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Re: VLF: Tweek mode resonances

To: "[email protected]" <[email protected]>
Subject: Re: VLF: Tweek mode resonances
From: Laurence KL7 L <[email protected]>
Date: Sun, 26 Jul 2015 23:24:22 -0800
In-reply-to: <94D7BD8F8ABB4AB798759379C79927F3@White>
References: <94D7BD8F8ABB4AB798759379C79927F3@White>
Reply-to: [email protected]
Sender: [email protected]

Really cool / im going to take a look down there and do battle with 60Hz harmonic monster that lurks

Laurence


On Jul 26, 2015, at 14:58, Markus Vester <[email protected]> wrote:

Friday night (July 24/25) was stunning. Coming home from a beer after local midnight, I noticed spectacular VLF spherics resonances on the DK7FC remote garden grabber (screenshot http://df6nm.bplaced.net/VLF/spherics/dk7fc_VLF_150725_0704.jpg from the morning). I decided to restart the dual-loop directional spectrogram, the one which sometimes feeds my VLF wideband grabber.
 
After a couple of minutes, the first spectrum appeared, and I was literally swept away: Fat resonance peaks at all multiples of 1.62 kHz up to the tenth harmonic, and up to 20 dB above the noise: http://df6nm.bplaced.net/VLF/spherics/vlfwide_150725_0700_cursor-2347.png. This was really surprising, as my suburban loops are usually very prone to local interference, and I had never seen or heard something like that before. The colour direction finder displayed the resonances in grey, which means "no direction", just as expected for a vertical-incidence circular polarized wave. 
 
http://df6nm.bplaced.net/VLF/spherics/tweek_resonances_dk7fc_df6nm_Blitzortung_150725_0700.png is a combined image showing two screenshots from Stefan in Heidelberg (top) and myself in Nuernberg (bottom), stacked above one another with identical timescales. The resonances in Heidelberg were strongest between 22 and 23 UT, became significantly weaker at 23:20, and died out about 0:10 UT. Resonances in Nuernberg came about an hour later, strongest at 23:40, and fadeout between 0:30 and 1:10 UT. Interestingly, the resonance frequency goes through a flat minimum at the time of maximum amplitude and sharpness, which is different for the two locations.
 
The spectrograms can be compared to a Blitzortung screenshot taken at 0:40 UT, inserted at the bottom left of the image. Blitzortung sorts displayed flashes in 20 minute bins, indicated by colours from red (oldest, 22:40 - 23 UT), through shades of orange, to white (last 20 minutes, i.e. 0:20 to 0:40 UT). For clarity, I repeated the Blitzortung colours in rectangles corresponding to the respective parts of the spectrogram timescale.
 
The position of the stormfront at the time of fadeout allows us to estimate the lateral range across which resonance amplification could be used to enhance communication. We find that tweek resonances were strong up to about 90 km away, and became very weak at about 180 km (indicated by blue circles around Heidelberg). Unfortunately this means that the distance between DK7FC and DF6NM (180 km) is probably already too large to benefit much from such a vertical resonance mode.
 
But on the other hand, it is known that tweeks have often been heard from great distances, up to 3000 or 6000 km over seawater. So why are the resonances restricted to a couple of 100 km? I think the answer for this is dispersion - the same effect that creates the characteristic chirp sound in distant tweeks. For a purely vertical wave, the time interval between successive hops is always 2h/c, about 0.6 ms with h = 90 km ionosphere height- This is the case if the source is close to the receiver. However at increased ranges, the path is initially at lower angle, and becomes steeper and steeper for later hops. Thus the time interval between successive pulses is initially smaller, and grows towards later (more vertical) hops. Source distance can be calculated from a tweek's knee in a spectrogram - far away sources have less rectangular, more rounded knees. In the resonance spectrum, this means that energy from for further away sources will be more spread out, and the maximum shifted to higher frequencies. This seems to be exactly what we have observed in the slow spectrograms: the sharpest resonances at the lowest frequencies occur when the storm passes just overhead.
 
At times, the resonances even appeared to split up. Look at the zoomed section around the fourth harmonic at the bottom: During the maximum in Nuernberg (23:45 UT), there are two distinct peaks, one at  6430 Hz and another at 6518 Hz. In the correponding orange Blitzortung crosses, we indeed find two separate sections of lightning activity, a smaller and nearer one passing south of Nuernberg, and another broader one passing north.
 
Let's take another look in time domain. http://df6nm.bplaced.net/VLF/spherics/dualloops_tweeks_scope.png are SpecLab scope shots from a couple of tweeks from recordings. For these, the frequency range had been restricted to 4 - 16 kHz to eliminate interference from 3 kHz railway noise and military MSK stations, and SpecLab's autonotch filter was engaged to suppress residual mains and railway harmonics. The two traces correspond to the two loops, red is the North-South loop (actually pointing 330° azimuth), and blue is East-West (60°). Tweek #3 shows a direct groundwave received only by the red channel, and a subsequent train of echoes which are received by both, with slightly increasing echo spacing. We find that undulations in the blue trace preceed those on the red one, showing that the circular polarized magnetic field vector rotates counterclockwise, i.e. from east to north.
 
There's also a couple of wav and ogg recordings in http://df6nm.bplaced.net/VLF/spherics/ - these nearby tweeks sound rather musical, like plucking on a short tight piece of guitar string.
 
All the best,
Markus (DF6NM)
 
PS: I started this thread on the rsgb LF reflector, but it may be appropriate to post to the VLF / Natural radio group  guess I'll just do both.
 
 
Sent: Saturday, July 18, 2015 6:50 AM
Subject: VLF: Tweek mode resonances

Waking up early this morning, I took a look at Stefan's garden grabber http://www.iup.uni-heidelberg.de/schaefer_vlf/DK7FC_remote_Grabber.html and was greeted by a fascinating display on his VLF panel.
 
The screenshot http://df6nm.bplaced.net/VLF/spherics/dk7fc_VLF_150718_1326.jpg shows a number of narrow tweek-mode resonances at multiples of 1.72 kHz. These are obviously spherics from nearby lightnings, bouncing multiple times vertically between the ionosphere and ground (much the same as clapping your hands between two parallel brick walls). The resonances are rather sharp, indicating a high Q-number (ie. around 100 bounces until decay). They are visible up to about 20 kHz, showing unusually small damping of vertical incidence reflections at these frequencies. There is a small variation of resonance frequency over time, reflecting the variable height of the ionospheric ceiling. The fundamental resonance at 1.7 kHz is probably not visible due to the frequency response of the loop and receiver.
 
A (somewhat late) screenshot from Blitzortung http://df6nm.bplaced.net/VLF/spherics/image_b_de_150718_0324.png shows the last red crosses between Wiesbaden and Stuttgart passing over Heidelberg at around 1:30.
 
The tweek resonances were received on the loop antenna in the garden but not on the E-field antenna of the (somewhat whitened out) city grabber http://df6nm.bplaced.net/VLF/spherics/dk7fc_wideband_150718_0330.jpg. This corroberates the notion of near vertical incidence and horizontal H-field polarisation. According to the literature, tweek tails are usually circular polarized as only one sense of rotation exhibits a high reflection coefficient. They are predominately excited by horizontal current components in intra-cloud lightnings.
 
Of course the resonances will also be there in quiet nights without spherics, so they could probably be employed to enhance fieldstrength (up to a factor of Q) for medium-range VLF communication experiments using magnetic transmit and receive antennas. When Stefan still had his earth dipole, we already discussed a 2 kHz tweek-mode experiment, which for various reasons hasn't taken place yet. Now it looks like one could even employ the fifth mode near 8.5 kHz...
 
All the best,
Markus (DF6NM)
 
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