LONG-TERM TRENDS IN THE SOLAR WIND PROTON MEASUREMENTS

Abstract

ABSTRACT We examine the long-term time evolution (1965–2015) of the relationships between solar wind proton temperature (T p) and speed (V p) and between the proton density (n p) and speed using OMNI solar wind observations taken near Earth. We find a long-term decrease in the proton temperature–speed (T p–V p) slope that lasted from 1972 to 2010, but has been trending upward since 2010. Since the solar wind proton density–speed (n p–V p) relationship is not linear like the T p–V p relationship, we perform power-law fits for n p–V p. The exponent (steepness in the n p–V p relationship) is correlated with the solar cycle. This exponent has a stronger correlation with current sheet tilt angle than with sunspot number because the sunspot number maxima vary considerably from cycle to cycle and the tilt angle maxima do not. To understand this finding, we examined the average n p for different speed ranges, and found that for the slow wind n p is highly correlated with the sunspot number, with a lag of approximately four years. The fast wind n p variation was less, but in phase with the cycle. This phase difference may contribute to the n p–V p exponent correlation with the solar cycle. These long-term trends are important since empirical formulas based on fits to T p and V p data are commonly used to identify interplanetary coronal mass ejections, but these formulas do not include any time dependence. Changes in the solar wind density over a solar cycle will create corresponding changes in the near-Earth space environment and the overall extent of the heliosphere.