heterosphere

The heterosphere is a region of Earth's atmosphere located above 100 km (about 60 miles). Unlike the lower homosphere, where gases are well-mixed, the heterosphere is characterized by the separation of atmospheric gases based on their molecular mass. Lighter gases like helium and hydrogen are concentrated in the highest layers, while heavier gases such as nitrogen decrease rapidly with altitude.1234

Above 1,000 km (about 600 miles), helium and hydrogen are the primary constituents. Nitrogen is found only in trace amounts above 500 km. This stratification is most pronounced during periods of low solar activity when temperatures in the heterosphere are cooler, causing heavier gases to settle more significantly. The transition zone between the homosphere and heterosphere, occurring around 100 km, is known as the turbopause.12