Twilight
What is twilight?
Twilight is the time when the Sun is below the horizon but still lighting the sky. This map draws it as a soft band: from the terminator down to where the Sun is 18° below — the edge of astronomical night.
Three common edges
Civil twilight: Sun 0° to 6° down — streetlights, a clear horizon. Nautical: 6° to 12° — the brighter stars, a usable sea horizon. Astronomical: 12° to 18° — the sky is not yet fully dark for faint objects. Past 18° down, night.
This map uses the far edge, −18°, so the band you see is the whole of twilight, not just the civil slice.
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Why summer nights stay pale
At mid-latitudes the Sun dives steeply. Twilight is short. Near the polar circles the Sun skims the horizon, so it takes hours to fall 18°. In June, much of the north never reaches astronomical night at all. That is a geometry fact, not a weather one.