Context. In "cool" spectral lines, the smoothed upper edge of the solar chromosphere is prolate in the South-North direction at the epoch of minimum solar activity and nearly spherically symmetric at the maximum phase. We attribute the effect to the dynamical nature of the upper chromosphere, which consists of a large number of small jet-like structures ascending into the corona. We could not resolve the source region of an individual jetlet, although similar but larger structures are visible, especially in EUV coronal lines. Aims. We consider the problem of the formation of an individual jet above the limb, assuming that a large number of jet-like events is responsible for the prolate solar upper chromosphere. We then assume that spicules, being the cool part of the phenomenon, behave similarly, and we will mainly concentrate the analysis on the magnetic origin of the event.Methods. Image processing is used to reveal the displacement of magnetic field tubes filled with coronal plasma and jet formation due to field aligned motion above the null point created in the corona by the emerging magnetic bipole. Results. The growth of the bipole leads to a reconnection of the field lines and to a specific plasma motion in the vicinity of the null point that results in a plasma flow along the spine line of the 3D null. We assume that similar but smaller processes could happen very often at a smaller scale in the chromosphere, near emerging magnetic ephemeral regions, forming numerous jetlets in the upper chromosphere. As the field aligned motion is guided by the magnetic field, at the epoch of low activity the large-scale structure of the polar magnetic field and the one of the quiet equatorial region is sufficiently different to explain the prolateness of the chromosphere.
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 Hα, 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 Hα 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 Hα 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.
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