Determining lightning polarity (positive or negative) by raw signal processing?

[ThunderStorm]

Hello, my name is Oana Liviu from Romania and I am currently operating the station number: 951 from Resita, Romania (system RED).

It is well known that if you use two perpendicular antennas, if you position them to N-S/W-E and you wire them correctly, you can get an estimate of the direction of the strike by processing the raw signal data (see attached image). This can be acheived by calculating the arctangent of the ratio between the signals coming from each antenna among some other calculations.

Let's say that one is able to determine a relatively correct direction of the signal.

My question is: if the calculated direction by the means posted above is somewhat the opposite of the real direction (the strike is located to the south-west, but the calculation yields a north-east direction), that means that that strike have a positive polarity? Considering the calculations have been correctly done?

I am considering some experiments based on this and I want to know if I'm heading in the right "direction".

Thanks

[RichoAnd]

If you know the right direction, the polarity tells you whether it is cloud-ground, or ground-cloud.

An E-field antenna tells you the polarity, but not the direction.

/Richo

[ThunderStorm]

Thanks for the reply!

So basically by processing the direction component of the signals, one can tell if the lightning originates from the ground, but only an E-field antenna could detect the positively-charged lightnings that occures between an anvil and the ground?

Best regards,

Liviu.

[pasense]

Richo,
I don't quite agree with you: cloud-ground lightning, CG for short, comes in 2 polarities: about 90% of the strokes carry negative charge to earth, they are -CG lightning, the remaining 10% carry positive charge to earth, they are +CG. So the polarity has nothing to do with the difference between lightning in or between the clouds (intra- or inter cloud ligthning) and CG lightning. Cloud lightning produces different signals from CG lighting, they are more complicated and usually weaker. All lightning detection systems detect almost exclusively CG lightning with high efficiency; cloud lightning is sometimes detected but with low efficiency. 
The best way to determine the polarity of CG lightning is to use the E field antenna. I use the H antennas  for triggering and the E field antenna only for the signal; the trigger treshold is set so high that E field noise only rarely triggers.

[RichoAnd]

When I write "cloud-ground, or ground-cloud." I'm talking about main discharge between cloud and ground.
...and YES...lightning physics is quite complicated - I still do not understand much :-)

/Richo

[pasense]

Yes, lightning physics is complicated and still an active field of research... I was surprised to learn that it is still not clear how lighting discharge starts at all.
For polarity detection the E field recording is needed, but many stations are not using the E field antenna, maybe because it generates to much false triggers, but it would be good if they could give it a try.

[kriu]

For example, LINET system uses loop antennas and can detect all types (and other data) of discharges: CG, IC, polarity and lightning current values.
Below antenna for LINET system.

[pasense]

Kriu,
you are right: they use only the loop antennas. The webpage of the company is: https://www.nowcast.de/.
The high accuracy of their detection (they claim 100 m to 75 m) allows them also to check for the height of the discharge, but I am not sure what their efficiency for intra- or inter cloud lightning really is - what do they use as reference??
Anyway, this appears to be the best current system for lightning detection.

[ThunderStorm]

Thanks for replies !

I will give a shot with e-field antenna, trying to mount it ASAP
On the other side, I think my proposed approach can detect the polarity, even though with a smaller detection ratio, because data for both of the channels must be sent to server. I can make even a comparision.
I will post some results here,

Best regards,

Liviu.

[Tobi]

Determining the polarity is easy in theory. What we need are a couple of stations where we now the parameters.

Here's that what you can do:

1. Select a stroke which is not too far and not too close (i.e. 100 - 300km) to your station. Too close one may cause some interference, too far might be a reflection.
a) The signal shape should be some sort of a single spike and not too weak. Don't use signals of GREEN stations.
b) The accuracy of the stroke should be good, let's say < 1km.
c) The signal should include an e-field channel and two horizontal H-field channels.
d) Check other stations signal of the stroke which have similar distances. They should be very similar shape.
d) Save the signal!

2. Search for the stroke in another archive like
a) Germany: https://kachelmannwetter.com/de/blitze
b) USA: https://weather.us/lightning
c) For other regions you can check the latest 60 minutes here https://weather.us/lightning/americas
--> Double check whether the selected stroke is really the one of the signal.

3. Try to match these parameters with your signals:
a) The strokes polarity should always match the polarity of the spike in the E-field channel (either always equal, or always the opposite). If you also consider the directions, you can check the H-field antennas as well (see first post).
b) The current should be propotional of the spikes amplitude multiplied with gain on equal distances from stroke to station.
c) The type (cloud to ground or intra cloud) might have some influence on the shape of the signal.

4. Post your results here.

After we have some results, we can implement algorithms to compute those parameters. Then signals of other stations can be used as well, as their antenna parameters can be calculated from the calibrated stations.

It is also possible to recompute past events again and again to improve all the algorithms. But we have to record them manually. For example, I have world wide signal data from 2017-08-18 15:00 UTC +6 hours which is around 60GB.

It is important not to change any antenna positions/directions or filter settings since you've recorded the signals.

Note: All this can take some long time! I hope that we can implement some of the things from above within the next months, but I can not guarantee it.