Field line resonances and waveguide modes at low latitudes: 1. Observations
Field line resonances and waveguide modes at low latitudes: 1. Observations
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低纬度地区的场线共振和波导模式： 1. 观测
DOI：
10.1029/1999ja900268
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发表时间：
2000
影响因子：
--
通讯作者：
B. J. Fraser
中科院分区：
文献类型：
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作者：
F. Menk;C. Waters;B. J. Fraser
磁力线共振（FLR）是产生Pc 3 -4（107 -100 mHz）地磁脉动的重要机制。在卫星和地面数据中，有相当多的观测证据表明在中纬度地区存在FLRs。然而，低纬度地区是这样的研究，因此，低纬度FLR的许多方面没有得到很好的理解。一个临时的12站磁强计阵列横跨澳大利亚东部从L= 1.3-2.0被用来调查的变化Pc 3 -4功率与纬度，FLR的性质和低纬度的限制，以及低于本地谐振频率的频谱成分的属性。实例介绍了代表性的日子。在这个纬度范围内，功率谱非常相似，并且经常表现出许多峰，这些峰之间的间隔为1.3 -5 mHz。使用交叉相位技术，我们发现谐振频率随着纬度的降低而增加到L <$1.6，然后在低纬度处降低。这是由于低海拔电离层重离子的影响。共振的特征尺寸为L <$0.15，共振Q在L=2.0时为1.2 -1.4，在L=1.3时为1.3-1.4，归一化阻尼因子γ/ωR <$0.2 -0.4。FLR的低纬度探测极限取决于许多因素，但在详细检查的一天，它是L = 1.4。对于低于本地谐振频率的信号，振幅在0.3 dB/0.1 L处随纬度降低。站间相位延迟是不一致的飞行时间的径向传播的快模式波在赤道面，虽然一个峰值发生在该地区的阿尔芬速度峰值。我们的结论是，这些结果是一致的，无论是与调制的传入快模波或腔或波导模式的存在，驱动离散强迫振荡的低纬度场线的频率范围内，并耦合到本地FLR的频率匹配。
Field line resonances (FLRs) are an important mechanism for the generation of Pc3–4 (∼7–100 mHz) geomagnetic pulsations. There is considerable observational evidence for the existence of FLRs at middle latitudes, both in satellite and ground data. However, the low-latitude regions are less accessible for such studies, and consequently many aspects of low-latitude FLRs are not well understood. A temporary 12-station magnetometer array spanning eastern Australia from L= 1.3–2.0 was used to investigate the variation in Pc3–4 power with latitude, the nature and low-latitude limit of FLRs, and properties of spectral components below the local resonant frequency. Examples are presented for representative days. Power spectra are remarkably similar over this range of latitudes and often exhibit a multitude of peaks separated by ∼3–5 mHz. Using cross-phase techniques, we find that the resonant frequency increases with decreasing latitude to L∼1.6, then decreases at lower latitudes. This is due to the effect of ionospheric heavy ions at low altitudes. The characteristic size of the resonances is L∼0.15, the resonance Q is ∼2 at L=2.0 and 1.3–1.4 at L=1.3, and the normalized damping factor γ/ωR∼0.2–0.4. The low-latitude detection limit of FLRs depends on a number of factors, but on a day examined in detail it was L∼1.4. For signals below the local resonant frequency, amplitude decreased with latitude at ∼3 dB/0.1 L. Interstation phase delays are not consistent with the time of flight of radially propagating fast-mode waves in the equatorial plane, although a peak occurs in the region where the Alfven velocity peaks. We conclude that these results are consistent either with modulation of the incoming fast-mode waves or the existence of cavity or waveguide modes which drive discrete forced oscillations of low-latitude field lines across a range of frequencies, and which couple to the local FLR where the frequencies match.