STATE OF WASHINGTON — 2024 was the year of the Aurora Borealis, and tonight another spectacular display is firing off around the world. The Northern Lights have been spotted as far south as Tucson, Arizona, so we should have a great display of the breathtaking waves of green and purple here in the Pacific Northwest as well. Puget Sound currently has partly cloudy skies; we know it can be even bleaker this time of year!

The reason 2024 was so active for aurora viewing is because we are at the peak of an 11-year cycle of solar activity. The last time we were in the middle of such a cycle, cell phones and social media weren’t as prevalent. These days, news of an aurora display is spreading quickly online, and most of us are armed with a phone camera that, when pointed at the night sky, can quickly make out the shimmering curtains of color that our naked eye can’t see.

Thursday evening Aurora Borealis

Thursday night’s storm is rated as a G4 with a corresponding Kp rating between 8 and 9 – similar to the severity of the storm that broke out in May. Many other locations around the world, including in the northern United States, have already captured incredible views of the Aurora Borealis.

Our show should be just as good! Don’t forget to step away from the city’s light pollution, look north and use your camera lens to better catch the undulating colors in the northwest night sky.

What are the Northern Lights?

Our dazzling aurora shows begin about 93 million miles away on the sun’s surface. Sometimes a sunspot cluster will erupt in a complex called a ‘solar storm’, emitting huge bursts of energy in the form of eruptions and ejections of magnetic field and plasma. These eruptions are known as coronal mass ejections, or CMEs.

The CMEs are pulling away from the sun and hurling into space. The size, speed and strength of a CME can vary widely. CMEs typically arrive on Earth in three to five days, but the fastest ones have been found to reach our planet in just 15 to 18 hours!

Once here, the emitted energy and plasma – and the embedded magnetic field – interact with our Earth’s atmosphere. This disturbance results in an exchange of energy from the solar wind to our Earth’s magnetosphere, and is known as a ‘geomagnetic storm’.

How are the strengths of solar storms rated?

The Space Weather Prediction Center (SWPC) is part of NOAA (National Oceanic and Atmospheric Administration). The SWPC monitors solar flares and geomagnetic storms and has developed a scale similar to that used for hurricanes and tornadoes to monitor these disturbances and communicate their intensity to the public.

Part of the scale is based on a ‘K’ index, also called a ‘Planetary-K’ index, stylized as ‘Kp’. This measurement quantifies the horizontal motion in Earth’s magnetic field over three-hour intervals, where 0 is completely calm (marked by green) and five or more indicates a geomagnetic storm is occurring (marked by red). The top of the scale is 9, which reflects an extreme storm.

As such, the colors, intensity, shape and opacity of an aurora borealis event correlate with the Kp index: Kp0 means we can see faint green, from Kp3 we can see some yellow, Kp4 brings pink, Kp5 brings blue and purple and at Kp8 red becomes part of the mix. Thus, green hues are common at lower Kp values, but the blue, purple and red hues only become visible during more intense geomagnetic storms.

During the severe geomagnetic storm that occurred in May 2024, SWPC’s planetary K-index showed the Kp index reaching between 8 and 9 in the early morning hours of May 11.

While the Kp index measures the movement in the magnetic field, the overall impacts of a geomagnetic storm, including disruptions to power grids, GPS and communications systems, are also measured by SWPC, with the ‘G scale’. The scale goes from G1-G5, where G1 corresponds to Kp5 and G5 to Kp9.