Objectives: To observe the evolution of active regions associated with micro-type III radio bursts.
Scientific Background: Micro-type III radio bursts, reported by Morioka et al. (2007), are the elements of the type III storm, characterized by (1) a very low intensity, (2) short durations, and (3) continuous emissions compared to the ordinary type III radio bursts. The micro-type III bursts are not necessarily accompanied by energetic phenomena like flares, different from the ordinary type III bursts. The existence of micro-type III bursts suggests that a very small fraction of electrons are continuously accelerated/heated, and escape from the Sun toward the interplanetary space. Morioka et al. found that the micro-type III bursts are generated from the active regions bordering on coronal holes. The interaction between active regions and coronal holes may be a key to accelerate the beam electrons that produce the micro-type III radio emissions. Therefore, the micro-type III burst would be a diagnostic phenomenon that indicates the existence of field-aligned electron acceleration in active regions. Recently, Minoshima et al. performed a drift kinetic Vlasov simulation of solar flares to understand the flare particle acceleration and transport mechanisms. From the numerical simulation, they propose a scenario for the creation of escaping electrons by a centrifugal acceleration taking place in curved open magnetic field lines. The mechanism requires only the bulk motion of curved magnetic field lines, not explosive phenomena such as magnetic reconnection. This mechanism most effectively works at the boundary between open and close field lines. In these two viewpoints, Minoshima et al. suggest this mechanism as a plausible candidate of the acceleration process of electrons for micro-type III bursts. For further understanding of the micro-type III bursts, the evolution of the burst-generating active regions should be observed in a high time resolution (<~10 s), which has not been implemented so far. The evolution of magnetic field lines at the boundaries between active regions and coronal holes is particularly interesting. Currently, several spacecraft such as Geotail, Wind, and STEREO continuously monitor solar radio waves at the MH/HF frequency range. Moreover, Iitate Planetary radio telescope (IPRT) of Tohoku University observes the type-I bursts, which are weak and rapid solar bursts appearing with micro-type III bursts in the UHF frequency range. Simultaneous observations of the micro-type III/type-I bursts by these spacecraft and ground-based radio telescope, and the Hinode observation of the corresponding active region are essential to understand how continuous electron beams are generated in the active region bordering coronal holes. Morioka et al. 2007, ApJ, 657, 567 Minoshima et al. in preparation. |
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