| The mesosphere and lower thermosphere(MLT)is a part of the middle atmosphere,which plays an important role in the energy exchange process between the sun and the earth.Complex physical and chemical processes are happening in this area all the time.However,due to the detection limitation,the understanding of the atmospheric environment in MLT region is still lacking.A full understanding of the MLT characteristics is not only conducive to the development and utilization of the space environment,but also has important technological significance for the nation's development.Although the existing ground-based detection equipment can obtain a continuous horizontal wind from 60 km to 100 km,it is difficult to detect accurate wind over 100 km.In addition,the impact of solar activities on human daily life and the earth's space environment monitoring is becoming more and more noticeable.Studying responses of MLT atmospheric environment to solar activities would help enrich our understanding of middle atmosphere and low ionosphere.It also has scientific significance for the research and improvement of atmospheric models.Based on the above two points,we carried out some research with Langfang medium frequency(MF)radar.The main content is as follows:(1)Based on MF radar raw data,we studied on the meteor observation system and made a scheme to extract underdense meteor echoes.An angle of arrival algorithm based on "Y" antenn arrangement was also proposed to avoid angular ambiguity.We conducted the first nighttime meteor observation with MF radar in mid-latitude China.After preprocessing,PEV detection and CEV confirmation,meteor echoes were successfully detected,as well as the height and angular distribution.The results show that the meteor echoes are concentrately distributed in the height range of 100?120 km with the maximum height of 141 km.The zenith angle is mostly from 10° to 30°,and the azimuth is towards southwest.In addition,we have proposed the best observation scheme to obtain the complete spatial distribution of meteor echoes,that is the combined vertical beam and oblique beam.We also find that the meteor echoes at 141 km could be successfully detected with the actrual sampling rate of only 10 Hz.It is suitable for most of the MF radar equipment which are currently in operation.(2)Based on the meteor echoes detected by MF radar,the bi-hourly averaged and night averaged horizontal wind from 96 km to 115 km are calculated.The wind detection height with MF radar is successfully extended to over 100 km.By comparison with the observation results of the co-located VHF all-sky meteor radar,we find that the night-averaged winds obtained by two radars are consistent in terms of wind direction and variation,and the errorbars are overlapped well,which verifies the feasibility and accuracy of MF radar meteor wind technique.In addition,the upper height limit of horizontal wind estimation by MF radar is about 115 km.It is almost 10 km heigher than traditional meteor radars.(3)Based on the electron density data of MF radar during solar flare events in September,2017,we studied the variation of electron density,the absorption of X/O waves,the relationship between electron density and electron generation,and the variation of equal electron density height in different solar flare levels.The following conclusions are obtained: 1.The electron density at lower heights can be obtained during solar flares,and the higher the flare levels,the lower the electron density height;2.The relationship between the electron density and the electron generation rate during different flare levels is the square relation;3.The reduction of equal electron density height has a weak linear relationship with solar flare levels.These conclusions have enriched our understanding of ionospheric D region,and are also of great value for the observation of solar flares and their earth effects,as well as the application of radio wave propagation..(4)Based on the wind data from 2009 to 2020 obtained by MF radar,we analysed the responses of atmospheric horizontal wind and tides at the height of 80?100 km to11-year solar activities.The correlation coefficients and confidence level between F10.7and winds are calculated in different seasons and at different heights.The conclusions are as follows: 1.The zonal wind is positively correlated with solar activities at 80-84 km in spring and at 80-82 km in summer.2.The meridional wind is positively correlated with solar activities at 84-88 km in spring and at 84-90 km in summer.3.The amplitude of annual and semiannual oscillations of the zonal or meridional wind is mainly negatively correlated with solar activities from 80 km to 90 km.In response to these conclusions,we proposed a mechanism that the solar activities could affect the stratospheric thermal and wind structure,leading to change of upward propagating gravity wave net flux,and finally affect the MLT atmospheric wind field. |
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