Chromospheric Prolateness: Solar Cycle Variations

Abstract

The solar prolateness (also known as Ovalisation, a french origin name) of the extended dynamical chromosphere is established from measurements performed above 2 Mm heights during the years of solar minimum, using the H$\alpha$, Ca II K and HeII 304 line emissions from both ground-based and space-based 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 H$\alpha$ and Ca II lines, do NOT show this Coronal Hole (CH) effect. Coronal lines and HeI (D3; 1083 nm) do show CHs but do not show the prolateness effect. We first 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 possibly related to the emergence of the small scale network and internetwork magnetic field towards the corona and small scale magnetic reconnections. 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 (Filippov and Koutchmy, 2000). We however note that polar regions are also showing different types of activity in the low corona, including small prominence eruptions seen e.g. in H$\alpha$ and linear jets seen in SXR and EUV as well as in W-L (eclipses). Some kind of dynamical 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 the extended in the horizontal direction lifting effect of a large part of the polar chromosphere.