The Solar Activity Dependence Of Nonmigrating Tides In Ionosphere And Thermosphere At Low And Middle Latitudes Observed By CHAMP And GRACE

The ionosphere-thermosphere(IT)system,which is a crucial area of the earth's space environment,is an important object for the study of space weather.Due to the influence of the solar radiation,the energy input from the solar wind,the fluctuation in the atmosphere and so on,the ionosphere-thermosphere system varies significantly with different temporal and spatial scales.The ionospheric electron density and the thermospheric total mass density are vital parameters to characterize the the ionosphere-thermosphere system.The investigation of their variations is important for understanding the coupling of the ionosphere and thermosphere.Because of the thermal sources and wave-wave interaction effects,tides are generally excited in the earth's atmosphere.By direct propagation or E region wind dynamo coupling mechanism,tides further propagate upward to the upper atmosphere and give rise to the longitudinal variations of the ionospheric and thermospheric quantities.In this paper,by using more than ten years of ionospheric electron density and thermospheric total mass density observations from CHAMP and GRACE satellite,we analyze the longitudinal variations of the electron density and total mass density at low and middle latitudes,on the basis of understanding the relationship of the background electron density and total mass density with the solar activity.For the next step the tidal signatures in the electron density and total mass density are obtained by the method of Fourier fitting.Based on the derived amplitude of various tides,the solar activity dependence of the nonmigrating tides in the electron density and total mass density at low and middle latitudes is investigated.The main results are summarized as follows:1.The zonal(longitudinally averaged)mean value of the electron density in the F region of the ionosphere and the daily averaged value of the total mass density in the thermosphere observed by CHAMP and GRACE satellites are closely related to the solar activity.In the years of higher solar activity,the electron density and thermospheric total mass density are higher,which are lower for the lower solar activity.2.The zonal mean value of the electron density in the ionospheric F region shows an obvious equatorial ionization anomaly(EIA).For the latitude variation,the EIA crests are usually located at 8-20°Mlat,which is somewhat closer to the magnetic equator during solar minimum years.The zonal mean values of ionospheric electron density show obvious seasonal characteristics,maximizing around equinox seasons and minimizing around June solstice.3.The longitudinal WN4(Wave number 4)pattern in the F region electron density during September equinox at low latitudes in the daytime,which disappears at night,is robust feature throughout the 23/24 solar cycles.With the decrease of solar activity,the amplitude of the longitudinal WN4 pattern decreases.4.During local summer at the southern/northern middle latitudes,the longitudinal WN1/WN2 patterns in the F-region electron density are very prominent,showing no local time difference.The WN1 pattern in thermosphere total mass density at middle latitudes is very clear.5.The absolute amplitudes of DE3 in the F-region electron density during September equinox at low latitudes,as well as DE1,DO and DW2 during local summer at middle latitudes are highly related to the solar activity,all show larger amplitudes during solar maximum years.While their relative amplitudes(normalized by the zonal mean electron density)show little dependence on the solar activity.The absolute amplitudes of D0,DW2 and SPW1 at middle latitudes in thermospheric total mass density,show larger amplitudes during solar maximum years.The relative amplitudesof these tides have no clear correlation with solar activity.6.At low latitudes during September equinox,a clear modulation by the QBO can be found in the relative amplitudes of DE3 in F region electron density both from CHAMP and GRACE observations,which is consistent with the QBO effect as reported for E region quantities.It also supports the strong coupling of the nonmigrating tides between the E and F region.However,no QBO effect can be found in the tidal components DEI,DO and DW2 in F region electron density at the middle altitudes,which implies that these tides are generated in situ at F-region altitudes.