Auroral kilometric radiation: Wave modes, harmonics, and source region electron density structures
Auroral kilometric radiation: Wave modes, harmonics, and source region electron density structures
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极光千米辐射：波模、谐波和源区电子密度结构
DOI：
10.1029/ja090ia03p02753
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发表时间：
1985
影响因子：
--
通讯作者：
R. Benson
中科院分区：
文献类型：
--
作者：
R. Benson
当源区等离子体与陀螺频率比fN/fH在0.1 ~ 1.3之间变化时，极光千米辐射（AKR）的特性发生了显著变化。其中最值得注意的是，随着fN/fH由小到大的变化，从右手极化异常(X)模式AKR优势到左手普通(O)模式AKR优势的变化。除了X和O模式的AKR外，还观察到Z （X模式的慢分支）和whistler (W)模式。在所有fN/fH值上都可以观察到Z模式，通常局限于fN和fH之间的频率区域，并且通常比O模式稍微弱一些。W模式被限制在小于1 /2的频率范围内，这表明它是从较低高度的源到达卫星的场对准导管信号的结果。谐波AKR波段，这似乎是自然的，而不是乐器的起源，似乎与O以及X模式有关，是常见的观察。二次谐波X模式似乎是由于传播信号，而高次谐波似乎被限制在低密度源区域。当fN/fH较大时，与O模相关的二次谐波主要出现在高于环境2fH值的频率处，而当fN/fH较小时，则出现在低于环境2fH的频率处。这些观测结果来自ISIS 1号卫星12次通过AKR源区域的约200个电离图。关于波模式识别的推论是基于将观测到的AKR频率与声诱发的等离子体共振和电离层反射迹线的波截止进行比较。从卫星高度到F层电离峰高度的电子密度等高线由每次通过的探测仪数据得到。几乎在所有这些通道上都进行了源接触，并确定了最小AKR源高度和相应的fN/fH值（对应于最大值）。推导出的（fN/fH）最大值总是小于0.4，在X型AKR的产生过程中通常小于0.2。另一方面，对于O型AKR， （fN/fH） max接近0.9。相应的AKR震源区最小高度延伸至2400 km。（fN/fH）最大值与先前发表的最大不稳定时间增长率的结果非常一致，该结果是由AKR多普勒移位回旋加速器机制获得的，作为fN/fH的函数。低密度区域，即Ne < 100 cm−3的AKR源区域沿卫星轨道（88°倾角）的纬度范围从几度到20°以上不等。在广泛的密度消耗中，即那些扩展了许多度的密度消耗中，环境fN/fH值没有明显的增强。这些结果受到上层探测器电离图间距对应的~ 100公里空间分辨率的限制，在更细的空间尺度（Ne为~ 10公里，ΔNe为1/10公里）上得到证实，使用了船上Langmuir探测器的现场数据，这些数据可用于12次中的10次。观察到的AKR源区密度腔内缺乏大密度增强提供了额外的信心，即观察到的强AKR是回旋加速器X模式辐射而不是等离子体频率O模式辐射。然而，观测到的Ne增强比支持源区反馈机制所需的要大。
A number of dramatic changes are observed in the characteristics of auroral kilometric radiation (AKR) as the source region plasma to gyro-frequency ratio fN/fH varies from 0.1 to 1.3. Most notable of these is a change from right hand polarized extraordinary (X) mode AKR dominance to left hand ordinary (O) mode AKR dominance as fN/fH varies from smaller to larger values. In addition to X and O mode AKR, Z (the slow branch of the X mode) and whistler (W) mode are also observed. The Z mode is observed over all fN/fH values, is often confined to the frequency region between fN and fH, and is typically slightly less intense than the O mode. The W mode is confined to frequencies less than fH/2, suggesting that it is the result of field-aligned ducted signals reaching the satellite from a source at lower altitudes. Harmonic AKR bands, which appear to be of natural rather than instrumental origin and appear to be associated with the O as well as the X mode, are commonly observed. The second harmonic X mode appears to be due to propagating signals, whereas the higher harmonics appear to be confined to low-density source regions. The second harmonic associated with the O mode is observed mainly at frequencies above the ambient 2fH value when fN/fH is large and below 2fH when fN/fH is small. The observations were obtained from some 200 ionograms from 12 passes of the ISIS 1 satellite through AKR source regions. Inferences concerning wave mode identification are based on comparisons of the observed AKR frequencies with sounder-induced plasma resonances and wave cutoffs of ionospheric reflection traces. Electron density contours from the satellite altitude down to the altitude of the F layer ionization peak were obtained from the sounder data on each pass. Source encounters were made on nearly all of these passes, and both the minimum AKR source altitudes and the corresponding fN/fH values (which correspond to maximum values) were determined. The deduced (fN/fH) max is always less than 0.4 and is typically less than 0.2 during the generation of X mode AKR. For O mode AKR, on the other hand, (fN/fH) max approaches 0.9. The corresponding AKR source region minimum altitudes extend down to 2400 km. The (fN/fH) max values are in excellent agreement with previously published results of the maximum instability temporal growth rates, obtained from the AKR Doppler-shifted cyclotron mechanism, as a function of fN/fH. The latitudinal extent of the low-density, i.e., Ne < 100 cm−3, AKR source regions ranged from a few degrees to more than 20° along the satellite orbit (88° inclination). Within the wide density depletions, i.e., those that extended over many degrees, there were no large enhancements of the ambient fN/fH value. These results, which were limited by the ∼100 km spatial resolution corresponding to the spacing between topside sounder ionograms, were substantiated on a much finer spatial scale (∼10 km for Ne or 1/10 km for ΔNe) using in situ data from the on-board Langmuir probe which were available for 10 of the 12 passes. The observed lack of large density enhancements within AKR source region density cavities provides additional confidence that the observed intense AKR is cyclotron X mode radiation rather than plasma frequency O mode radiation. The observed Ne enhancements, however, were larger than those required to support a feedback mechanism in the source region.