Atmospheric oscillations in solar magnetic flux tubes - I. Excitation by longitudinal tube waves and random pulses

The response of exponentially spreading magnetic flux tubes embedded in an isothermal solar atmosphere to the propagation of longitudinal tube waves and random pulses produced in the solar convection zone is studied analytically. General solutions are presented and applied to solar tube models. It is shown that free atmospheric oscillations inside these flux tubes are generated with oscillation periods near 3 min, which are essentially identical to the oscillation periods observed in the interior regions of supergranulation cells. The observed oscillations are therefore consistent with processes in magnetic tubes as well as in nonmagnetic regions. Stochastic perpetual excitation is expected to keep these flux tube oscillations present at all times. These oscillations are inconsistent with the observed 7-min oscillations in the chromospheric network.