Ionospheric conductivity modulation in ULF pulsations
Ionospheric conductivity modulation in ULF pulsations
复制标题
ULF 脉动中的电导率调制
DOI：
10.1029/1998ja900180
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发表时间：
1999
影响因子：
--
通讯作者：
K. Glassmeier
中科院分区：
文献类型：
--
作者：
S. Buchert;R. Fujii;K. Glassmeier
利用非相干散射雷达和磁强计测量，我们报告了在下午地磁Pc5脉动期间，除了电场和磁场的高幅度变化外，粒子降水和电离层电导率还发生了2倍的调制。因此，这一事件似乎比之前研究的事件要复杂得多，在以前的研究中，关于电导率的信息大多是不可用的。我们的地面数据集为我们提供了几条关于磁层过程的线索。电导率变化的原因似乎是热电子周期性地被调制扩散到损耗锥中，而这又是由环电流不稳定性引起的。观测到的扰动的相位传播方向也与环形电流源的假设一致。根据电离层的电子密度，我们可以粗略地估计出赤道相空间的扩散速率，这似乎是相对较高的。此外，强烈的电场和Pointing通量的变化表明，在磁层中发生了与剪切Alfven模的强烈耦合。功率和波极化的纬向变化具有场线共振的特征。此外，电导率、电场和磁场的功率谱分析表明，在所有三个参数中都存在类似湍流的背景，这源于磁层，但受到电离层的影响。电导谱的功率率斜率与电场的功率率斜率相当，而由于小尺度电流结构的屏蔽，地面磁场随频率的下降幅度更大。电导率与向东的电场之间明显的反相关关系被解释为电离层极化效应，它将阿尔芬波从电离层向上传播。最后，我们发现，由于电导率的时变，只有霍尔电流的左手和右手分量之比与磁场的左手和右手分量之比非常接近，而电场具有明显不同的极化。
Using incoherent scatter radar and magnetometer measurements, we report that during terrestrial magnetic Pc5 pulsations in the afternoon sector, a modulation of particle precipitation and ionospheric conductivities by a factor of 2 occurs in addition to high-amplitude variations of electric and magnetic fields. The event thus seems to be considerably more complicated than previously studied ones where information about conductivities was mostly not available. Our ground-based data set gives us several clues about magnetospheric processes. The origin of the conductivity variations seems to be periodically modulated diffusion of hot electrons into the loss cone that is in turn caused by a ring current instability. The direction of the phase propagation of the observed disturbances is also consistent with the hypothesis of a ring current source. From the ionospheric electron densities we can roughly estimate the equatorial phase space diffusion rate which seems relatively high. In addition, strong electric field and Poynting flux variations suggest that intense coupling to shear Alfven modes happens in the magnetosphere. The latitudinal variation of power and wave polarization shows features of a field line resonance. Furthermore, power spectral analysis of conductivities, electric and magnetic fields, reveals that there is a turbulent-like background in all three parameters, which is of magnetospheric origin but modified by the ionosphere. The power law slope of the conductivity spectra is comparable to that of the electric field, while the ground magnetic field shows a steeper decrease with frequency because of the shielding of small-scale current structures. A clear anticorrelation between conductivities and the eastward electric field is interpreted as an ionospheric polarization effect, which transmits Alfven waves from the ionosphere upward. Finally, we show that due to the time-varying conductivities only the handedness (ratio of left- and right-handed components) of the Hall current is very close to that of the magnetic field, while the electric field has a significantly different polarization.