| This thesis reports on the physical mechanism of the anomalous variation of electron temperature in the ionospheric F region during magnetic storms,the formation mechanism of the ionospheric additional layer(F3 layer),and the seasonal variation of helium ions(He+)in the upper ionosphere as well as the physical mechanism of the vertical distribution based on the measurement of the incoherent scattering radar at the Arecibo Observatory in Puerto Rico(18.3°N,66.7°W).The primary results are summarized as follows:The response of the low-latitude ionosphere F region electron temperature(Te)to the strong magnetic storm occurred on August 5,2011,with the measurements of Arecibo dual-beam incoherent scattering radar.After the magnetic storm on August 5,a prompt electron temperature enhancement of 2300 K,with a rapid increase by nearly 800 K within an hour was observed over Arecibo.To our knowledge,this is the first time that such a strong response of the electron temperature to a magnetic storm has been observed over Arecibo.The perturbation values of Te and Ne also illustrate a prominent anticorrelation during the daytime.The correlation coefficients of Te and Ne during the daytime are-0.88 and-0.91,respectively.The dominance of the effect of photo-ionization compared to other heat sources leads to the anti-correlation between Te and Ne.The depletion in the ratio of[O]/[N2]leads to a decrease in Ne.Using the observation Arecibo ISR combined with TIMED/GUVI satellite,the plasma vertical upward drift and a significant decrease in[O]/[N2]after the commencement of the magnetic storm was observed.The variation of neutral particle composition leads to a corresponding decrease in Ne,which causes a rapid increment in electron temperature after the magnetic storm.The F3 layer,which refers to the additional layer above the peak ionospheric height(HmF2),was first reported over Arecibo at the geomagnetic mid-latitudes,and the vertical structure of the F3 layer was analyzed and its formation mechanism was discussed.Previous studies have suggested that the F3 layer occurs mainly during the daytime at low geomagnetic latitudes and is mainly caused by photoionization and vertical upward ion drift.The F3 layer reported in this thesis appears at geomagnetic mid-latitudes and occurs during nighttime and its formation mechanism is mainly due to the westward electric field.The westward electric field results in the vertical gradient of ion vertical drift and leads to the accumulation of Ne near 400 km,which indicates the formation of the F3 layer.We further validated this theoretical mechanism with numerical simulations and confirmed the important role of vertical ion drift in the formation of the F3 layer.The interconnection between helium ion concentration([He+]),ion temperature Ti,and plasma drift is investigated.We observe that the helium ion fraction[He+]/Ne presents a prominent anti-correlation with the perturbation change of Ti and a seasonal variation of[He+]/Ne in autumn and winter.The anti-correlation feature between the perturbation of[He+]/Ne and Ti is analyzed and well explained by the diffusive equilibrium between[He+]and[O+].Since the diffusive equilibrium occurs only below the peak altitude of[He+],the presence of ion drift downward during nighttime in Arecibo descends the peak altitude of[He+]and breaks the diffusive equilibrium in the upper ionosphere.The non-equilibrium consequently weakens the anti-correlation of[He+]/Ne and Ti during nighttime.Since winter has a more continuous and stronger ion downward drift,which prolongs the duration of the non-diffusive equilibrium,resulting in a higher[He+]/Ne and causing the seasonal variation of[He+]. |
