Big aurora displays tend to get attributed to dramatic solar eruptions, and often rightly so — but a large share of the more modest, moderately active nights that regularly light up the sky at higher latitudes come from something quieter and steadier: a coronal hole.
What a coronal hole actually is
The Sun’s corona is normally held in place by closed magnetic field loops that arc from one point on the surface to another, trapping hot plasma within them. A coronal hole is a region where those field lines instead open up and stretch out into interplanetary space rather than looping back down to the surface. Solar material can stream freely along those open field lines, escaping the Sun far more easily than it can from a closed-loop region, where it would otherwise be held in place.
The result is a high-speed solar wind stream — plasma flowing outward from the coronal hole at speeds often well above the solar wind’s typical background pace, sometimes exceeding 700-800 km/s compared with a quieter background of around 300-400 km/s. In visible light and in extreme ultraviolet imagery, coronal holes appear as darker patches on the Sun (hence “hole”) simply because there is less hot, dense plasma there to emit light — they aren’t holes in any physical sense, just cooler, less dense regions of open magnetic field, typically tens of degrees across and most commonly, though not exclusively, found near the Sun’s poles.
How a coronal hole stream reaches Earth
Unlike a CME, which is a discrete blob of ejected material, a coronal hole produces a continuous stream that rotates around with the Sun, spiralling outward in a curved path (the Parker spiral) shaped by the combination of the outflowing wind and the Sun’s own rotation — much like water from a rotating garden sprinkler traces a curved rather than straight path. If the hole is positioned so that its stream sweeps across Earth’s orbit, Earth spends some time — often a day or more — immersed in that faster solar wind.
As the fast stream catches up to the slower, ambient solar wind ahead of it (itself often left over from a quieter region of the Sun that rotated past days earlier), the two compress together at the boundary, creating a region of enhanced density and magnetic field generally called a stream interaction region, or, once it has recurred across more than one solar rotation, a corotating interaction region (CIR). That compressed boundary often arrives at Earth first, sometimes producing a brief density and temperature spike, followed by the sustained high-speed stream itself arriving behind it. This compressed boundary and the stream behind it are what actually disturb Earth’s magnetosphere and can push Kp up for an extended period — often a day or two, rather than the sharper few-hour spike typical of a CME impact.
A worked example
Suppose a mid-sized equatorial extension of a polar coronal hole rotates into an Earth-facing position, producing a stream that reaches roughly 650 km/s at its peak, compared with a background of about 350 km/s in the days before. As the stream’s leading edge compresses against the slower wind ahead of it, density and Bt both rise for several hours — this compression region is where Bz often becomes erratic and can turn briefly but meaningfully southward, sometimes producing a period of G1-level activity even before the bulk of the fast stream itself arrives. Once the compression region passes, the sustained 650 km/s stream itself follows for a day or more, typically producing lower but more consistent geomagnetic activity — often enough on its own to bring a solid display to flagship, oval-latitude destinations such as Abisko or Kiruna, even without a dramatic Bz spike, simply because the stream sustains a moderately disturbed magnetosphere for many hours running rather than one short burst.
Why they recur roughly every 27 days
The Sun rotates roughly once every 27 days as viewed from Earth (its true rotation period is close to 25 days, but Earth’s own orbital motion around the Sun stretches the apparent period as seen from here — this is called the synodic rotation period, to distinguish it from the shorter sidereal period measured against the fixed background of distant stars). Coronal holes are often long-lived structures, persisting for multiple solar rotations before they close up or reorganise, sometimes lasting many months at a time, particularly large polar holes during the declining phase of the solar cycle.
A coronal hole that faces Earth today and drives a period of elevated Kp may well still be there — largely unchanged in size and position — 27 days later, rotating back into an Earth-facing position and producing another, often similar, round of activity. This is why experienced aurora watchers sometimes track “27-day recurrence”: if a coronal hole produced a good few nights of moderate activity last month, it’s worth checking whether the same region is due to face Earth again. It’s not a guarantee — coronal holes evolve, shrink, and sometimes close entirely between rotations, and the stream’s exact speed and the compression region’s Bz behaviour can vary meaningfully from one rotation to the next — but it is one of the more genuinely predictable rhythms in space weather, in contrast to the much less predictable, episodic nature of CMEs.
Coronal holes across the solar cycle
Coronal holes exist throughout the roughly 11-year solar cycle, but their prominence changes. Near solar maximum, the Sun’s magnetic field is at its most complex and chaotic, sunspot activity is at its highest, and CME activity from active sunspot regions tends to dominate the geomagnetic picture, often overshadowing coronal-hole effects entirely. In the years following the peak, as the Sun’s activity declines and its magnetic field gradually reorganises toward the simpler configuration typical of solar minimum, coronal holes — especially large, stable ones near the poles that can extend down to lower solar latitudes — become a more prominent and often more persistent source of geomagnetic activity. See solar cycle 25/26 and what they mean for aurora chasers for how this rhythm shapes the years ahead, including why the declining phase of a cycle is sometimes underrated as an aurora-watching period.
What this means in practice for aurora watching
A coronal-hole-driven period of activity tends to feel different from a big CME storm: less dramatic, more sustained, and often producing a run of several moderately active nights in a row rather than one intense night followed by quiet. For flagship destinations well inside the auroral oval — places like Tromsø, Abisko, Whitehorse or Edmonton — this kind of steady, moderate activity is often enough for a solid display on its own, without needing a headline-making storm. For mid-latitude, storm-only cities, a coronal hole stream alone is rarely strong enough; those locations generally still need the sharper kick of a well-aimed, southward-oriented CME (see what is a CME) to bring the oval far enough south to matter. Either way, watching the real-time solar wind data — described in how to read a solar wind plot — is how you tell which kind of event you’re actually looking at as it develops: a gradual speed rise sustained over many hours points to a coronal hole stream, while a sharp, sudden jump across speed, density and field strength together more often marks a CME’s arrival.
Frequently asked questions
Are coronal-hole storms weaker than CME storms?
Usually, yes — coronal-hole-driven activity tends to produce moderate, sustained geomagnetic disturbance (often G1-G2) rather than the sharp, severe spikes a fast, well-aimed CME can cause, though there is real overlap and exceptions do happen.
Why do coronal holes recur roughly every 27 days?
That figure is the Sun's average rotation period as seen from Earth. A coronal hole that is Earth-facing today, if it survives long enough, rotates back into an Earth-facing position roughly 27 days later, giving its high-speed stream another chance to reach us.
Can a coronal hole be predicted in advance?
Reasonably well over the short term. Because they rotate with the Sun and tend to persist for weeks to months, forecasters can watch a coronal hole on one solar rotation and reasonably expect a similar, if evolving, high-speed stream on the next one — recurrent storms are one of the more forecastable parts of space weather.
Do coronal holes only appear during certain parts of the solar cycle?
They can appear throughout the cycle, but they are particularly prominent in the declining phase after solar maximum, when coronal holes tend to grow larger and more stable, making recurrent storms a notable feature of the years following the cycle's peak.
Why is the region where a fast and slow stream collide sometimes called something different (a "stream interaction region")?
Researchers distinguish a corotating interaction region (CIR) -- one that has persisted long enough to complete at least one full solar rotation, becoming a recurring feature -- from the more general term stream interaction region (SIR), which describes the same fast-slow collision physics whether or not it has recurred yet. In casual usage the terms are often used loosely, but a CIR specifically implies the 27-day repeat pattern.
Do coronal holes only occur at the Sun's poles?
No, though large, long-lived polar coronal holes are the most common source of the steady high-speed streams that matter most for recurrent storms. Coronal holes can also form at lower solar latitudes, sometimes as extensions reaching down from a polar hole, and these lower-latitude holes tend to have a more direct, better-aimed effect on Earth simply because they rotate through a position more squarely facing our planet.
How can I tell, from a live solar wind plot, that a coronal hole stream has arrived rather than a CME?
A coronal hole stream typically shows a more gradual rise in speed over many hours, often with density and temperature changes at the leading edge (the compressed interaction region) followed by a longer plateau of sustained high speed -- in contrast to a CME, which more often shows a sharper, more sudden jump in speed, density and field strength together, sometimes preceded by a distinct shock.