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

Abstract

Standing oscillations with multiple periods have been 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, P1/2P2, is important in applications of solar magneto-seismology. We examine how field-aligned flows impact P1/2P2 of standing modes in solar magnetic cylinders. For coronal loops, the flow effects are significant for both fast kink and sausage modes. For kink modes, they reduce P1/2P2 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 P1/2P2 due to flow is typically ≲ 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 P1/2P2 is not as strong. However, when applied to sausage modes, introducing field-aligned flows offers more possibilities in interpreting the multiple periods that have recently been measured. We conclude that field-aligned flows should be taken into account to help better understand what causes the departure of P1/2P2 from unity.