Coronal holes are regions on the Sun’s surface where the corona, the outermost layer of the Sun’s atmosphere, appears darker and cooler than its surroundings. These areas are characterized by low-density plasma (the matter the Sun is made up of) and are a common feature of the Sun’s magnetic field.

Coronal Hole – NASA SDO

Coronal holes are believed to be the source of high-speed solar wind particles that are constantly emitted into space. When the Sun’s magnetic field lines in these regions open-up, they allow charged particles to stream out from the corona and into space. These charged particles, mostly electrons and protons, make up the solar wind.

Coronal holes can vary in size and location on the Sun’s surface. They tend to be larger and more frequent near the solar poles, where the magnetic field lines are more open and less tightly packed. The size and frequency of coronal holes are also linked to the solar cycle, which follows an 11-year pattern of magnetic activity and sunspot activity.

When a coronal hole is facing the Earth, we can expect to experience increased geomagnetic activity within 2-3 days of the hole rotating onto the Earth facing side of the Sun. Increased geomagnetic activity is another term for increased potential of seeing the northern lights. Due to the increase in the solar wind speed, more charged particles within the solar wind will enter the atmosphere of the Earth, they will then react with oxygen and nitrogen molecules in the atmosphere and create the aurora borealis.

Image Credit – Gaute Bruvik and Visit Norway

So, if you spot a coronal hole on one of the many space weather warning websites, start getting excited.

Despite being relatively cool compared to their surroundings, coronal holes are still extremely hot, with temperatures in the range of 1 to 2 million degrees Celsius. This is because the temperature of the corona is not determined by its density, but by the activity of the Sun’s magnetic field.

Scientists study coronal holes to better understand the Sun’s magnetic field and its impact on the Earth and the rest of the solar system. By analysing the solar wind particles emitted from coronal holes, researchers can gain insight into the Sun’s magnetic activity and its effects on the Earth’s environment, including the formation of auroras and the impact of solar storms on communication and power systems.