A statistical study of large-scale traveling ionospheric disturbances using the GPS network in Japan



[1] Large-scale traveling ionospheric disturbances (LSTIDs) were statistically studied using total electron content (TEC) data from the GPS Earth observation network (GEONET) in Japan during 45 months from April 1999 to December 2002. One hundred fifty-four LSTIDs propagating southward were identified with time sequences of two-dimensional TEC maps. Fifty LSTIDs were observed in 2000, 45 in 2001, and 38 in 2002. Their occurrence rate (occurrence probability of one LSTID per 3 hours) increased as Kp value increased, that is, 1% at Kp = 4 and 75% at Kp = 9. The disturbed-time LSTIDs were frequently observed over Japan in spring and autumn, which is consistent with the seasonal dependence of the geomagnetic disturbances. On the other hand, the number of LSTIDs under quiet conditions, Kp ≤ 3, reached 43, that is, 28% of all the LSTIDs. The wave parameters, such as the damping rate, horizontal velocity and direction, and period of the clear 58 LSTIDs were precisely determined. The amplitude of the quiet-time LSTIDs decreased during their passage over Japan for every event, while that of the disturbed-time LSTIDs not only decreased but also increased. The LSTIDs can be classified from these observational results into the following three types: the disturbed-time damping LSTIDs (DD-LSTIDs), the disturbed-time growing LSTIDs (DG-LSTIDs), and the quiet-time damping LSTIDs (QD-LSTIDs). The occurrence number of DD-LSTIDs, DG-LSTIDs, and QD-LSTIDs was 35 (60%), 11 (19%), and 12 (21%), respectively. The mean horizontal velocity, period, wavelength, and propagation direction of all LSTIDs were 475 ± 171 m/s, 80 ± 29 min, 2131 ± 863 km, and 3 ± 19° east from south, respectively. Both the growth and decrease rates of the LSTIDs were correlated with their propagation velocities. An examination of the relation between the damping rates and the vertical propagation direction, θ, of atmospheric gravity waves (AGWs) which was derived from the horizontal velocity and the period of LSTIDs using the AGWs' dispersion relation, revealed that their damping and growth rates showed a clear correlation with θ. This correlation is due to the ion-drag effect, which is directly dependent on the angle between AGWs' propagation directions and the geomagnetic field. Considering the inclination of the geomagnetic field over Japan, both the damping and growing LSTIDs could be explained by the upward and downward propagating AGWs, respectively. The QD-LSTIDs had smaller θ which resulted from slower velocities and longer periods than those for the disturbed-time LSTIDs. These different characteristics would reflect those of the source mechanisms of the three types of LSTIDs.