Heavy ion abundances evolve with solar activity

¹ Heliophysics Science Devision, NASA Goddard Space Flight Center, 8800 Greenbelt, Road, Greenbelt, MD 20771, USA
² Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
³ University of Michigan Department of Climate and Space Sciences & Engineering, Climate and Space Research Building, 2455 Hayward St., Ann Arbor, MI 48109, USA
⁴ INAF – Institute for Space Astrophysics and Planetology, Via Fosso del Cavaliere, 100 00133 Rome, Italy

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

Received: 27 February 2025
Accepted: 24 April 2025

Abstract

Context. When observed at 1 AU, solar wind traveling at slow speeds (v_sw ≲ 500 km s⁻¹) is typically considered to have originated in source regions with magnetic topologies that are intermittently open to the heliosphere. Solar wind with fast speeds (v_sw ≳ 500 km s⁻¹) is generally believed to have originated in source regions that are continuously open to the heliosphere, such as coronal holes. The evolution of the solar wind helium abundance (A_He) with solar activity is likely driven by the evolution of different solar wind source regions. The change in the gradient of A_He and heavier elements with increasing v_sw can be used to identify characteristic speeds at which the dominant source of solar wind transition from source regions that have intermittently to continuously open magnetic topologies. However, these observations are typically limited to slow and intermediate speed solar wind (≲ 600 km/s) because slow wind is observed in the ecliptic more often than fast wind.

Aims. We aim to increase the maximum speed above which such analyses of the association between solar wind abundances and solar activity can be performed, extending it up to 800 km s⁻¹. This stands as a rough upper limit on non-transient solar wind speeds when observed near 1 AU. We also aim to characterize the evolution of heavy element abundances (X/H):(X/H)_photo with solar activity. This analysis provides insight into the evolution of solar wind source regions with solar activity.

Methods. We separate the solar wind into “fast” and “slow” for each element’s abundance based on the characteristic speed previously derived for it. We analyzed the evolution of helium and heavy element abundances with solar activity using ACE/SWICS observations in each speed interval and correlated these abundances with solar activity, as indicated by the 13-month smoothed sunspot number and a normalized version that accounts for the sunspot number’s amplitude in each cycle. We normalize the sunspot number to its maximum in each solar cycle to convert it to an amplitude-independent clock for timing the phase of solar activity. Finally, a comparison of the SWICS abundances with A_He derived from Wind/SWE observations offers a validation of our work.

Results. We show that (1) A_He is strongly correlated with sunspot number in the slow and fast wind; (2) the average non-transient solar wind A_He is limited to 51% of its photospheric value; (3) slow-wind heavy element abundances (with the exception of C) do evolve significantly with solar activity; (4) fast-wind heavy element abundances do not evolve with solar activity to a significant extent; (5) the correlation coefficient with sunspot number of elemental abundances for species heavier than He monotonically increases with increasing mass; and (6) the correlation coefficients between the in situ observations and the normalized sunspot number are stronger than those using the unnormalized sunspot number. We also report that the minimum in heavy element abundances may be closer to the rapid depletions and recoveries of A_He that precede and predict sunspot minima (i.e. the helium shutoff). However a higher time-resolution analysis is necessary to properly characterize this signature.

Conclusions. We infer that (1) the sunspot number is indeed a clock timing the solar cycle, but not a driver of the physical process underlying the evolution of A_He and heavy element abundances with solar activity; (2) this underlying process is likely related to the energy available to accelerate the solar plasma from the chromosphere and transition region or low corona into the solar wind; and (3) the differences between the evolution of slow and fast solar wind A_He and heavy element abundances are similarly related to the energy available to accelerate the elements at these heights above the Sun’s surface.