The period ratio for standing kink and sausage modes in solar structures with siphon flow. I. magnetized slabs

TL;DR

This study investigates how field-aligned flows influence the period ratios of standing kink and sausage modes in solar magnetic slabs, revealing significant effects especially in coronal conditions, which are crucial for solar magneto-seismology.

Contribution

It provides a detailed numerical analysis of flow effects on mode period ratios in magnetized slabs, highlighting their importance for accurate solar atmospheric seismology.

Findings

Flow reduces period ratio P1/2P2 significantly in coronal slabs.

Flow increases the threshold aspect ratio for sausage modes, affecting their detectability.

Standing kink modes show deviation from unity in period ratio even at zero width.

Abstract

In the applications of solar magneto-seismology(SMS), employing the ratio of the period of the fundamental mode to twice the one of its first overtone, $P_1/2P_2$, plays an important role. We examine how field-aligned flows affect the dispersion properties, and hence the period ratios, of standing modes supported by magnetic slabs in the solar atmosphere. We numerically solve the dispersion relations and devise a graphic means to construct standing modes. For coronal slabs, we find that the flow effects are significant, for the fast kink and sausage modes alike. For the kink ones, they may reduce $P_1/2P_2$ by up to 23% compared with the static case, and the minimum allowed $P_1/2P_2$ can fall below the lower limit analytically derived for static slabs. For the sausage modes, while introducing the flow reduces $P_1/2P_2$ by typically $\lesssim 5$ % relative to the static case, it significantly increases the threshold aspect ratio only above which standing sausage modes can be supported, meaning that their detectability is restricted to even wider slabs. In the case of photospheric slabs, the flow effect is not as strong. However, standing modes are distinct from the coronal case in that standing kink modes show a $P_1/2P_2$ that deviates from unity even for a zero-width slab, while standing sausage modes no longer suffer from a threshold aspect ratio. We conclude that transverse structuring in plasma density and flow speed should be considered in seismological applications of multiple periodicities to solar atmospheric structures.