Observational evidence of very long period pulsations of solar coronal microwave radiation

On 1998 September 23, a rare event was observed with the microwave (1.0–2.0 GHz and 2.6–3.8 GHz) radio spectrograph of National Astronomical Observatories. It was a complex type-IV burst accompanied by multi-periodic pulsations, type-III bursts and type-I like noise storms. This paper focuses on the pulsation components with multiple long periods (about 7.3, 4.9, 3.7, 1.2 and 0.4 minutes). The pulsations may be due to standing waves in closed loops. Magnetohydrodynamic standing Alfven waves driven by the photospheric velocity field penetrate across the magnetic field, and this leads to the modulation of the radio radiation. These pulsations have downward motions, so it is not excluded that disturbances propagating in a closed or an open field structure may have caused the pulsations. The resonance of Alfven waves is concerned with the photospheric 5-minute oscillations, hence pulsations of very long periods may be assumed to come from driving sources in the photosphere. This can imply a possible mutual coupling between coronal magnetic loops and the photosphere.