On the transition from Slow to Fast Wind as Observed in Composition Observations

Abstract

The solar wind is typically categorized as fast and slow based on the measured speed ($v_{\mathrm{sw}}$). The separation between these two regimes is often set between 400 and 600 km/s without a rigorous definition. Observations of the solar wind's kinetic signatures, chemical makeup, charge state properties, and Alfvénicity suggest that such a two-state model may be insufficiently nuanced to capture the relationship between the solar wind and its solar sources. We test this two-state fast/slow solar wind paradigm with heavy ion abundances (X/H) and characterize how the transition between fast and slow wind states impacts heavy ion in the solar wind. We show that (1) the speed at which heavy ion abundances indicate a change between fast and slow solar wind as a function of speed is slower than the speed indicated by the helium abundance; (2) this speed is independent of heavy ion mass and charge state; (3) the abundance at which heavy ions indicate the transition between fast and slow wind is consistent with prior observations of fast wind abundances; (4) and there may be a mass or charge-state dependent fractionation process present in fast wind heavy ion abundances. We infer that (1) identifying slow solar wind as having a speed $v_{\mathrm{sw}}\lesssim$ 400 km/s may mix solar wind from polar and equatorial sources; (2) He may be impacted by the acceleration necessary for the solar wind to reach the asymptotic fast, non-transient values observed at 1 AU; and (3) heavy ions are fractionated in the fast wind by a yet-to-be-determined mechanism.

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Cited by in corpus (4)

• Cross Helicity and the Helium Abundance as an in situ Metric of Solar Wind Acceleration
• The Evolution of Heavy Ion Abundances with Solar Activity
• Characterizing the Impact of Alfvén Wave Forcing in Interplanetary Space on the Distribution of near-Earth Solar Wind Speeds
• On the Regulation of the Solar Wind Helium Abundance by the Hydrogen Compressibility