The Sun has been observed for millennia, but systematic and quantitative records of its activity started being collected with the advent of telescopic observations in the early 17th century. These early observations revealed key signatures of solar magnetic activity such as sunspots and faculae. Subsequent advances in instrumentation and observing techniques enabled the study of different layers of the solar atmosphere and led to the establishment of numerous long-term observational programs. These records underpin our current understanding of solar variability across timescales from days to centuries. In this review, I survey the principal long-term historical archives of solar observations and their analyses, where “historical” refers to datasets spanning roughly 50 years or more. Because many of these observations were obtained when both instrumentation and the physical understanding of the Sun were evolving, their interpretation presents significant challenges. Modern analysis methods and improved calibration techniques, however, now allow these legacy datasets to be re-examined and homogenised, enabling the extraction of information that was not fully exploited at the time of their acquisition. I also discuss the main solar activity and variability indices derived from historical observations, as well as reconstructions of other solar parameters based on these records.
We review major achievements in our understanding of multiphase coronal plasma, where cool-dense and hot-tenuous matter coexists, brought about by advances in modeling and theory, inspired by observations. We give an overview of models that self-consistently form solar (or stellar) prominences and filaments, or (postflare) coronal rain, and clarify how these different phenomena share a common physical origin, relating radiative losses and coronal heating. While we do not fully understand the coronal heating, multi-dimensional models of solar prominence and rain formation demonstrate how thermal instability triggers condensations, and how their morphology may reveal aspects of the applied heating at play. We emphasize how the many pathways to linear instability due to combined ingredients of heat-loss, gravity, flows, and magnetic topologies are all involved in the resulting nonlinear magnetohydrodynamics. We provide some challenges to future model efforts, especially concerning prominence fine structure, internal dynamics, and their overall lifecycle.
The Sun provides nearly all the energy powering the Earth’s climate system, far exceeding all other energy sources combined. The incident radiant energy, the “total solar irradiance,” has been measured by an uninterrupted series of temporally overlapping precision space-borne radiometric instruments since 1978, giving a record spanning more than four 11-year solar cycles. Short-term total-irradiance variations exceeding 0.1
Mean-field dynamo theory, describing the evolution of large-scale magnetic fields, has been the mainstay of theoretical interpretation of magnetism in astrophysical objects such as the Sun for several decades. More recently, three-dimensional magnetohydrodynamic simulations have reached a level of fidelity where they capture dynamo action self-consistently on local and global scales without resorting to parametrization of unresolved scales. Recent global simulations also capture many of the observed characteristics of solar and stellar large-scale magnetic fields and cycles. Successful explanation of the results of such simulations with corresponding mean-field models is a crucial validation step for mean-field dynamo theory. Here the connections between mean-field theory and current dynamo simulations are reviewed. These connections range from the numerical computation of turbulent transport coefficients to mean-field models of simulations, and their relevance to the solar dynamo. Finally, the most notable successes and current challenges in mean-field theoretical interpretations of simulations are summarized.
Coronal dimmings associated with coronal mass ejections (CMEs) from the Sun have gained much attention since the late 1990s when they were first observed in high-cadence imagery of the SOHO/EIT and Yohkoh/SXT instruments. They appear as localized sudden decreases of the coronal emission at extreme ultraviolet (EUV) and soft X-ray (SXR) wavelengths, that evolve impulsively during the lift-off and early expansion phase of a CME. Coronal dimmings have been interpreted as “footprints” of the erupting flux rope and also as indicators of the coronal mass loss by CMEs. However, these are only some aspects of coronal dimmings and how they relate to the overall CME/flare process. The goal of this review is to summarize our current understanding and observational findings on coronal dimmings, how they relate to CME simulations, and to discuss how they can be used to provide us with a deeper insight and diagnostics of the triggering of CMEs, the magnetic connectivities and coronal reconfigurations due to the CME as well as the replenishment of the corona after an eruption. In addition, we go beyond a pure review by introducing a new, physics-driven categorization of coronal dimmings based on the magnetic flux systems involved in the eruption process. Finally, we discuss the recent progress in studying coronal dimmings on solar-like and late-type stars, and how to use them as a diagnostics for stellar coronal mass ejections and their properties.
