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Solar Physics: Overview  

E.R. Priest

Solar physics is one of the liveliest branches of astrophysics at the current time, with many major advances that have been stimulated by observations from a series of space satellites and ground-based telescopes as well as theoretical models and sophisticated computational experiments. Studying the Sun is of key importance in physics for two principal reasons. Firstly, the Sun has major effects on the Earth and on its climate and space weather, as well as other planets of the solar system. Secondly, it represents a Rosetta stone, where fundamental astrophysical processes can be investigated in great detail. Yet, there are still major unanswered questions in solar physics, such as how the magnetic field is generated in the interior by dynamo action, how magnetic flux emerges through the solar surface and interacts with the overlying atmosphere, how the chromosphere and corona are heated, how the solar wind is accelerated, how coronal mass ejections are initiated and how energy is released in solar flares and high-energy particles are accelerated. Huge progress has been made on each of these topics since the year 2000, but there is as yet no definitive answer to any of them. When the answers to such puzzles are found, they will have huge implications for similar processes elsewhere in the cosmos but under different parameter regimes.


Solar Prominences  

Duncan H. Mackay

Solar prominences (or filaments) are cool dense regions of plasma that exist within the solar corona. Their existence is due to magnetic fields that support the dense plasma against gravity and insulate it from the surrounding hot coronal plasma. They can be found across all latitudes on the Sun, where their physical dimensions span a wide range of sizes (length ~60–600 Mm, height ~10–100 Mm, and width ~4–10 Mm). Their lifetime can be as long as a solar rotation (27 days), at the end of which they often erupt to initiate coronal mass ejections. When viewed at the highest spatial resolution, solar prominences are found to be composed of many thin co-aligned threads or vertical sheets. Within these structures, both horizontal and vertical motions of up to 10–20 kms−1 are observed, along with a wide variety of oscillations. At the present time, a lack of detailed observations of filament formation gives rise to a wide variety of theoretical models of this process. These models aim to explain both the formation of the prominence’s strongly sheared and highly non-potential magnetic field along with the origin of the dense plasma. Prominences also exhibit a large-scale hemispheric pattern such that “dextral” prominences containing negative magnetic helicity dominate in the northern hemisphere, while “sinistral” prominences containing positive helicity dominate in the south. Understanding this pattern is essential to understanding the build-up and release of free magnetic energy and helicity on the Sun. Future theoretical studies will have to be tightly coordinated with observations conducted at multiple wavelengths (i.e., energy levels) in order to unravel the secrets of these objects.