Solar flares trigger an increase in plasma density in the ionosphere including the D region, and cause the absorption of radio waves, especially in high-frequency (HF) ranges, called short-wave fadeout (SWF). To evaluate the SWF duration and absorption statistically, we analyze long-term (36 years) ionosonde data observed by the National Institute of Information and Communications Technology (NICT). The minimum reflection frequency, f min, is used to detect SWFs from 15-min-resolution ionosonde observations at Kokubunji, Tokyo, from 1981 to 2016. Since f min varies with local time (LT) and season, we refer to d f min, which is defined as f min subtracted by its 27-day running median at the same LT. We find that the occurrence of SWFs detected by three criteria, (i) d f min ≥ 2.5 MHz, (ii) d f min ≥ 3.5 MHz, and (iii) blackout, during daytime associated with any flare(s) greater than the C1 class is maximized at local noon and decreases with increasing solar zenith angle. We confirm that the d f min and duration of SWFs increase with the solar flare class. We estimate the absorption intensity from observations, which is comparable to an empirical relationship obtained from sudden cosmic noise absorption. A generalized empirical relationship for absorption from long-distance circuits shows quantitatively different dependences on solar flare flux, solar zenith angle, and frequency caused by different signal passes compared with that obtained from cosmic noise absorption. From our analysis and the empirical relationships, we estimate the duration of extreme events with occurrence probabilities of once per 10, 100, and 1000 years to be 1.8–3.6, 4.0–6.8, and 7.4–11.9 h, respectively. The longest duration of SWFs of about 12 h is comparable to the solar flare duration derived from an empirical relationship between the solar flare duration and the solar active area for the largest solar active region observed so far.

中文翻译：

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用于极端空间天气事件估计的短波衰落统计分析

太阳耀斑引发电离层（包括 D 区）等离子体密度的增加，并导致无线电波被吸收，尤其是在高频 (HF) 范围内，称为短波衰减 (SWF)。为了从统计上评估 SWF 持续时间和吸收，我们分析了美国国家信息和通信技术研究所 (NICT) 观测到的长期（36 年）离子探空仪数据。最小反射频率 f min 用于从 1981 年至 2016 年东京 Kokubunji 的 15 分钟分辨率离子探空仪观测中检测 SWF。由于 f min 随当地时间 (LT) 和季节而变化，我们指的是 df min，其定义为 f min 减去其在相同 LT 下的 27 天运行中位数。我们发现通过三个标准检测到 SWF 的出现，(i) df min ≥ 2.5 MHz，(ii) df min ≥ 3.5 MHz，以及 (iii) 停电，在白天，与任何大于 C1 级的耀斑相关的耀斑在当地中午最大化并随着太阳天顶角的增加而减少。我们确认 SWF 的 df min 和持续时间随着太阳耀斑等级的增加而增加。我们从观测中估计吸收强度，这与从突然的宇宙噪声吸收中获得的经验关系相当。长距离电路吸收的广义经验关系表明，与从宇宙噪声吸收获得的结果相比，不同信号通过引起的对太阳耀斑通量、太阳天顶角和频率的依赖性在数量上有所不同。根据我们的分析和经验关系，我们估计每 10、100 和 1000 年发生一次的极端事件的持续时间为 1.8-3.6、4.0-6.8 和 7.4-11.9 小时，分别。SWFs 的最长持续时间约为 12 小时，与根据太阳耀斑持续时间与迄今为止观察到的最大太阳活动区域的太阳活动面积之间的经验关系得出的太阳耀斑持续时间相当。