Spacecraft observations of the interaction of active comets with the solar wind

Abstract

In 1985 and 1986, six spacecraft encountered two comets and obtained a wealth of new data about the plasma processes at work in the interaction of an active comet with the solar wind. This paper summarizes those data and their interpretations. The interaction process starts millions of kilometers from the nucleus where the solar wind first picks up cometary ions. The cometary ions not only slow down the wind but also create disturbances which are ultimately responsible for the surprisingly high fluxes of energetic particles detected at comets. After passing through a structure that is arguably called a bow shock, the plasma continues to slow down as it accretes more cometary ions. The deceleration of the wind causes the interplanetary magnetic field to pile up and drape around the comet's ionosphere, as predicted by Alfvén in 1957. It is this draped field that defines the geometry of a comet's plasma tail. At Halley the Giotto spacecraft detected a well‐defined boundary separating the mixture of solar wind and cometary plasmas and the interplanetary magnetic field from the field‐free, nearly pure cometary plasma in the inner coma. Other unexpected features in the inner coma were a flux of fast “granddaughter” ions and high densities of negative ions. For all the comet encounters, the region between the bow shock and the field‐free cavity was distinguished by several sudden changes in the properties of the plasma; the natures and causes of these features are subjects of intense debate.