The evolution of plasma parameters on a coronal source surface at 2.3 Rs during solar minimum

TL;DR

This study maps and analyzes coronal outflow velocities and densities at 2.3 solar radii during solar minimum, revealing how polar hole shrinkage affects solar wind expansion and flux tube geometry.

Contribution

It provides the first detailed global maps of coronal outflow velocities and densities at 2.3 Rs during solar minimum, highlighting changes in flux tube expansion related to polar hole evolution.

Findings

Polar hole areal expansion factors decrease from super-radial to near radial during solar minimum.

Largest flux tube expansion factors are near the coronal hole/streamer interface.

Coronal outflow velocities and densities are mapped during solar minimum, showing stability and subsequent changes.

Abstract

We analyze data from the Solar and Heliospheric Observatory to produce global maps of coronal outflow velocities and densities in the regions where the solar wind is undergoing acceleration. The maps use UV and white light coronal data obtained from the Ultraviolet Coronagraph Spectrometer and the Large Angle Spectroscopic Coronagraph, respectively, and a Doppler dimming analysis to determine the mean outflow velocities. The outflow velocities are defined on a sphere at 2.3 Rs from Sun-center and are organized by Carrington Rotations during the solar minimum period at the start of solar cycle 23. We use the outflow velocity and density maps to show that while the solar minimum corona is relatively stable during its early stages, the shrinkage of the north polar hole in the later stages leads to changes in both the global areal expansion of the coronal hole and the derived internal flux tube expansion factors of the solar wind. The polar hole areal expansion factor and the flux tube expansion factors (between the coronal base and 2.3 Rs) start out as super-radial but then they become more nearly radial as the corona progresses away from solar minimum. The results also support the idea that the largest flux tube expansion factors are located near the coronal hole/streamer interface, at least during the deepest part of the solar minimum period.