Mariner 2 observations of the solar wind: 1. Average properties

This paper summarizes the properties of the positive-ion component of the solar wind observed during the four months of the Mariner 2 flight to and past Venus in 1962. The protons' average velocity and temperature were approximately 500 km/sec and 1.7×1050K, respectively. Several streams of hot, high-velocity plasma were observed to recur at 27-day intervals, with peak velocity and temperature values of ∼ 830 km/sec and 9×1050K. One of these streams probably existed for at least 18 months. Between streams the velocity dropped as low as 307 km/sec, while the temperature was ∼ 3×1040K. Near 1 AU the average number density was approximately 5 protons/cm3. The density was usually highest at the leading or western edge of each stream, with a maximum value of ∼80 protons/cm3. Otherwise, the density varied inversely with the plasma velocity. The ions' velocity, temperature, and density were calculated from ∼35,000 energy/charge spectrums by fitting the data to isotropic Maxwell-Boltzmann distributions in a reference frame moving away from the sun at the solar-wind velocity. A model in which the protons and α particles had equal thermal velocities gives a better fit to the observed spectrums than does an equal-temperature model. The spectrums usually had high-energy tails, which became more pronounced at the higher plasma velocities. The velocity, temperature, and high-energy tail were not strongly dependent on distance from the sun, whereas the density varied approximately as the inverse square of this distance.