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晚上翻了翻arrl的handbook，其中p892的一段对电离层的描述证实js的推断应该是正确的，横轴表示的是频率，单位是mhz，这个频率应该指的是当前时间的电离层临界频率，临界频率和路径传输最高频率muf应该是两个概念，muf有时会数倍于当前电离层临界频率。
实际上图中显示的应该是好几层电离层的参数。
图表最下面两行d代表是距离，下面一行muf对应该距离的路径最高可用频率。因为不同距离的muf也是不同的，距离越远，muf就会高一些。
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谢谢ba6iv及各位。我在ba6iv的提示下，也查了查2008年的arrl handbook，在第二十章“propagation of rf signals”中，有如下文字。按此，图中的曲线应该表示电离层各层垂直反射的频率。左边是根据探测得出的电离层各项参数。右边的不同色块，应该表示反射强度或者自由电子密度之类的。
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iionospheric refraction
the refractive index of an ionospheric layer decreases as the density of free-moving electrons increases (this is opposite from the refractive index in the troposphere since the ionosphere is a dispersive medium). in the densest regions of the f layer,that density can reach a trillion electrons per cubic meter (1012 e/m3). even at this high level, radio waves are refracted gradually over a considerable vertical distance, usually amounting to tens of km. radio waves become useful for terrestrial propagation only when they are refracted enough to bring them back to earth. see fig 20.6.
although refraction is the primary mechanism of ionospheric propagation, it is usually more convenient to think of the process as a reflection. the virtual height of an ionospheric layer is the equivalent altitude of a reflection that would produce the same effect as the actual refraction. the virtual height of any ionospheric layer can be determined using an ionospheric sounder, or ionosonde, a sort of vertically oriented radar. the ionosonde sends pulses that sweep over a wide frequency range, generally from 2 mhz to 6 mhz or higher, straight up into the ionosphere. the frequencies of any echoes are recorded against time and then plotted as distance on an ionogram. fig 20.7 depicts a simple ionogram.
the highest frequency that returns echoes at vertical incidence is known as the vertical incidence or critical frequency. the critical frequency is almost totally a function of ion density. the higher the ionization at a particular altitude, the higher becomes the critical frequency. physicists are more apt to call this the plasma frequency, because technically gases in the ionosphere are in a plasma, or partially ionized state. f-layer critical requencies commonly range from about 1 mhz to as high as 15 mhz.
