Ionospheric Climatological Variations In The Electron Density Profile And Its Responses To Transient Solar Irradiation Events

The ionospheric electron density can be usually described by the characteristic parameters of the ionospheric profile,such as the peak parameters of E,F1 and F2 layer of the ionosphere.Studying the morphological structure and variation of the ionospheric electron density profile can help us to understand the photochemical and dynamic processes of the ionosphere and further serves for communications,navigation and the space weather’s modeling and forecast.In the past,due to the lack of full spatial and temporal coverage of the ground-based observations,especially in the oceanic areas and the southern hemisphere,the global characteristics of the ionospheric electron density profiles are not fully understood.In this paper,we have used different observational data,including data from the Constellation Observing System for Meteorology,Ionosphere,and Climate observations(COSMIC),ionosonde,and the incoherent scatter radar(ISR)and so on,along with the empirical models such as the IRI and the theoretical model(Thermosphere Ionosphere Electrodynamics General Circulation Model(TIEGCM)),to explore the ionospheric climatological variations in the electron density profile and its responses to transient solar irradiation events.And the underlying physical mechanisms have been discussed.The main results are given as follows.(1)Study on longitudinal variations of the occurrence probability of the ionospheric Fi layer peak at middle and high latitudesThe Fi layer peak parameter is an important shape factor of the ionospheric profile.The previous empirical models usually show changes with latitude for the ionospheric F1 layer peak occurrence probabilities,but do not consider their longitudinal structure.This study has focused on the longitudinal variations of the ionospheric F1 layer peak occurrence probabilities at middle to high latitudes(40-70°)in the summer daytime under low solar activity.The ionospheric electron density profiles retrieved from COSMIC observations are used.This paper further defined the relative F1 layer peak depth(r),which is the ratio between the absolute depth of the F1-F2 valley and the F1 layer peak density,can represent the occurrence probability of the F1 layer peak.The TIEGCM model generally reproduced the observed longitudinal pattern of the r ratio,and hence the occurrence probabilities well.Further TIEGCM simulations show that the higher F1 layer occurrence probabilities can be largely explained by longitudinal variations of the O/N2 ratios and also that the dynamic processes,such as the neutral winds above the F1 layer,have additional contributions.These results reveal that the low O/N2 ratios deplete the ionospheric electron density above the F1 layer at middle and high latitudes and thus make the F1 layer stand out.(2)Study on the global distribution characteristics of the peak density of the ionospheric F1 layerThere are few studies about the longitudinal variation of the peak density of the F1 layer in the past.We have collected the global observations of ionosonde from 2006 to 2010 and studied how the peak density(NmF1)of the F1 layer change with latitude and longitude.The ionosonde observations and the results of IRI are consistent in their latitudinal dependence,while not in their longitudinal variations.The observations show obvious longitudinal variations in NmF1.During summer of each hemisphere,NmF1 is higher within 90°E-150°E.Using results of MSIS,HWM and ISR ion drift model,we have shown that the longitudinal variations of NmFl in the northern hemisphere are similar with the corresponding longitudinal patterns in the background oxygen atom O,while the longitudinal variations of the vertical plasma drift associated with neutral winds and electric field drifts in the southern hemisphere play a major role,that is,the influence of transport processeses could contribute largely to the longitudinal variations of NmF1 in the southern hemisphere.(3)Study on altitudinal variations and the time delay of the electron density profiles in response to the solar eclipseThe ionospheric responses to the solar eclipse at different altitudes and the possible mechanisms involved are not fully understood.Also the different responses at different cases need further study.In this work,we have studied the ionospheric response to the two different solar eclipse cases occurring in Asia and America by combining the TIEGCM model simulation with the the ionosonde observations.The results show that the variations of the electron density in E and F1 layers are synchronous with that of the solar eclipse.Moreover,the relative variations of the electron density in the E and F1 layers during eclipse have a positive correlation with the solar radiation flux changes.These results are consistent with the previous observations.Moreover,we found that the height of the maximum response of electron density during the solar eclipse is generally a few tens of kilometers higher than the F1 layer peak location.This is not consistent with the expectation.In theory,due to the altitudinal changes of the chemical loss rate,the relative decrease of the electron density in response to the eclipse will increase with altitude below about 200 km.Further,through the TIEGCM simulations,we have found that the delay time of the maximum electron density decrease during the solar eclipse around the the F2 layer peak height not only changes with altitude,but also varies with latitude and longitude obviously.Further,by using the term analysis of the O+ continuity equation,we have addressed the relative effects of the photo-chemical and transport processes during the solar eclipse,which are not well understood before.The results show that the longer delay time of the ionospheric electron density around F2 layer peak after the eclipse is related to the stronger plasma transport processes due todiffusion andplasma drifts.However,the contributions of the dynamical processes to the electron density reduce in the eclipse vary with latitude and longitude,so that the delay time of the ionospheric electron density decrease of the F2 layer depends on latitude and longitude during the solar eclipse.(4)Study on the response of the ionospheric electron density profile to the solar flareIn this study,we have combined the ISR observations at Millstone Hill and TIEGCM simulations to examine the response of the electron density profiles during three different solar flares.We also have compared the ionospheric responses to flares with the effect of solar eclipse.The results show that the enhancement of electron density during the solar flares mainly takes place at low altitudes below 150 km and its magnitude becomes relatively small in the F layer.The TIEGCM simulations show that the response time of the electron density changes with altitude during the flare.The delay time of the flare response of the electron density is very short below 200 km(E and F1 layers),and above 200 km it gradually increases with altitude.The change of the delay time with altitude could be associated with the different loss rates at different altitudes.We have found that the response of the ionospheric elelctrons between eclipses and flares are different in terms of the delay time and changing magnitudes.This is due to different changes of solar radiation during solar eclipses and flares,and the associated ionospheric response in these two kinds of cases cannot be simplify considered asopposite.