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

Journal Article · Thu Nov 01 00:00:00 EST 1990 · Journal of Geophysical Research; (United States)

DOI:https://doi.org/10.1029/JA095iA11p18745· OSTI ID:5049636

Neugebauer, M ^[1]; Coates, A J ^[2]; Neubauer, F M ^[3]

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1. California Inst. of Tech., Pasadena (United States)
2. Univ. Coll. London, (England)
3. Univ. Koeln, Cologne (West Germany)

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 {times} 10{sup 6} km, while the WG mass density exceeded the proton mass density when r < 6 {times} 10{sup 6} 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 {times} 10{sup 6} km, but the proton anisotropy did not drop below 10:1 until the spacecraft entered the foreshock region at r = 1.4 {times} 10{sup 6} 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 and with increasing angle {alpha} between the interplanetary field and the solar wind velocity vector. 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.

• OSTI ID: 5049636
• Journal Information: Journal of Geophysical Research; (United States), Vol. 95:A11; ISSN 0148-0227
• Country of Publication: United States
• Language: English

Related Subjects

71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
HALLEY COMET
SOLAR WIND
INTERACTIONS
ACCELERATION
ADIABATIC PROCESSES
ANISOTROPY
CORRELATIONS
DATA
ION DENSITY
IONIC COMPOSITION
IONIZATION
MASS SPECTROMETERS
MASS TRANSFER
MOLECULAR IONS
PROTONS
SHOCK WAVES
SPACE VEHICLES
VARIATIONS
BARYONS
CHARGED PARTICLES
COMETS
ELEMENTARY PARTICLES
FERMIONS
HADRONS
INFORMATION
IONS
MEASURING INSTRUMENTS
NUCLEONS
SOLAR ACTIVITY
SPECTROMETERS
VEHICLES
640107* - Astrophysics &amp; Cosmology- Planetary Phenomena