A special summer midday population of echoes has been reported by high‐frequency (HF) radars at ranges less than 300 km. These echoes come from a narrow altitude region near 100 km. Their spectral width indicates that they decay at a rate controlled by ambipolar diffusion. We label the echo regions as “far‐aspect angle irregularity regions” by contrast with “high‐aspect angle irregularity regions” (HAIR) documented in the past. Both types are from structures that decay after an initial growth phase. We show that each type decays at a rate controlled by the altitude where it is produced. The large aspect angles and decaying process come from a monotonic aspect angle increase due to a weak altitude dependence of the “eigenfrequency.” HAIR echoes are produced by the decay of otherwise standard E ‐region irregularities. However, HAIR echoes are usually not far‐aspect angle irregularity regions because the latter are the result of a Gradient Drift Instability in the bottomside of the E ‐region where the vertical density gradients can sometimes be so strong that a less than 10 mV/m northward electric field is able to destabilize 10‐m structures in the afternoon sector. Owing to smaller dip angles, the Gradient Drift Instability requirement is smaller at midlatitudes although it still needs a combined electron drift and eastward neutral wind contribution of the order of 100 m/s. With northward electric fields of 2 to 3 mV/m, the minimum midlatitude eastward wind requirement is of the order of 50 m/s.

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关于HF雷达在100 km高度看到的远视角度不规则区域的起源

高频（HF）雷达报告了在不到300 km的范围内有一个特殊的夏季正午回声。这些回声来自接近100 km的狭窄海拔区域。它们的光谱宽度表明它们以双极性扩散控制的速率衰减。与过去记录的“高视角不规则区域”（HAIR）相比，我们将回波区域标记为“远视角不规则区域”。两种类型都来自在初始生长阶段后衰减的结构。我们表明，每种类型的衰变速率均受其产生的海拔高度控制。大的外观角度和衰减过程来自于单调的外观角度的增加，这是由于“特征频率”对海拔高度的依赖性较弱。HAIR回声是由标准E的衰减产生的区域违规行为。但是，HAIR回波通常不是远视角度不规则区域，因为后者是E区域底部的梯度漂移不稳定性的结果，在该区域中垂直密度梯度有时会非常强，以至于小于10 mV / m北向电场能够破坏下午部分10米的结构。由于倾角较小，尽管在中纬度仍需要结合电子漂移和向东中性风贡献约100 m / s的速度，但对中纬度的梯度漂移不稳定性要求较小。在北向电场为2到3 mV / m的情况下，中纬度向东的最小风速要求约为50 m / s。