Short answer: Red auroras occur when charged solar particles excite oxygen atoms at very high altitudes, where the air is thin. The result is crimson light that often appears at the upper edges of the display and during strong solar activity. Details

• What causes color in the aurora: The light comes from excited atoms in Earth’s atmosphere. Oxygen and nitrogen emit different colors when struck by solar particles. The specific color depends on the altitude and the gas involved. Red light comes from excited oxygen atoms at high altitudes, green from oxygen at lower altitudes, and blue/purple from nitrogen at the lowest altitudes. This is why red is less common and typically seen during intense activity or at higher elevations.

• Altitude dependence: Green auroras predominantly appear around 100–150 km altitude, red auroras appear higher, roughly 150–400+ km, where oxygen is less dense, allowing red emission to dominate in the right conditions. The upper-edge glow can be visible even when the main body is green.

• Observability: Red auroras are often only visible with strong solar storms or from vantage points where the high-altitude emissions can be seen against the night sky. They may appear alongside green auroras or on the fringe of the display.

If you’d like, I can tailor this to your location and current solar conditions, or pull a few recent sources that explain red auroras in more depth.