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• The electron density in the D region obtained by Kunming (25.6N,103.8E) MF radar is used in this study. • The strong positive correlation of electron density in the D region with solar activity. • A empirical model of D region is established based on the correlation between D ionosphere and F107. The electron density (N e) in the D region, although its value is much smaller than that in the F region, has a highly important effect on electromagnetic wave propagation by energy absorption due to high-frequency collisions between electron-molecular, electron-ionic, molecular-ionic and so on. Solar activity is the main source of ionization in the D region of the ionosphere. The observed N e during 2008–2015 by the Kunming station (25.6°N, 103.8°E) medium radar (MF radar) is utilized to analyze its strong relationship with solar activity. Then, the relationship between them is employed to establish an empirical model in this study. The N e in the D region is enhanced with increasing height and time towards noon, and the solar zenith angle is increasingly smaller towards noon. The averages of N e at several heights below and above 76 km show that the D region ionosphere up to 76 km is more strongly controlled by solar radiation and less strongly controlled below 76 km. According to the above, an empirical model is established based on the relationship between the D-region ionosphere and solar activity index F 107. [ABSTRACT FROM AUTHOR]

Gravity wave (GW) activities play a prominent role in the complex coupling process of wave–wave and wave–background circulation around mid-low-latitude and equatorial areas. The wavelengths are wide, from about 10 m to 100 km, with a period from minutes to hours. However, the oscillations of GW activities are apparently different between the period bands of 0.1 to 1 h (HF) and 1 to 5 h (LF). To further understand the characteristics of GW activities, the neutral winds during 2008–2009 with a resolution of 3 min obtained from a medium-frequency (MF) radar in Kunming (25.6° N, 103.8° E) were analyzed. Using two numerical filters, the HF and LF GWs were estimated. Interestingly, the power spectral density grows larger as the frequency increases. It linearly falls with decreasing frequency when the period is less than 2 h. The seasonal variations in both HF and LF GWs are strongly demonstrated in August–September, November, and February–March with maximum meridional variances of 1100 m2 s−2 and 500 m2 s−2 and maximum zonal variances of 800 m2 s−2 and 350 m2 s−2 in, respectively. The turbulent velocity was also calculated and shows similar oscillations with GW activities. Furthermore, the GW propagation direction exhibits strong seasonal variations, which may be dependent on the location of the motivating source and background wind. [ABSTRACT FROM AUTHOR]

So far, the least is known about the D-region ionosphere out of the entire ionosphere due to the lack of a conventional detecting method and continuous data accumulation. Medium frequency (MF) radar is an important conventional tool for understanding the D-region ionosphere by measuring the electron density (Ne) within the height range of 60–90 km. To investigate the statistical variation of the D-region, especially at the mid-low latitude area, this study presents the statistical variations in the D-region Ne with the solar zenith angle (SZA), season, and altitude observed by Kunming MF radar (25.6° N, 103.8° E) under low solar activity (2008–2009). The diurnal variation of Ne behaves like typical diurnal changes, which are closely consistent with the SZA. The outstanding feature, the diurnal asymmetry phenomenon, significantly appears in different seasons and at different altitudes. The Ne has obvious semi-annual characteristics, and is larger in summer and fall and the smallest in winter. Compared to other seasons, the variation in the Ne with altitude is the most stable in summer. Due to the impacts of the highest SZA, the value of Ne in winter is the smallest, with a maximum value of less than 300 electrons/cm3, and the largest in summer and fall, with a maximum of 472 electrons/cm3. Particularly, the peaks of Ne above 76 km do not always appear at the time when the SZA is the smallest (at noon). Both the simulations by the International Reference Ionosphere (IRI2016) and observations using MF radar present a strong positive correlation with solar radiation. Meanwhile, it cannot be ignored that there were still large differences between the simulations and observations. To quantitatively analyze the differences between the observations and simulations, the observed value was subtracted from the simulated value. The results show that the maximum value between them was up to 350 electrons/cm3, and the minimum difference appeared at around 72 km, with a value less than 100 electrons/cm3. However, below 66 km, the observations were larger than the simulations, which were, on the contrary, above 76 km. [ABSTRACT FROM AUTHOR]

We have investigated the variation of mean wind almost continuously monitored by the Kunming (25.6°N, 103.8°E) MF radar over the period from August 2008 to August 2010. The mean circulation in zonal winds presents a obvious annual variation above 90 km and semiannual variation below 85 km, but has different direction up 90 km and down 85 km. An eastward flow is prevailing in spring and autumn below 85 km and in spring and summer up 90 km with amplitudes as large as 24 m/s and 21 m/s, respectively. But a westward flow dominates in summer and winter below 85 km and in autumn and winter up 90 km with amplitudes as large as −12 m/s and −10 m/s, respectively. Meanwhile the eastward wind dominates for all months at 85–90 km. The annual variation also appears in meridional wind below 90 km, which means a poleward flow occurs from September to April with amplitude maximum of 11 m/s, while the equatorward wind dominates almost for two years with maximum of −26 m/s above 90 km. Comparison of the wind vertical profile along with standard errors with the HWM07 model winds shows that the zonal wind agrees well with model in winter, autumn and summer, but the meridional wind in spring, winter and summer. [ABSTRACT FROM AUTHOR]

Abstract Electron density ( N e ) and effective reflection height ( H ' , where the electron density reaches its peak value) in the D region observed by medium frequency (MF) radar at Kunming (25.6°N, 103.8°E) were utilized to quantitatively study the ionospheric response to solar flares. It was found that the H ' and N e had profound responses to solar flares. The results showed a sharp decrease in the H ' from 76 to 80 km during the non-flare period to nearly 58 km with flares, and a sudden increase in the N e from below 4 × 10 8 m3 in non-flare period to 1.8 × 10 9 m3 during 13 M-level flares. The behavior of N e was strongly and positively correlated with the variation of the X-ray flux, with the largest coefficient being 0.93, while the changes of H ' exhibited a strongly negative correlation of −0.96 with that of the X-ray flux. Moreover, variations of H ' and Δ N e depended on not only the flare level, but also upon the onset time and duration of flare. Highlights • N e and H ' from MF radar are used to quantify the ionospheric D region response to solar flares. • During solar flare periods, the H ' delines rapidly from near 76 km–58 km, and Δ N e is greater than 8 × 10 8 m3, up to 1.7 × 10 9 m3. • Ionospheric response to solar flares depends on the level, onset and duration of the solar flare. [ABSTRACT FROM AUTHOR]