Twilight
What is twilight?
Twilight is the light in the sky before sunrise and after sunset. The Sun is below the horizon, but its light still reaches the atmosphere above you.
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.