Monitoring nighttime medium-scale traveling ionospheric disturbances using the GPS network over North America

Using the total electron content (TEC) data observed from a dense GPS network in North America from June 2007 to May 2008, we analyzed the propagation characteristics of the nighttime medium-scale traveling ionospheric disturbances (MSTIDs) over North America. The results show that, the nighttime MSTIDs occur frequently between the local time of 21 : 00LT (05 : 00UT) and 02 : 00LT (10 : 00UT), which can be characterized by the maximum amplitude of 0. 45 similar to 0. 6TECU and the average period of 20 similar to 40 minutes in the TEC perturbation series. The nighttime MSTIDs tend to propagate southwestward at the horizontal velocities of 100 similar to 200 m/s covering large area in the range of magnetic latitude (24 degrees N similar to 44 degrees N) and longitude (130 degrees W similar to 70 degrees W). The statistical results indicate that, the nighttime MSTIDs have a clear semiannual variation, with the maximum amplitudes around equinox and the smallest ones around solstice; the horizontal velocities in summer are about 20% larger than that in winter. Further studies have revealed that, the semiannual variation of nighttime MSTIDs is caused by the semiannual variation of background TEC, while the seasonal variation of the horizontal velocity is closely related with the seasonal variation of the thermospheric wind.