Investigation Of The Weather And Climatology Phenomena In The MLT Region Using The Meteor Radars

The mesosphere and low thermosphere(MLT)region is an interesting region where the neutral atmosphere changes to the ionization atmosphere.The MLT region can be disturbed by the upward momentum deposition of atmospheric waves,including planetary waves,tides,and gravity waves from lower atmosphere.In addition to coupling from the lower atmosphere,the MLT region is also influenced by geomagnetic effects produced by solar activity through the interaction between the solar wind and Earth's magnetosphere-ionosphere-thermosphere(MIT)system.The knowledge of wind field,temperature,pressure,density and airglow in the MLT region is essential for understanding the coupling between the lower,middle and upper atmosphere.However,the lack of the continuous measurements of the MLT region makes the studying of the dynamics and climate of the MLT region relatively scarce.In this thesis,a global mesospheric densities and temperatures were derived using data from a existing global distribution of meteor radars.Using the meteor radar-derived density and temperature,we represent an observation and analysis of the short-term weather changes,and the long-term climatology variations in the MLT region.The following is the main aspects of the thesis.Neutral mesospheric densities at a low latitude have been derived during April 2011 to December 2014 using data from the Kunming meteor radar in China(25.6°N,103.8°E).The daily mean density at 90 km was estimated using the ambipolar diffusion coefficients from the meteor radar and temperatures from the Sounding of the Atmosphere using Broadband Emission Radiometry(SABER)instrument.The seasonal variations of the meteor radar derived density are consistent with the density from the Mass Spectrometer and Incoherent Scatter(MSIS)model,show a dominant annual variation,with a maximum during winter,and a minimum during summer.A simple linear model was used to separate the effects of atmospheric density and the meteor velocity on the meteor radar peak detection height.We find that a 1 km/s difference in the vertical meteor velocity yields a change of approximately 0.42 km in peak height.The strong correlation between the meteor radar density and the velocity corrected peak height indicates that the meteor radar density estimates accurately reflect changes in neutral atmospheric density,and that meteor peak detection heights,when adjusted for meteoroid velocity,can serve as a convenient tool for measuring density variations around the mesopause.A comparison of the ambipolar diffusion coefficient and peak height observed simultaneously by two co-located meteor radars indicates that the relative errors of the daily mean ambipolar diffusion coefficient and peak height should be less than 5%and 6%,respectively,and that the absolute error of the peak height is less than 0.2 km.Neutral mesopause temperatures over a low latitude station were derived by applying the temperature gradient model technique to data from a meteor radar installation located in Kunming.The estimated temperatures are in good agreement with SABER temperatures and exhibit clear seasonal and interannual variations with dominant spectral peaks at annual,semiannual,quasi 90-day and terannual oscillations.However,the amplitudes of the temperature fluctuations and the dominant spectral peaks are larger than those from SABER.An improved method that accounts for the temperature sensitivity of the slope estimated from the meteor radar data was developed to calibrate the larger fluctuations obtained using the temperature gradient model technique.The resulting calibrated temperatures are more consistent with SABER observations,and the accuracy of the derived temperatures is significantly improved.Temperatures were estimated from the FWHM of the meteor height distribution and compared with those derived from the gradient technique and SABER measurements.The results of the comparison show that the performance of the temperatures estimated from FWHM by using KMMR data at low latitudes is not as good as at high and near-high latitudes(Lee at al.,2016;Liu et al.,2017),suggesting a latitudinal dependence of FWHM temperatures.In addition,we applied the temperature gradient method to estimate the mesopause temperature using an Antarctica Davis meteor radar and two Arctic Svalbard and Tromso meteor radars over 12 years data.The temperatures derived from the polar meteor radars have a strong linear correlation with MLS temperature.The existing distribution of meteor radars,located from high to low-latitude regions,permits an estimate of global neutral mesosphere density,and provides a favorable temporal and spatial coverage for examining the climatology of global mesospheric density.In this chapter,we report a climatology of mesopause density estimated using data from meteor radars at Davis Station(68.6°S,77.9°E),Svalbard(78.3°N,16°E)and Troms(?)(69.6°N,19.2°E),which are located in high-latitudes and have more than 12 years data;the Mohe(53.5°N,122.3°E),Beijing(40.3°N,116.2°E)and Wuhan(30.5°N,114.6°E)meteor radars,located in mid-latitudes;the Kunming(25.6°N,103.8°E),Fuke(19.5°N,109.1°E)and Darwin(12.4°S,130.8°E)meteor radars,located in low-latitudes.The daily mean density was estimated using the ambipolar diffusion coefficients from meteor radars and temperature from the Microwave Limb Sounder(MLS)on the Aura satellite.The seasonal variations of meteor radar derived density are compared with the density from the Mass Spectrometer and Incoherent Scatter(MSIS)model,as well as the MLS density calculated from the MLS temperature and the geopotential height measurements.The seasonal variations of the Davis meteor radar densities in southern polar mesosphere mainly show an annual variation,with a maximum during late spring and a minimum during early winter.The seasonal variations of the northern polar mesosphere mainly show an annual and a weak semiannual variation,with a maximum in the spring and a minimum in the summer.The seasonal variations of southern and northern polar mesospheric densities show a clear seasonal asymmtry.As the latitude changes from high latitudes to middle latitudes,the annual variations become weaker,the semiannual variation stronger.And the latitude changes from middle latitudes to lower latitudes,the semiannual variations become weaker,and the annual variations become dominate at low latitudes.We compare the meteor radar derived densities with the MSIS model density,we find that the seasonal variations of the MSIS densities in Southern Polar mesosphere are close to the meteor radar densities,however,in the north hemisphere,the seasonal variation of the meteor radar densities are much more complex than that of the MSIS densities.We present the first analysis of 6.75,9 and 13.5 day periodic oscillations observed in the neutral mesospheric density in the declining phase of solar cycles 23 and 24.Mesospheric densities near 90 km are derived using data from the Davis meteor radar(68.5°S,77.9°E;magnetic latitude,74.6°S),Antarctica.The 9 day oscillation in the polar mesosphere density is the strongest feature in solar cycle 23,especially.However,the 13.5 day oscillation in the polar mesospheric density is the primary feature in this solar cycle and may be gradually strengthening as the minimum of solar cycle 24 approaches.Spectral analysis indicates that the pronounced periodicities of 6.75,9 and 13.5 days observed in the mesosphere densities are associated with variations in solar wind high-speed streams and recurrent geomagnetic activity.Neutral mesospheric winds and temperatures,simultaneously measured by the Davis meteor radar,also exhibit 9 and 6.75 day periodicities.A Morlet wavelet analysis shows that the time evolution of the 6.75,9 and 13.5 day oscillations in the neutral mesosphere densities and winds are similar to those in the solar wind and in planetary magnetic activity index,Kp and in auroral electrojet index,AE.The periodic oscillation in density shows a strong anticorrelation with periodic changes in the auroral electrojet index.These results indicate that a significant decrease in neutral mesospheric density as the geomagnetic activity enhances.These results provide new insight into solar-terrestrial coupling at mesospheric heights,as well as providing a new explanation of some "planetary wave period(2-20 day)"oscillations in the mesosphere.We report the first observations of a high-and middle-latitude neutral mesospheric density response to geomagnetic storms.Interhemisphere mesospheric densities are estimated using data from meteor radars at Davis Station(68.6°S,77.9°E),Svalbard(78.3°N,16°E)and Tromse(?)(69.6°N,19.2°E),which are located under the auroral zone;the Mohe(53.5°N,122.3°E),and Beijing(40.3°N,116.2°E)meteor radars,located in northern midlatitudes,and the Microwave Limb Sounder(MLS)on the Aura satellite.Both case studies and a superposed epoch analysis indicate that geomagnetic storms can significantly influence mesospheric density,causing a greater than?10%decrease in the auroral zones and a-5%decrease at higher midlatitudes.This is the first time interhemisphere observations of a neutral mesospheric density response to geomagnetic storms have been reported and reveals a direct coupling between the Sun and the Earth's mesosphere through an interaction between the solar wind and the Earth's MIT system.Considering the more than?10%decrease in polar mesospheric density during storms,it is reasonable to expect that geomagnetic storms influence the mesospheric dynamics.This raises a challenge to modeling work aimed at understanding the possible physics and chemical connection in the mesosphere and MIT coupling during geomagnetic storms.In this thesis,we estimate the global mesospheric densities and temperatures using the data from the global existing distribution of meteor radars and prove that the meteor radar can be used to provide a favorable temporal and spatial coverage for studying the variability of the MLT region.Using the meteor radar derived mesospheric density and temperature,we find some new short-term weather changes and long-term climatology variation in MLT region.These findings improve the understanding the coupling in the Earth's atmosphere between the different layers.