Statistical Analysis of the Structure and Dynamics of Coronal Hole Magnetic Fields

Irina Bilenko (Sternberg Astronomical Institute)


Coronal holes are usually associated with the unipolar magnetic field regions, and are believed to be the sources of the solar wind high-speed streams. However, not every passage of an isolated coronal hole over the solar disk causes high-speed solar wind stream. The problem of the formation and acceleration of the solar wind is one of the most important of the solar physics. The regions of open configurations of the magnetic fields related to coronal holes are believed to be the source of formation of high-speed solar wind streams.
SOHO observations from 1996 to 2011 were used to investigate the spatial and temporal evolution of the magnetic field and EUV intensity distribution within coronal holes. An algorithm for coronal hole automatic detection and identification were developed. Using solar wind measurements the streams of the fast and slow solar wind at 1 AU were connected with their source regions. It was found that high-speed streams of the solar wind do not concentrate to the center of a coronal hole registered at coronal level. The structure of the photosphere magnetic field associated with such a region differs significantly from that of the other parts of the coronal hole and that of quiet photospheric regions. Coronal hole structure also changes with height.
It seems that the deepest layers of the solar atmosphere down to the photosphere are involved in the formation of the high-speed streams. The maximum values of the solar wind speed are observed in the cases when the regions of the coronal holes registered in the HeI line and in all EUV observed levels coincide. In these cases during the whole period of the coronal hole passage the solar disk an increase in the solar wind speed up to the maximum values and density decrease are observed. The maximum solar wind velocity is also observed when a coronal hole adjoins an active region. The presence of the adjoining active region is of a great importance for the magnetic field configuration and distribution in a coronal hole.
Probably high-speed streams of the solar wind are generated directly in the chromospheric network at the basis of coronal holes and possibly highfrequency waves are generated in the process of a small-scale reconnection. At the same time, low speed streams are formed high in the corona in the top parts of arch structures.
A possible mechanism of high-speed solar wind stream formation in coronal holes is discussed.

Paper ID: P012

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