Responses Of The Upper Atmosphere And The Ionosphere To Sudden Stratospheric Warmings

An extreme wintertime polar meteorological phenomenon,known as sudden stratospheric warming(SSW),has been known to influence the entire Earth's atmosphere.It is generally attributed to the sudden increase in the North Pole stratospheric temperature and a weakening or even reversal of zonal mean wind in the North Pole.The SSW event can affect the Mesosphere and Lower Thermosphere(MLT),even the ionosphere via atmospheric circulation.In this thesis,the neutral wind data obtained from meteor and medium frequency radars,the ionospheric critical frequency in the F2 region(foF2),and GEO total electron content(TEC)data were ultilized to study the response of the upper atmosphere and the ionosphere to SSWs and discuss theirs coupling processes.The major results are as follows:(1)Investigation of the variations of the MLT neutral winds and tides during the 2013 SSW eventThe neutral winds obtained within the latitudinal range of 18°-53°N from a meteor radar chain from December 2012 to February 2013 were decomposed to study the response of the neutral wind and tidal waves in the MLT region to the 2013 SSW event.The results show that the zonal wind reversed from eastward to westward.The westward wind was observed to be larger at middle latitudes than that at low latitudes during the SSW events.Meanwhile,a sharp increase in the northward wind was descerned at all stations.The amplitude of the diurnal and semidiurnal tides decreased from January 5 to 13 and increased subsequently.In addition,the terdiurnal tides increased in amplitude,especially at Beijing,Wuhan and Kunming stations.Moreover,the wavelet analysis revealed that the quasi 6.5-day wave oscillations dominated the zonal and meridional winds at Kunming and Sanya stations,while quasi 16-day waves dominated the zonal wind at Beijing and Wuhan and the quasi 10-day as well as 16-day waves were prevailing in the meridional winds at Beijing and Wuhan,respectively.In the same vein,the quasi 16-day and quasi 10-day waves were prominent in the zonal and meridional winds during January 1-18 at Mohe.(2)The coupling between the MLT and the ionospheric electrodynamics in response to the SSW events during 2009-2017The foF2 data during 2009-2017 and D region electron density during 2009-2015 at Kunming,and the neutral winds data at Kunming are statistically analyzed to study the coupling process between the lower atmosphere and the ionosphere during 9 SSW events.During the SSW events time frame,foF2 enhanced or decreased,and the peak of ?foF2 is up to 200%.Moreover,in the 9 SSW events considered,the semidiurnal signature of ?foF2 was clearly observed and the local time of the peak value shifted to later local time.These features were clearly seen in October 2013(before the 2014 SSW).We further studied the coupling mechanisms during the 2009 SSW by deriving and decomposing the equatorial electrojet(EEJ)current into tidal modes over the East Asia sector.We found the solar and lunar semidiurnal tides in EEJ showed significant variations,and the variation of lunar semidiurnal tide is sometimes greater than that of the solar semidiurnal tide.In contrast,the tidal variabilities in EEJ current and ?foF2 showed that the lower atmospheric wave and planetary wave interaction modulate the?foF2 via the E-region dynamo effect.These results further confirmed that the diurnal and terdiurnal tides play a significant role in this coupling process.The variations of D-region electron density showed significant enhancement during some SSW events as well.(3)Exploration of the responses of the MLT neutral winds and the ionosphere to the 2019 Antarctic SSW eventThe foF2 data from August to October 2019 at Kunming,Guangzhou and Sanya as well as the neutral wind data obtained from Kunming meteor radar are utilized to study the responses of the MLT and the ionosphere to the 2019 Antarctic polar SSW.We found a significant difference between the MLT and ionospheric variations.The increases in the foF2 at Kunming and Guangzhou showed the periodic oscillations,which were not evident in Haikou.The peaks of the foF2 at these three stations during the 2019 Antarctic polar SSW occurred between 14-19 LT.The semidiurnal amplitude signature in the changes of foF2(i.e.,?foF2)shifted to a later local time,which was more notable at Haikou than at other two stations.Further,the wavelet analysis of?foF2 at 15 LT demonstrated the amplitudes of planetary waves with the periods of quasi-6.5 and quasi-10 were strong at three stations.The tidal analysis of wind data revealed that the dominant tidal waves were zonal and meridional lunar semidiurnal tides.The GEO TEC analysis during the 2019 Antarctic polar SSW revealed strong latitudinal variation of TEC,?TEC showed pronounced semidiurnal signature at EIA peak and became stronger in the southern hemisphere.The dynamical changes in the neutral winds and foF2 have great significance for in-depth understanding of the MLT and ionospheric responses to SSW events.Further study on the coupling processes between the MLT and the ionosphere during SSW events is required as the present observational capabilities are unlikely to fully capture of the MLT and the ionosphere in response to the SSW.For the same reasons,numerical studies using high-resolution models are required to fully characterize the dynamics accompanying the coupling system of the lower atmosphere-ionosphere leading to neutral winds and ionospheric changes,including the statistical analysis of multiple SSW events.