2D and 3D Polar Plume Analysis from the Three Vantage Positions of STEREO/EUVI A, B, and SOHO/EIT     

Judith de Patoul  Bernd Inhester  Li Feng  Thomas Wiegelmann 《Solar physics》2013,283(1):207-225

Polar plumes appear as elongated objects starting at the solar polar regions. Here we analyze these objects from a sequence of images taken simultaneously by the three spacecraft telescopes STEREO/EUVI A and B, and SOHO/EIT. We establish a method capable of automatically identifying plumes in solar EUV images close to the limb at 1.01?–?1.39?R _⊙ in order to study their temporal evolution. This plume-identification method is based on a multiscale Hough-wavelet analysis. Then two methods to determine the 3D localization and structure of the plumes are discussed: first, tomography using filtered back-projection and including the differential rotation of the Sun and, second, conventional stereoscopic triangulation. We show that tomography and stereoscopy are complementary for studying polar plumes. We also show that this systematic 2D identification and the proposed methods of 3D reconstruction are well suited to identify plumes individually and also to analyze the distribution of plumes and inter-plume regions. Finally, the results are discussed, focusing on plume position and cross-sectional area.  相似文献   

Chromite in the Platreef (Bushveld Complex, South Africa): occurrence and evolution of its chemical composition     

Marina A. Yudovskaya  Judith A. Kinnaird 《Mineralium Deposita》2010,45(4):369-391

The regional distribution and chemical composition of massive and disseminated chromitites through a Platreef sequence and along a strike distance of over ∼20 km were investigated to correlate them both within the framework of the northern limb and to the eastern and western limbs of the Bushveld Complex. The chromitite layers and seams of the Platreef form two main chromite-bearing zones: the Upper Chromitite that occurs as two to three discontinuous seams in feldspathic pyroxenite at approximately 20 m below the Platreef top contact and the Lower Chromitite that is composed of multiple seams in feldspathic harzburgite, pyroxenite and norite close to the bottom contact of the Platreef with footwall. Electron micro-probe analyses reveal that the chemical composition of chromite depends on the host rock type. Norite and pyroxenite host chromite with the highest Cr₂O₃ content while harzburgite-hosted chromites are Cr and Mg poor. The wide range in chromite compositions is explained by the influence of late-magmatic processes including post-cumulus growth and re-equilibration, interaction with fluid- and sulphide-saturated magmatic liquid and contact metamorphism. Each of these processes is characterised by its own distinct geochemical signature, but generally they lead to a decrease in Mg and Al and an increase in both di- and tri-valent Fe in the chromite. The occurrence of chromitite locally on the different distance from the contact between the upper Platreef sills and the overlying Main Zone magma suggests erosion of the upper Platreef by the Main Zone as it was emplaced. The localisation of chromitites supports an independent development of the northern limb during the Critical Zone emplacement although the chemical composition of chromite and co-existing silicates from ultramafic rocks suggest a Critical Zone affinity with the eastern and western limbs of the Bushveld Complex.  相似文献