[1] The global ionospheric map (GIM) is used to observe variations in the total electron content (TEC) of the global positioning system (GPS) associated with 35 M ! 6.0 earthquakes that occurred in China during the 10-year period of 1 May 1998 to 30 April 2008. The statistical result indicates that the GPS TEC above the epicenter often pronouncedly decreases on day 3-5 before 17 M ! 6.3 earthquakes. The GPS TEC of the GIM and electron density profiles probed by six microsatellites of FORMOSAT3/ COSMIC (F3/C) are further employed to simultaneously observe seismoionospheric anomalies during an M w 7.9 earthquake near Wenchuan, China, on 12 May 2008. It is found that GPS TEC above the forthcoming epicenter anomalously decreases in the afternoon period of day 6-4 and in the late evening period of day 3 before the earthquake, but enhances in the afternoon of day 3 before the earthquake. The spatial distributions of the anomalous and extreme reductions and enhancements indicate that the earthquake preparation area is about 1650 km and 2850 km from the epicenter in the latitudinal and longitudinal directions, respectively. The F3/C results further show that the ionospheric F 2 peak electron density, N m F 2 , and height, h m F 2 , significantly decreases approximately 40% and descends about 50-80 km, respectively, when the GPS TEC anomalously reduces.

This paper provides the first three‐dimensional description of the ionospheric density structure of the Weddell Sea Anomaly (WSA). The WSA is characterized by a nighttime ionospheric density greater than that in daytime in the Weddell Sea region around the Antarctic Peninsula during the southern hemisphere summer. It was first observed by ground‐based ionosondes located in the Antarctica back in the 1950s and was further investigated by two‐dimensional maps over the oceans using TEC measurements collected by the TOPEX/Poseidon recently. Although these TEC maps have provided two‐dimensional views for tracking the time‐evolution and spatial coverage of the WSA, the vertical distribution of this peculiar feature is still unavailable. With the vertical ionospheric density profiles observed by the FORMOSAT‐3/COSMIC, three‐dimensional density structure of the WSA is presented here for the first time. Meanwhile, a similar WSA signature is observed in the northern and eastern hemisphere during June solstice by both the GPS‐TEC and the FORMOSAT‐3/COSMIC electron density observations. From the observed altitudinal structure of the WSA during 1800–2400 LT and the similar feature occurred in the opposite hemisphere suggest that the southward offset of the magnetic equator with respect to the geographic equator plays a major role for the WSA formation.

Ionospheric signatures possibly induced by the Nepal earthquake are investigated far outside the epicentral region in Taiwan (~3700 km distance from the epicenter) and in the Czech Republic (~6300 km distance from the epicenter). It is shown that the ionospheric disturbances were caused by long period,~20 s, infrasound waves that were excited locally by vertical component of the ground surface motion and propagated nearly vertically to the ionosphere. The infrasound waves are heavily damped at the heights of F layer at around 200 km, so their amplitude strongly depends on the altitude of observation. In addition, in the case of continuous Doppler sounding, the value of the Doppler shift depends not only on the advection (up and down motion) of the reflecting layer but also on the compression/ rarefaction of the electron gas and hence on the electron density gradient. Consequently, under significant differences of reflection height of sounding radio waves and partly also under large differences in plasma density gradients, the observed ionospheric response at larger distances from the epicenter can be comparable with the ionospheric response observed at shorter distances, although the amplitudes of causative seismic motions differ by more than one order of magnitude.

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Theoretical model simulations for an intense magnetic storm show the creation of a low‐latitude electron density arch aligned along the geomagnetic field created by strong uplift of the F2 layer that is driven by the penetration electric field. When the arch forms during the day, a new F2 layer is created at the original altitude by photoionization, and a density hole can be created between this new F2 layer and the arch. When the arch forms during the night, the F2 layer is not recreated and no hole forms. In a vertical profile of electron density, the daytime elevated ionospheric layer can appear distinctly from the recreated F2 layer, in which case the elevated layer is called the F3 layer. A latitude cut through the night‐side arch shows the characteristics of an equatorial electron density trough, bounded to the north and south by enhanced densities associated with plasma that has diffused down along the geomagnetic field from the elevated layer. It is pointed out that the signature of the night‐side arch has been seen in low Earth orbit spacecraft data, but has not been previously associated with an arch structure.

[1] We examine ionospheric electron temperatures (T e ) observed by HINOTORI satellite during three earthquakes; M6.6 occurred in November 1981, M7.4 and M6.6 in January 1982 over Philippine, respectively. It is found that T e around the epicenters significantly decreases in the afternoon periods within 5 days before and after the three earthquakes. The region of ionosphere disturbance extends to 80-120 degrees in longitude. A tendency exists that duration of the disturbance becomes longer as the increase of earthquake magnitude. F 2 peak frequency, f o F 2 and virtual height, h'F from a chain of 4 ionosonde stations located in the longitude zone of 120°E-130°E are used together with electron density(N e ), that is observed simultaneously onboard HINOTORI satellite to find possible cause mechanisms of the abnormal reduction of electron temperatures. Behavior of HINOTORI T e /N e and ionosonde f o F 2 /h'F implies the existence of westward electric field over epicentre. Our finding suggests that simple two plasma instruments might be able to play a fundamental role to study ionosphere disturbance associated with earthquake, if the constellation of small/mini satellites is organized and the orbits are properly chosen.

[1] A new type of the additional layer is predicted to occur in the low-latitude F region ionosphere during evening hours of a major magnetic storm. By using the coupled NCAR Thermosphere Ionosphere Electrodynamic General Circulation Model (TIEGCM) and Sheffield University Plasmasphere Ionosphere Model (SUPIM) runs during the major magnetic storm of 29-30 October 2003, we present a new type of additional layer which occurs at the equatorial ionization anomaly (EIA) by a new physical mechanism. This mechanism requires that storm time meridional neutral wind surges travel from high to low latitudes and cross into the opposite hemisphere. The wind surges modify the field-aligned plasma velocities in the EIA regions significantly after uplift of the ionospheric layer by a penetration electric field and interact with the downward field-aligned plasma velocities of the enhanced equatorial fountain. The combined storm effects of the enhanced plasma fountain and the neutral wind surges result in plasma convergence in altitude and form the additional layers underneath the EIA crests.