[TangerineSDR] Lightning Stroke/TLE Analysis with VLF Receiver

[Jonathan]

Sometimes lightning strokes produce what are called transient luminous
events, or TLEs. They are commonly known as sprites and jets. Not much is
known about them but they are continuously studied. Columns and regions of
atmosphere are ionized so much by the electric potentials caused by the
lightning stroke that the air glows red. It’s present for a split second
when the stroke occurs, so you need a quick eye and a camera to capture an
image. Many enthusiasts who capture these beautiful events often use timed
video and photography and use lightning stroke data to identify the
specific lightning stroke, it's channel current, and polarity. The
"channel" is the conductive channel of ionized air where lightning current
either flows upwards or downwards. VLF receivers detect the radio emissions
from lightning strokes, called sferics. The sferic signal characteristics
are fed into models that calculate stroke polarity and channel current.

An enthusiast and photographer, Paul, captured a double TLE, showing both a
sprite (the dendritic structure) and an ELVE (the upper dim region of red
glowing air above the sprite). ELVEs are often, but not always, indicative
of what is called a "continuing current", or a residual current flowing
through the channel with ELF frequency components.
[image: Inline image]

Using a VLF receiver connected to a soundcard and vlfrx-tools software, it
is possible to look at the sferic's impulse in a time domain plot. A
continuing current will often show up as an "ELF tail" right after the
initial impulse. This tail has ELF frequency components and is indicitive
as that "little wavy line" after the sferic's impulse. The plot below
show's the sferic from the stroke that created the TLE above. The ELF tail
is hard to see because it has some high frequency components on it, but it
is there. Running the following signal processing chain in vlfrx-tools
software produced the plot below:

vtread -T2022-12-14_02:09:20,+30s /data/vlf_96k | vtfilter -a th=5 |
vtresample -r32000 | vtcat -T2022-12-14_02:09:38.2,+0.1 | vtplot -t "+266kA
Stroke Nice Sprite/ELVE Combo"
[image: Inline image]

The farther the VLF receiver is from the lightning stroke, the longer the
ELF tail is. If this stroke occurred in Europe, it would be much longer,
but it was captured in the US, closer to the VLF receiver. Here is another
example of an ELVE captured by Paul:
[image: Inline image]

This is the time domain plot with the ELF tail easier to see because there
are much less high frequency components:
[image: Inline image]

In recording these millisecond events, it is essential to use precision
timing, which is why I use a GPS receiver to enable accurate and precision
timestamping. I used the signal processing chain above to pull the spectrum
data from the data store, filter out mains hum, resample to 32k to remove a
lot of high frequency components, then feed the specific spectrum chunk
into the plotting program.

With vlfrx-tools software and a network of VLF receivers, you can do
lightning location as well. Here is a lightning map from a network of VLF
receivers in India:
[image: Inline image]

The red dots indicate the location of a stroke and the circles indicate VLF
receiver locations.

Eventually, I would like to have a network of VLF receivers collecting
sferic data for lightning location.

Jonathan
KC3EEY
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