Period ratios for standing kink and sausage modes in magnetized structures with siphon flow on the Sun

TL;DR

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

Contribution

It provides the first detailed analysis of flow effects on period ratios in solar magnetic cylinders, highlighting their importance in interpreting observed oscillations.

Findings

Flow reduces period ratio $P_1/2P_2$ by up to 17% in coronal loops.

Flow effects are less significant in photospheric tubes.

Flow can alter the threshold aspect ratio for trapped sausage modes.

Abstract

Standing oscillations with multiple periods were found in a number of atmospheric structures on the Sun. The ratio of the period of the fundamental to twice the one of its first overtone, $P_1/2P_2$, is important in applications of solar magneto-seismology. We examine how field-aligned flows impact $P_1/2P_2$ of standing modes in solar magnetic cylinders. For coronal loops, the flow effects are significant for both fast kink and sausage modes. For kink ones, they reduce $P_1/2P_2$ by up to 17\% relative to the static case even when the density contrast between the loop and its surroundings approaches infinity. For sausage modes, the reduction in $P_1/2P_2$ due to flow is typically $\lesssim 5.5\%$ compared with the static case. However, the threshold aspect ratio, only above which can trapped sausage modes be supported, may increase dramatically with the flow magnitude. For photospheric tubes, the flow effect on $P_1/2P_2$ is not as strong. However, when applied to sausage modes, introducing field-aligned flows offers more possibilities in interpreting the multiple periods recently measured. We conclude that field-aligned flows should be taken into account to help better understand what causes the departure of $P_1/2P_2$ from unity.