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Paper: The Chromospheric Prolateness and its Variations
Volume: 346, Large-scale Structures and their Role in Solar Activity
Page: 269
Authors: Vilinga, J.; Koutchmy, S.
Abstract: The chromospheric prolateness (also called ovalisation) of the extended dynamical chromosphere was established from measurements performed above 2Mm heights during the years of solar minimum, using the Hα, Ca II K and HeII 304 line emissions from both groundbased and spacebased observations. Coronal X-EUV emissions usually penetrate deep enough into the chromosphere to completely mask this effect on transition region lines and produce the so-called coronal hole effect. However, cool lines like Hα and Ca II lines, do not show this CH effect. Coronal lines and HeI (D3; 1083 nm) do show coronal holes but no prolateness effect. We briefly review different methods which can potentially be used to measure the prolateness. Further we note the similarity of the geometric behaviour of the prolateness and its variation along the solar cycle compared to the behaviour of the fast solar wind. It suggests the same origin related to the emergence of the small scale network and internetwork magnetic field towards the corona. A simple geometric model was proposed to explain the effect of the prolateness of the solar chromosphere by considering that the specific dynamical part of the solar atmosphere above the 2 Mm level, being a mixture of up and down moving jets of chromospheric matter with the coronal plasma between them, is responsible for the solar prolateness. We however note that polar regions are showing different types of activity in the low corona, including prominence eruptions seen in Hα and jets seen in SXR and EUV. Some kind of dissipation of the newly emerged magnetic field is needed. More systematic measurements should be done to build a more complete, possibly 3D, picture to explain this lifting effect of a large part of the chromosphere.
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