Observing Coronal Microscales and their Connection with Mesoscales

Abstract

It has been known since the dawn of modern eclipse observations that the solar corona is structured on all scales from the solar radius to the limit of ground-based imaging (until the advent of adaptive optics, about 1 arcsec). Early space-based imaging at soft x-ray and extreme ultraviolet (EUV) wavelengths confirmed pervasive structure in the hot coronal plasma and added diagnostic power. During the same period, magnetograms revealed structure in the photospheric magnetic field at all observable scales and spawned the study of magnetic flux tubes. Because magnetic pressure dominates gas pressure in the corona (low β), it was clear that the magnetic field guides coronal structure. However, that alone does not dictate the density and temperature structure of the corona or determine on what spatial scales inhomogeneity will occur. It was equally clear that the physical mechanism(s) responsible for heating the corona ultimately determine those scales, and, consequently, that observing them would be key to understanding coronal heating. This leads immediately to the question: What spatial and temporal scales are present, and what scales are theoretically anticipated? In a seminal paper, Parker introduced the idea of topological dissipation: that magnetic flux tubes rooted in the photosphere, jostled by turbulent convection, will progressively develop fine structure down to scales that ultimately result in magnetic reconnection [1]. Parker further developed his theory and coined the term "nanoflare" for such small-scale heating events [2]. Notwithstanding the still-debated applicability of topological dissipation in the corona [3], it seems highly likely that random motions of magnetic footpoints in the photosphere will inevitably lead to very fine-scale structure in the coronal magnetic field. Thus, whether reconnection, wave dissipation, or both are responsible for local heating, the heated volumes are expected to possess structure on similarly fine scales.

A Strategy to Close Key Questions about the Middle Solar Corona During this Decade

“…Likewise, coronagraphic observations must be expanded to lower heights (See WP by Rabin et al, 2022). This can only be achieved with much-improved stray-light rejection.…”

On Solving the Outstanding Questions in Solar Wind Physics

“…See Seaton et al 2022and Vourlidas et al 2022 for more details on the science questions relevant to coverage of the middle corona. Coronal Microscale Observatory, Rabin et al 2022, is a possible mission concept that measures cross-regional (low-to-mid corona) and cross-scale (micro-to-meso-toglobal) coupling, and see Casini et al 2022 for using H I Ly-α polarization diagnostics as an enabling new technology for making plasma measurements through the middle corona. Solving the questions of solar wind physics is linked to solving the coronal heating problem.…”