Journal of Geophysical Research: Space Physics Observation of Ionospheric Alfvén Resonances at 1–30 Hz and Their Superposition With the Schumann Resonances
Journal of Geophysical Research: Space Physics Observation of Ionospheric Alfvén Resonances at 1–30 Hz and Their Superposition With the Schumann Resonances
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地球物理研究杂志：1-30 Hz 电离层阿尔文共振的空间物理观测及其与舒曼共振的叠加
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发表时间：
2018
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通讯作者：
andM. Musur
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作者：
C. D. Beggan;andM. Musur
自 2012 年 9 月以来，英国埃斯克代尔缪尔天文台对高频磁场（0.1–100 Hz）进行了长期测量。我们分析了五年的动态频谱图，以检查舒曼和电离层阿尔文共振 (IAR) 和 Pc1 脉动的发生和行为。以漫带形式观察到的共振是由地球-电离层空腔内的能量反射和电离层的非线性电导率梯度引起的。舒曼共振 (SR) 连续发生，但观察到 IAR 在数据集中约 50% 的白天中出现在当地夜间。通常，IAR 的频率为 1-8 Hz，但我们发现它们延伸至 30 Hz，并且在大约 9% 的时间里强烈叠加在前三个舒曼共振上。这些现象包括相长和相消干扰、几个小时内的非线性频率变化以及极性增强。此外，IAR 的幅度并不像人们经常提出的那样随频率而迅速下降。我们发现 IAR 及其与 SR 的叠加受到季节和地磁活动的强烈控制。我们将数据集中最不寻常的 IAR 行为的 6 天与电离探空仪测量的 f0F2（电离层电导率的代表）进行比较，但发现几乎没有相关性。我们建议，由于当前的理论模型无法解释这些观察结果，因此需要进一步的工作来理解它们是如何产生的。简单语言摘要 对地球磁场快速变化（每秒变化 1 到 50 次）的测量显示出非常微弱的重复模式。它们是由赤道附近雷暴中的雷击产生的磁场引起的。闪电的强度足以在全球范围内反复“回响”几秒钟，然后消失，类似于共鸣钟的声音。这些模式以每秒约 8、14 和 21 次的固定周期重复，称为舒曼共振。在夜间，测量中会出现其他模式，这些模式是由暂时困在 100 至 1,000 公里高度之间的高层大气（称为电离层）中的磁波引起的。这些模式被标记为电离层阿尔文共振，是地球磁场的一个相对未经研究的特征。我们研究了过去 5 年从英国埃斯克代尔缪尔天文台收集的磁场数据中的这些模式。我们检查了这些类型的模式发生的频率，并发现了与季节以及磁场总体活跃程度的联系。我们还发现了不寻常且目前无法解释的模式，包括舒曼共振和电离层阿尔文共振之间的干扰。
Long-term measurements of the high-frequency magnetic field (0.1–100 Hz) have been made at Eskdalemuir Observatory in the United Kingdom since September 2012. We analyze five years of dynamic spectrograms to examine the occurrence and behavior of the Schumann and ionospheric Alfvén resonances (IAR) and Pc1 pulsations. The resonances, observed as diffuse bands, arise from reflections of energy both within the Earth-ionosphere cavity and from the nonlinear conductivity gradient of the ionosphere. Schumann Resonances (SR) occur continuously but IAR are observed to arise at local nighttime in ∼50% of days in the data set. Typically, IAR are found at frequencies of 1–8 Hz, but we find them extending out to 30 Hz and strongly superimposing over the first three Schumann resonances around 9% of the time. These phenomena include constructive and destructive interference, nonlinear frequency changes over the span of several hours, and polarity enhancements. In addition, the magnitude of the IAR does not decline rapidly with frequency as often proposed. We find that the IAR and their superposition with SR are strongly controlled by season and geomagnetic activity. We compare 6 days with the most unusual IAR behavior in the data set to ionosonde measurements of f0F2, a proxy for ionospheric conductivity but find little correlation. We suggest that, as current theoretical modeling does not account for these observations, further work is needed to understand how they arise. Plain Language Summary Measurements of the very rapid changes of the Earth’s magnetic field (changes at a rate of between 1 and 50 times per second) show very weak repeating patterns. They are caused by magnetic fields from lightning strikes in thunderstorms near the equator. The lightning strikes are strong enough to “echo” around the globe repeatedly for a few seconds before fading, similar to the sound from a resonating bell. These patterns repeat at fixed periods of around 8, 14, and 21 times per second and are called the Schumann resonances. At night time, other patterns appear in the measurements caused by magnetic waves temporarily trapped in the upper atmosphere (called the ionosphere) between heights of 100 and 1,000 km. These patterns are labeled the ionospheric Alfvén resonances are a relatively unstudied feature of the Earth’s magnetic field. We looked at these patterns in magnetic field data collected from Eskdalemuir Observatory in the United Kingdom over the past 5 years. We checked how often these types of patterns occurred and found a link to the seasons and how active, in general, the magnetic field is. We also found unusual and currently unexplained patterns including interference between the Schumann and ionospheric Alfvén resonances.