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LF: Re: Ghost bands created by VLF ionospheric heating

To: <[email protected]>, <[email protected]>, <[email protected]>
Subject: LF: Re: Ghost bands created by VLF ionospheric heating
From: "Markus Vester" <[email protected]>
Date: Sat, 25 May 2013 21:25:40 +0200
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Three days ago (May 22), the powerful French Navy station HWU shifted its operating frequency from 21.75 to 22.6 kHz. This is closer to 23.4 kHz from German DHO, and consequently the intermodulation bands created by joint ionospheric heating have moved and become more prominent due to the lower audio beat (0.8 kHz). shows crossmodulation impressed on HGA22, a fat red band centered on 136.33 kHz (ie. 135.43 + 23.4 - 22.6). There is also a weaker band visible on 136.73 kHz (135.43 + 23.4 - 22.1) from DHO with GQD, which appears much more rarely here. Observers in Western Europe might also see crossmodulation from other VLF combinations, eg. GQD - HWU = 0.5 kHz.
There are some more explanations in this thread:
Best 73,
Markus (DF6NM)
Sent: Sunday, February 10, 2013 2:07 AM
Subject: Ghost bands created by VLF ionospheric heating

On my LF grabber there is again a ~ 200 Hz wide red noise band centered on 137.08 kHz. It has been observed several times before but has been absent for a few months. Sometimes it is accompanied by a weaker "brother", centered on 137.18 kHz with purple (ie. northerly) colour. The structure is reminiscent of an MSK signal ("the ghost of CFH"). The bands were seen regularly but are stronger in nights with strong Luxemboug effect. 
After a lot of head scratching, a while ago I have found a really astonishing explanation for this effect: It is ionospheric cross modulation, with a pair of strong VLF transmitters acting together as a modulated heater.
DHO38 on 23.4 kHz and HWU (which has recently reappeared on 21.75 kHz) have a difference frequency of 1.65 kHz, with the sum of the two FSK shifts. Similar to an AM demodulator, the beat frequency between the two transmitters modulates the electron temperature in the D-layer, and the variable absorption creates crossmodulation on the carriers of HGA22 (135.43 + 1.65 kHz) and DCF39 (138.83 - 1.65 kHz). This constellation is rather unusual not only in that the heating is done by VLF signals. What is even more peculiar is that unlike a simple AM transmitter, the two non-colocated heaters must produce an inhomogeneous and fast moving heating pattern.
If you look at the Twente WebSDR now, you can currently see and hear prominent 1.65 kHz FSK sidebands around DCF39 and also DCF49 (128.93 kHz). However HGA seems less affected at Twente, presumably because the easterly reflection area is not so well illuminated for that path.
Best 73,
Markus (DF6NM)
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