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date: 12 February 2025

The Solar Coronalocked

The Solar Coronalocked

  • Richard MortonRichard MortonNorthumbria University

Summary

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).

Subjects

  • Cosmology and Astrophysics
  • Plasma Physics

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