As the CH HSS begins to arrive at Earth, solar wind speed and temperature increase, while particle density begins to decrease. The CIR can result in particle density enhancement and interplanetary magnetic field (IMF) strength increases preceding onset of the CH HSS. From the perspective of a fixed observer in interplanetary space, the CIR will be seen to lead the coronal hole high speed stream (CH HSS). As the high speed stream interacts with the relatively slower ambient solar wind, a compression region forms, known as a co-rotating interaction region (CIR). Persistent coronal holes are long-lasting sources for high speed solar wind streams. It is also possible for coronal holes to develop in isolation from the polar holes or for an extension of a polar hole to split off and become an isolated structure. Coronal holes are most prevalent and stable at the solar north and south poles but these polar holes can grow and expand to lower solar latitudes. The more persistent coronal holes can sometimes last through several solar rotations (27-day periods). This open, magnetic field line structure allows the solar wind to escape more readily into space, resulting in streams of relatively fast solar wind and is often referred to as a high speed stream in the context of analysis of structures in interplanetary space.Ĭoronal holes can develop at any time and location on the Sun, but are more common and persistent during the years around solar minimum.
They appear dark because they are cooler, less dense regions than the surrounding plasma and are regions of open, unipolar magnetic fields. Coronal holes appear as dark areas in the solar corona in extreme ultraviolet (EUV) and soft x-ray solar images.
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