The ionospheric mid-latitude trough (IMT) is the electron density depletion phenomenon in the F region during nighttime. It has been suggested that the IMT is the result of complex plasma processes coupled to the magnetosphere. In order to statistically investigate the characteristics of the IMT, we analyze topside sounding data from Alouette and ISIS satellites in 1960s and 1970s. The IMT position is almost constant for seasons and solar activities whereas the IMT depth ratio and the IMT feature are stronger and clearer in the winter hemisphere under solar minimum condition. We also calculated transition heights at which the densities of oxygen ions and hydrogen/helium ions are equal. Transition heights are generally higher in daytime and lower in nighttime, but the opposite aspects are seen in the IMT region. Utilizing the Incoherent Scatter Radar (ISR) electron temperature measurements, we find that the electron temperature in the IMT region is enhanced at night during winter. The increase of electron temperature may cause fast transport of the ionospheric plasma to the magnetosphere via ambipolar diffusion, resulting in the IMT depletion. This mechanism of the IMT may work in addition to the simply prolonged recombination of ions proposed by the traditional stagnation model.

1. INTRODUCTION
2. DATA AND ANALYSIS
    2.1 Topside Sounding Ionospheric Data
    2.2 Incoherent Scatter Radar Data
3. RESULTS
4. DISCUSSION
5. SUMMARY AND CONCLUSION
REFERENCES

• Junseok Hong(Department of Astronomy, Space Science and Geology, Chungnam National University/Korea Astronomy and Space Science Institute)
• Yong Ha Kim(Department of Astronomy, Space Science and Geology, Chungnam National University) Corresponding Author
• Young-Sook Lee(Department of Astronomy, Space Science and Geology, Chungnam National University)