Showing papers by "H. Rosenbauer published in 1986"

Ion composition and dynamics at comet Halley

Abstract: The Giotto space probe's ion mass spectrometer has obtained data on the composition and velocity distributions of cometary ions at distances of between 7.5 million and 1300 km from the comet Halley nucleus. Solar wind He(2+) was found throughout the coma, as close as 5000 km, with the He(+) produced by charge exchange being within about 200,000 km. A pile-up of heavy cometary ions was found at about 10,000 km from the nucleus. Inside the contact surface, which was found at about 4600 km, ion temperatures as low as about 340 K and outflow velocities of about 1 km/sec were found.

Ion flow at comet Halley

Abstract: The three-dimensional positive ion analyzer aboard the Giotto spacecraft has been used to study the interaction between protons and alpha-particles in the solar wind and positive ions from comet Halley Although the first impression of the overall structure is that the plasma flow evolves smoothly as the nucleus is approached, three sharp transitions of relatively small amplitude can be identified on both the inbound and outbound legs of the trajectory The outermost one, at about one million km from the nucleus, appears to be a multiple crossing of a weak bow shock The innermost one, at 80,000 km, is the boundary where the flowing plasma becomes depleted On a microscopic scale, the turbulence created by the interaction between the two ion populations extends to a distance of several million km from the nucleus At Giotto's closest approach to the nucleus, the plasma produced around the spacecraft by dust and gas impacts was much more energetic than had been expected

Coulomb collisional domains in the solar wind

Abstract: The role of Coulomb collisions in the solar wind, their number of occurrence for protons, and the correlation between collisional effects and the stream structure and large-scale interplanetary magnetic field are investigated. Helios plasma data, displayed in Carrington rotation plots, clearly show that the relative number of collisions (ratio of expansion over collision time scale) maximizes near the heliospheric current sheet, which is revealed as the main collisional domain of the solar wind. Statistical results indicate that for about 20% of the time the low-speed solar wind plasma is influenced strongly by Coulomb collisions.

An interpretation of the ion pile-up region outside the ionospheric contact surface. [Halley's comet]

Abstract: 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

Abstract: 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.

The Giotto Ion Mass Spectrometer

Abstract: The Giotto Ion Mass Spectrometer (IMS) consists of two sensors: one optimized for the outer and the other for the inner coma, with each obtaining complementary information in the region for which it is not optimized. The outer coma is characterized by the interaction between solar wind and comentary plasmas, the inner coma by the outflow of cometary neutrals and their ionization products. Both sensors feature mass imaging characteristics, permitting simultaneous measurements of several ion species by multidetector arrays. Resultant mass-per-charge resolution is greater than or = 20. Energy per charge, and the elevation and aximuth of incident ions are measured. Calibration and in-flight solar-wind data show that the IMS will meet its scientific goals for the Halley encounter.

Hot ions observed by the Giotto ion mass spectrometer inside the Comet Halley contact surface

Abstract: 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.

Ion temperature and flow profiles in Comet Halley's close environment

Abstract: The Giotto high intensity spectrometer identified the contact surface 4800 km from the comet nucleus. This boundary is clearly seen by a drastic drop in the temperatures of different ion species from 2000 K outside to values as low as 300 K inside. Inside the contact surface outflow speed = > 1 km/sec, in contrast to a value around 0 right outside. 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. At 27,000 km distance there is again a rather abrupt jump to significantly higher temperatures, higher outflow speeds, and lower densities.

Giotto-IMS observations of ion flow velocities and temperatures outside the contact surface of Comet Halley

Abstract: 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.