Long-Term Trends In The Solar Wind Proton Measurements

TL;DR

This study analyzes four decades of solar wind proton data near Earth, revealing long-term trends and cycle-dependent relationships between proton properties and solar activity, with implications for space weather modeling.

Contribution

It identifies long-term variations in proton temperature and density relationships with solar wind speed, highlighting cycle-dependent changes and the importance of time-dependent empirical models.

Findings

Proton temperature-speed slope decreased from 1972 to 2010 and increased afterward.

The proton density-speed relationship follows a power law with an exponent correlated to the solar cycle.

Proton density in slow wind correlates with sunspot number with a ~4-year lag.

Abstract

We examine the long-term time evolution (1965-2015) of the relationships between solar wind proton temperature (Tp) and speed (Vp) and between the proton density (np) and speed using OMNI solar wind observations taken near Earth. We find a long-term decrease in the proton temperature-speed (Tp-Vp) slope that lasted from 1972 to 2010, but has been trending upward since 2010. Since the solar wind proton density-speed (np-Vp) relationship is not linear like the Tp-Vp relationship, we perform power law fits for np-Vp. The exponent (steepness in the np-Vp 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 np for different speed ranges, and found that for the slow wind np is highly correlated with the sunspot number with a lag of ~4 years. The fast wind np variation was less, but in phase with the cycle. This phase difference may contribute to the np-Vp exponent correlation with the solar cycle. These long-term trends are important since empirical formulas based on fits to Tp and Vp data are commonly used to identify ICMEs, 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.