Ionospheric responses to sudden stratospheric warming (SSW) are not well understood, particularly in the midlatitudes and under high solar conditions. During the 2013 SSW, ionospheric disturbances were observed in eight locations on the meridional chain from 30.5°N to 42.8°N in northern China. The midlatitude ionosphere responded strongly to the SSW despite being under high solar flux. The F2 layer maximum electric density increased by more than 80%, and the peak height was elevated more than 60 km. Well‐set and organized semidiurnal variations were recorded in early and middle January during the SSW in eight observation locations. The expected foF2 decrease in the afternoon hours was not clearly discernible; however, nighttime enhancements occurred frequently. The time‐period spectra of the average foF2 and zonal winds and meridional winds at altitudes of 86–95 km presented quasi‐16 day planetary wave‐like oscillations during the warming event. The coupling between the atmosphere and ionosphere may be strengthened by the quasi‐16 day waves. The amplified diurnal, semidiurnal, and terdiurnal tides in foF2 were also recorded during the warming, in good agreement with earlier observations. Importantly, the variations in the semidiurnal tides included a 16 day periodic component, indicating that the modulated semidiurnal tides may transmit these 16 day planetary wave‐like oscillations to the F region through wind dynamo. Although the PW‐tide interaction theory is not novel, it is of significance in the midlatitude ionospheric response to SSW.

The driving mechanism of the midlatitude field-aligned irregularities (FAIs) has been in dispute for many years. The experimental observations were carried out during the solar eclipse of 22 July 2009 in Wuhan, China, to study the possibility of the wave-driven irregularities. A high-frequency coherent scatter radar was used to detect the E region irregularities. An ionosonde was applied to record the trace of gravity waves in ionosphere. The E region FAIs occurred at the end of the solar eclipse with fluctuant Doppler. At the same time, the oscillations on the fE s (maximum reflecting frequency of E s ) curve and in the Doppler velocity of the echoes from the E s layer were also recorded. The data analysis and comparison show that the gravity waves and the FAIs occurred at the same time with the in-phase variations in amplitude and phase. Thus, the solar eclipse and the gravity waves may play important roles in the occurrence of the irregularities. A schematic diagram of one-period gravity wave is used to explain the possible gravity wave-driven mechanism and the Doppler fluctuation in the irregularities. The daytime FAI in midlatitude is a rare phenomenon, even in the condition of solar eclipse. There are only four cases of the E region FAIs observed during a solar eclipse, including our observations. The unique feature of our observations is the synchronized oscillations in the irregularities and in the E s layer, which will help address the outstanding question of the source of the midlatitude E region FAIs.

[1] The solar eclipse on 15 January 2010 traversed Asia and completed its travel on the Shandong Peninsula in China at sunset. Two vertical incidence ionosondes at Wuhan and Beijing and the oblique incidence ionosonde network in North China were implemented to record the ionospheric response to the solar eclipse. Following the initial electron density decrease caused by the eclipse, the ionosphere was characterized by a strong premidnight enhancement, and a subsequent ionospheric decay, and a~10 h later postmidnight enhancement. Neither geomagnetic disturbance occurred during the eclipse day nor did obvious nighttime peak appear for the 10 day mean of the F2-layer critical frequency (foF2). The electron density profilogram of the Beijing ionosonde indicates that the two enhancements were the result of the plasma flux downward from the top ionosphere, possibly due to the steep decrease of the ionospheric electron density and plasma temperature during the solar eclipse. The two-dimensional differential foF2 maps present the regional variations of the nighttime electron density peaks and decay. Both the pre-and postmidnight enhancements initially appeared in a belt almost in parallel with the eclipse track and then drifted southward. The different magnitudes of greatest eclipse in the umbra and outside tend to account for the different occurrence times of the plasma flux. The ionospheric decay following the premidnight enhancement is also considered as a consequence of the eclipse shade.

The solar eclipse effects on the ionosphere are very complex. Except for the ionization decay due to the decrease of the photochemical process, the couplings of matter and energy between the ionosphere and the regions above and below will introduce much more disturbances. Five ionosondes in the Northeast Asia were used to record the midlatitude ionospheric responses to the solar eclipse of 20 May 2012. The latitude dependence of the eclipse lag was studied first. The f o F 2 response to the eclipse became slower with increased latitude. The response of the ionosphere at the different latitudes with the same eclipse obscuration differed from each other greatly. The plasma flux from the protonsphere was possibly produced by the rapid temperature drop in the lunar shadow to make up the ionization loss. The greater downward plasma flux was generated at higher latitude with larger dip angle and delayed the ionospheric response later. The waves in the f o E s and the plasma frequency at the fixed height in the F layer are studied by the time period analytic method. The gravity waves of 43-51 min center period during and after the solar eclipse were found over Jeju and I-Cheon. The northward group velocity component of the gravity waves was estimated as~108.7 m/s. The vertical group velocities between 100 and 150 km height over the two stations were calculated as~5 and~4.3 m/s upward respectively, indicating that the eclipse-induced gravity waves propagated from below the ionosphere.

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