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.
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
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.
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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