Cometary pick‐up ions observed near Giacobini‐Zinner

In‐situ measurements of the rest‐frame distribution functions of cometary pick‐up water‐group ions are used to derive the number and energy density of these ions as a function of distance from the Comet Giacobini‐Zinner. The observed density profiles are found to be the same in the inbound and outbound passes and their shape is consistent with predictions of a simple model of pick‐up ion production from neutrals of a given type which flow radially outward at ∼1 km/s and are lost at a rate λ. Comparing predictions of this model with the measured density profiles we find that λ = 2·10 −6 sec −1 and that the production rate of water‐group cometary molecules is 2.6·10 28 /sec. The measured distribution functions of the pick‐up water‐group ions may be represented by a two‐segment power‐law with a break at an energy E B ∼20 keV, corresponding to a speed, V B = 438(E/M) 1/2 , of ∼ 500 km/s at distances greater than ∼10 5 km. The shapes of the distribution functions indicate that the interaction process of the solar wind with cometary ions is predominantly stochastic.