[TangerineSDR] Lightning Stroke/TLE Analysis with VLF Receiver

[Bob Stricklin]

Johnathan,

The Global Meteor Network (GMN)

Has a few TLE researches involved. They have posted on this topic in the past as well.

See if this link will work for you without being listed in the Groups.io<http://Groups.io>:

https://globalmeteornetwork.groups.io/g/main/topic/91046167#6266

The GMN cameras are excellent for recording these and you can get the time data needed.

My VLF system with be sitting next to my cameras.

This is an image captured by Damir Šegon in France on On Fri, May 13, 2022 at 6:41 PM. If the image is used in a publication then we should get his permission.

[cid:B0E77621-B45C-42AC-991B-0B2DE80B9C6C at StricklinRanch.com]


the image has aircraft meteors and probably some bugs.

There has been additional discussion on this but it is a minor topic on GMN. The GMN cameras have good information on the exact location (especially if multiple cameras)  and brightness of the light produced as well.

Bob N5BRG




On Jan 27, 2023, at 7:41 AM, Jonathan via TangerineSDR <tangerinesdr at lists.tapr.org<mailto:tangerinesdr at lists.tapr.org>> wrote:

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.
[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"
[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:
[Inline image]

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