Abstract
Oscillatory power is pervasive throughout the solar corona, and magnetohydrodynamic (MHD) waves may carry a significant energy flux throughout the Sun’s atmosphere. As a result, over much of the past century, these waves have attracted great interest in the context of the coronal heating problem. They are a potential source of the energy required to maintain the high-temperature plasma and may accelerate the fast solar wind. Despite many observations of coronal waves, large uncertainties inhibit reliable estimates of their exact energy flux, and as such, it remains unclear whether they can contribute significantly to the coronal energy budget. A related issue concerns whether the wave energy can be dissipated over sufficiently short time scales to balance the atmospheric losses. For typical coronal parameters, energy dissipation rates are very low and, thus, any heating model must efficiently generate very small-length scales. As such, MHD turbulence is a promising plasma phenomenon for dissipating large quantities of energy quickly and over a large volume. In recent years, with advances in computational and observational power, much research has highlighted how MHD waves can drive complex turbulent behaviour in the solar corona. In this review, we present recent results that illuminate the energetics of these oscillatory processes and discuss how transverse waves may cause instability and turbulence in the Sun’s atmosphere.
Highlights
Dissimilar to many other astrophysical plasmas, the Sun is relatively close to Earth and, its atmosphere can be studied in high detail with both ground- and space-based telescopes
If the characteristic time scales of the photospheric velocities are short in comparison to the travel time, we typically find direct current (DC) heating
It may be of little surprise that recent modelling, alongside previous analytical results, has suggested that coronal loops need little encouragement to become turbulent
Summary
In the decades since Hannes Alfvén published their pioneering analysis on magnetohydrodynamic (MHD) wave modes [1], the Sun’s atmosphere has been extensively used as a laboratory for observing, describing and understanding oscillatory behaviour in magnetised fluids. Continuous monitoring with a high cadence and high resolution, imaging and spectroscopy has identified a plethora of different wave modes permeating the photosphere (e.g., [2,3]), the chromosphere (e.g., [4,5]), the corona (e.g., [6,7]) and solar wind (e.g., [8,9]). The ubiquity of this periodic behaviour provides a range of questions for solar physicists, including: 1. This review focuses on one key aspect of oscillatory behaviour, which is pertinent to all three of these points; namely, to what extent do transverse MHD waves drive turbulent behaviour in the solar corona?
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