The vertical propagation of coseismic ionospheric disturbances (CIDs) associated with the foreshock of the Tohoku earthquake was examined using an HF Doppler sounding system. The sounding system used in this study received radio waves at four different frequencies (5.006, 6.055, 8.006, and 9.595 MHz), indicating that ionospheric disturbances can be observed at up to four altitudes. CIDs were observed at these four frequencies in association with the foreshock of the Tohoku earthquake. From the propagation speed of the coseismic disturbances, it is assumed that the ground motion caused by the Rayleigh wave propagating from the epicenter generates the acoustic mode wave propagating upward, causing the neutral atmospheric particles to move vertically. To examine the characteristics of the acoustic wave, we determined the ratio of the vertical speed of the neutral particles to that of the ground motion observed by a seismometer. The vertical profiles of the ratio are roughly explained in terms of the characteristics of the acoustic mode wave resulting from the ground motion. However, the absolute values of the ratio are smaller than the theoretical ratio estimated by the atmospheric mass density and the attenuation model considering the viscosity, thermal conductivity, and relaxation losses. This result implies that the CIDs in this event may be affected by the nonlinear behavior of the acoustic mode wave.

中文翻译：

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与使用高频多普勒探测观测到的东北地震前震相关的同震电离层扰动的垂直传播

使用高频多普勒探测系统检查了与东北地震前震相关的同震电离层扰动 (CID) 的垂直传播。本研究中使用的探测系统接收四种不同频率（5.006、6.055、8.006 和 9.595 MHz）的无线电波，表明可以在多达四个高度观察到电离层扰动。在与东北地震的前震相关的这四个频率上观察到 CID。从同震扰动的传播速度来看，假设由从震中传播的瑞利波引起的地面运动产生向上传播的声模波，导致中性大气粒子垂直运动。为了检查声波的特性，我们确定了中性粒子的垂直速度与地震仪观测到的地面运动的垂直速度之比。该比值的垂直分布是根据地震动产生的声波的特性来粗略解释的。然而，考虑到粘度、热导率和弛豫损失，该比率的绝对值小于由大气质量密度和衰减模型估计的理论比率。该结果意味着该事件中的 CID 可能会受到声模式波非线性行为的影响。该比值的绝对值小于由大气质量密度和考虑粘度、热导率和弛豫损失的衰减模型估计的理论比值。该结果意味着该事件中的 CID 可能会受到声模式波非线性行为的影响。该比值的绝对值小于由大气质量密度和考虑粘度、热导率和弛豫损失的衰减模型估计的理论比值。该结果意味着该事件中的 CID 可能会受到声模式波非线性行为的影响。