STATE OF WASHINGTON – 2024 was the year of the Aurora Borealis, and tonight another spectacular display is taking place around the globe. The Northern Lights have already been spotted as far south as Tucson, Arizona, so we should also see a great display of the stunning waves of greens and purples here in the Pacific Northwest. Puget Sound is currently experiencing partly cloudy skies – we know it could be a lot rougher this time of year!

The reason 2024 has been 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 a cycle like this, neither cell phones nor social media were as widespread. These days, news of an aurora apparition spreads quickly online, and most of us are armed with a phone camera that, when pointed at the night sky, can quickly spot the shimmering curtains of colors our naked eyes can't see.

The Aurora Borealis on Thursday evening

Thursday night's storm is classified as G4 with corresponding Kp ratings between 8 and 9 – similar to the severity of the storm that struck in May. Many other locations around the globe, including the northern United States, have already captured incredible views of the Aurora Borealis.

Our performance should be just as good! Remember to stay away from city light pollution, look north, and use your camera lens to better capture the billowing colors in the northwest night sky.

What are the Northern Lights?

Our brilliant aurora shows begin about 150 million kilometers away on the surface of the sun. Sometimes a sunspot cluster erupts into a complex called a “solar storm,” which emits huge bursts of energy in the form of bursts and ejections of magnetic fields and plasma. These eruptions are called coronal mass ejections, or CMEs.

The CMEs move away from the Sun and slingshot into space. The size, speed and strength of a CME can vary greatly. CMEs typically reach Earth within three to five days, but the fastest ones have been shown to reach our planet in just 15 to 18 hours!

Here, the emitted energy and plasma – and the embedded magnetic field – interacts with the Earth's atmosphere. This disturbance results in an exchange of energy from the solar wind into our Earth's magnetosphere and is called a “geomagnetic storm.”

How is the strength of a solar storm assessed?

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

Part of the scale is based on a “K” index, sometimes called a “Planetary K” index, stylized as “Kp”. This value quantifies the horizontal movement in the Earth's magnetic field over three-hour intervals, with 0 indicating complete calm (marked green) and five or more indicating that a geomagnetic storm is occurring (marked red). The top of the scale is 9, indicating an extreme storm.

The colors, intensity, shape and coverage of an Aurora Borealis event correlate with the Kp index as such: Kp0 means we could see faint green, from Kp3 we could see some yellows, Kp4 brings pink, Kp5 brings blue and purple and at Kp8 Red becomes part of the mix. Therefore, shades of green are common at lower Kp values, but shades of blue, purple, and red only become visible during more intense geomagnetic storms.

In the May 2024 severe geomagnetic storm, the SWPC planetary K index showed that the Kp index reached a value between 8 and 9 in the early hours of May 11th.

While the Kp index measures movement in the magnetic field, the overall impact of a geomagnetic storm, including disruption to power grids, GPS and communications systems, is also measured by SWPC using the “G scale”. The scale ranges from G1-G5, where G1 corresponds to Kp5 and G5 corresponds to a Kp9.