How far south is the aurora tonight?

On this grid the 10% edge over central North America (100W to 85W) is absent. Over Europe (0 to 30E) it is absent.

The dashed line on the map sits 9.0 degrees of latitude south of the solid edge. That offset is SWPC's 1000 km divided by 111.2 km per degree. It marks how far south a bright display could appear on the northern horizon under ideal conditions, and it is labeled a best-case horizon because it is one.

The OVATION grid has 360 columns of longitude. For each column, Glow walks north from the equator and stops at the first cell whose value is at or above the threshold you pick. That latitude is the edge for that column. Joining all 360 gives the solid line on the map.

A column with no cell at the threshold gets no point, and the line breaks there instead of guessing. At 50% the line may break into short pieces.

The sector table reduces the line to five regions: North America west (125W to 110W), central (100W to 85W) and east (80W to 65W), Europe (0 to 30E) and Siberia (90E to 150E). Each sector reports the southernmost edge found in its columns.

The threshold is the minimum OVATION value you count as aurora. At 10% the edge marks the outer fringe the model sees. At 30% and 50% it traces the brighter core of the oval, which sits farther north.

Comparing the three is more useful than any one of them. When the 10% and 50% lines sit close together, the oval is narrow. When the 10% line runs far south of the 50% line, the model sees a wide band of weaker aurora.

SWPC states on its forecast page and its viewing tips that bright aurora can be seen up to about 1000 km (600 miles) away with a clear view to the north. Glow converts that to 9.0 degrees of latitude and draws it as the dotted line.

Treat the dotted line as the far limit, not the expected one. It assumes a bright display, a high vantage point, an open northern horizon and a dark, clear sky. On many nights at least one of those fails.