solar wind upstream from the bow shock of a comet is well-established both for comet Giacobini-Zinner and for comet Halley. Whereas previous reports have been concerned either with the magnetic field observations or with plasma observations, here observations of the magnetic field with the solar wind proton and alpha particle distributions are combined. This allows the three possible modes of propagation for these waves to be separated. The magnetic component is predominantly transverse to the magnetic field and linearly polarized. The flow vector also has a substantial amount of power parallel to the magnetic field. An examination of the pressure variations shows that slow magnetosonic waves are more common than the fast mode.
Gaseous material expanding from the nucleus of Comet Halley into space forms the neutral coma around the comet. Ionization in the solar UV radiation removes particles from the coma and injects them into the solar wind plasma. These freshly created ions are accelerated by the interplanetary electric field on cycloidal trajectories with gyrocenters moving with the speed of the magnetic field lines. In the solar wind frame of reference these particles move along the magnetic field lines with a fixed pitch angle. Pitch-angle scattering and energy diffusion reduce quickly the initial energy anisotropy which is associated with the narrow pick-up structures. First observations of heavy cometary pick-up ions (water group ions) at the bow shock are presented. The evolution of the distribution function in the vicinity of the shock and radial density profiles are discussed.
It is shown that a decrease of the keV particle fluxes in the dayside magnetosphere near the geosynchronous orbit is characteristic of the first several hours of magnetospheric disturbances. After some hours newly injected plasma from the nightside reaches the 'evacuated' regions of the dayside magnetosphere and strong flux increases are observed. The 'evacuation' of the dayside magnetosphere is interpreted in terms of a change in the convection pattern associated with an increase of the large scale electric field at the onset of the disturbance. The model presented is capable of accommodating all characteristics of the observational data, such as the temporal and spatial distributions, energy and pitch angle characteristics, and differences between electrons and protons.