Magnetohydrodynamic Fast Sausage Waves in the Solar Corona

TL;DR

This review discusses the properties, recent theoretical advances, and observational implications of magnetohydrodynamic fast sausage modes in the solar corona, highlighting their importance for coronal seismology and flare studies.

Contribution

It synthesizes recent theoretical and modeling developments on coronal FSMs, expanding understanding of their dispersion and observational signatures beyond canonical models.

Findings

Better understanding of FSM dispersion in canonical configurations

Identification of qualitative differences in non-canonical equilibrium dispersions

Highlighting the importance of FSMs in interpreting solar flare pulsations

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.