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Massive "coronal hole" in the Sun prompts aurora alert for southern states

A high-speed stream from a coronal hole is expected to lead to G2 (Moderate) geomagnetic storming on December 4 (UTC Day) and G1 (Minor) storming on December 5, 2023, according to an alert this morning from the NOAA Space Weather Prediction Center (SWPC).

Coronal holes are striking features of the Sun, often appearing as dark regions in the solar corona when observed in extreme ultraviolet (EUV) and soft X-ray solar images. These areas are notable for being cooler and less dense compared to the surrounding plasma. The darkness of coronal holes is attributed to their nature as regions of open, unipolar magnetic fields.

Role of magnetic fields in coronal holes

The open magnetic field line structure of coronal holes is a critical aspect of their behavior. This structure allows the solar wind, a stream of charged particles emitted by the Sun, to escape more easily into space. The result is a stream of relatively fast solar wind, often referred to as a high-speed stream in interplanetary space analysis.

Interestingly, coronal holes can develop at any time and place on the Sun. However, they are more common and persistent around the years of solar minimum. The more persistent coronal holes can last through several solar rotations, each spanning about 27 days.

Prevalence and stability of coronal holes

Coronal holes are most prevalent and stable at the solar north and south poles. These polar holes can grow and expand to lower solar latitudes. Additionally, coronal holes can develop in isolation from polar holes, or an extension of a polar hole can split off to become an isolated structure. Persistent coronal holes serve as long-lasting sources for high-speed solar wind streams.

As the high-speed stream interacts with the slower ambient solar wind, a compression region forms, known as a co-rotating interaction region (CIR). From the perspective of a fixed observer in interplanetary space, the CIR will lead the coronal hole high-speed stream (CH HSS).

Effects of CIR on planetary conditions

The CIR can cause particle density enhancement and interplanetary magnetic field (IMF) strength increases before the onset of the CH HSS. As the CH HSS approaches Earth, solar wind speed and temperature rise, while particle density begins to decrease. After the passage of the CIR and the transition into the CH HSS flow, the overall IMF strength typically starts to weaken.

Coronal holes located near the solar equator are most likely to affect Earth with CIR passage and/or higher solar wind speeds. Strong CIRs and the faster CH HSS can significantly impact Earth’s magnetosphere, causing geomagnetic storms to the G1-G2 levels (Minor to Moderate) on the NOAA Space Weather Scale.

In rarer cases, stronger storming may occur. The larger and more expansive coronal holes can be a source of high solar wind speeds that affect Earth for extended periods.

Forecasting a coronal hole

Forecasters closely analyze coronal holes for their potential to cause escalated geomagnetic activity and possible storming. They are noted on the daily synoptic drawing.

The Space Weather Prediction Center (SWPC) forecasters consider the effects of CIR and CH HSS activity when forecasting the anticipated levels of overall planetary geomagnetic response for each 3-hour synoptic period over the next three days, as detailed in the 3-day forecast. Predicted CIR or CH HSS influences are further elaborated in the forecast discussion.

Coronal holes and auroras

Coronal holes play a significant role in the creation of auroras on Earth. These dark areas on the Sun’s surface, characterized by open magnetic fields, allow solar winds to escape more easily into space. When these high-speed solar winds, often emanating from coronal holes, reach Earth, they can interact with the planet’s magnetosphere. This interaction involves the solar wind’s charged particles colliding with Earth’s magnetic field and atmosphere.

As the solar wind penetrates the magnetosphere, it can cause geomagnetic storms, especially if the incoming solar wind is strong and fast. These storms disturb the magnetosphere and lead to the acceleration of electrons and protons. When these accelerated particles collide with gases in Earth’s upper atmosphere, such as oxygen and nitrogen, they excite these gases, causing them to release photons – the basic units of light. This release of photons results in the shimmering lights known as auroras.

The intensity and frequency of auroras often depend on the strength of the solar wind and the geomagnetic storms it induces. Coronal holes, being significant sources of fast solar winds, can enhance the likelihood and intensity of auroras. During times when Earth encounters streams of solar wind from these coronal holes, the chances of observing auroras at higher and sometimes lower latitudes increase. This phenomenon makes coronal holes an essential factor in predicting and understanding auroral activities on Earth.

In summary, understanding coronal holes and their associated phenomena is crucial for anticipating and mitigating the impacts of solar activity on Earth’s magnetosphere and space weather conditions.

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