[1] The ionosphere responses to a solar flare observed by using ground-based receivers of the global positioning system (GPS) are investigated in this paper. Two quantities, the total electron content (TEC) and its time rate of change (rTEC), can be derived from the receivers. The theoretical studies show that the rTEC is related to the frequency deviation of the GPS signals. Meanwhile, worldwide ground-based GPS receivers are employed to derive the TEC and associated rTEC to monitor the ionospheric solar flare effect on 14 July (Bastille Day) 2000. It is found that ionospheric solar flare effects can be observed from predawn to postdusk regions, and the most pronounced signatures appear in the midday area. The agreement between theoretical predications and observations demonstrates that the TEC is suitable to monitor the overall variations of flare radiations while the rTEC is capable to detect sudden changes in the flare radiations.

Data recorded by the Geostationary Operational Environmental Satellites, frequency deviations derived from a CW‐HF Doppler sounding system, and geomagnetic field strength variations obtained from ground‐based magnetometers were analyzed to study the ionospheric and geomagnetic solar flare effects. A model of flare radiation was constructed from synthetic flux intensities and satellite X ray observations, and corresponding ionospheric frequency deviation and electron density at various altitudes were calculated. The evolution of solar EUV and X ray radiation and associated maximum values responsible for frequency deviation and geomagnetic field strength for various flux intensities of the excess solar radiation were studied in detail. It was found that during a solar flare, not only the magnitude of solar radiation, but also the rate of its change dramatically affects the maximum ionospheric frequency deviation. However, the results confirmed that only the intensity of solar radiation determines the magnitude of geomagnetic field strength.

[1] During geomagnetic activities, perturbed electric fields at middle and low latitudes of the ionosphere may result from the effect of prompt penetration from high latitudes and the disturbance dynamo mechanism, respectively. The polarity of penetration electric fields depends on the orientation of the interplanetary magnetic field (IMF) Bz, and that of electric fields associated with the disturbance dynamo is almost opposite to that in the quiet time condition. Although a few hours are required to build up the perturbed electric fields through the disturbance dynamo mechanism, the dynamo electric fields can persist for several hours after geomagnetic activities cease. It turns out that the low-latitude electric field disturbances associated with the disturbance dynamo mechanism should be more persistent than that associated with the direct penetration from the polar cap, especially for the period in the recovery phase. Model results show a significant, westward disturbance dynamo electric field at period close to sunset, but it seems not important in the empirical model. This perturbed electric field at sunset will weaken the intensity of the prereversal enhancement and of the following fountain effect in the storm time.Relative to the quiet time condition, it produces an enhancement in the total electron content. This phenomenon is seasonal. It only occurs at period close to equinox, when the hemispheric wind (summer to winter) is minimal. These theoretical results have been substantiated by model results of the National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model and by the global integration and modeling in this study. Both results show that the positive phase of the ionospheric storm at the magnetic equator close to sunset is produced by the westward disturbance dynamo electric field.

One of the possible candidates which modifies the ionosphere before large earthquake is electric field. We presume that the electric field associated with large earthquakes is generated in the ionosphere dynamo region (100–120 km). This paper tries to identify the evidence of the contribution of the neutral atmosphere in the dynamo region. The relationship between the critical frequency at the F2 peak (foF2) and the height profile of the neutral atmosphere temperature was studied for two large earthquakes: Wenchuan, 2008 and Pingtung Doublet, 2006. It is found that the wave amplitude of the vertical wavelength 20–30 km which is usually superposed on the height profile of the neutral atmosphere temperature enhances when the foF2 increases. The correlation between the wave amplitude and foF2 is found better along a longitudinal direction than along latitude direction