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Magnetohydrodynamic Fast Sausage Waves in the Solar Corona

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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.

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Notes

  1. The ambient is taken to be always static in this review.

  2. Kink modes are addressed only when necessary in this review. For more details, see the companion reviews by Nakariakov et al. (in preparation, this issue) and Van Doorsselaere et al. (2020, this issue).

  3. In the ER83 setup, the equilibrium quantities are transversely structured in a piece-wise constant manner, meaning that the subscript \(\mathrm{i}\) (\(\mathrm{e}\)) applies to the entire interior (exterior). We choose to introduce the subscripts this way such that we can avoid re-introducing them when discussing the equilibria with continuous transverse structuring.

  4. The first several zeros are \(j_{0, 1} = 2.4048\), \(j_{0, 2} = 5.5201\), \(j_{1, 1} = 3.8317\), and \(j_{1, 2} = 7.0156\).

  5. As detailed in Sect. 2.2, much can be learned for the dispersive properties of FSMs by examining the slab counterpart of the ER83 equilibrium. In the slab case, the analytical study by Karamimehr et al. (2019) showed that leaky FSMs of different transverse orders (\(l\)) behave differently with decreasing \(k\). Relative to its transverse harmonics (\(l \ge 2\)), the transverse fundamental (\(l = 1\)) is characterized by a faster decrease in \(\omega _{\mathrm{R}}\) accompanied by a more rapid increase of \(\omega _{\mathrm{I}}\).

  6. In the slab case, it is possible to analytically establish that the damping time experiences its shortest value at the long wave-length limit when the density ratio of the internal and external medium is close to unity, while experiencing its highest value just below the cut-off value when the density ratio possesses its highest value (Karamimehr et al. 2019).

  7. These two papers found that the damping time \(\tau \) (or equivalently \(\omega _{\mathrm{I}}\)) at \(kR \to 0\) does not depend on the transverse order. In addition, \(\tau /P\) is proportional to \(\sqrt{\rho _{\mathrm{i}}/ \rho _{\mathrm{e}}}\) rather than \(\rho _{\mathrm{i}}/\rho _{\mathrm{e}}\) for sufficiently large density contrasts. These differences from the cylindrical case are not regarded as “primary”, though.

  8. This is not to say that Fig. 3 contradicts Figure 3a in Roberts et al. (1984), which sketches the three-phase scenario for impulsively generated wavetrains. It is just that there are certain requirements for the sketch to apply, with the need to sample a wavetrain at a distance sufficiently far from the exciter already implied in Equation (7). On top of that, some further time-dependent simulations have suggested that the duration of the exciter needs to be \(\lesssim R/v_{\mathrm{Ai}}\) (Goddard et al. 2019), and the spatial extent of the exciter needs to be comparable to the waveguide radius (Yu et al. 2017). We take these requirements as encouraging rather than discouraging for one to further examine impulsive wavetrains, because the deviations of the signatures of observed wavetrains from the sketch actually encode a rich set of seismic information on, say, the exciters. For more details, see Sect. 5.2.

  9. Crazy tadpoles were found in nonlinear sausage wavetrains in density-enhanced slabs as well (Pascoe et al. 2017c). Nonlinear waves, however, are beyond the scope of this review.

  10. If the width of the current sheet (CS) is taken to vanish, then the slab results directly apply (e.g. Feng et al. 2011, appendix). If the CS width is taken to be finite, then the Alfvén speed vanishes somewhere in the CS, and the characteristic timescales of FSMs tend to be comparable to the transverse sound time evaluated with the internal sound speed (Edwin et al. 1986, Fig. 2). Impulsive wavetrains, however, remain qualitatively the same in the sense of MHD (e.g., Jelínek and Karlický 2012; Mészárosová et al. 2014), which is understandable because Features 4 and 5 persist therein. We note that the Morlet spectra of the modulated radio fluxes further depend on the radiation mechanism that bridges the MHD results and measurables. The spectra may be similar across a substantial range of radio frequencies, as in the first identification of tadpoles in decimetric type IV bursts (Mészárosová et al. 2009b). Their morphology may be frequency-dependent at lower radio frequencies, resulting in the “drifting tadpoles” named by Mészárosová et al. (2009a).

  11. Implied here is that the magnetic trap forms first, and FSMs are excited in this preset host by some continuation of the energy release. The energy release is nonetheless “impulsive” or thrust-like, by which Zaitsev and Stepanov (1982) meant that its duration is shorter than the periods of the FSMs. Intuitively speaking, the expanded segment may shrink again, provided that the magnetic field therein is not that weak. FSMs may therefore develop around some new quasi-equilibrium, in conjunction with the response of the segment to the “thrust” imparted by the energy release. Thrust-excited FSMs were numerically explored by Pascoe et al. (2009) for preset expanded loops that are in force balance with the environment. However, to our knowledge, one has yet to quantitatively examine the excitation of FSMs during the restoration processes of expanded loops that are not in force balance with their ambient. At this point, we note also that the subsecond-period oscillations recently detected in decimetric radio bursts by Yu and Chen (2019) actually agree with the interpretation of thrust-excited FSMs in post-reconnection loops (see Fig. 11 therein). However, different from the Zaitsev and Stepanov (1982) scenario is that the modulation to the radio flux was attributed to the trapping and/or acceleration of energetic electrons within the time-varying wavepackets rather than the modulation of the mirror ratio of the local trap by the time-varying FSMs.

  12. Recall that the period \(P\) is of the order of the transverse Alfvén time (\(R/v_{\mathrm{Ai}}\), Equation (5)). Adopting the radiative loss function given by Rosner et al. (1978), one finds that the associated damping timescale \(\tau _{\mathrm{rad}}\) is given by \(3600 T_{6}/n_{9}\) sec when \(0.56< T_{6} <2\) and \(2200 T_{6}^{5/3}/n_{9}\) sec when \(2 < T_{6} <10\), where \(T_{6}\) (\(n_{9}\)) is the loop temperature (electron density) in \(10^{6}\) K (\(10^{9}\) cm−3). Typically \(\tau _{\mathrm{rad}} \gg R/v_{\mathrm{Ai}}\).

  13. In this context, we note that impulsively generated wavetrains have also been modeled for curved slabs by Nisticò et al. (2014) with applications to AR loops in mind. If we extend the idea to non-straight equilibria rather than specifically curved loops, then magnetic funnels were modeled by Pascoe et al. (2013b) and coronal holes by Pascoe et al. (2014). These studies demonstrate that leaky components can form quasi-periodic wavetrains in the external medium. Unlike the simple straight slab geometry, the propagation of these wavetrains is no longer necessarily perpendicular to the waveguide. For example, the refraction due to the nonuniform external medium can allow the wavetrain to propagate in the direction of the structure, despite that the wavetrain itself is outside the structure and not structure-guided.

  14. The fluctuating plasma velocity was not considered because it does not affect the gyrosynchrotron emission.

  15. For trapped FSMs, Fig. 1 indicates that when \(\rho _{\mathrm{i}}/\rho _{\mathrm{e}}\) is given, \(v_{\mathrm{ph}}/v_{\mathrm{Ai}}\) decreases with increasing \(kR\). This is not to be confused with that \(v_{\mathrm{ph}}/v_{\mathrm{Ai}}\) for a relatively small \(kR\) shows little dependence on \(\rho _{\mathrm{i}}/\rho _{\mathrm{e}}\). When evaluating \(v_{\mathrm{ph}}= 2L/P\), KMR12 somehow adopted twice the measured period. KMR12 further supposed that \(v_{\mathrm{ph}}\) is close to the external Alfvén speed \(v_{\mathrm{Ae}}\), and employed the smallness of \(v_{\mathrm{Ae}}\) to argue that it is difficult to reconcile the measured period with an FSM in an ER83 equilibrium. We note that, the insensitivity of \(v_{\mathrm{ph}}/v_{\mathrm{Ai}}\) to \(\rho _{\mathrm{i}}/\rho _{\mathrm{e}}\) for a relatively small \(kR\) means that \(v_{\mathrm{ph}}/v_{\mathrm{Ae}}\) is approximately \(\propto \sqrt{\rho _{\mathrm{e}}/ \rho _{\mathrm{i}}}\), and hence \(v_{\mathrm{Ae}}\) can be much larger than the measured \(v_{\mathrm{ph}}\) for large \(\rho _{\mathrm{i}}/\rho _{\mathrm{e}}\). Despite that, their argument remains valid.

  16. Practical seismological diagnostics using kink oscillations therefore includes using additional observables such as the shape of the damping profile (Hood et al. 2013; Pascoe et al. 2013a, 2016, 2019), independent sources of information such as the EUV intensity (Pascoe et al. 2017b, 2018), and Bayesian analysis (Arregui et al. 2013b; Pascoe et al. 2017a,d) including MCMC sampling (Anfinogentov et al. 2020) which permits constraints to be calculated without the need for a unique solution.

  17. Without imaging information, this study assumed that \(L/R \gg 1\), and derived \(R/v_{\mathrm{Ai}}\) rather than \(v_{\mathrm{Ai}}\).

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Acknowledgements

We thank the reviewers for their constructive comments and suggestions, which helped improve this manuscript substantially. We gratefully acknowledge ISSI-BJ for supporting the workshop on “Oscillatory Processes in Solar and Stellar Coronae”, during which this review was initiated. BL was supported by the National Natural Science Foundation of China (41674172, 11761141002, 41974200). PA acknowledges funding from his STFC Ernest Rutherford Fellowship (No. ST/R004285/2). AAK was supported by budgetary funding of Basic Research program II.16. DJP and TVD were supported by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 724326) and the C1 grant TRACEspace of Internal Funds KU Leuven.

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Oscillatory Processes in Solar and Stellar Coronae

Edited by Valery M. Nakariakov, Dipankar Banerjee, Bo Li, Tongjiang Wang, Ivan Zimovets and Maurizio Falanga

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Li, B., Antolin, P., Guo, MZ. et al. Magnetohydrodynamic Fast Sausage Waves in the Solar Corona. Space Sci Rev 216, 136 (2020). https://doi.org/10.1007/s11214-020-00761-z

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