Magnetohydrodynamic Fast Sausage Waves in the Solar Corona

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,

A New View of the Solar Interface Region from the Interface Region Imaging Spectrograph (IRIS)

“…The first fully resolved standing fundamental fast sausage mode was detected by Tian et al (2016b) during an M1.6 flare, characterized by the π/2 phase difference between the Doppler velocity and the intensity, with a short period around 25 seconds. Such waves are leading candidates to explain the quasi-periodic pulsation (QPP) phenomena and represent unique seismological tools of flaring loops (see also Li et al, 2020a). Interestingly, an increase of the wave period with time was detected, suggesting a continuous generation of fast sausage modes in the gradually longer flaring loops issued from the reconnection process.…”

Solar Cycle Dependence of ICME Composition

“…The so-called ionization equilibrium is realized when τ dyn is much longer than the timescales characterizing the ionization/recombination processes (τ ion ). If accepting that the ionization is primarily due to electron impact (and possibly autoionizations) and the recombination largely involves radiative and dielectronic processes, then the ionization/recombination frequencies (ν ion ≡ 1/τ ion ) are proportional to the electron density N e as well as the ionization and recombination rate coefficients, which in turn depend essentially only on the electron temperature (T e ) (e.g., Owocki, Holzer, and Hundhausen, 1983;Del Zanna and Mason, 2018;Shi et al, 2019;Li et al, 2020). Ionization equilibrium is expected when a fluid parcel just leaves its source regions given the pertinent high values of N e .…”

Chromospheric Heating by Magnetohydrodynamic Waves and Instabilities

“…How can we determine the isolated symmetric modes of flux tubes (see sausage waves in sub‐section 3.2) in the solar chromosphere when these are strongly structured in the radial direction? (e.g., Chen et al., 2016; Li et al., 2018, 2020; Lopin & Nagorny, 2015)…”

“…(sub-section 3.1, and, e.g., Goossens et al 2011) (iii) How can we determine the isolated symmetric modes of flux tubes (see sausage waves in sub-section 3.2) in the solar chromosphere when these are strongly structured in the radial direction? (e.g., Lopin and Nagorny 2015;Chen et al 2016;Li et al 2018Li et al , 2020.…”