Not every disturbance heading toward Earth starts with a bang. Some of the most reliably recurring geomagnetic activity comes from something that looks, in solar imagery, like nothing at all — a dark, quiet patch on the Sun called a coronal hole. It's not an eruption. It's an opening, and what streams out of it can disturb Earth's magnetic field for days at a stretch.

A coronal hole is a region of the Sun's corona that appears dark in extreme ultraviolet (EUV) and soft X-ray images, because it's genuinely cooler and less dense than the plasma around it. That lower density exists because the magnetic field in a coronal hole is open and unipolar — the field lines stretch outward into space rather than looping back down to the surface, giving charged particles a clear path to escape rather than being trapped in a closed magnetic loop.

That open structure is precisely what makes coronal holes matter for space weather: it lets solar wind escape far more freely than from surrounding regions, producing a stream of unusually fast solar wind, typically 500 to 800 km/s, aimed in whatever direction the hole happens to be facing.

Coronal holes are most persistent and stable at the Sun's north and south poles, where large polar holes are a near-permanent feature. They can also develop in isolation away from the poles — either forming independently or splitting off as an extension of a polar hole that drifts to lower latitude — and these isolated holes are the ones most likely to end up Earth-facing. Coronal holes are generally more common and long-lived during the years around solar minimum, though isolated holes regularly appear during solar maximum too, simply competing for attention with the flares and CMEs that dominate the active phase.

A persistent coronal hole's fast wind runs into the slower, more typical solar wind ahead of it, and because the fast stream is catching up rather than starting fresh, the two don't simply pass through each other — they pile up into a compressed, turbulent boundary called a corotating interaction region (CIR). The CIR arrives first, characterized by rising particle density and a stronger interplanetary magnetic field, followed by the actual high-speed stream itself, marked by rising solar wind speed and temperature as density drops back off. This handoff — density and field strength first, then speed — is the signature forecasters look for to confirm a CIR is underway rather than some other disturbance.

Because the Sun completes a full rotation roughly every 27 days, a persistent coronal hole doesn't just affect Earth once — it rotates back into an Earth-facing, or "geoeffective," position on the same rough schedule, sometimes for several solar rotations in a row. This is the most predictable pattern in all of space weather: once a hole has proven geoeffective, forecasters can reasonably expect its next pass roughly four weeks out, well before it happens.

Coronal hole high-speed streams and their leading CIRs typically produce G1 to G2 (minor to moderate) geomagnetic storming, though rarer, stronger events are possible, especially when a CIR interacts with an unrelated CME arriving around the same time. The magnetic polarity of the stream (whether it's classified positive or negative) also shapes how it interacts with Earth's field, alongside the more familiar Bz orientation that determines the strength of any resulting storm.

A G1-G2 storm from a coronal hole stream can produce weak power grid voltage fluctuations, minor satellite drag and surface charging, and aurora visibility extending to around 60° geomagnetic latitude or, during stronger G2 conditions, further south still — into the northern-tier US states, the British Isles, and central Scandinavia. These are the same category of established effects covered in this wiki's geomagnetic storms entry, just typically at the milder end of the scale, sustained over a longer stretch of days rather than concentrated into a single sharp event.