Relationships between quasi-periodic VLF emission and geomagnetic pulsation

Abstract

Relationships between quasi-periodic VLF emissions (QP) and geomagnetic pulsations are examined, based on observations at Syowa Station, Antarctica. It is confirmed that QP emissions can be classified into two types according to whether or not QP emissions are accompanied by geomagnetic pulsations with approximately the same period. One type, associated with concurrent pulsations, is a daytime phenomenon and often occurs in moderate magnetic activity. The QP period ranges mostly from 20 to 60 s. The spectral form depends significantly on the modulation period. When the period is longer than 30–40 s, the upper frequency limit of the emission band varies almost synchronously with the change in signal strength and the pulsation wave form. The rising characteristic of the QP component which comprises diffuse noises occasionally with discrete emissions, is observed when the period is shorter than 30–40 s. It is very likely that the QP emission of this type is caused by the interaction between whistler turbulence and compressional hydromagnetic waves in the outer magnetosphere. Another type, without any corresponding pulsation has a broad maximum of occurrence in the afternoon-evening and is observed in quiet geomagnetic conditions. The QP component, consists of either diffuse noises or discrete periodic emissions or a combination of both, and generally shows a midfrequency rise. These characteristics are very similar to those of QP emissions observed at Eights ( L ≅ 4). It is suggested that this type of QP emission is a phenomenon occurring near the plasmapause and that is closely related to the bulge or detached regions of the plasmasphere.