E. G. Shelley

Evidence for methane and ammonia in the coma of comet P/Halley

Methane and ammonia abundances in the coma of Halley are derived from Giotto IMS data using an Eulerian model of chemical and physical processes inside the contact surface to simulate Giotto HIS ion mass spectral data for mass-to-charge ratios (m/q) from 15 to 19. The ratio m/q = 19/18 as a function of distance from the nucleus is not reproduced by a model for a pure water coma. It is necessary to include the presence of NH3, and uniquely NH3, in coma gases in order to explain the data. A ratio of production rates Q(NH3)/Q(H2O) = 0.01-0.02 results in model values approximating the Giotto data. Methane is identified as the most probable source of the distinct peak at m/q = 15. The observations are fit best with Q(CH4)/Q(H2O) = 0.02. The chemical composition of the comet nucleus implied by these production rate ratios is unlike that of the outer planets. On the other hand, there are also significant differences from observations of gas phase interstellar material.

The pick-up of cometary protons by the solar wind

The HERS detector of the Ion Mass Spectrometer on the Giotto spacecraft measured the 3-dimensional distribution of picked-up cometary protons over a distance of ~8 million km upstream of the bow shock of comet P/Halley. The protons were observed to be elastically scattered out of their original cycloidal trajectories such that they were nonuniformly distributed over a spherical shell in velocity space. The shell radius (relative to its expected radius) and thickness increased as the bow shock was approached. Down-stream of the shock, the cometary protons could not be distinguished from the heated solar wind protons.

Ion temperature and flow profiles in comet P/Halley’s close environment

The HIS instrument of the ion mass spectrometer on board the Giotto spacecraft identified the contact surface at 4800 km distance from the comet nucleus. This boundary is clearly seen by a drastic drop in the temperatures of different ion species from about 2000 K outside to values as low as 300 K inside. Also, inside the contact surface an outflow speed of > 1 km s −1 was measured, in contrast to a value around zero right outside. We discuss how these numbers might be affected by a potential charge-up of the spacecraft. Outside the contact surface, the ion temperature rises gradually with increasing distance. Between 9000 and 10 000 km distance the ion density increases by a factor of 4. This “ion pile-up” is not yet explained uniquely. Between 25 000 and 28 000 km distance there is again a rather abrupt jump to significantly higher temperatures and higher outflow speeds.

An Interpretation of the ion pile-up region outside the ionospheric contact surface

An analysis interrelating some of the published results of plasma measurements by the Giotto spacecraft in the inner coma of comet P/Halley suggests that the formation of the plasma pile-up region at a distance of 10,000 – 30,000 km may be the result of accumulation of ion density in a stagnant flow region with a sharp electron temperature transition forming a recombination front at about 10,000 km.

The composition and dynamics of cometary ions in the outer coma of comet P/Halley

During its flyby at comet Halley, the Giotto spacecraft encountered high densities of cometary ions inside approximately 200 000 km from the nucleus. Their properties changed drastically as the comet was approached. We present here density profiles of solar wind alpha particles and of the major cometary ions as obtained by the IMS-HERS sensor between 340 000 km and 60 000 km from the nucleus. Typical mass spectra at various distances are presented and angular and velocity distributions of the cometary ions are discussed.

Charge exchange of solar wind ions in the coma of comet P/Halley

In addition to solar wind pick-up of cometary ions, charge exchange between solar wind ions and cometary neutrals is, potentially, an important comet-solar wind interaction closer to the nucleus. One monitor of the charge exchange rate is the relative abundances of solar wind He2+ and He+ Radial profiles of these two ions measured by the Giotto IMS/HERS instrument on the inbound trajectory to comet Halley are presented. Results indicate that charge exchange may be an important solar wind loss process in the comet Halley coma at distances ~ 105 km from the nucleus.

Giotto-IMS observations of ion-flow velocities and temperatures outside the magnetic cavity of comet P/Halley

Fluid parameters for He’ ions obtained from the Giotto Ion Mass Spectrometer HERS sensor are presented; also investigated are proton densities and velocities and thermal speeds of protons, alpha particles, and heavy ions in the hour before closest approach. A bow shock transition lasting ten minutes is observed. A region of enhanced He’ ion densities, and velocity, and decreased temperature is observed from 20:26 to 21:45. Sharp decreases in the proton density are observed at 23:30 and at 23:41; there are also sharp drops in alpha particle density and temperature at 23:30. There is a relative flow velocity between alpha particles and oxygen ions of approximately 20 km s−1 during a period roughly from 22:55 to 23:10; by 23:30 the difference in flow velocity is less than the experimental uncertainities.Fluid parameters for He(++) ions obtained from the Giotto ion mass spectrometer are presented. Proton densities and velocities and thermal speeds of protons, alpha particles, and heavy ions in the hour before closest approach are discussed. A region of enhanced He(++) ion densities, and velocity, and decreased temperature is observed from 20:26 to 21:45. Sharp decreases in the proton density are observed at 23:30 and at 23:41. There is a relative flow velocity between alpha particles and oxygen ions of 30 km/sec during a period from 22:55 to 23:30; the difference in flow velocity is less than the experimental uncertainities. The flow properties of protons observed during this period are also discussed.

Hot ions Observed by the Giotto Ion Mass Spectrometer Inside the Comet Halley Contact Surface

Just inside the contact surface (~ 4700 km) the High Energy Range Spectrometer (HERS) sensor of the Giotto Ion Mass Spectrometer (IMS) detected a sudden, intense burst of ions that lasted until the HERS sensor ceased transmitting data at a distance of ~ 3000 km from comet P/Hallley. During this brief interval ions with M/Q= 1, 2,12,14,16–19, 24, and 28 were observed. The heavier ions appear in two populations (in the S/C frame): 1) a very low energy, almost omnidirectional distribution, and 2) a more energetic (~ ram speed) population coming from the ram direction. In this paper we investigate the possible origin(s) of these two populations. In particular, we discuss whether the low energy ions belong to the natural Halley enviroment or are generated at the spacecraft by dust and gas bombardment.