Search Results (7)

High-frequency (HF) ocean radars have become essential tools for monitoring surface currents, offering real-time, wide-area coverage with cost-effectiveness. This study compares the compact CODAR system (MABT, 13 MHz) and the square-array phased-array radar (KNTN, 8 MHz) deployed at Cape Maobitou, Taiwan. Radial velocity measurements were evaluated against data from the Global Drifter Program (GDP), and a quality control (QC) mechanism was applied to improve the data’s reliability. The results indicated that KNTN provides broader spatial coverage, whereas MABT demonstrates higher precision in radial velocity measurements. Baseline velocity comparisons between MABT and KNTN revealed a correlation coefficient of 0.77 and a root-mean-square deviation (RMSD) of 0.23 m/s, which are consistent with typical values reported in previous radar performance evaluations. Drifter-based velocity comparisons showed an initial correlation of 0.49, with an RMSD of 0.43 m/s. In more stable oceanic regions, the correlation improved to 0.81, with the RMSD decreasing to 0.24 m/s. To clarify, this study does not include multiple environmental scenarios but focuses on cases where both radar systems operated simultaneously and where surface drifter data were available within the overlapping area. Comparisons are thus limited by these spatiotemporal conditions. Radar data may still be affected by environmental or human factors, such as ionospheric variations, interference from radio frequency management issues, or inappropriate parameter settings, which could reduce the accuracy and consistency of the observations. International ocean observing programs have developed quality management procedures to enhance data reliability. In Taiwan, the Taiwan Ocean Research Institute (TORI) has established a data quality management mechanism based on international standards for data filtering, noise reduction, and outlier detection, improving the accuracy and stability of radar-derived velocity measurements.To eliminate the effects caused by different center frequencies between MABT and KNTN, this study used the same algorithms and parameter settings as much as possible in all steps, from Doppler spectra processing to radial velocity calculation, ensuring the comparability of the data. This study highlights the strengths and limitations of compact and phased-array HF radar systems based on co-observed cases under consistent operational conditions. Future research should explore multi-frequency radar integration to enhance spatial coverage and measurement precision, improving real-time coastal current monitoring and operational forecasting. Full article

In this work, local time variations of the response of the ionospheric F layer radial current (IRC) to southward and northward IMF turning events at low and high solar activity are investigated for the first time using Challenging Minisatellite Payload (CHAMP) observations from 2001 to 2010. The response strength of disturbed IRC to the southward and northward IMF turnings does not show any preference for low or high solar activity. At low and high solar activity, the IRC increases in the upward (downward) direction in the daytime (nighttime) within 1.5 h after a sudden southward IMF turning. Conversely, the IRC increases in the downward (upward) direction in the daytime (nighttime) within 1.5 h after a sudden northward IMF turning. The response of zonal wind is insignificant or opposite to that of the IRC. F region electron density may only contribute to the response of the IRC in certain local time sectors. This work indicates that the enhanced convection electric field induced by southward IMF turnings and the reduced convection electric field combined with the overshielding electric field during northward IMF turnings impact the prompt penetration electric field from high latitudes to low latitudes and cause local time differences in the responses of the IRC. Full article

An earthquake is a seismic event resulting from a sudden release of energy in the lithosphere, which produces waves that can propagate through the atmosphere into the ionosphere, causing ionospheric disturbances, and excites an additional electric field in the lower ionosphere. Two large-scale traveling ionospheric disturbances (LSTIDs) at daytime Turkey longitudes were found, with phase speeds of 534 and 305 m/s, respectively, after the second strong earthquake at 10:24 UT on 6 February 2023. During strong earthquakes, the equatorial ionospheric currents including the E-region equatorial electrojet (EEJ) and F-region ionospheric radial current (IRC) might be perturbed. At the Tatuoca station in Brazil, we observed a stronger-than-usual horizontal magnetic field associated with the EEJ, with a magnitude of ~100 nT. EEJ perturbations are mainly controlled by neutral winds, especially zonal winds. In the equatorial F-region, a wave perturbation of the IRC was caused by a balance of the electric field generated by the zonal winds at ~15° MLat, the F-region local winds driven by atmospheric resonance, and the additional polarization electric field. Our findings better the understanding of the complex interplay between seismic events and ionospheric current disturbances. Full article

High-frequency radars (HFRs) are important for remote sensing of the marine environment due to their ability to provide real-time, wide-coverage, and high-resolution measurements of the ocean surface current, wave height, and wind speed. However, due to the intricate multidimensional processing demands (e.g., time, Doppler, and space) for internal data and effective suppression of external noise, conducting quality control (QC) on radar-measured data is of great importance. In this paper, we first present a comprehensive quality evaluation model for both radial current and synthesized vector current obtained by direction-finding (DF) HFRs. In the proposed model, the quality factor (QF) is calculated for each current cell to evaluate its reliability. The QF for the radial current depends on the signal-to-noise ratio (SNR) and DF factor of the first-order Bragg peak region in the range–Doppler (RD) spectrum, and the QF for the synthesized vector current can be calculated using an error propagation model based on geometric dilution of precision (GDOP). A QC method is then proposed for processing HFR-derived surface current data via the following steps: (1) signal preprocessing is performed to minimize the effect of unwanted external signals such as radio frequency interference and ionospheric clutter; (2) radial currents with low QFs and outliers are removed; (3) the vector currents with low QFs are also removed before spatial smoothing and interpolation. The proposed QC method is validated using a one-month-long dataset collected by the Ocean State Monitoring and Analyzing Radar, model S (OSMAR-S). The improvement in the current quality is proven to be significant. Using the buoy data as ground truth, after applying QC, the correlation coefficients (CCs) of the radial current, synthesized current speed, and synthesized current direction are increased by 4.33~102.91%, 1.04~90.74%, and 1.20~62.67%, respectively, and the root mean square errors (RMSEs) are decreased by 2.51~49.65%, 7.86~27.22%, and 1.68~28.99%, respectively. The proposed QC method has now been incorporated into the operational software (RemoteSiteConsole v1.0.0.65) of OSMAR-S. Full article

In this work, the temporal variations in the ionospheric F region current (FRC) during the 2022 volcanic eruption in Tonga are reported for the first time. The FRC increased and underwent several polarity reversals following the eruption, as observed by the dual Swarm satellites. By combining neutral wind observations derived from the Ionospheric Connection Explorer (ICON) satellite, we attempt to reveal the potential physical mechanisms responsible for these variations. The ICON and Swarm satellites were located at almost the same distance from the epicenter of the volcanic wave, thus providing a good opportunity for analyzing coordinated observations. The F-layer neutral wind data reflect eastward and westward fluctuations that are basically consistent with the polarity reversals identified in the FRC. The E-layer neutral wind also exhibits polarity reversals that affect the equatorial ionosphere electron density anomaly. These results show that the atmospheric fluctuations caused by the analyzed volcanic eruption affected the wind pattern distribution in the E-F region, thus further affecting the spatial distribution of the FRC and electron density in the F layer. Full article

Solar flare effects (Sfe) are magnetic variations caused by solar flare events. They only show up in the illuminated hemisphere. Their detection is a difficult task because they do not have a definite pattern and, additionally, they must be separated from other magnetic perturbations. However, we attempted to automatize these detections by using two different strategies. The first strategy takes advantage of one of the Sfe characteristics, as they usually have a rapid rise, followed by a smooth decay, which typically produces a crochet-like shape in the magnetograms. Thus, we created several morphological models for each magnetic component. Then, we identified a definite Sfe time interval by setting the conditions for various parameters, such as the correlations of the measured data with the models, or the model similarities among the different components. In the second stage of this strategy, we observed clusters of time intervals. Each of these clusters were attributed to a timespan of event possibility. We found the statistical optimal value of the correlation parameters by using the ROC curve method and Youden index. The second strategy was based on some of the properties of Sfe ionospheric electric currents, such as their spherical symmetry around the vortex. Here, the algorithm calculated the derivative of the data in order to avoid contamination of the daily variation $S_q$, and, by means of trigonometric formulas, computed the magnetic radial component relative to the Sfe current vortex (the focus). It then created an Sfe index with this data. A prior assumption of the focus position in a preceding work is no longer needed since we made a wide patrol of the space area to find it. Through a progressive thresholding process, we found its statistical optimal value (0.4 nT min⁻¹) again by using the ROC curve method and Youden index. For both of the strategies, we have made a large calculation of Sfe detection (for the period of 2000–2020), which included 33 Sfe. Finally, we combined the results of both methods—which in fact are complementary—and obtained a unified list that gave a higher hit ratio than those that were obtained separately. This unified method gave promising results towards the possibility of Sfe automatic detection. Full article

Ionospheric clutter is one of the main problems for high-frequency surface wave radars (HFSWRs), as it severely interferes with sea surface state monitoring and target detection. Although a number of methods exist for ionospheric clutter suppression, most are suitable for radars with a large-sized array and are inefficient for small-aperture radars. In this study, we added an auxiliary crossed-loop antenna to the original compact radar antenna, and used an adaptive filter to suppress the ionospheric clutter. The experimental results of the HFSWRs data indicated that the suppression factor of the ionospheric clutter was up to 20 dB. Therefore, the Bragg peaks that were originally submerged by the ionospheric clutters could be recovered, and the gaps in the current maps can, to a large extent, be filled. For an oceanographic radar, the purpose of suppressing ionospheric clutter is to extract an accurate current speed; the radial current fields that were generated by our method showed an acceptable agreement with those generated by GlobCurrent data. This result supports the notion that the ionospheric suppression technique does not compromise the estimation of radial currents. The proposed method is particularly efficient for a compact HFSWRs, and can also be easily used in other types of antennas. Full article