On the Dynamic Nature of the Prolate Solar Chromosphere

The upper edge of the solar chromosphere looks not like perfect circle in some spectral lines. It is prolate in the South-North direction at the epoch of minimum solar activity and nearly spherically symmetrical at the maximum phase. We attribute the effect to the dynamical nature of the upper chromosphere, which consists of a large number of small cool jets (jetlets) ascending into the corona. A proposed simple geometric model can explain the effect of the prolateness of the solar chromosphere. Due to the dynamic nature of the solar atmosphere above the 2 Mm level, the magnetic field is considered to play a very important role in the density distribution with the height, guiding the mass flows along the field lines. The difference of the magnetic field topology in the polar and the equatorial regions leads to different heights of the chromospheric limb. We could not resolve a source region of an individual jetlet, however similar but larger structures are visi
ble in EUV coronal lines. We present an example of the jet formation obtained by TRACE in the 171 Å channel. Field aligned motion arises above the null point created in the corona by the emerging magnetic bipole. The scale of bipole is large enough to recognize the saddle structure around the 3D null point in the TRACE images. We believe that similar but smaller processes could happen very often at smaller scale in the chromosphere near emerging magnetic ephemeral regions forming numerous jetlets of the upper chromosphere.