The Tale of Two Minima and a Solar Cycle in Between: An Ongoing Fast Solar Wind Investigation

TL;DR

This study compares the physical properties of polar coronal holes during two solar minima using UVCS/SOHO data, revealing changes in the solar wind and providing empirical constraints for coronal heating and acceleration models.

Contribution

It presents the first direct comparison of polar coronal hole properties between two solar minima, enhancing understanding of solar wind variations over the solar cycle.

Findings

Differences in oxygen line intensities and profiles between minima.

Variations in outflow velocities and temperature distributions.

Implications for models of coronal heating and solar wind acceleration.

Abstract

We have measured the physical properties of polar coronal holes from the minimum activity phase of solar cycle 23 (1996-1997) to the present minimum of solar cycle 24 (2007-2009) using the UVCS instrument on SOHO. Observations in H I Lyman alpha (121.6 nm) and O VI (103.2, 103.7 nm) provide spectroscopic diagnostics of proton and O5+ bulk outflow velocities and velocity distributions as a function of heliocentric distance above the poles of the Sun. These observations have allowed us to follow the changes in the physical properties of the polar coronal holes during solar cycle 23 and its approach to the current minimum. Recent ground- and space-based observations have reported a variety of phenomena associated with the current minimum. We present the comparison of observed oxygen line intensities, line ratios, and profiles for polar coronal holes at both minima and during solar cycle 23 and show how this new minimum manifests itself in the ultraviolet corona. The comparison of the physical properties of these two minima as seen by UVCS in the extended corona, now possible for the first time, may provide crucial empirical constraints on models of extended coronal heating and acceleration for the fast solar wind.