The serial number assigned to 27-day rotation periods of solar and geophysical parameters. Rotation 1 in this sequence was assigned arbitrarily by Bartels to begin in January 1833, and the count has continued by 27-day intervals to present. The 27-day period was selected empirically from the observed recurrence of geomagnetic activity attributed to corotating features on the sun. A collisional shock wave in front of the magnetosphere arising from the interaction of the supersonic solar wind with earth's magnetic field. Coronal holes are largest and most stable at or near the solar poles, and a source of high-speed solar wind.
Coronal holes are visible in several wavelengths but most notably in solar x-rays.
They are associated with the large-scale, closed magnetic structures in the corona. The average heliocentric width is about 45 degrees. The largest geomagnetic storms are caused by coronal mass ejections. The line in the corona that separates solar magnetic fields of opposite polarity. It is calculated from solar observations of the photospheric magnetic field.