[1] The 15-month climatology of medium-scale traveling ionospheric disturbances (MSTIDs) during a solar minimum period has been constructed from observations of a dense GPS receiver array in Central China. In total, 793 MSTID events are identified, with peaks in occurrence at 1500 LT and 0100 LT. The occurrence of MSTIDs decreases following an increase in geomagnetic activity, with 46% of the MSTIDS occurring in the daytime. Daytime MSTIDs are characterized by a major occurrence maximum around the winter solstice and by an equatorward propagation direction. The period, phase velocity, azimuth, and amplitude of daytime MSTIDs are 20-60 min, 100-400 m/s, 130°-270°, and 0.8-1.5%, respectively. The remaining 54% of the MSTIDs occurred at night, and were characterized by a peak in occurrence at the summer solstice and by a southwestward propagation direction. The period, phase velocity, azimuth, and amplitude of nighttime MSTIDs are 20-70 min, 50-230 m/s, 170°-300°, and 2-7%, respectively. The propagation directions and the seasonal behaviors support the view that daytime MSTIDs are an ionospheric manifestation of atmospheric gravity waves from the lower atmosphere, while a possible excitation mechanism of nighttime MSTIDs is the electrodynamics process caused by plasma instability in the F layer.

[1] On 12 May 2008 at 0628 UT a major earthquake M s = 8.0 struck Wenchuan County (31.0°N, 103.4°E) in southwest China. The maximum ionospheric electron density at F 2 peak (N m F 2 ) recorded an unusual large enhancement during the afternoon-sunset sector by the Chinese ionosondes over Wuhan (30.5°N, 114.4°E) and Xiamen (24.4°N, 123.9°E), which are close to the earthquake epicenter. An averaged increase at these two stations is about 2 times on a geomagnetic quiet day, 9 May (Kp 2), 3 days prior to the earthquake, relative to the median value of 1-12 May, whereas the increase was much less significant over Yamagawa (31.2°N, 130.6°E) and Okinawa (26.7°N, 128.2°E) in Japan. Combining the data from the network of 58 global positioning system receivers around China and the global ionospheric map, the variations of the total electron content reveal the region where enhancement persisted for a long period to be within longitudes 90°-130°E. Our results suggest that this abnormal enhancement is most possibly a seismo-ionospheric signature.

No abstract

Based on the linear theory, the detailed growth rate expression of plasma waves for hot ions with ring velocity distributions is derived, which can be directly used to study the wave growth with satellite data. Using linear growth rate calculations with simple magnetic field and background plasma density models, the roles of five parameters (Ne,ηH + : ηHe + : ηO +, Er, Tperp, and Tpara) playing in the excitation of O+ band electromagnetic ion cyclotron (EMIC) waves driven by hot H+ with ring velocity distributions are investigated. Our calculations demonstrate that ring distributions with sufficiently large ring velocity Vr, small perpendicular thermal spread V⊥, and appropriate parallel thermal spread V// could enhance the wave growth of oblique O+ band EMIC waves. Moreover, our results show that O+ band EMIC waves considered here are mainly unstable with large wave normal angles in a dense and rich O+ background plasma, in consistent with the statistical characteristics of O+ band EMIC waves previously observed by satellites. Therefore, it is indicated that hot H+ with ring velocity distributions should play an important role in the excitation of O+ band EMIC waves, as a necessary and useful supplement to ring current ions with temperature‐anisotropy bi‐Maxwellian distributions.