Sunspots are the most prominent manifestations of magnetic fields on the visible surface of the Sun (photosphere). While historic records mention sunspot observations by eye more than two thousand years ago, the physical nature of sunspots has been unraveled only in the past century starting with the pioneering work of Hale and Evershed. Sunspots are compact magnetic-field concentrations with a field strength exceeding 3,000 G in their center, a horizontal extent of about 30 Mm and typical lifetimes on the order of weeks. Research during the past few decades has focused on characterizing their stunning fine structure that became evident in high-resolution observations. The central part of sunspots (umbra) appears, at visible wavelengths, dark due to strongly suppressed convection (about 20% of the brightness of unperturbed solar granulation); the surrounding penumbra with a brightness of more than 75% of solar granulation shows efficient convective energy transport, while at the same time the constraining effects of magnetic field are visible in the filamentary fine structure of this region. The developments of the past 100 years have led to a deep understanding of the physical structure of sunspots. Key developments were the parallel advance of instrumentation; the advance in the interpretation of polarized light, leading to reliable inversions of physical parameters in the solar atmosphere; and the advance of modeling capabilities enabling radiation magnetohydrodynamic (MHD) simulations of the solar photosphere on the scale of entire sunspots. These developments turned sunspots into a unique plasma laboratory for studying the interaction of strong magnetic field with convection. The combination of refined observation and data analysis techniques provide detailed physical constraints, while numerical modeling has advanced to a level where a direct comparison with remote sensing observations through forward modeling of synthetic observations is now feasible. While substantial progress has been made in understanding the sunspot fine structure, fundamental questions regarding the formation of sunspots and sunspot penumbrae are still not answered.
12
Article
Sunspots
M. Rempel and J.M. Borrero
Article
The Partonic Content of Nucleons and Nuclei
Juan Rojo
Deepening our knowledge of the partonic content of nucleons and nuclei represents a central endeavor of modern high-energy and nuclear physics, with ramifications in related disciplines, such as astroparticle physics. There are two main scientific drivers motivating these investigations of the partonic structure of hadrons. On the one hand, addressing fundamental open issues in our understanding of the strong interaction, such as the origin of the nucleon mass, spin, and transverse structure; the presence of heavy quarks in the nucleon wave function; and the possible onset of novel gluon-dominated dynamical regimes. On the other hand, pinning down with the highest possible precision the substructure of nucleons and nuclei is a central component for theoretical predictions in a wide range of experiments, from proton and heavy-ion collisions at the Large Hadron Collider to ultra-high-energy neutrino interactions at neutrino telescopes.
Article
The Solar Corona
Richard Morton
The solar corona is the hot, tenuous outer layer of the Sun’s atmosphere. The coronal plasma is roughly around 1 mega-Kelvin (MK) but can reach temperatures of around 10 MK in certain regions. Due to this high temperature, the plasma in the corona emits electromagnetic radiation predominantly at Extreme Ultraviolet (EUV) and X-ray wavelengths. The corona’s appearance and dynamic behavior is defined by a complex network of magnetic fields that thread the fully ionized plasma. Due to an excess of magnetic pressure compared to gas pressure, the magnetic field is able to control the flow of plasma in the corona, leading to a wide variety of different structures. The appearance of the corona undergoes small variations over hours, and changes dramatically over the course of the Sun’s magnetic cycle. At the large scales are vast patches of open magnetic fields known as coronal holes, in which the fast, hot solar wind originates. The coronal holes are most prominent at the minimum stage of the Sun’s activity cycle. The other large-scale feature is coronal streamers, which transition from closed to open magnetic field with height and extend out as elongated rays into the heliosphere. The streamers are always present and are a source of transient events in the solar wind. The inner corona is dominated by closed loops of plasma, which are structured on the smallest resolvable scales and are the result of emerging bipolar patches of magnetic field.
The Sun’s corona is also the source of a long-standing question in astrophysics. First raised in the 1940s, scientists are still trying to understand how energy is deposited in the coronal plasma, causing the heating of the plasma to millions of degrees and the acceleration of the solar wind. It is clear that the magnetic field plays a substantial role, but being able to pin down the mechanisms of energy release is challenging. Modern observations have revealed signatures of magnetohydrodynamic waves and magnetic reconnection throughout the corona, both of which are leading explanations. However, there are still many unknowns about their contributions to energy deposition throughout the corona. Revealing the nature of coronal energy deposition is not only important for understanding the Sun but is also key for understanding the evolution of planetary systems around solar-like stars (which get bombarded with EUV/X-ray radiation and stellar winds) and the evolution of solar-like stars themselves (including mass and angular momentum loss).
12
