# Day-to-Day Boundary Fluctuations in Coronal Holes: Causes and Consequences

## TL;DR

Coronal hole boundaries on the Sun change daily due to supergranular convection and magnetic reconnection, affecting the slow solar wind and its variability.

## Contribution

The paper identifies supergranular convection and interchange reconnection as key drivers of coronal hole boundary fluctuations.

## Key findings

- Coronal hole boundary fluctuations occur even without nearby sunspots or transient activity.
- Boundary changes are caused by supergranular convection and magnetic reconnection, influencing the slow solar wind.
- A sudden long-lived coronal hole formed via interchange reconnection with the north polar hole.

## Abstract

Extreme-ultraviolet (EUV) images from the Atmospheric Imaging Assembly on the Solar Dynamics Observatory and the EUV Imager on the Solar Terrestrial Relations Observatory show that coronal hole boundaries often change from one day to the next on spatial scales up to several supergranules. Such changes may occur even in the absence of nearby sunspots or transient activity. We attribute the fluctuations to the action of supergranular convection, which continually rearranges the photospheric flux distribution both near and far from the hole boundaries. The boundary displacements may exceed a supergranular diameter because, in addition to simple advection, the open magnetic flux may undergo interchange reconnection with the long closed loops rooted just outside the boundary. This injects streamer material into the heliospheric plasma sheet but does not lead to a mixing of open and closed flux, whose interface remains clearly defined in EUV images and qualitatively consistent with current-free extrapolations of the (instantaneous) photospheric field. However, the boundary fluctuations are likely to be a major cause of the well-known variability of the slow solar wind, with the footpoint locations of the wind intercepted by a given spacecraft continually changing relative to the hole boundary on timescales of a day or less. This variability reflects the steep increase in the rate of flux-tube divergence toward the boundary, which leads to rapid changes in the measured wind speeds and densities. We also describe an unusual case in which a long-lived coronal hole forms suddenly without any nearby flux emergence, apparently as a result of transient-driven interchange reconnection with the north polar hole.

The online version contains supplementary material available at 10.1007/s11207-026-02610-8.