During the peak of solar activity, the "ionosphere" with intensified disturbances is not simple

Have you ever witnessed the aurora? Do you know the "behind-the-scenes director" of this magical spectacle?

This "director" is a special region spanning 60–1000 km above the Earth’s surface, essential for daily communications, broadcasting, navigation, and positioning: the ionosphere. As the closest space layer to us, it profoundly impacts radio communications, satellite navigation and positioning, radar detection, and more—making it deeply intertwined with both technological activities and daily life. Take satellite signals, for example. When propagating through the ionosphere, it ceases to act as a transmission medium and instead becomes a pure "disruptor". For instance, GNSS equipment suffers reduced positioning accuracy due to ionospheric refraction errors, and RTK systems may only yield float solutions instead of fixed ones.

The core driver of intensified ionospheric disturbances is sunspot activity, which follows an 11.2-year cycle. 2025 marks the peak year of Solar Cycle 25. Over the next year or two, as residual solar activity persists, geomagnetic storms and auroras triggered by strong solar events will remain frequent. Now is the time to deepen our understanding of this increasingly active ionosphere.

Earth’s "Protective Umbrella" Between Earth and Space

Located 60–1000 km above the surface, this layer of charged particles acts as a "protective umbrella," ionized by solar ultraviolet radiation. In the thin upper atmosphere, neutral particles interact and mix. The upper atmosphere and ionosphere continuously change in response to solar flares, high-altitude winds, and dynamic electric fields. These variations generate turbulence within the ionosphere, causing ionospheric "scintillation". Beyond disrupting satellite orbits, scintillation also interferes with radio wave navigation and communication systems—particularly in low-latitude regions near the equator.

Ionospheric "Scintillation" Phenomenon

Generated by solar radiation acting on electrons in the ionosphere, the concentration of charged atmospheric particles (plasma) fluctuates with the sun throughout the day-night cycle. Due to solar UV ionization, combined with high-altitude winds and the geomagnetic field, ion density increases above and below the geomagnetic equator (which does not perfectly align with the geographic equator) during daytime. At night, this density decreases as ions recombine with free electrons, manifesting as the disturbances we commonly observe.

Global Ionospheric Irregularities

Particles flowing from the Sun to Earth are split by intense solar UV radiation into positively charged ions and negatively charged electrons, forming an ocean of charged particles: the ionosphere. As solar radiation causes the ionosphere to expand, its size, shape, and intensity change throughout the day. Similarly, hurricanes carrying electric charges generate currents and magnetic fields that induce ionospheric irregularities. Generally, the ionosphere’s total extent is larger during daytime than at night. Despite some patterns, its pronounced irregularities make monitoring challenging.

Vertical Layering and Horizontal Structure of the Ionosphere

• D-layer (60–90 km): Denser atmosphere with high collision frequency between electrons, neutral particles, and ions. Radio waves experience severe attenuation here.
• E-layer (90–160 km): Electron density and altitude vary with solar zenith angle and sunspot number. Peak density occurs near 110 km. Primary ionization sources are solar UV and soft X-rays.
• F-layer: Region of highest electron density, extending above 160 km.
  • *F1-layer (160–180 km)*: Prominent in summer. Primarily ionized by solar UV radiation at 30.4 nm, strongly controlled by the geomagnetic field.
  • *F2-layer (persistent)*: Peak electron density around 300 km. Ionized mainly by solar extreme UV radiation, heavily influenced by the geomagnetic field.

Horizontal Structure: Variations in solar zenith angle across longitude, latitude, and local time dictate the geographic distribution of solar radiation intensity. This shapes the ionosphere’s horizontal electron density pattern—higher by day and at low latitudes, lower by night and at high latitudes. The unique structure of the geomagnetic field near the equator (nearly horizontal) also creates the "equatorial anomaly."

Ionospheric Delay: Impacting Space Measurement Technologies

The time delay of electromagnetic waves passing through the atmosphere due to charged particles is called ionospheric delay. Refraction, diffraction, and scattering by charged particles alter wave propagation speed and direction, introducing errors. Delay intensity correlates strongly with ionospheric electron density, which varies with sunspot activity, geographic location, season, and time of day. While these variations follow certain patterns and cycles—providing empirical references—increasing irregularity during solar maxima necessitates large-scale, long-term observational data for calibration.

The ionosphere above our sky profoundly influences daily communications, broadcasting, navigation, and positioning. We need comprehensive understanding and research to harness its potential with technology—optimizing the ionosphere and refining "algorithms" for better utility.