Review and Interpretation of Particle Measurements Made by the Vela Satellites in the Magnetotail

Abstract

The Vela nuclear test detection satellites are in near-circular orbits around the earth at a geocentric distance of about 17R _E. This paper discusses data obtained from four of these satellites — Vela 2A and 2B, launched in July 1964, and Vela 3A and 3B, launched in July 1965. The orbits of these four satellites lie in a plane inclined about 60° with respect to the ecliptic plane. The satellites spend roughly $1\over 3$ of their time (about $1{1 \over 2}$ days per orbit) in the magnetotail. The orientation of the orbital plane in inertial space is such that the satellites penetrate the ecliptic plane near the noon and midnight meridians in mid-April and mid-October and near the dawn and dusk meridians in mid-July and mid-January. The satellites are spin-stabilized with their spin axes inclined ∼ 60° to the ecliptic plane and lying approximately in the orbital plane. Each satellite carries, among other instruments, a Geiger counter sensitive to electrons of energy > 45 keV and a hemispherical plate electrostatic analyzer which detects electrons or protons of energy between 200 eV and 20 keV. The apertures of these detectors point at 90° relative to the spin axis. On satellites 2A and 2B the analyzer obtains a complete 14-point energy spectrum of electrons or protons in each of eight directions relative to the sun in 128 sec. Five of these directions lie within ±12° of the sun and the other three are at approximately 90°, 180° and 270° relative to the sun. The analyzer on satellites 3A and 3B obtains a 64-point energy spectrum of electrons or protons in the same eight directions every 256 sec. The Geiger counters on all four satellites make a measurement once per second. More complete descriptions of the instruments on these satellites have been presented by Singer (1965) and by Bame et al. (1967).