Persistent Coronal Holes: What Are They and Why Do They Matter?

Coronal holes are regions on the Sun's surface where the magnetic field is open, allowing hot plasma to escape into space. These holes are the source of the solar wind, a stream of charged particles that constantly flows from the Sun and interacts with the Earth's magnetosphere. Coronal holes can have a significant impact on space weather, affecting satellite communications, navigation systems, and power grids.

While coronal holes can develop at any time and location on the Sun, they are more common during the years around solar minimum, the period of low solar activity in the 11-year solar cycle. Some coronal holes can persist for several solar rotations, lasting for months or even years. These persistent coronal holes are most prevalent at the poles, but they can also expand to lower latitudes or form independently from the polar holes.

In this blog post, we will explore the development, characteristics, and role of persistent coronal holes in the solar wind and space weather.

Development and Persistence of Coronal Holes

Coronal holes can form at any time and location on the Sun's surface, depending on the configuration of the magnetic field. The Sun's magnetic field is constantly changing, influenced by the differential rotation of the Sun and the emergence and decay of magnetic flux. During solar minimum, the Sun's magnetic field is less complex and more stable, resulting in fewer sunspots and flares, but more coronal holes.

Some coronal holes can persist for multiple 27-day solar rotations, the time it takes for the Sun to complete one full rotation as seen from Earth. These persistent coronal holes are usually associated with large-scale magnetic structures that remain stable over long periods of time. For example, the **South Pole Coronal Hole** (SPCH) was first observed in 2014 and lasted until 2019, spanning over five years and more than 60 solar rotations.

 Polar Coronal Holes

Polar coronal holes are the most stable and prevalent type of persistent coronal holes. They are located near the north and south poles of the Sun, where the magnetic field lines are mostly radial and open. Polar coronal holes are present throughout the solar cycle, but they are more prominent and extensive during solar minimum, when the polar magnetic field is stronger and more uniform.

Polar coronal holes can sometimes expand to lower latitudes, reaching the equator or even crossing it. This can happen when the polar magnetic field weakens or reverses, as part of the solar cycle. For instance, in 2019, the **North Pole Coronal Hole** (NPCH) extended to the southern hemisphere, creating a large trans-equatorial hole that covered almost a quarter of the Sun's surface.

 Independent Formation of Coronal Holes

Apart from the polar coronal holes, it is also possible for a coronal hole to develop independently from the polar regions. These coronal holes can form at any latitude, often as a result of the interaction between different magnetic domains on the Sun. For example, a coronal hole can form when a new active region emerges near an existing coronal hole, creating a gap in the magnetic field.

Sometimes, an extension of a polar coronal hole can split off, forming an isolated structure. This can happen when the polar coronal hole is distorted by the differential rotation of the Sun, or when it encounters a region of opposite magnetic polarity. For example, in 2017, a **Small Isolated Coronal Hole** (SICH) detached from the NPCH and rotated around the Sun for several weeks.

 Role as Long-lasting Sources for Solar Wind Streams

Persistent coronal holes are important sources for the solar wind, especially the high-speed solar wind streams that can reach speeds of over 800 km/s. These streams are generated by the fast and steady outflow of plasma from the coronal holes, driven by the high temperature and low density of the corona.

The high-speed solar wind streams can interact with the ambient solar wind, which is slower and more variable. This interaction can create compression regions, where the plasma density and pressure increase, forming shock waves and turbulence. These compression regions can enhance the geomagnetic activity on Earth, causing auroras, geomagnetic storms, and radio blackouts.

 Conclusion

Coronal holes are regions on the Sun where the magnetic field is open, allowing plasma to escape into space. They are the source of the solar wind, a stream of charged particles that affects the Earth's magnetosphere and space weather. Coronal holes can develop at any time and location on the Sun, but they are more common during solar minimum, the period of low solar activity in the solar cycle. Some coronal holes can persist for several solar rotations, lasting for months or even years. These persistent coronal holes are most prevalent at the poles, but they can also expand to lower latitudes or form independently from the polar holes. Persistent coronal holes are important sources for the high-speed solar wind streams, which can interact with the ambient solar wind and create compression regions that enhance the geomagnetic activity on Earth.

We hope you enjoyed this blog post about persistent coronal holes and learned something new about the Sun and its influence on our planet.