Research Of Measurement Method Of Metastable Helium Density In The Thermosphere And Exosphere

Helium is an important atmospheric species in the range of the thermosphere and exosphere.Probing the density of helium helps people understand space environment of the thermosphere and exosphere better.This dissertation firstly presents the research background and the scientific significance of probing the metastable helium(He(23S))density in thermosphere.And then details of principle of resonance fluorescence Lidar for measurements of the middle and upper atmosphere are given in section 2,such as,energy levels of Helium,the metastable helium lines properties,metastable helium density model,background noise in twilight,the Lidar equation,the effective backscatter cross-section for the He(23S)and so on.To achieve detection of the density of helium atoms of the atmosphere at altitude of 200 km-1000 km,we designed a He Lidar system which includes a 60W CW laser transmitter,a telescope array consists of six 1 m diameter telescope,single photon counting system and data acquisition system.The photon counts profile versus altitude is simulated under different He density and integration time.The results show that,when integration time is 5 min,and the density profile of He(23S)in summer(the density is about 103 m-3)is used,SNR will be better than 20 at attitude between 300-600km.and the density of He(23S)in winter(the density is about 105 m-3)is used,SNR will better than 100 at attitude higher than 400 km.A 1083.0 nm CW seed laser is used to actively lock the outgoing laser frequency to one of the He(23S)→He(23P)transitions.So the production of metastable helium atoms,the measurement of helium saturation absorption spectra,and the experimental method of modulation transfer spectrum method are introduced in detail in this work.The Helium resonance fluorescence lidar as a novel type of lidar,is now restricted by laser and infrared detection technology and difficult to implement.To the author’s knowledge,there is no formed He lidar system all over the world.This work firstly proposed a He lidar project in China,which helps make groundbreaking measurements in the thermosphere and exosphere.