[P-EM12] Coupling Processes in the Atmosphere-Ionosphere System

[PEM12-P27] Analysis of neutral wind using meteor echoes observed by HF Doppler sounding

*Hiroki Saito¹, Hiroyuki Nakata¹, Hiroyo Ohya¹, Keisuke Hosokawa² (1.Graduate School of Science Engineering, Chiba University, 2.Department of Communication Engineering and Informatics, University of Electro-Communications)

It is known that ionization occur when meteors enter the Earth's atmosphere at an altitude of 80 ~ 120 km. The plasma associated with meteors entering is called a meteor trail. Meteor echoes are observed because the electrons in meteor trail scatter radio waves. Since most of the meteor radio observations have been performed using VHF band radio waves, meteor radio waves were observed using radio waves in the HF band in this study. Meteor echoes reflect radio waves in the incident direction when radio waves are incident from the direction vertical to the trail. On the other hand, the meteor echo is reflected under the condition that the incident angle is equal to the reflection angle when radio waves are incident obliquely. When the echo is alive longer, it may not be possible to approximate the trail of the meteor with a straight line because of the deformation of meteor trails. We have examined meteor echoes observed by the HF Doppler sounding system utilized by the University of Electro-Communications and four institutions. The transmitter of this system is located the Chofu Campus of the University of Electro-Communications. In this HF Doppler observation, radio waves are reflected in the ionosphere just above the midpoint between the transmitting point and receiving point, and the vertical movement of the ionosphere is observed from the difference between the receiving frequency and the transmitting frequency (Doppler shift) of the radio wave. However, since meteor trails are located at altitude of 80 to 120 km, radio waves are reflected at these altitudes, which is lower than the usual reflections. We used Doppler data received at the Fujisawa, Sugito, Kakioka, Orai, and Kashima stations using 8 MHz radio waves transmitted from the Chofu campus. The event investigated in this study occurred over the Kanto area at 20:30:27 JST on October 25, 2014. We determined the reflection points on the meteor trail. In this event, the Doppler frequency changed from a positive value to a negative value for more than 10 minutes. From this timescale, the main change in Doppler frequency is considered to be the effect of neutral wind. Since neutral wind is mainly horizontal wind, we estimated the wind speed of the neutral wind assuming that the meteor trail moved only in the horizontal direction. Changing the azimuth angle by 1°the moving speed of each reflection point using the Doppler shift. The speed of the neutral wind at 3, 6, 9, and 12 minutes after the start time of the echo was estimated that it moved in the direction with the smallest coefficient of variation. The wind speed was almost constant around 10 m/s at each time. In addition, the direction of movement of trail changed from northeast to south. Therefore, the direction of the neutral wind also changed similarly.