Study of VLF band electromagnetic waves in the Antarctica observed by polar patrol balloons

The two large scientific balloons (PPB: polar patrol balloons) were launched on January 13th, 2003 at Syowa Station in Antarctica. The balloons reached the altitude of 33 km, and observed various data for about two weeks. We developed the wideband electromagnetic wave receiver onboard PPB and observed ELF/VLF waves in Antarctica. VLF band waves observed in Antarctica are frequently modulated in frequency. Among these frequency modulations, the modulations of about 20 sec are considered to have relations with the compressional Alfven waves. By taking cross correlations between data observed by two balloons, we can identify the direction from which VLF waves propagate as well as the source region where these waves are excited. We compared one event observed by both balloons on January 17th, and found the time when the strong VLF wave observed by the 8th PPB was 3 seconds earlier than that observed by the 10th PPB. This result indicates that this VLF wave propagates with the velocity of about 67 km/s. The projection of this velocity on the equator plane is about 1800 km/s, which is almost the same order as the estimated velocity of the compressional Alfven waves in this region.