Solar radiation, which varies over multiple temporal scales, modulates remarkably the evolution of the ionosphere. The solar activity dependence of the ionosphere is a key and fundamental issue in ionospheric physics, providing information essential to understanding the variations in the ionosphere and its processes. Selected recent studies on solar activity effects of the ionosphere are briefly reviewed in this report. This report focuses on (1) observations of solar irradiance at X-ray and extreme ultraviolet wavelengths and the outstanding problems of solar proxies, in the view of ionospheric studies, (2) new findings and improved representations of the features of the solar activity dependence of ionospheric key parameters and the corresponding physical processes, (3) possible phenomena in the ionosphere under extremely high and low solar activity conditions that are unique, as indicated by historical solar datasets and the deep solar minimum of solar cycle 23/24, and (4) statistical studies and model simulations of the ionosphere response to solar flares. The above-mentioned studies provide new clues for comprehensively explaining basic processes in the ionosphere and improving the prediction capability of ionospheric models and related applications.

The daily averaged Solar EUV Monitor (SEM)/Solar Heliospheric Observatory (SOHO) EUV measurements, solar proxies, and foF2 data at 20 ionosonde stations in the east Asia/Australia sector are collected to investigate the solar activity dependences of the ionospheric peak electron density (NmF2). The intensities of solar EUV from the SEM/SOHO measurements from 1996 to 2005 show a nonlinear relationship with F107, and the SEM/SOHO EUV can be better represented by a solar activity factor P = (F107 + F107A)/2. Seasonal and latitudinal dependences are found in the solar activity variation of NmF2 in the east Asia/Australian sector. The slope of NmF2 with P in the linear segment further shows similar annual variations as the background electron densities at moderate solar activity. Observations show a nonlinear dependence of NmF2 on solar EUV (the saturation effect of NmF2 for high solar EUV). On the basis of a simple model of photochemistry, taking the neutral atmospheric consequences into account, calculations at fixed height simulate the saturation effect of NmF2, but the observed change rate of NmF2 with P is inadequately reproduced. Calculations taking into account the influence of dynamics (via a simple model of the solar EUV dependence of the ionospheric height) tend to reproduce the observed change rate of NmF2. Results indicate that besides solar EUV changes, the influence of dynamics and the atmospheric consequences should substantially contribute to the solar activity variations of NmF2.

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 F₂ peak (N_mF₂) 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.

The global ionospheric maps (GIMs) produced by JPL are used to investigate the longitudinal structure of the low latitude ionosphere. As a proxy of the ionization parameter at low latitudes, the latitudinally integrated total electron content (ITEC) is first extracted from low latitude GIMs and then Fourier filtered to obtain the wavenumber‐4 components. We then study in detail the diurnal, seasonal and solar cycle variations of the wave patterns. It is found that the wavenumber‐4 patterns are intense and well developed in boreal summer and early boreal autumn, but quite weak in boreal winter. This seasonal variation is consistent with that of the zonal wind of the non‐migrating tide mode DE3. We also found that the wavenumber‐4 patterns shift eastward with a shifting speed that is smaller in daytime than at night. This is attributed to the contribution of both the eastward propagation of DE3 in E‐region and the zonal E ×B ion drifts in F‐region. Our results support the suggestion that the longitudinal wavenumber‐4 structure of the low latitude ionosphere should be originated from the non‐migrating tide mode DE3.

Measurements from the incoherent scatter radar (ISR) and ionosonde over Millstone Hill (42.6°N, 288.5°E) are analyzed to explore ionospheric temporal variations. The F₂ layer peak density N_mF₂, peak height h_mF₂, and scale height H are derived from a Chapman α layer fitting to observed ISR electron density profiles. Diurnal, seasonal, and solar activity variations of the ionospheric characteristics are presented. Our study on the solar activity dependence of N_mF₂, h_mF₂, and H indicates that the peak parameters (N_mF₂ and h_mF₂) of the F₂ layer increase with daily F_10.7 index and saturate (or increase with a much lower rate) for very high F_10.7; however, they show almost linear dependence with the solar proxy index F_10.7p = (F_10.7 + F_10.7A)/2, where F_10.7A is the 81‐day running mean of daily F_10.7. This suggests that the overall effect of solar EUV and neutral atmosphere changes on the solar activity variation of ionospheric ionization is linear with F_10.7p. The rate of change in the ionospheric characteristics with solar activity exhibits a seasonal and local time variation. Over Millstone Hill, N_mF₂ in summer is characterized by the evening peak in its diurnal variation, and N_mF₂ exhibits winter anomaly under low and high solar activity levels. The temporal variations of the topside effective scale height H₀ can be explained in terms of those in the slab thickness. The IRI model overestimates the N_e effective topside scale height over Millstone Hill; therefore our analysis for the effective topside scale height from the Millstone Hill measurements might help to improve the IRI topside profiles at middle latitudes.

The responses of Equatorial Ionization Anomaly (EIA) to the superstorms of October-November 2003 were investigated using the total electron content (TEC) measured with global positioning system (GPS) receivers in China, Southeast Asia, Australian (CSAA), and the American regions. Enhanced EIA was seen to be correlated with the southward turning of the interplanetary magnetic field B_z. In both the CSAA and American regions, EIA was intensified, corresponding to a large increase in the F-layer peak height (hmF2) measured by ionosonde and digisonde at middle and equatorial latitudes. However, the enhanced EIA was shown to be more significant during the daytime in the American region, which was associated with a series of large substorms when B_z was stable southward. The prompt penetration electric field and the wind disturbances dynamo electric field are suggested to be responsible for this observation according to current theory, although some features cannot be totally decipherable. Both the ionogram and magnetometer data show the existence of a weak shielding effect whose effect still needs further study. A clear asymmetric ionospheric response was shown in our TEC observations, even though it was only one month after autumnal equinox. The southern EIA crest was totally obliterated on 29 and 30 October in the CSAA region and on 31 October in the American region. Ion temperatures from the Defense Meteorological Satellite Program (DMSP) spacecraft revealed that the unequal energy injection at the polar region might be the reason for this effect. It is concluded that different physical processes have varying degrees of importance on the evolution of EIA in the CSAA and American regions.