As you know, when you climb up a high mountain, the atmospheric pressure and temperature decrease with altitude. However, the temperature does not monotonically decrease with altitude because ultraviolet (UV) radiation from the sun is absorbed in specific regions in the atmosphere. One is an ozone layer in the stratosphere, and the other is an ionosphere at the top of the atmosphere (80-1000km). In this region, the atmospheric particles are partially ionized, so that HF radio wave can be reflected to travel long distance. The ionosphere is a transition region from Earth’s atmosphere to the space, in other words, the entrance to the outer space (Fig. 1).
Fig.1: Atmospheric structure from ground to space
Plasma density in the ionosphere is determined by the product of neutral atmospheric density (pressure) and the UV radiation intensity. Because the atmospheric density decreases and the UV radiation increases with altitude, the plasma density has a peak at altitudes of 300-400 km where many satellites such as International Space Station are orbiting. Radio waves transmitted from satellites at much higher altitudes such as GPS satellites must propagate through the ionosphere before reaching the ground. When the ionosphere is under disturbed condition, it may degrade the amplitude and phase of the radio waves and cause severe error in GPS signals (Fig. 2). Therefore, it is very important to monitor and understand the ionospheric condition continuously.
Fig. 2: Ionospheric effect on radio propagation
Plasma density in the ionosphere is horizontally stratified under the quiet condition. The vertical structure and seasonal/local time variations are empirically known. By using the empirical models, ionospheric effects on radio propagation and GPS signal can be estimated to some extent.
However, if the ionospheric density suddenly increases or decreases, the empirical models cannot be applied to such phenomena, resulting in degradation of the radio signal. Such phenomenon is called an ionospheric storm. The ionospheric storm has a variety of scale sizes; a global-scale storm affects the whole Earth, and a small-scale storm affects only a specific region. It is important to understand various phenomena in the ionosphere for space utilization.
We are conducting observations and researches of the ionosphere for several decades as a part of space weather forecast. The ionospheric forecast is used by various communities such as airline companies and ham radio fans.
Although there are many phenomena causing a localized ionospheric storm, it has been very difficult to predict when and where such storm will occur. One of the most severe storms tends to occur in the magnetic equatorial region where the geomagnetic field line is parallel to the ground surface.
As the ionosphere has a peak plasma density at around 300-400km, a light fluid is supporting a heavy fluid just below the peak altitude. In the evening, plasma density in lower altitudes decreases by recombination, so that the vertical plasma density gradient becomes unstable (Rayleigh-Taylor instability). This phenomenon is called “plasma bubble” because lower density region grows from the bottomside through the peak altitude like bubble.
The density inside the bubble is so low that it severely affects radio propagation and degrades signal quality. The fully-grown plasma bubble extends in north-south direction along geomagnetic field lines and propagates eastward during nighttime, resulting in ionospheric storms in the Japanese meridian sector. We have installed multiple instruments in Southeast Asia under collaboration with research institutes in these countries (Fig. 3).
Fig. 3: Ionospheric observatories in Japan and Southeast Asia
It is important to understand the mechanism of plasma bubble generation and to forecast its occurrence from the space weather point of view. However, it has been difficult to forecast the occurrence of the plasma bubble only from observational data because the plasma bubble has a large day-to-day variability in the occurrence pattern, and the precursor of the plasma bubble before evening is still undetermined. Therefore, it is necessary to develop computer simulation models to study ionospheric physics as meteorologists forecast the weather by using such model.
Fig. 4 shows the growth of plasma bubble reproduced by a supercomputer at NICT. Large-scale initial perturbation at the bottomside of the ionosphere gradually develops into bubble structure, and penetrates into the topside with very turbulent internal structures.
The calculation is conducted in 3D spatial domain, so that north-south structure can also be captured clearly in the model. Integrating this model with the other global-scale numerical model and real-time observational results will lead us to make the ionospheric storm forecast possible in the near future.
Fig. 4: Modeling results of plasma bubble in the ionosphere.
Senior Researcher Yokoyama and parabolic antenna for solar observation satellites
Yoshihisa IrimajiriRemote Sensing Laboratory
Shoichiro KojimaRemote Sensing Laboratory
Tatsuhiro YokoyamaSpace Environment Laboratory
Miho FujiedaSpace-Time Standards Laboratory
Kensuke SasakiElectromagnetic Compatibility Laboratory
Makoto AokiRemote Sensing Laboratory
Seiji KawamuraRemote Sensing Laboratory
Aoi NakamizoSpace Environment Laboratory
Maya MizunoElectromagnetic Compatibility Laboratory
Koki WakunamiElectromagnetic Applications Laboratory