VHF Radar Operation Starts in Thailand for Observation of Plasma Bubble Causing Radio Interference

Abstract

The National Institute of Information and Communications Technology (NICT, President: Hideyuki Tokuda, Ph.D.), in collaboration with King Mongkut's Institute of Technology Ladkrabang (KMITL, President: Prof. Dr. Suchatvee Suwansawat), starts operation of a radar installed at KMITL Chumphon campus in Thailand to observe plasma bubbles that cause radio propagation disturbances. Chumphon is close to the magnetic equator where plasma bubbles are expected to be generated, and is the best place to observe plasma bubbles from their initial generation stage. In the vicinity of the plasma bubble, radio propagations are disturbed, which may degrade satellite-based navigation and communications. For this reason, continuous observation and forecast of plasma bubbles have been strongly desired in recent years. Routine observation of plasma bubbles by this radar is expected to be a major advancement in expanding the use of precise satellite-based navigation in the field of agriculture, construction, etc.

Background

NICT has been conducting research and development related to monitoring and forecasting of space weather including ionospheric and solar activities for the purpose of providing the information on forecasts and warnings about radio propagation conditions. NICT has been delivering the "Space Weather Forecast" every day since 1988. In recent years, the need for space weather information has increased in various fields. In particular, in the aviation industry, the Global Space Weather Center of International Civil Aviation Organization (ICAO) has started providing space weather information services on November 7, 2019. NICT also provides information on communications, satellite-based navigation, and radiation exposure as a member of the center.

One of the most significant effects of space weather on our social life is the degradation of satellite-based navigation, such as GPS, due to ionospheric disturbances. One of the intense ionospheric disturbances is "plasma bubbles" that occur around the magnetic equator, which is the equator of the Earth's magnetic field. In the vicinity of the plasma bubble, the radio propagations are disturbed, which may reduce the accuracy of satellite-based navigation or disable them in the worst case.

Recently, advanced satellite-based navigation technology, such as unmanned auto-driving system in the field of agriculture and construction, has been available, and is already an indispensable infrastructure in our society. Since plasma bubbles cause degradation of satellite-based navigation not only at low latitudes near the magnetic equator, but also at mid-latitudes such as Japan, continuous observation and forecast of plasma bubbles have been strongly desired in recent years.

Achievements

Figure 1 Chumphon VHF radar system for monitoring plasma bubble

NICT and KMITL collaboratively have installed a radar system for observation of plasma bubbles in the KMITL Chumphon campus and start operation on January 17, 2020 (see Figure 1). Chumphon is close to the magnetic equator where plasma bubbles are expected to be generated, and is the best place to observe plasma bubbles immediately after their generation (see Figure 2).

The radar system consists of 18 antennas equidistantly installed in the east-west direction at spacing of about 5m, using radio waves in the VHF band (39.65 MHz). The radio waves transmitted from the radar are reflected by the plasma irregularity caused by plasma bubbles in the ionosphere. By observing the reflected radio waves, the location and velocity of the plasma bubbles can be monitored (see Figure 3).

Plasma bubbles generally travel eastward after their generation and sometimes develop to mid-latitudes, causing radio propagation disturbances in the southern part of Japan. The radar will enable routine observation of plasma bubbles, which will lead to a significant progress in expanding the use of high-accuracy satellite-based navigation in the fields of agriculture, construction, etc. in Japan and Southeast Asia.

Future Prospects

For future work, using GPS receiver networks in Japan and Thailand in addition to this radar system, we will investigate the effects of plasma bubbles on various satellite-based navigation methods and conduct research to reduce the effects. These results will be provided as one of the space weather forecast services.

Figure 3 Schematic illustration of monitoring plasma bubbles by VHF radar system

Chumphon VHF radar system for monitoring plasma bubble

Glossary

Plasma Bubble

A plasma bubble is a phenomenon in which the plasma density in the ionosphere is locally decreased and appears as a "bubble" in the ionospheric plasma. In general, plasma bubbles generate after sunset and travel eastward while stretching northward and southward along the Earth’s magnetic field lines. The plasma density changes greatly inside and around plasma bubbles, which affects radio propagation of satellite-based navigation and communications passing in the vicinity of plasma bubbles. This results in degradation of satellite-based navigation and communications.

Generation and propagation of plasma bubbles
Movie: https://www.youtube.com/watch?v=FDhZF9-ixv8

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Magnetic Equator

The north and south poles of the Earth's magnetic field is called the geomagnetic pole. It is off by about 10 degrees from the north and south poles of the earth's rotation. Therefore, the equator based on the magnetic pole (called the geomagnetic equator) deviates from the geographic equator. The geomagnetic equator is north of the geographic equator in Africa, Asia, and the Western Pacific, and south in the Eastern Pacific and near the Americas. The real Earth's magnetic field has a complex distribution rather than a simple one represented by a bar magnet. More realistically, it is necessary to consider a line connecting the points where the magnetic field is horizontal to the ground (called the magnetic equator).

Locations of the magnetic equator based on IGRF-12 model

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Space Weather

Space weather is natural phenomena that occur in the vicinity of the earth, including the ionosphere and magnetosphere, mainly due to solar activity. Space weather affects communication/broadcasting using HF radio and satellite, satellite-based navigation, operation of satellites and international space stations, etc.