Meridian Project Phase II Low-Latitude High-Frequency Radar Observations Yield Initial Results

The Equatorial Plasma Bubble (EPB) is a highly significant phenomenon in the low-latitude equatorial space environment, capable of causing severe ionospheric scintillation, which can lead to the fading or even complete disruption of space-to-Earth radio signals, thereby affecting the reliability of navigation and positioning systems. The airspace over the southern part of mainland China, the South China Sea, and the surrounding Indian Ocean and western Pacific Ocean are severely impacted by plasma bubbles. Historically, due to the difficulty of deploying ground-based observation equipment over the ocean, there has been a lack of effective observation of the full process of plasma bubble generation and evolution in these regions. To address this issue, the national major scientific and technological infrastructure project, the "Ground-based Space Environment Monitoring Network (Meridian Project Phase II)," has constructed the world's first low-latitude high-frequency radar. This radar was developed and built by the Institute of Geology and Geophysics, Chinese Academy of Sciences, and is deployed at the Dongfang Station in Hainan Province.

Figure 1: Antenna array of the low-latitude high-frequency radar and a typical detection field of view from the Meridian Project Phase II

Compared to traditional line-of-sight ionospheric observation radars, the low-latitude high-frequency radar of the Meridian Project Phase II uses radio waves that undergo multiple ground-ionosphere reflections and refractions. The complex and variable propagation paths are influenced by anomalies and irregularities in the low-latitude ionosphere. Achieving over-the-horizon detection of plasma bubbles presents significant technical challenges in both radar system design and the analysis and identification of plasma bubble targets. Since the project's initiation at the end of 2019, the research team has actively tackled these challenges by conducting radio ray tracing and over-the-horizon detection condition simulations in low-latitude regions, designing a bidirectional over-the-horizon detection antenna array, and developing long-distance non-uniform coding processing technology. The equipment construction was completed at the Dongfang Station in Hainan in 2023, and continuous observation experiments have been underway.

The low-latitude high-frequency radar of Meridian Project Phase II operates at frequencies between 8-22 MHz and utilizes a fully digital phased array system. It comprises two radar subsystems, eastward and westward, each containing 24 transceiver circuits and 24 sets of transceiver antennas. The radar offers flexible experimental parameter settings, allowing for adjustments in detection frequency, detection range, scanning field of view, and radar coding according to actual needs. In a typical detection mode, the low-latitude high-frequency radar has a detection range of 4500 km in both east and west directions and a scanning field of view angle of 48°, covering an area from the Indian sector in the west to the western Pacific over a range of 9000 km (Figure 1).

Figure 2: Plasma bubble phenomena detected by the low-latitude high-frequency radar of the Meridian Project Phase II (blue and red regions indicate plasma bubble irregularity scattering echoes).

To obtain observation results as soon as possible, the research team conducted detection experiments alongside the debugging process during the radar development. In April 2023, using the low-latitude high-frequency radar for bidirectional continuous detection, multiple plasma bubble echoes were detected at different distances (Figure 2). Plasma bubble echoes were detected at a distance of 3500 km (approximately at 80°E longitude) in the westward direction and at a distance of 2000 km (approximately at 125°E longitude) in the eastward direction.

As the world's first low-latitude high-frequency radar, to validate the effectiveness of its over-the-horizon detection of plasma bubbles, the research team compared the radar's detection results with monitoring data from the IONISE network and other monitoring setups previously established in Hainan Island and surrounding areas of Southeast Asia. Figure 3 shows that the plasma bubble structures detected by the low-latitude high-frequency radar are consistent with the monitoring results of the IONISE network. Multiple plasma bubble structures drift from west to east, being first detected by the westward radar of the low-latitude high-frequency radar, then by the IONISE network as they approach and pass over Hainan Island, and finally by the eastward radar of the low-latitude high-frequency radar as they move away from Hainan Island.

Figure 3: Validation of observation results from the low-latitude high-frequency radar of the Meridian Project Phase II and the IONISE network.

Traditional ground-based monitoring equipment leaves large observational gaps over the ocean. The over-the-horizon detection capability of the low-latitude high-frequency radar can cover areas that traditional ground-based monitoring equipment cannot observe, providing unique continuous observational data for studying ionospheric environmental changes over the Pacific and Indian Oceans. In practical applications, the long-distance detection of plasma bubbles and their drift velocity information by the low-latitude high-frequency radar can be used to predict the probability and timing of plasma bubbles affecting the low-latitude region of China. This information is crucial for forecasting and warning about ionospheric scintillation from the Indian Ocean to the Western Pacific.

Currently, the research team is leveraging the wide detection range and flexible parameter settings of the low-latitude high-frequency radar to design various detection modes and conduct observational experiments. Preliminary observation results have been obtained for ultra-long-distance plasma bubble detection across continents and for detecting lower atmospheric fluctuations and typhoons. Data analysis is actively ongoing.

The development and preliminary observation results of the low-latitude high-frequency radar of the Meridian Project Phase II have been published in the international professional academic journal of space physics, JGR: Space Physics (Lianhuan Hu, Guozhu Li*, Baiqi Ning, Wenjie Sun, Haiyong Xie, Xiukuan Zhao, Yi Li, Guofeng Dai. Development of Low Latitude Long Range Ionospheric Radar for Observing Plasma Bubble Irregularities and Preliminary Results [J]. Journal of Geophysical Research: Space Physics, 2024, 129(3): e2023JA032099. DOI: 10.1029/2023JA032099).

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