[1] Couplings between the ionosphere and meteorological events have been studied widely. However, most of them are individual case studies or correlation analyses, and few are aiming at the full morphological features of the ionospheric response processes. In this paper, complete records of 24 strong typhoons from 1987 to 1992 were collected, and comparison was made with corresponding ionospheric HF Doppler shift data. The main purpose of the present work is to find the temporal evolution of these responses and their common features by the merit of the continuities of HF Doppler shift observation in time. On the basis of the statistical analyses, this paper reveals the common features of ionospheric responses to typhoon. A summary of these characteristics is as follows:(1) During the existing time of a typhoon, there are almost always medium-scale traveling ionospheric disturbances (TIDs) in the ionosphere, especially when a strong typhoon is landing or near the coast of a mainland. (2) These TIDs show quite clear periodicity and their periods vary with time and gradually grow longer. (3) After sunset, the wavelike disturbances with large magnitudes often excite the midlatitude spread-F. (4) The intense typhoon can cause the wavelike records of the Doppler shift to show the S-shaped echo tracing, which means that the amplitudes of those waves are sufficiently large, and (5) the sunrise-like phenomena often appear in nonsunrise time during the period the typhoon exists. The phenomena mentioned above are generally in agreement with the linear propagation theories of the acoustic-gravity waves (AGWs) in the atmosphere. A typhoon is surely one of the important ground sources of the wavelike disturbances in troposphere; this source is very effective especially when a typhoon is landing on or near a mainland coast. Of course, the morphological details of the ionospheric response to typhoon can by no means be completely identical every time. In this study, except for TIDs that almost always appear during all the typhoon events, the other common features are not seen every time. However, we are certain that the phenomena summarized above are statistically the manifestation of the ionospheric response to typhoon since they appear much more frequently during periods influenced by typhoon.

[1] A global ionospheric total electron content (TEC) model based on the empirical orthogonal function (EOF) analysis method is constructed using the global ionosphere maps provided by Jet Propulsion Laboratory during the years 1999-2009. The importance of different types of variation to the overall TEC variability as well as the influence of solar radiation and geomagnetic activity toward TEC can be well represented by the characteristics of EOF base functions E k and associated coefficients P k . The quick convergence of EOF decomposition makes it possible to use the first four orders of the EOF series to represent 99.04% of the overall variance of the original data set. E 1 represents the essential feature of global spatial and diurnal variation of the TEC. E 2 contains a hemispherically asymmetric pattern manifesting the summer-to-winter annual variation. E 3 and E 4 can well reflect the equatorial anomaly phenomenon. P 1 contains an obvious solar cycle variation pattern as well as annual and semiannual variation components. P 2 mainly includes an annual fluctuation component. P 3 has a strong annual variation and a weak seasonal variation pattern. P 4 has both evident annual and semiannual oscillation components. The Fourier series as a combination of trigonometric and linear functions are used to represent the solar cycle, annual, and semiannual variation of the coefficients. Therefore the global TEC model is established through incorporating the modeled EOF series. The accuracy and quality of the model have been validated through the model-data comparison, which indicates that the model can reflect the majority of the variations and the feature of temporal-spatial distribution of the global ionospheric TEC.

Longitudinal differences of low‐latitudinal ionospheric responses during five major, four minor, and a none‐stratospheric sudden warming (SSW) winters from 2009 to 2018 between East Asian and American sectors are studied with total electron content (TEC). The time‐shifted semidiurnal (TS) pattern and the amplitude (AM2), phase angle (PAM2), and relative strength (RSM2) of the lunar semidiurnal tide (M2) harmonic in TEC are compared between the two sectors. Main results are as follows: (1) TS patterns, AM2,and RSM2 tend to be more discernable or larger in the American sector than in the East Asian sector. (2) TS patterns and PAM2 correspond well with the moon phase, and the occurrence of TS patterns coincides well with the enhancement of AM2 and RSM2. (3) Such patterns sometimes occur before the polar peak warming and experience several cycles during one event, but the most significant one tends to follow the peak warming. These characteristics are most distinct during major events with low solar activities. Our results support the mechanism that TS patterns in low‐latitudinal ionosphere parameters are due to enhanced lunitidal effects on the E region dynamo. Besides, changes in temperature and wind during SSWs can also contribute to the generation of TS signatures. Longitudinal differences in TEC suggest that the M2 influence on the low‐latitudinal ionosphere tends to be more prominent in the American sector than in the East Asian sector during SSWs. These differences probably result from a combined effect of the longitudinal variety in atmospheric (especially tidal) and electrodynamic processes.

[1] On the basis of ionospheric total electron content (TEC) enhancement over the subsolar region during flares, and combined with data of the peak X-ray flux in the 0.1-0.8 nm region, EUV increase in the 0.1-50 and 26-34 nm regions observed by the SOHO Solar EUV Monitor EUV detector, also with the flare location on the solar disc, the relationship among these parameters is analyzed statistically. Results show that the correlation between ionospheric TEC enhancement and the soft X-ray peak flux in the 0.1-0.8 nm region is poor, and the flare location on the solar disc is one noticeable factor for the impact strength of the ionospheric TEC during solar flares. Statistics indicate clearly that, at the same X-ray class, the flares near the solar disc center have much larger effects on the ionospheric TEC than those near the solar limb region. For the EUV band, although TEC enhancements and EUV flux increases in both the 0.1-50 and 26-34 nm regions have a positive relation, the flux increase in the 26-34 nm region during flares is more correlative with TEC enhancements. Considering the possible connection between the flare location on the solar disc and the solar atmospheric absorption to the EUV irradiation, an Earth zenith angle is introduced, and an empirical formula describing the relationship of TEC enhancement and traditional flare parameters, including flare X-ray peak and flare location information, is given. In addition, the X-ray class of the flare occurring on 4 November 2003, which led the saturation of the X-ray detector on GOES 12, is estimated using this empirical formula, and the estimated class is X44.

In situ electron density measurements by the CHAllenging Minisatellite Payload and the Defense Meteorological Satellites Program F17 satellites show that the midlatitude ionization at altitudes of ∼350 and 850 km is enhanced in the late evening. The enhancements increase to maximum around midnight and are clearly observed till early morning as the equatorial ionization decays to minimal level. They appear in the winter hemisphere during June and December solstices and in both hemispheres during equinox. The enhancements are well confined between ±30° and ±50° magnetic latitude, with the magnetic flux tubes of L = 1.3 − 2.4 connecting to the plasmasphere. Furthermore, coincident longitudinal variations exist in both the ionospheric enhancements and the plasmaspheric total electron content, especially during the solstice months. The coincidence may suggest essential plasma transport between the ionosphere and the plasmasphere. These facts support the idea that the plasmasphere provides extra plasma to the midlatitude ionosphere through downward plasma influx along the magnetic field lines to form the nighttime ionization enhancements when the sunlight is absent.