Comparative Analysis of Four Neural Network Methods for TEC Modeling during Ionospheric Magnetic Storms
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摘要: 电离层总电子含量(TEC)是描述电离层的重要参数, 然而电离层经验模型对于磁暴期TEC的预测精度不够理想. 针对这个问题, 使用LSTM、BiLSTM、结合注意力机制的卷积神经网络-长短期记忆(CNN-LSTM-Attention)以及结合注意力机制的卷积神经网络–双向长短期记忆(CNN-BiLSTM-Attention)神经网络模型进行电离层TEC预报的研究. 根据Dst ≤ –30 nT的标准, 确定了2004-2022年发生磁暴的时间段. 在此基础上, 结合多种时空参数(UTS与UTC, SA与AA, CHS与SHS), 得到了四种适用于磁暴期TEC的预测模型. 利用2023年磁暴期的TEC对四种模型的精度和可靠性进行验证. 结果表明, CNN-BiLSTM-Attention模型的预测效果明显优于其他三种模型, 其绝对平均误差(MAE)为0.882~5.270 TECU, 均方根误差(RMSE)为1.175~6.983 TECU, 且预测结果与参考值之间存在较强的相关性, 决定系数(R2)均在0.7以上. 此外, 该模型拟合函数的斜率整体上最接近于1, 同样优于其他三种模型的拟合效果.Abstract: The Total Electron Content (TEC) of the ionosphere is an important parameter for describing the ionosphere activities, and much research has been done for the accurate methods for the ionospheric TEC prediction. However, the prediction accuracy of ionospheric empirical models for TEC during geomagnetic storms is still not ideal. To address this issue, this paper aims to assess the performance of ionospheric TEC predicting methods, which involve the LSTM, the BiLSTM, the convolutional neural network-long short-term memory combined with attention mechanism (CNN-LSTM-Attention), and the convolutional neural network-bidirectional long short-term memory combined with attention mechanism (CNN-BiLSTM-Attention). At first, the geomagnetic storm periods are identified by comparing with the threshold of Dst index (≤−30 nT), during the years from 2004 to 2022. Then, four neural network models for the ionospheric TEC prediction are formed, through the combinations of multiple spatiotemporal parameters, such as UTS, UTC, SA, AA, CHS, and SHS. Finally, the accuracy and reliability of the four neural network models are assessed using the reference TEC dataset collected during geomagnetic storms in 2023, and three statistical index, Mean Absolute Error (MAE), The Root Mean Square Error (RMSE), and coefficient of determination R2, are utilized. The results show that, the performance of the CNN-BiLSTM-Attention model is superior to the other three models, with MAE ranging from 0.882 to 5.270 TECU, RMSE between 1.175 and 6.983 TECU, and R2 values exceeding 0.7. In order to better describe the difference between the predicted values and the reference values, the scatter plots of two datasets are plotted for the fitting of linear regression equations. The slope of fitted function from CNN-BiLSTM-Attention model is very close to the ideal value 1, also indicating a better performance compared to the other models.
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表 1 Dst指数与地磁活动关系
Table 1. Relationship between Dst index and geomagnetic activity
Dst指数/nT 地磁情况 Dst > –30 地磁平静期 –50 < Dst ≤ –30 微小磁暴 –100 < Dst ≤ –50 中等磁暴 –200 < Dst ≤ –100 大磁暴 Dst ≤ –200 特大磁暴 表 2 低纬度测站模型预测TEC的精度统计
Table 2. Accuracy of model prediction TEC at low latitude stations
站点 评价指标 LSTM BiLSTM CNN-LSTM-Attention CNN-BiLSTM-Attention AS00 Q站 MAE/TECU 5.013 5.257 6.725 3.637 RMSE/TECU 6.856 7.014 8.756 5.013 R2 0.871 0.865 0.790 0.931 CAJ2 M站 MAE/TECU 4.126 4.304 3.666 3.303 RMSE/TECU 5.124 5.472 4.604 4.116 R2 0.888 0.890 0.917 0.931 FZA0 M站 MAE/TECU 5.809 6.156 5.931 5.270 RMSE/TECU 7.602 7.976 7.695 6.983 R2 0.920 0.912 0.918 0.932 表 3 中纬度测站模型预测TEC的精度统计
Table 3. Accuracy of model prediction TEC at middle latitude stations
站点 评价指标 LSTM BiLSTM CNN-LSTM-Attention CNN-BiLSTM-Attention AT138站 MAE/TECU 2.150 1.896 2.001 1.482 RMSE/TECU 2.672 2.399 2.528 2.132 R2 0.938 0.950 0.945 0.961 BC840站 MAE/TECU 2.385 2.856 2.533 1.494 RMSE/TECU 3.522 3.832 3.752 2.158 R2 0.897 0.878 0.883 0.961 RL052站 MAE/TECU 2.082 3.702 1.774 1.529 RMSE/TECU 2.778 5..089 2.284 2.177 R2 0.617 0.363 0.759 0.765 表 4 高纬度模型预测TEC的精度统计
Table 4. Accuracy of model prediction TEC at high latitude stations
站点 评价指标 LSTM BiLSTM CNN-LSTM-Attention CNN-BiLSTM-Attention GA762站 MAE/TECU 1.291 1.472 1.116 1.033 RMSE/TECU 1.765 2.069 1.489 1.341 R2 0.790 0.711 0.850 0.883 TR169站 MAE/TECU 1.288 1.233 1.254 0.882 RMSE/TECU 1.750 1.678 1.709 1.175 R2 0.868 0.878 0.874 0.940 -
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