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ATPL Instruments & Nav Aids Question

Hi All,

Forgive me if this has been asked before, but I wasn't able to find anything relevant.

I've had the below question in a practice exam, for ATPL Instruments & Nav Aids (NZ). The answer given in the marking schedule is A.

Quote:

An aircraft flying westbound between Auckland and Sydney is midway across the Tasman Sea at dusk. The enroute navigation chart lists the following HF frequencies: 13261, 8867, 5643, 3467. Radio propagation is reported as normal.

Which of the frequencies would be most suitable for communication with Sydney ahead and Auckland behind?

(a) 8.867 MHz 5.643 MHz
(b) 5.643 MHz 8.867 MHz
(c) 5.643 KHz 3.467 KHz
(d) 8.867 KHz 5.643 KHz

I understand that the frequencies given should be MHz, eliminating (c) and (d), but how should you select from the other options? Is this something to do with the doppler effect? Should a higher frequency be selected for the station in front? Why not use 13261/3467 instead?

Any input is appreciated.

Use high frequencies during day. Low frequencies during night.
Use high frequencies to call far away places and low frequencies to call close radios.
This all is related with the ionosfere. The HF waves uses reflection (or refraction-don't know much about that) At night we only have one layer. With day light we have two layers.
So, to call sydney I would use the higher one(8967) as it must be daylight there and Auckland 5643 as it must be already night.

a) 8.867 MHz 5.643 MHz

Hope this helps.
cheers