Imaging Preflare Broadband Pulsations in the Decimetric-metric Wavelengths

Abstract

Abstract Preflare activities contain critical information about the precursors and causes of solar eruptions. Here we investigate the characteristics and origin of a group of broadband pulsations (BBPs) in the decimetric-metric wavelengths that took place during the preflare stage of the M7.1 flare on 2011 September 24. The event was recorded by multiple solar instruments, including the Nançay Radioheliograh, that measure the properties of the radio source. The BBPs started ∼24 minutes before the flare onset, extending from <360 to above 800 MHz without a discernible spectral drift. The BBPs consisted of two stages. During the first stage, the main source remained stationary, and during the second stage, it moved outward along with a steepening extreme-ultraviolet (EUV) wave driven by the eruption of a high-temperature structure. In both stages, we observe frequent EUV brightenings and jets originating from the flare region. During the second stage, the BBPs became more frequent and stronger in general, and the polarization level gradually increased from <20% to >60% in the right-handed sense. These observations indicate that the steepening EUV wave is important to the BBPs during the second stage, while the preflare reconnections causing the jets and EUV brightenings are important in both stages. This is the first time that such a strong association of an EUV wave with BBPs is reported. We suggest a scenario in which reconnection occurs together with a shock that sweeps across the loops as the cause of the BBPs.