Impulsively generated sausage waves in coronal tubes with transversally continuous structuring

Abstract

The frequency dependence of the longitudinal group speeds of trapped sausage waves plays an important role in determining impulsively generated wave trains, which have often been invoked to account for quasi-periodic signals in coronal loops. We examine how the group speeds ($v_{\mathrm{gr}}$) depend on angular frequency ($\omega$) for sausage modes in pressureless coronal tubes with continuous transverse density distributions by solving the dispersion relation pertinent to the case where the density inhomogeneity of arbitrary form takes place in a transition layer of arbitrary thickness. We find that in addition to the transverse lengthscale $l$ and density contrast ${\rho }_{\mathrm{i}}/{\rho }_{\mathrm{e}}$, the group speed behavior depends also on the detailed form of the density inhomogeneity. For parabolic profiles, $v_{\mathrm{gr}}$ always decreases with $\omega$ first before increasing again, as happens for the much studied top-hat profiles. For linear profiles, however, the behavior of the $\omega -v_{\mathrm{gr}}$ curves is more complex. When ${\rho }_{\mathrm{i}}/{\rho }_{\mathrm{e}}\lesssim 6$, the curves become monotonical for large values of $l$. On the other hand, for higher density contrasts, a local maximum $v_{\mathrm{gr}}^{\max }$ exists in addition to a local minimum $v_{\mathrm{gr}}^{\min }$ when coronal tubes are diffuse. With time-dependent computations, we show that the different behavior of group speed curves, the characteristic speeds $v_{\mathrm{gr}}^{\min }$ and $v_{\mathrm{gr}}^{\max }$ in particular, is reflected in the temporal evolution and Morlet spectra of impulsively generated wave trains. We conclude that the observed quasi-periodic wave trains not only can be employed to probe such key parameters as density contrasts and profile steepness, but also have the potential to discriminate between the unknown forms of the transverse density distribution.

35 pages, 15 figures, accepted for publication in ApJ

References in corpus (5)

• Advances in Observing Various Coronal EUV Waves in the SDO Era and Their Seismological Applications (Invited Review)
• Quasi-periodic Fast-mode Wave Trains Within a Global EUV Wave and Sequential Transverse Oscillations Detected by SDO/AIA
• Propagation and dispersion of sausage wave trains in magnetic flux tubes
• Effect of a sausage oscillation on radio zebra-pattern structures in a solar flare
• Standing sausage modes in coronal loops with plasma flow

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• Quasi-Periodic Radio Bursts Associated with Fast-mode Waves near a Magnetic Null Point
• Coronal Quasi-periodic Fast-mode Propagating Wave Trains
• Impulsively Generated Wave Trains in Coronal Structures: I. Effects of Transverse Structuring on Sausage Waves in Pressureless Tubes
• Fast magnetoacoustic wave trains: from tadpoles to boomerangs
• Standing Sausage Perturbations in solar coronal loops with diffuse boundaries: An initial-value-problem perspective