Envelope soliton in solar radio emission

Several envelope soliton fine structures have been observed in solar radio metric-wave emission. We present a model of longitudinal modulational insta bility to explain these fine structures. It is found that this instability can only occur in the condition of sound velocity being larger than Alfven velocity in corona. Therefore, the envelope soliton fine structures should display in the coronal region with high temperature and low magnetic field, which corresponds to the solar radio emission in the legion of meter and d ecameter wavelength.