Reconstruction Of Three Dimensional Density Distribution Of The Earth Plasmasphere By CT Technology

The Earth Plasmasphere is a donut-shaped region surrounding Earth and extending to distances of about 5 Earth radii (Re). The plamasphere may influence the control and security of spacecraft which should be given adequate attention. Scientists found that under the solar radiation, He+ ion constituting 20% of the plasma population resonantly scatters isotropic EUV radiation with the wavelength of 30.4 nm. In 2000 the American IMAGE satellite was launched which contained EUV sensor to study the distribution of cold plasma in Earth’s plasmasphere by imaging the distribution of the He+ ion through its emission at 30.4 nm. The images collected by IMAGE satellite provide a lot of useful data for our further study.Each EUV sensor unit recorded the integrated column density of He+ along the line of sight through the plasmasphere which is very like a computer tomography detector. This provides us with the possibility to map the three dimensional density distribution of the earth plasmasphere using two dimensional EUV data by CT technology. After in-depth study of the IMAGE satellite and EUV data, we devise a modified ART method to reconstruct the earth plasmasphere, during the reconstruction we also use the total variation (TV) regularization and boundary constraint to restrain the iteration. We use both the simulation data and real EUV data for the reconstruction and the result verifies our hypothesis. We know that the EUV data is very different from the data sampled by traditional CT, which brought three significant problems in our study. They are the earth shelter problem, the limited-angle and sparse sample problem and the motion problem. To resolve the earth shelter problem, we modify the back-project path during the ART iteration. For the limited-angle and sparse sample problem we use two constraint methods to regularize the iteration, that is: 1. Produce the plasmasphere density map in the geomagnetic equatorial plane under the assumption of weakening of the earth dipole magnetic field, and use the geomagnetic equatorial plane map to produce a boundary constraint; 2. The TV regularization. For the motion problem, we analyze maps of the density distribution of plamasphere in the geomagnetic equatorial plane to calculate the manner of motion. To verify above mentioned solutions of the problems, we have built a dipole magnetic field phantom. A group of simulation data were obtained and used for the reconstruction. The result verifies the correction of our solutions. In the end we reconstruct the 3-D density distribution of the earth plasmasphere using the real EUV data and obtain the first map of the earth plasmasphere.This paper shows a new way to the study of the earth plasmasphere, and we can expect to go further in the study of this field since more reliable and complete data of plasmasphere may be acquired in the future due to the development of space technology of our country.