Z‐mode sounding within propagation “cavities” and other inner magnetospheric regions by the RPI instrument on the IMAGE satellite
Z‐mode sounding within propagation “cavities” and other inner magnetospheric regions by the RPI instrument on the IMAGE satellite
复制标题
IMAGE 卫星上的 RPI 仪器在传播“空腔”和其他内部磁层区域内进行 Z 模式探测
DOI：
--
复制
发表时间：
2003
期刊：
影响因子：
--
通讯作者：
D. Gallagher
中科院分区：
文献类型：
--
作者：
D. L. Carpenter;T. Bell;U. Inan;R. Benson;V. Sonwalkar;B. Reinisch;D. Gallagher
[1] 当IMAGE卫星上的射电等离子体成像仪（RPI）在等离子体层内部以及在两极上空的中低高度运行时，在RPI 3 kHz至3 MHz探测频率范围低端发射的脉冲能够以Z模式以及哨声模式传播。在等离子体层内的中等高度，经常观测到在IMAGE朝向地球一侧有转折点的离散Z模式回波，其形式与在顶部探测仪记录中发现的常规和倾斜Z模式回波类似。在极区，通常满足$f_{pe}/f_{ce}<1$，Z模式回波往往是弥散的，并表现出一些特性，比如在$f_{ce}$的当地值处强度降低，此时Z模式折射率面的拓扑结构随频率有显著变化。在极区或近极区高达数千千米的高度，Z模式探测经常提供关于$f_{pe}$当地值的信息，而依赖于上混合共振频率$f_{uh}$指示的被动记录通常无法做到这一点。在从极光区到等离子体层的过渡区域内以及在等离子体层自身内部，在大约2000千米及以上的高度，我们发现了Z模式俘获现象的证据，这种现象先前在与极区纬度检测到的自然波发射相关的研究中被提及。这种现象涉及从探测仪朝向地球一侧更密集区域的转折点以及从上方更稀薄区域返回的回波。在Z模式“腔”内，发现离散的Z模式回波沿着地磁场对齐的路径在上部和下部高度反射点之间来回传播。回波呈现出独特的形式，这取决于IMAGE是位于Z模式截止频率高度剖面的最小值之上还是之下。通过一个反演过程，基于仅约20到30 kHz宽的频带内的回波延迟信息，这种回波使得确定沿地磁场线在IMAGE位置上方数千千米处的电子密度剖面成为可能。这种密度剖面的形式总体上与从更高高度进行的常规RPI X模式探测所报告的结果一致，也与基于等离子体层场线密度扩散平衡模型的预测一致。
[1] When the Radio Plasma Imager (RPI) on the IMAGE satellite operates in the inner plasmasphere and at moderate to low altitudes over the poles, pulses emitted at the low end of the RPI 3 kHz to 3 MHz sounding frequency range can propagate in the Z mode as well as the whistler mode. At medium altitudes within the plasmasphere, discrete Z-mode echoes with turning points Earthward of IMAGE are often observed, analogous in form to the regular and oblique Z-mode echoes found on topside sounder records. In the polar regions, where fpe/fce < 1 usually obtains, the Z-mode echoes tend to be diffuse and to exhibit properties such as an intensity decrease at the local value of fce, where there is a significant change with frequency in the topology of the Z-mode refractive index surface. At altitudes up to a few thousand km in polar or near-polar regions, Z-mode soundings regularly provide information on the local value of fpe, whereas passive recordings that depend upon indications of the upper hybrid resonance frequency fuh usually fail to do so. Within the transition region from the auroral zone to the plasmasphere and within the plasmasphere itself, at altitudes from ≈2000 km upward, we find evidence of the Z-mode trapping phenomenon, noted previously in connection with natural wave emissions detected at polar latitudes. This phenomenon involves echoes returning both from turning points in denser regions Earthward of the sounder and from more tenuous regions above. Within Z-mode “cavities,” discrete Z-mode echoes are found to propagate back and forth along geomagnetic-field-aligned paths between upper and lower altitude reflection points. The echoes present unique forms, depending upon whether IMAGE is located above or below a minimum in the altitude profile of the Z-mode cutoff frequency. Through an inversion process, such echoes make possible determination of the electron density profile along geomagnetic field lines to distances of thousands of km above the location of IMAGE, all on the basis of echo delay information within frequency bands only ≈20 to 30 kHz wide. The forms of such density profiles are in general agreement with results reported from conventional RPI X-mode sounding from higher altitudes, as well as with predictions based upon a diffusive equilibrium model of density along plasmasphere field lines.