Magnetohydrodynamic Fast Sausage Waves in the Solar Corona
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Magnetohydrodynamic Fast Sausage Waves in the Solar Corona B. Li1 · P. Antolin2 · M.-Z. Guo1 · A.A. Kuznetsov3 · D.J. Pascoe4 · T. Van Doorsselaere4 · S. Vasheghani Farahani5
Received: 5 September 2020 / Accepted: 30 October 2020 © Springer Nature B.V. 2020
Abstract Characterized by cyclic axisymmetric perturbations to both the magnetic and fluid parameters, magnetohydrodynamic fast sausage modes (FSMs) have proven useful for solar coronal seismology given their strong dispersion. This review starts by summarizing the dispersive properties of the FSMs in the canonical configuration where the equilibrium quantities are transversely structured in a step fashion. With this preparation we then review the recent theoretical studies on coronal FSMs, showing that the canonical dispersion features have been better understood physically, and further exploited seismologically. In addition, we show that departures from the canonical equilibrium configuration have led to qualitatively different dispersion features, thereby substantially broadening the range of observations that FSMs can be invoked to account for. We also summarize the advances in forward modeling studies, emphasizing the intricacies in interpreting observed oscillatory signals in terms of FSMs. All these advances notwithstanding, we offer a list of aspects that remain to be better addressed, with the physical connection of coronal FSMs to the quasi-periodic pulsations in solar flares particularly noteworthy. Keywords Sausage modes · Coronal seismology · Quasi-periodic pulsations · Solar flares Oscillatory Processes in Solar and Stellar Coronae Edited by Valery M. Nakariakov, Dipankar Banerjee, Bo Li, Tongjiang Wang, Ivan Zimovets and Maurizio Falanga
B B. Li
[email protected]
1
Shandong Provincial Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, Institute of Space Sciences, Shandong University, Weihai 264209, China
2
Department of Mathematics, Physics and Electrical Engineering, Northumbria University, Newcastle upon Tyne NE1 8ST, UK
3
Institute of Solar-Terrestrial Physics, Irkutsk 664033, Russia
4
Centre for Mathematical Plasma Astrophysics, Mathematics Department, KU Leuven, Celestijnenlaan 200B bus 2400, 3001 Leuven, Belgium
5
Department of Physics, Tafresh University, Tafresh 39518-79611, Iran
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1 Introduction Despite the considerable advances in modern instrumentation, it remains challenging to directly measure some key parameters of the highly structured solar corona, the magnetic field being particularly noteworthy (e.g., Cargill 2009). Coronal seismology has proven valuable for indirectly probing coronal structures by combining the measurements of the hosted low-frequency waves with continuously refined theories of magnetohydrodynamic (MHD) waves in an inhomogeneous medium (Roberts et al. 1984, see also the reviews by e.g., Roberts 2000, Nakariakov and Verwichte 2005, Nakariakov and Kolotkov 2020). The terminology in modern coronal seismology comes primarily from the theoretical work by
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