A comparison of overshoot modelling with observations of polar mesospheric summer echoes at radar frequencies of 56 and 224 MHz
A comparison of overshoot modelling with observations of polar mesospheric summer echoes at radar frequencies of 56 and 224 MHz
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DOI：
10.5194/angeo-33-737-2015
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发表时间：
2015-06
影响因子：
1.9
通讯作者：
O. Havnes;H. Pinedo;C. L. Hoz;A. Senior;T. Hartquist;M. Rietveld;M. Kosch
中科院分区：
文献类型：
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作者：
O. Havnes;H. Pinedo;C. L. Hoz;A. Senior;T. Hartquist;M. Rietveld;M. Kosch
抽象的。我们比较了雷达观测的极地中层夏季回波（PMSEs）调制的人工电子加热，在224 MHz（EISCAT VHF）和56 MHz（MORRO）的频率。我们集中观察了1天，持续约3.8小时。具有更宽波束的MORRO雷达始终观察到一个或多个PMSE层，而VHF雷达在69%的时间内观察到PMSE。从统计学上看，MORRO和VHF雷达后向散射对加热器循环（加热器打开48秒，加热器关闭168秒）的反应有明显的差异。当加热器打开时，MORRO通常会增加其后向散射水平，正如Scales和Chen（2008）所预测的那样，VHF雷达几乎总是看到“正常”的VHF过冲行为，最初的后向散射迅速减少。然而，在某些加热器循环中，我们确实看到VHF反向散射在其初始降低到高于加热器开启之前的几倍的水平之后有了实质性的恢复。当加热器被关闭时，几乎所有VHF情况下的反应都明显过冲，但MORRO情况下的反应较少。后向散射值随时间变化的单个曲线的比较显示，至少对于这一特定的一天，在~ 85 km高度以上的高层中，两个雷达都看到了加热器打开时后向散射的减少，在加热器打开时间内几乎没有恢复。这些变化很好地描述了目前的模型。另一方面，在81-82公里的低层的后向散射可以是完全不同的，与温和的或没有减少后向散射的加热器被打开，随后由一个强大的恢复两个雷达的水平几倍以上的未受干扰的PMSE。在两个非常不同的雷达频率下，这种同时的、几乎相同的行为并没有被目前的建模很好地描述。
Abstract. We have compared radar observations of polar mesospheric summer echoes (PMSEs) modulated by artificial electron heating, at frequencies of 224 MHz (EISCAT VHF) and 56 MHz (MORRO). We have concentrated on 1 day of observation, lasting ~ 3.8 h. The MORRO radar, with its much wider beam, observes one or more PMSE layers all the time while the VHF radar observes PMSEs in 69% of the time. Statistically there is a clear difference between how the MORRO and the VHF radar backscatter reacts to the heater cycling (48 s heater on and 168 s heater off). While MORRO often reacts by having its backscatter level increased when the heater is switched on, as predicted by Scales and Chen (2008), the VHF radar nearly always sees the "normal" VHF overshoot behaviour with an initial rapid reduction of backscatter. However, in some heater cycles we do see a substantial recovery of the VHF backscatter after its initial reduction to levels several times above that just before the heater was switched on. For the MORRO radar a recovery during the heater-on phase is much more common. The reaction when the heater was switched off was a clear overshoot for nearly all VHF cases but less so for MORRO. A comparison of individual curves for the backscatter values as a function of time shows, at least for this particular day, that in high layers above ~ 85 km height, both radars see a reduction of the backscatter as the heater is switched on, with little recovery during the heater-on time. These variations are well described by present models. On the other hand, the backscatter in low layers at 81–82 km can be quite different, with modest or no reduction in backscatter as the heater is switched on, followed by a strong recovery for both radars to levels several times above that of the undisturbed PMSEs. This simultaneous, nearly identical behaviour at the two very different radar frequencies is not well described by present modelling.