Geomagnetic conjugate observations of large-scale traveling ionospheric disturbances using GPS networks in Japan and Australia

The geomagnetic conjugacy of large-scale traveling ionospheric disturbances (LSTIDs) was studied using total electron content (TEC) data derived from GPS networks in Japan and Australia. The number of simultaneous occurrences of LSTIDs within +/- 1 hour in both hemispheres was 5 out of 20 (21) events in Japan (Australia) when Kp >= 5- and 0 out of 15 (10) when Kp <= 4+. As for the LSTIDs observed simultaneously in both hemispheres, the propagation velocities of equatorward LSTID were comparable between the two hemispheres, with differences of 10-40%. The crossing times at 30 degrees geomagnetic latitude of the simultaneous LSTIDs over Japan and Australia were also different by several tens of minutes for all five events. These observational results indicate that the LSTIDs observed almost simultaneously in both hemispheres are not connected electromagnetically through the geomagnetic field but are generated by atmospheric gravity waves propagating to the equator independently in the two hemispheres.