The ionosphere isn't anything like a perfect mirror.
RF at or below the MUF still penetrates, some fo it
refracts.
A friend just received her PHD in astrondomy and her
thesis was based on observations on the 2 HF
radio astronomy allocations.
Even with the MUF right above either of these 2 freqs,
signals stll penetrated the ionosphere.

I will ask Ms C how mcuh attenuation there was.
From conversations, it wasn't as much as I thought.
It seems that even at best, less then 10% of the RF
refracts in the ionosphere, the rest "punches" through.

Terry