## 5 Ultra-Wide Ionospheric Monitoring
### Inversion of the Backscatter Ionograms  
The inversion of the backscatter ionograms is based on the QP model [31; 32]. The initial parameters of QP model are determined by the International Reference Ionosphere (IRI) model [33], as well as the real-time recorded electron density profiles of Wuhan digisonde. The leading edges of  
Figure 11: Swept-frequency oblique backscatter ionograms of WIOBSS-AA with five different beams, **(a)** 285\({}^{\circ}\), **(b)** 300\({}^{\circ}\), **(c)** 315\({}^{\circ}\), **(d)** 330\({}^{\circ}\), and **(e)** 345\({}^{\circ}\) in a clockwise manner. The operating frequency is from 6.6 to 13.2 MHz with 100 kHz step. The detecting range is from 100 to 1400 km with 3.84 km range resolution. The first swept-frequency ionogram began to be recorded at 9:40 L.T. and the last ionogram was obtained at 9:56 L.T. The color bar represents the signal to noise ratio (SNR) of the recorded echoes.  
Figure 12: The leading edges of the oblique backscatter swept-frequency echoes of the five beams.  
backscatter ionograms are segmentally fitted to obtain the ionospheric characteristics on each radio path as realistically as possible. The inversion method makes full use of the recorded backscatter ionograms, which are based on the simulated annealing algorithm [34]. By dividing the leading edge into several groups, this method can extract ionospheric information of different areas along the sounding path and eventually provide useful details of ionospheric parameters [35].  
As shown in Figure 13, the inverted electron density profiles distributed along the detecting distance of the five beams are very different. The five electron density profiles of the same longitude and different latitude are selected from Figure 13 and displayed in Figure 14. We find that in the experimental period the electron density at the 112.77\({}^{\circ}\) E longitude increased with decreasing latitude.  
The maximum electron density in the two-dimensional ionospheric electron density plots of the five beams is extracted and then used to compose the maximum electron density map over a fan-shaped region as shown in Figure 15a by applying a two-dimensional interpolation algorithm. Over the observing region, lower maximum ionospheric electron density is found in the more northerly region. A longitude difference in the electron density distribution is not obvious. Using the IRI-2012 model data, the similar result is also gotten, as shown in Figure 15b. The variation range of ionospheric electron density in the F region is smaller in the IRI-2012 model than that in the inversion results. The IRI model is based on experimental evidence using all available ground and space data sources. Although there are no ionospheric stations in the observing area of WIOBSS-AA, IRI model is useful for the evolving theoretical understanding of ionosphere. The F2 layer equatorial anomaly, the \"fountain  
Figure 13: The two-dimensional ionospheric electron density distributing plots of five different directions corresponding to the five beams of the antenna array. The electron density distributing plots (**a–e**) is inverted by the leading edge of the beam of \(345^{\circ}\), \(330^{\circ}\), \(315^{\circ}\), \(300^{\circ}\), and \(285^{\circ}\), respectively.  
effect\", is strong in the Asia region and the \"northern crest\" is at 20\({}^{\circ}\)-30\({}^{\circ}\) N. That is the most possible reason why the electron density drops for higher latitude values in Figure 15.