A STATISTICAL ANALYSIS OF 50 YEARS OF DAILY SOLAR WIND VELOCITY DATA

Abstract

ABSTRACT Daily mean value of solar wind velocity measured by various spacecraft near the Earth’s orbit from 1963 November 27 to 2013 December 31 given by OMNIWeb is utilized to investigate its characteristics through statistical analyses. The percent probability distribution of solar wind velocity can be described well by the Γ distribution function with the most probable velocity to be 373 km s − 1 . It is found that solar wind could be statistically classified into three groups: (1) the low-velocity wind, v < 450 km s − 1 , which positively responds to, and in the cycle phase lags, the solar activity cycle; (2) the high-velocity wind, 450 ≤ v < 725 km s − 1 , which negatively responds to, and in the cycle phase leads, the solar activity cycle, but in a short timescale (one-rotation-period) lags the solar activity cycle; and (3) the extreme-high-velocity wind, v ≥ 725 km s − 1 , which positively responds to, and in cycle phase leads, the solar activity cycle. A period of about 27 days is determined for solar wind in the first two groups, so that solar wind in the groups is modulated by the solar rotation and related with solar long-life magnetic structures. Solar wind of extreme high velocity appears mainly at the descending phases of the solar cycles.