The Picket Fence: The Vertical Green Rays That Accompany STEVE

When STEVE appears in the sky, it's often not alone. Below the narrow mauve ribbon, a series of evenly spaced vertical green rays sometimes develops — a structure that aurora photographers have named the picket fence for its resemblance to a row of fence posts. It's one of the more visually striking aurora-adjacent formations, and its relationship to STEVE adds an interesting layer to the physics of what's happening overhead.

What the Picket Fence Is

The picket fence is a series of discrete, vertical green rays that appear below a STEVE arc, typically extending downward from the STEVE ribbon toward the horizon. The rays are evenly or semi-regularly spaced, brighter and more defined than the diffuse green that sometimes underlies STEVE, and they appear and disappear on timescales of minutes.

Despite appearing alongside STEVE, the picket fence is driven by a different physical process. Where STEVE is produced by thermal emission from fast-moving hot plasma, the picket fence rays are associated with conventional auroral electron precipitation — the same mechanism that produces structured aurora borealis. The two structures co-occur because the same geomagnetic conditions that produce the fast-moving plasma stream driving STEVE also create conditions favorable for electron precipitation in the adjacent region.

What helped me picture the relationship: STEVE and the picket fence are like two performers sharing a stage but playing different instruments. They appear together because they're part of the same event, but the sound each one makes — the physical process producing each — is distinct.

What the Picket Fence Means for Aurora Travelers

The picket fence is relatively rare — it occurs during a subset of STEVE events rather than every time STEVE appears. Its appearance alongside STEVE during periods of elevated Kp and active substorm conditions makes high-latitude destinations during active solar periods the most likely place to encounter it. Like STEVE itself, it tends to appear equatorward of the main auroral oval — which means it can sometimes be seen from locations that don't regularly experience conventional aurora overhead.

For travelers specifically hoping to see the picket fence, the primary strategy is the same as for STEVE: be outside during active geomagnetic periods, watch to the south of the main aurora display for unusual structures, and photograph anything that doesn't match conventional aurora forms. Citizen science reporting through platforms like Aurorasaurus helps researchers track these events and refine understanding of when and where they occur. For more on the citizen science connection to STEVE and related phenomena, see our Citizen Science page.

What the Picket Fence Means for Photographers

The picket fence is a photogenic structure — its vertical, evenly spaced rays create a strong geometric element that contrasts with both the smooth ribbon of STEVE above and the broader aurora forms that may be present elsewhere in the sky. Capturing the picket fence alongside STEVE in a single frame requires a wide-angle lens with enough vertical field of view to include both the STEVE ribbon and the rays extending below it.

Because the picket fence is green — the standard oxygen emission color of conventional aurora — it can be somewhat difficult to identify visually against a background of diffuse green glow. In photographs, its discrete, evenly spaced vertical structure distinguishes it clearly from the more continuous luminosity of diffuse aurora or the horizontal bands of the main aurora display. Reviewing images in the field during a STEVE event is the most reliable way to identify whether picket fence structure is present.

Exposures in the 8–15 second range are generally appropriate for capturing both STEVE and the picket fence together, assuming the structures are not moving too rapidly. Shooting continuously during a STEVE event with an intervalometer gives the best coverage of a phenomenon whose appearance and disappearance can be difficult to time manually.

Return to the full Northern Lights Glossary to continue through the Aurora Visual Forms and Phenomena section.