tokens: consistent with previous studies that show an anticorrelated relationship between the magnetic field strength and the ionopause altitude (Elphic et al. 1980 ; Russell Vaisberg 1983 ; Phillips et al. 1985 ; Angsmann et al. 2011 ). The ionospheric thermal pressure balances the magnetic pressure difference at the ionopause. To sustain stronger magnetic pressure, it needs higher thermal pressure, and thus higher plasma density in the ionosphere. Therefore, the ionopause would be pushed down to a lower altitude with higher plasma density. However, it is important to point out that this process is based on an assumption that the ionization of the ionosphere is not significantly changed. Figure 4. Left to right: the magnetic field strength near the periapsis prior to, during, and after the passage of the ICME on November 4, November 5, and November 6, respectively. The IMA measurements of heavy ion density are overplotted (red dots). The heavy ion density on November 6 is not available. Nevertheless, the altitude of the ionopause during the strong ICME (Figure 4 (b)) did not decrease at all considering that the magnetic field strength increased a lot. The magnetic pressure differences () across the ionopause on November 4 and 6 are 0.7, and 1.1 × 10 −8 dyn cm −2 , respectively. But on November 5, the magnetic pressure difference reaches 2 × 10 −7 dyn cm −2 ( from 250 to 116 nT) or 9.0 × 10 −8 dyn cm −2 even if we use 200 nT as the magnetic strength in the ionosphere. To balance such atypically higher magnetic pressure, the plasma density of the ionosphere must be correspondingly much greater. The heavy ion densities measured by the IMA instrument are also shown in Figure 4 . Although the resolution of IMA data is low, we can still easily see that the ionospheric plasma density was over 1000 cm −3 during the strong ICME with respect to that less than 400 cm −3 during quiet solar wind. The much higher ionospheric plasma shows that the ionosphere had been greatly excited and perhaps entirely expanded under the impact of the strong ICME. The strengthened ionosphere during the passage of ICME has been evidenced by the fact that the magnetized ionosphere can be demagnetized by some ICMEs during solar minimum (Zhang et al. 2008a ). Due to the limitation of measurements, we have no ideas about the reason why the ionosphere was greatly enhanced by the ICME. However, previous study of the Venus and Mars ionosphere has determined that the solar energetic particles (SEPs) could cause extra ionization of the neutral particles in exosphere through impact ionization and charge-exchange reactions (Futaana et al. 2008 ). Simulations also show that the SEPs can effectively increase the ionization rate of the Venusian atmosphere (Plainaki et al. 2016 ). 4. Discussions and Conclusions We have presented very detailed observations of the induced magnetosphere and ionosphere during the passage of the extremely strong ICME on 2011 November 5. From our observations, a clear
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