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Characteristics of type III solar radio bursts are studied using high-frequency resolution of the SFA (sweep-frequency analyzer) of the Plasma Wave Instrument (PWI) on board the Geotail spacecraft. We often observe abnormal type III bursts, which have separated frequency bands or have prolonged tails at particular frequencies. These observations provide observational clues to detect density inhomogeneities in the upstream interplanetary medium. We propose possible models of interplanetary density structures which can account for some type III spectrum structures observed.