The interaction of the solar wind with cometary plasma is studied in the framework of a collisionless 2D bi‐ion fluid plasma model which allows for different bulk velocities of the light and heavy ions. It is shown that an ion composition boundary is formed which is impenetrable for the protons but presents no barrier to the magnetic field flux. The magnetic field is transported further downstream by the charge‐neutralizing electrons which follow the heavy ions through the boundary. We believe that this new type of plasma boundary can explain the observed ion composition boundary at comets (cometopause, pile‐up boundary), at Mars (planetopause, massloading boundary) and at Venus (rarefaction wave, intermediate transition).
Abstract.Ionic charge state distributions for a variety of species, such as C, O, Ne, Mg, Si and Fe were obtained with the Solar Energetic Particle Ionic Charge Analyzer (SEPICA) on ACE for the strongest of a series of energetic particle events after the November 4 and 7, 1997, flares. The capabilities of SEPICA allow a much more detailed analysis of the charge distributions than previous instrumentation. Over the energy range from--0.2 to 1 MeV/Nuc a trend is observed that shows charge states increasing with energy, in particular for Mg, Si and Fe. In addition, for Fe a mixed charge state distribution with a distinct peak at lower charge states (10-14) is observed simultaneously with a tail reaching to charge states up to --20. This may be an indication of a mixture of different energetic particle populations.
Abstract. He + pickup ions as observed with SOHO CELIAS CTOF have been analyzed for the time period DOY 160 -190, 1996. During this time of the year the Earth is on the upwind side of the interstellar gas flow with respect to the sun. The high-speed cut-off in the frame of the sun is significantly higher v/V,w = 2, predicted for pickup ions. The difference increases with lower solar wind speeds. This behavior is interpreted as an effect of the local interstellar gas flow velocity (inflow at large distances including gravitational acceleration by the sun)on the pickup ion distribution. The neutral velocity is added to the solar wind velocity in the determination of the pickup ion cut-off on the upwind side and subtracted on the downwind side of the gas flow. This new observation will provide a valuable tool to determine the interstellar gas flow and will thus complement direct neutral gas measurements.
This paper discusses modifications of a bow shock ahead of an obstacle in the solar wind (SW) which can occur when the flow consists of a proton plasma and a secondary ion populations. The secondary species may be composed of alpha particles, which are a natural part of the ambient SW, or of heavier particles which are picked up by the solar wind in source regions, such as at comets or Mars. By using a 2D collisionsless bi‐ion fluid model which treats protons and heavy ions as distinct and which assumes that the two fluids communicate with each other by means of electromagnetic forces only, it is shown that for high enough value of the heavy ion mass density a ‘splitting’ of the bow shock takes place. Downstream from the proton bow shock, where differential streaming between ion species arises, a second discontinuity is formed which resembles a shock‐like transition for the heavy ion flow. This plasma boundary, called the heavy‐ion discontinuity (HID), causes also a distinct deflection of the proton flow and significant magnetic field variation. The results seem to be of importance for different types of SW obstacles, especially for planetary objects where massloading of the SW plays a dominant role in bow shock formation, as at comets and probably at Mars. It is suggested that the ‘massloading boundary (MLB)' found in the magnetosheath of Mars and the ‘mysterious boundary’ detected in the cometosheath of Halley and Grigg‐Skejllerup are HID's of the described nature.
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