F2Schicht

The F2-Schicht, or F2 layer, is the highest-density region of the ionosphere’s F layer, located at altitudes roughly between 250 and 400 kilometers above the Earth's surface. It is primarily formed by solar extreme ultraviolet and X-ray radiation ionizing atomic oxygen, creating a layer with the greatest electron density in the ionosphere. The peak electron density within this layer is described by the parameter hmF2, and the maximum plasma frequency, foF2, determines the highest radio frequency that can be reflected back to Earth from the layer.

The F2 layer is highly variable. During daytime, solar radiation increases ionization and electron density, raising foF2 and shifting hmF2 upward. At night, recombination reduces electron density, weakening the layer. Variations also follow the solar cycle: higher solar activity generally enhances ionization and raises foF2 values. Seasonal and geographic differences exist, with distinct patterns at equatorial, mid-latitude, and high-latitude regions, and the layer can exhibit irregularities during geomagnetic disturbances.

Because radio waves in the high-frequency band reflect from the F2 layer, it plays a crucial role in long-distance HF communications. Predictions of propagation conditions rely on measurements of foF2 and hmF2, typically obtained from ionosondes and related instruments. The F2 layer’s behavior is a common subject of space weather monitoring and ionospheric modeling, informing navigation, communication planning, and understanding of upper-atmosphere dynamics.