A coronal hole is a region on the sun’s surface where the magnetic field opens outward into space instead of looping back down. This open configuration allows solar plasma to stream away freely, creating what we know as the fast solar wind. Coronal holes appear as dark patches in ultraviolet and X-ray images of the sun, and they are one of the primary drivers of space weather that affects Earth.
Why Coronal Holes Look Dark
If you’ve ever seen a close-up image of the sun taken by a space telescope, you may have noticed large, irregular dark regions that look almost like holes burned into the surface. These are coronal holes. They appear dark not because material is missing, but because the plasma inside them is significantly cooler and thinner than the surrounding corona. A typical coronal hole has a temperature around 700,000 degrees Kelvin and a density roughly four times lower than the quiet corona around it, which sits closer to 1 million degrees Kelvin.
This difference makes coronal holes especially visible at specific ultraviolet wavelengths. NASA’s Solar Dynamics Observatory (SDO) captures them most clearly at 193 angstroms, a wavelength in the extreme ultraviolet range. Earlier observations relied on ground-based telescopes measuring a specific wavelength of helium light, but modern space-based instruments have made detection far more precise and continuous.
The Magnetic Field That Creates Them
The defining feature of a coronal hole is its magnetic field structure. In most of the corona, magnetic field lines arch out from the sun’s surface and curve back down, forming closed loops that trap hot plasma in place. Inside a coronal hole, the field lines extend straight out into interplanetary space without returning. This “open” magnetic configuration means there’s nothing holding the plasma in, so it escapes along those field lines and accelerates outward.
Coronal holes tend to rotate more rigidly than the rest of the sun’s surface, which is unusual because the sun rotates faster at its equator than at its poles. Scientists believe this rigid rotation happens because the magnetic field lines are constantly reconnecting, essentially reshaping and stabilizing the hole’s boundaries even as the surrounding plasma swirls at different speeds. The strength and geometry of the magnetic field inside a coronal hole directly influence how fast and dense the resulting solar wind will be.
Coronal Holes and the Fast Solar Wind
The solar wind comes in two varieties. The slow solar wind, which originates from the edges or interiors of dense magnetic structures called streamers, travels at roughly 400 kilometers per second. The fast solar wind, which pours out of coronal holes, moves at around 750 kilometers per second, nearly twice as fast.
This speed difference matters because when a stream of fast solar wind catches up to slower-moving wind ahead of it, the collision creates a compressed region of turbulent plasma called a corotating interaction region. These compressed zones are what ultimately interact with Earth’s magnetic field and can trigger geomagnetic disturbances. Because coronal holes can persist for weeks or even months, the same hole can send a fast wind stream toward Earth every 27 days or so as the sun completes one rotation, making the effects somewhat predictable.
How They Shift With the Solar Cycle
Coronal holes don’t stay in the same place. Their size and location follow the sun’s roughly 11-year activity cycle in a distinctive pattern. During solar minimum, when sunspot activity is low, the sun typically has one large coronal hole at each pole. These polar holes are enormous, stable features that dominate the solar wind environment for years.
As the cycle ramps up toward solar maximum, those polar holes shrink and eventually disappear. In their place, smaller coronal holes pop up at lower latitudes, sometimes right at the sun’s equator. Observations during the 1999 to 2002 solar maximum tracked this progression in detail: polar holes gave way to equatorial holes, which were then joined by holes at mid and high latitudes as the sun’s magnetic poles reversed polarity. This migration matters because equatorial coronal holes are far more likely to send fast solar wind directly at Earth, which orbits in the sun’s equatorial plane.
Effects on Earth
When fast solar wind from a coronal hole reaches Earth (typically two to four days after leaving the sun), it compresses and disturbs Earth’s magnetosphere. The resulting geomagnetic storms are generally moderate. NOAA rates geomagnetic storms on a scale from G1 (minor) to G5 (extreme). Coronal hole streams most commonly produce G1 or G2 storms, though particularly large or well-positioned holes can occasionally push activity to G3.
At the G1 level, effects include weak power grid fluctuations and minor disruptions to satellite operations. G2 storms can trigger voltage alarms in high-latitude power systems and, during long-duration events, may stress transformers. The more severe G4 and G5 ratings, which bring risks of widespread blackouts and transformer damage, are almost always caused by coronal mass ejections (CMEs) rather than coronal hole streams.
One area where coronal holes punch above their weight is aurora production. While CMEs tend to cause brief, intense auroral displays, the fast wind from coronal holes interacts with Earth’s magnetic field over much longer periods, typically three to four days at a stretch. This makes coronal holes a reliable source of sustained auroral activity, especially in the auroral zones at high latitudes. For aurora watchers, a large coronal hole facing Earth is one of the best predictable opportunities to see the northern or southern lights.
Coronal Holes vs. Coronal Mass Ejections
People sometimes confuse coronal holes with coronal mass ejections because both produce space weather effects, but they work very differently. A CME is a sudden eruption that hurls a massive blob of magnetized plasma into space, often triggered by a solar flare. It’s a one-time event that can hit Earth with tremendous force but passes relatively quickly. A coronal hole, by contrast, is a persistent structure that produces a continuous stream of fast solar wind for as long as it exists.
The practical difference is predictability. CMEs are hard to forecast more than a day or two in advance. Coronal holes, because they can last for multiple solar rotations, allow forecasters to anticipate recurring geomagnetic activity weeks ahead. If a coronal hole produced a geomagnetic storm this month, there’s a good chance it will do so again about 27 days later when the sun’s rotation brings it back into alignment with Earth.