Coronal Holes: The Sun's Recurring Gift to Aurora Watchers

Not all elevated aurora activity comes from dramatic solar eruptions. Some of the most reliable periods of geomagnetic activity are driven by a quieter, more predictable feature of the sun — the coronal hole. For aurora travelers and photographers who want to plan further ahead than a 3-day forecast allows, coronal holes are worth understanding in some detail.

What a Coronal Hole Is

The sun's outer atmosphere, the corona, is threaded with magnetic field lines. In most places, those field lines loop back to the surface, trapping hot plasma close to the sun. In a coronal hole, the magnetic field instead opens outward into space, allowing plasma to escape freely. The result is a faster, denser stream of solar wind flowing out from that region — what forecasters call a high-speed stream.

What helped me picture this: imagine the sun's corona like a field covered in garden sprinklers, most of which spray in arcs that fall back to the ground. A coronal hole is a sprinkler where the nozzle points straight up and the water goes much higher — and farther. When Earth passes through the path of that elevated spray, geomagnetic activity picks up.

For more context on how coronal holes fit within broader solar activity patterns, see our overview of solar cycles and the northern lights.

Why the 27-Day Cycle Matters for Aurora Travelers

The detail that makes coronal holes particularly useful for planning is their recurrence. The sun rotates approximately once every 27 days as seen from Earth. A coronal hole that faces Earth this week will face Earth again in roughly 27 days — and the high-speed stream it produces will arrive at Earth on a similar schedule, offset by the 1 to 4 days it takes to travel from the sun.

This predictability shows up in NOAA's 27-day geomagnetic outlook, which forecasters use to identify likely activity windows associated with known coronal holes. It doesn't guarantee aurora on a specific night, but it does give travelers a credible basis for identifying weeks that are statistically more likely to produce elevated activity than others.

For anyone planning a northern lights trip, aligning your travel window with an active coronal hole passage is a reasonable strategy — particularly when combined with a location like Fairbanks that sits beneath the auroral oval and produces worthwhile displays even at moderate activity levels. Our Northern Lights Tour in Fairbanks runs during the prime winter viewing season, when dark skies and clear weather combine with active solar conditions. For more on seasonal timing, see our guide on the best time to see the northern lights in Alaska.

Coronal Holes vs. CMEs: Different Drivers, Different Characters

Aurora driven by a coronal hole high-speed stream tends to have a different character than CME-driven activity. CME storms often arrive sharply — a sudden intensification that can produce dramatic displays over a concentrated window of hours. Coronal hole activity tends to be more gradual and sustained, sometimes elevating geomagnetic indices over 2 to 3 consecutive days as Earth moves through the stream.

For travelers with several nights in the field, a coronal hole passage can be more consistently productive than waiting for a single CME event. The activity may not reach the same peak intensity, but it offers more opportunities across a longer window.

What Coronal Holes Mean for Photographers

Coronal hole-driven activity is generally more predictable in pace than CME activity, which has practical benefits in the field. Aurora during a high-speed stream event tends to be active but not frantically fast-moving, giving photographers more time to work with structure before it shifts. Shutter speeds of 5 to 12 seconds on a wide-angle lens are often effective, depending on how elevated activity becomes.

The multi-night nature of coronal hole passages also gives photographers something valuable: multiple attempts. If the first night is cloudy or activity peaks while you're still setting up, there's a reasonable probability that conditions remain elevated the following night. That redundancy reduces the pressure of any single session and allows for more deliberate compositional choices.

Tracking a coronal hole passage in real time is straightforward. Solar imagery from NASA's Solar Dynamics Observatory shows coronal holes as dark regions on the sun's disk. When a large coronal hole rotates into an Earth-facing position, the associated high-speed stream typically arrives 1 to 4 days later. Pairing that arrival with real-time solar wind speed data from NOAA gives a clear picture of when conditions are developing.

Return to the full Northern Lights Glossary to continue through the Solar Physics and Space Weather section.