Uncovering the global appearance of the ionospheric "saturation effect", the Taiwanese and Japanese team's research was published in an international journal

"The stronger the sun is, the more active the ionosphere is." This has always been many people's intuitive impression of space weather, but the latest research has found that this is not the case. Chair Professor Liu Zhengyan of the Department of Space at Chuo University, Professor Chen Yuying of the Institute of Statistics, and Dr. Huang Junyan of Kyoto University collaborated to analyze 25 years of global ionospheric observation data and confirmed for the first time that the ionosphere has a "saturation effect" on solar activity. That is, when solar activity increases to a certain level, the electron content of the ionosphere no longer increases proportionally, but will gradually level off. The research results were published in the international journal "Geoscience Letters".

The research team used more than 20 years of global ionospheric map data from 1999 to 2024 to analyze the total electron content above the Earth, which is the number of electrons that GPS signals encounter when passing through the ionosphere. The team combined empirical mode decomposition and two-stage regression methods to find out from long-term data when the ionosphere began to change from "increasing with solar activity" to "gradually saturated".

It was found that the saturation effect does not only appear in a certain region, but is a global ionospheric phenomenon, but its location and time are not uniform. Low latitude areas, especially near the magnetic equator, are more prone to saturation during the spring and autumnal equinoxes; while in the summer hemisphere, obvious saturation characteristics also appear in mid- and high-latitude areas. Research also points out that these phenomena are related to known ionospheric structures such as equatorial ionospheric anomalies, Weddell Sea anomalies, and Yakutsk anomalies.

The importance of this discovery is that it reminds scientists that the ionosphere is not a simple system controlled only by the intensity of solar radiation. In addition to the ionization caused by the sun's extreme ultraviolet rays, neutral winds in the high-altitude atmosphere, the ratio of oxygen and nitrogen molecules, and the movement of plasma along magnetic field lines may all affect whether the ionosphere enters a saturated state. That is, the sun is important, but not the only controlling factor.

The research team pointed out that understanding the saturation effect can help improve ionospheric models and space weather forecasts, especially during peak solar activity periods. If the model still assumes that "the stronger the sun is, the ionosphere will become stronger in proportion", it may overestimate or misjudge the actual changes in the ionosphere. Through this research, scientists can more accurately understand the global response of the ionosphere during periods of high solar activity, further improving the reliability of GPS positioning, satellite communications and aviation navigation.

This research also continues Taiwan's long-standing advantages in ionosphere and space weather research. From observing plasma density holes during magnetic storms in the past to establishing long-term statistical images of the global saturation effect this time, the research team gradually uncovered how the Earth's high-altitude plasma environment is affected by solar activity and laid the foundation for future space weather monitoring and early warning technology.