Comparison of picked‐up protons and water group ions upstream of comet Halley's bow shock

Abstract

Data are presented on the properties of picked‐up cometary protons and water group (WG) ions observed upstream of the bow shock of comet Halley by the ion mass spectrometer and Johnstone plasma analyzer experiments on the Giotto spacecraft. The number of WG ions exceeded the number of cometary protons at cometocentric distances r < 1.3×106 km, while the WG mass density exceeded the proton mass density when r < 6×106 km. The small scale variations of proton and WG densities were well correlated, which argues against the "expanding halo" model which has been used to explain the quasi‐periodicity of energetic particles observed by VEGA and Giotto. Two parameters are used to describe the pitch angle distributions of the picked‐up ions: a 10% width, which is the full angular width at 10% of the maximum of the pitch angle distribution, and a mean width. The cometocentric distance profiles of both parameters exhibited a great deal of scatter, and both parameters showed a steeper average radial gradient for WG ions than for protons. The 10% widths of WG ions were consistent with less than 10∶1 anisotropies for r < 2.5×106 km, but the proton anisotropy did not drop below 10∶1 until the spacecraft entered the “foreshock” region at r = 1.4×106 km. After subtraction of the variation in field direction, the mean width of the proton pitch angle distribution was nearly independent of distance everywhere outside the shock. The WG mean width, on the other hand, increased with increasing WG density (as expected) and with increasing angle α between the interplanetary field and the solar wind velocity vector (an unexpected result). No increase in shell radii, due to either adiabatic compression or first‐order Fermi acceleration, could be discerned for either ion species until the spacecraft was very close to the bow shock. The thickness of the picked‐up proton shell slowly increased as r decreased inside 2.5×106 km, whereas the water group shell thickness remained fairly constant until just outside the shock. While some of the differences between the two ion species can be attributed to the greater ionization scale length of cometary H atoms compared to WG atoms and molecules, the very different dependences of their pitch angle and energy diffusion rates on ion densities and on α are difficult to understand in terms of presently available theories and models.