Type‐A (weaker and wider Doppler spectra) and Type‐B (stronger and narrower Doppler spectra) echoes are inherent to the puzzling 150‐km echoing phenomenon. In this paper, we investigate the characteristics and possible origin of the Type‐A and Type‐B 150‐km echoes using high‐resolution 53‐MHz radar and Digisonde observations from Gadanki. Results show that the Type‐A echoes dominate the echoing phenomenon and they always precede the Type‐B echoes. Intriguingly, the Type‐B echoes are fundamentally not narrower than the Type‐A echoes as was originally thought. The primary reason that distinguishes the Type‐B echoes from the rest is the confinement of a few high PSD values at the center of the spectra, resulting in narrow spectral width. Spectral spreads of both types of echoes are found to increase with echo signal‐to‐noise ratio (SNR), and in fact, the spectral spreads of the Type‐B echoes are higher than those of the Type‐A echoes. Observations also show spectral evolution displaying the occurrence of Type‐A and Type‐B echoes in tune with the quasiperiodicity in the echo strength (SNR). We propose that both types of echoes primarily are of common origin, presumably linked with naturally enhanced incoherent scattering. Looking at the simultaneously observed short period variations in the lower F region electron density, we propose a gravity wave induced pathway as a plausible mechanism accounting for echo intensity modulation as well as spectral transition.

中文翻译：

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关于A型和B型150公里雷达回波

令人困惑的150公里回声现象固有有A型（多普勒频谱变弱和变宽）和B型（多普勒频谱变强和变窄）回波。在本文中，我们使用高分辨率的53 MHz雷达和Gadanki的Digisonde观测资料，研究了Type-A和Type-B 150 km回波的特征和可能的成因。结果表明，Type-A回波在回波现象中占主导地位，它们始终在Type-B回波之前。有趣的是，B型回波从根本上不比最初想象的要窄。区分B型回声与其余回声的主要原因是在光谱中心限制了一些较高的PSD值，从而导致了较窄的光谱宽度。发现两种回波的频谱扩展都随回波信噪比（SNR）的增加而增加，实际上，B型回波的频谱扩展高于A型回声的频谱扩展。观测结果还显示频谱演变，显示出A型和B型回波的发生与回波强度（SNR）的准周期性一致。我们建议这两种回波主要是共同起源的，大概与自然增强的非相干散射有关。观察下层同时观测到的短期变化在F区电子密度方面，我们提出了引力波诱导的途径，作为考虑回波强度调制以及光谱跃迁的合理机制。