Geomagnetic field line resonances at low latitudes: Pulsation event study of 16 August 1993
Geomagnetic field line resonances at low latitudes: Pulsation event study of 16 August 1993
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低纬度地磁场线共振：1993 年 8 月 16 日的脉动事件研究
DOI：
10.1029/2001ja900123
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发表时间：
2002
影响因子：
--
通讯作者：
B. Zolesi
中科院分区：
文献类型：
--
作者：
M. Vellante;M. Lauretis;M. Förster;S. Lepidi;B. Zieger;U. Villante;V. Pilipenko;B. Zolesi
[1]本文分析了在意大利中部（L = 1.55）的梯度装置和在高纬度站（匈牙利瑙吉岑克，L = 1.88）记录的脉动事件。这一事件发生在1993年8月16日，当时正值一场地磁暴的主要阶段。通过使用不同的频谱技术，包括标准方法，如站间振幅比、相位差和单站H/D振幅比，以及更先进的振幅-相位方法，在梯度装置处识别了场线共振特征。通过不同的技术确定的本地共振频率fr的能力进行比较，特别是，我们发现，H/D振幅比未能提供在某些情况下的估计fr。一个明确的清晨减少fr都在L = 1.55和L = 1.88已被发现，确认以前的实验和理论结果。在L = 1.55的估计fr值显着高于通常在这些纬度报告，这表明在此事件的电离层等离子体系统的特殊条件。使用标准经验中性气体模式MSIS-86和经验中性风模式HWM 90数值计算了两个纬度的环形模式超低频脉动本征频率的当地时间变化。进一步的数值模拟运行涉及适合暴风雨时期的特定受扰中性气体成分和改进的经向风，估计为观察到的hmF 2值的最佳近似值。计算出的本征频率与实验观察到的值在L = 1.88，但小于在L = 1.55的清晨观察到的值。E × B垂直漂移的影响可能是造成这种差异的原因。在L = 1.55处发现了迄今为止从未报道过的二次谐波信号，我们估计了谐波比f_2/f_1 = 1.7-1.8，与理论模型一致。在梯度安装的高频分辨率分析证实了先前的建议，存在一个离散的频谱驱动的一些腔/波导模式的场线谐振。
[1] The analysis of a pulsation event recorded at a gradient installation in central Italy (L = 1.55) and at a higher latitude station (Nagycenk, Hungary, L = 1.88) is presented. The event occurred on 16 August 1993 during the main phase of a geomagnetic storm. Field line resonance signatures have been identified at the gradient installation by using different spectral techniques, comprising standard methods, such as interstation amplitude ratio, phase difference, and single station H/D amplitude ratio, and more advanced amplitude-phase methods. The capacity of determining the local resonant frequency fr by the different techniques is compared; in particular, we find that the H/D amplitude ratio fails to provide in some cases an estimate of fr. A clear early morning decrease of fr both at L = 1.55 and at L = 1.88 has been found, confirming previous experimental and theoretical results. The estimated fr values at L = 1.55 are significantly higher than those usually reported at these latitudes, suggesting peculiar conditions of the ionosphere-plasmasphere system during this event. The local time variation of the toroidal mode ULF pulsation eigenfrequencies has been numerically calculated at both latitudes using the standard empirical neutral gas model MSIS–86 and the empirical neutral wind model HWM90. Further numerical simulation runs involved a particular disturbed neutral gas composition which is appropriate for stormy periods and an improved meridional wind estimated as best approximation to the observed hmF2 values. The calculated eigenfrequencies agree with the experimentally observed values at L = 1.88 but are smaller than the values observed in the early morning hours at L = 1.55. The effect of the E × B vertical drift is suggested to be the cause of the discrepancy. Possible signatures of the 2nd harmonic at L = 1.55, never reported so far at such low latitudes, have been found. We estimated a harmonic ratio f2/f1 ∼ 1.7–1.8 in agreement with theoretical models. A high-frequency resolution analysis at the gradient installation confirms previous suggestions of the existence of a discrete spectrum of field line resonances driven by some cavity/waveguide modes.