The transition from slow to fast wind as observed in composition observations

¹ Heliophysics Science Devision, NASA Goddard Space Flight Center, 8800 Greenbelt, Road, Greenbelt, MD 20771, USA
² Space Science, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238, USA
³ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁴ University of Michigan, Department of Climate and Space Sciences & Engineering, Climate and Space Research Building, 2455 Hayward Street, Ann Arbor, MI 48109, USA

^⋆ Corresponding author; b.l.alterman@nasa.gov

Received: 17 July 2024
Accepted: 2 December 2024

Abstract

Context. The solar wind is typically categorized as fast and slow based on the measured speed (v_sw). The separation between these two regimes is often set between 400 and 600 km s⁻¹ without a rigorous definition. Observations with v_sw above this threshold are considered “fast” and are typically considered to come from polar regions, that is, coronal holes. Observations with v_sw below this threshold speed are considered “slow” wind and typically considered to originate outside of coronal holes. 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. As heavy ion composition ratios are unchanged once the solar wind leaves the Sun, they serve as a key tool for connecting in situ observations to their solar sources. Helium (He) is the most abundant solar wind ion heavier than hydrogen (H). Long-duration observations from the Wind Solar Wind Experiment (SWE) Faraday cups show that the solar wind helium abundance has two distinct gradients at speeds above and below ∼400 km s⁻¹. This is a key motivator for identifying the separation between fast and slow wind at such a speed.

Aims. 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.

Methods. We study the variation in the gradients of the helium and heavy ion abundances as a function of the solar wind speed and characterize how the gradient of each abundance changes in fast and slow wind. We calculate v_sw as the proton or hydrogen bulk speed. The work uses Advanced Composition Explorer (ACE) heavy ion observations collected by the Solar Wind Ion Composition Spectrometer (SWICS) from 1998 to 2011. We compare the helium abundance observed by ACE/SWICS to the helium abundance observed by Wind/SWE to show that the results are consistent with prior work.

Results. 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.

Conclusions. We infer that (1) identifying slow solar wind as having a speed of v_sw ≲ 400 km s⁻¹ 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.