Magnetosphere - Ionosphere Coupling Field To The Current Study

Field-Aligned Currents (FACs, also called the Birkeland currents) are important for the magnetosphere-ionosphere coupling system. Till 80's of the last century, almost all the studies were made with one satellite at low-altitude which could only assume that current structures move quite slowly compared to the spacecraft speed. But at high altitude, we at least need two satellite data to calculate the current intensity because the time variation and spatial variation could not be resolved by the single satellite measurements.With one or two satellite data, we could not calculate the current density directly; however, it could be calculated through the estimated thickness of current sheet. The Cluster with 4-point measurement provides us a very good opportunity to calculate the current density by the so called curlometer technique with the magnetic field data in different space region and to study the FACs statistical properties. In this thesis, we used the PP data of the Fluxgate Magnetometer on board the Cluster, to study the features of the FACs distribution in the Plasma Sheet Boundary Layer (PSBL). Our study include investigating the feature of the FACs distribution in plasma sheet boundary layer in the magnetotail; investigating FACs at plasma sheet boundary layer in the magnetotail are controlled by interplanetary magnetic field (IMF) and solar wind; investigating the current density of FACs and relationship between the FACs density and AE, AU, AL indices in the storm time. In this thesis, we also study the FACs in TC-2's orbit that is at the middle altitude above the polar region. Our study mainly included the contents, which are as follows:(1) We statistically investigated the feature of the FACs distribution in plasma sheet boundary layer in the magnetotail, including the studies of FACs spatial distribution and density distribution. The results show that spatial distribution of the FACs has dawn-dusk asymmetry, that is, earthward FACs frequently appear in the dawn sector and tailward FACs frequently appear in the dusk sector. It appears that the field-aligned currents are concentrated in the plasma sheet boundary layer in the magnetotail and their directions are generally consistent with those of the region 1 currents. In the dawn or dusk sectors, the density of tailward FACs is higher than the density of earthward FACs. The density of earthward or tailward FACs in the dawn sector is higher than the density of earthward or tailward FACs in the dusk sector. Spatial distribution of the FACs also has south-north hemisphere asymmetry. FACs at the plasma sheet boundary layer map to ionospheric altitudes and coincides with auroral oval, the shape is an oval in the northern hemisphere and has round shape in the southern hemisphere. In the northern hemisphere, the number of earthward FACs is more than the number of tailward FACs in the dawn sector, and in the dusk sector the number of tailward FACs is more than the number of earthward FACs. But it is not the same for the southern hemisphere, where the number of tailward FACs is more than the number of earthward FACs in both the dawn and the dusk sectors. The FACs occurrence versus its density has a Gaussian distribution with added exponential decrease at higher density values. Most probable value of the earthward FACs density is 3.38 pT/km, and for the tailward is -3.75 pT/km.(2) FACs at plasma sheet boundary layer in the magnetotail are controlled by interplanetary magnetic field (IMF) and solar wind. The results show that the occurrence of the FACs and IMF By,Bz component are relevant and their density is controlled by the solar wind. FACs are easier to exist under strong southward IMF. The probability of the observation of FACs is higher when IMF is southward and are easier to exist when the dusk component of IMF increases. The density of FACs and solar wind velocity or the dynamic pressure have a close correlation. The density of FACs increases monotonically with the solar wind dynamic pressure, the stronger the solar wind, the bigger influence on the FACs. The solar wind velocity increases monotonically with the earthward FACs density. Although the variation of the solar wind velocity with the tailward FACs density is complex, but it shows similar behavior as that of solar wind and earththward FACs.(3) In this study, we consider the FACs at the Plasma Sheet Boundary Layers in the storm time and two cases are chosen for analysis. One (August 17, 2001) falls from sudden commencement to main phase, and the other (October 1, 2001) in the main phase to recovery phase. We study the current density of FACs and relationship between the FACs density and AE index in the storm time. The results show that: In the storm time, there is an evident increase in the FACs density. From the sudden commencement to the initial stage of the main phase, the FACs increases with decreasing AE index and decreases with increasing AE index; From the main phase to initial stage of the recovery phase, the FACs increases with increasing AE index and decreases with decreasing AE index; In the later stage of the recovery phase, the disturbance of the FACs is not so violent and the FACs variation with the AE index is also not very evident. We also study the relationship between the FACs density and AU, AL indices in storm time. The relationship between the FACs density and AL index is comparatively obvious. From the sudden commencement to the initial stage of the main phase, the FACs density increases with increasing AL index and decreases with decreasing AL index. From the main phase to initial stage of the recovery phase, the FACs increases with decreasing AL index. It can be noted that FACs density has strong relation with AL index. Amongst the AE, AL and AU indices, the relationship between the FACs density and AE index is most pronounced, while AU index has no clear relationship with FACs.Double Star and the four Cluster satellites have been working together, making an unprecedented study of the Sun-Earth connection from six different viewpoints in space. Simultaneous observation of the Double Star Program (DSP) and the Cluster mission provides us a very good opportunity to study FACs in different space conditions. In this thesis, with the magnetic field measurements by FGM instrument on board the DSP TC-2 satellite, we also study the FACs in TC-2's orbit that is at the middle altitude above the polar region. We establish a field-aligned coordinate system by using the IGRF(International Geomagnetic Reference Field) model and identify the FACs by subtracting the model field from the measured field. By calculation, we get some FACs cases and make a distinction between region 1 and region 2 FACs with the detrended data in the field-aligned coordinate system. Based on the selected FACs, the properties of the FACs at mid-altitude will be compared with that at low-altitude and high-altitude in the future work.