Solar coronal holes are regions of low-density and low-temperature material that form where the magnetic field lines are open or at least divergent. An understanding of their evolution is important because they are a source of the solar wind that impacts communications on Earth. Data from the solar and heliospheric observatory (SOHO) have shown that coronal holes with fast solar wind speeds tend to concentrate near the poles during solar minimum (1996 to 1997), whereas coronal holes with lower wind and outflow speeds tend to occur at all latitudes during solar maximum (2000 to 2001).
Now, Miralles et al. have observed the growth of a north pole coronal hole in February 2001, just as the sun was reversing its magnetic polarity and beginning its return to a solar minimum. Spectra of O+5 taken with the ultraviolet coronagraph spectrometer on SOHO show that the hole is broadened by a fast perpendicular ion stream flow and that the solar wind speeds are higher than those of solar maximum coronal holes. This solar minimum coronal hole may represent the first observation of the new negative polarity of the Sun. In addition, the timing of the growth of a solar minimum hole coincident with the Sun beginning to flip its magnetic field provides an important dynamic for large-scale solar processes that will need to be considered in models. -- LR
Astrophys. J. 560, rapid release 29 September (2001).
