VHF-UHF Propagation by Lightning Flash

Ian Roberts ZS6BTE 21 December 2015

By using the ionisation following lightning flash, it is possible for communications through VHF into the microwave frequencies to take place over similar distances available from Sporadic E propagation or meteor scatter (700-800 km or more). This is possible due to the height attained by lightning discharges and their associated ionisation effects, red sprites and blue jets.

Lightning scattering

A search on-line for information produced information either downright incorrect, or at the very least, highly discouraging:

"Lightning scattering has sometimes been observed on VHF and UHF over distances of about 500 km. The hot lightning channel scatters radio-waves for a fraction of a second. The RF noise burst from the lightning makes the initial part of the open channel unusable and the ionization disappears quickly because of recombination at low altitude and high atmospheric pressure. Although the hot lightning channel is briefly observable with microwave radar, no practical use for this mode has been found in communications." https://en.wikipedia.org/wiki/Radio_propagation#Lightning_scattering

This statement is in direct conflict with my observations made over several years using UHF TV transmitters as beacons received at distances exceeding 550 km.

From another source providing some technical information:

"A typical cloud to ground lightning flash culminates in the formation of an electrically conducting plasma channel through the air in excess of 5 kilometres (3.1 mi) tall, from within the cloud to the ground's surface. The actual discharge is the final stage of a very complex process. At its peak, a typical thunderstorm produces three or more strikes to the Earth per minute. Lightning primarily occurs when warm air is mixed with colder air masses, resulting in atmospheric disturbances necessary for polarizing the atmosphere."

"In general, cloud-to-ground (CG) lightning flashes account for only 25% of all total lightning flashes worldwide."

Fig 1: Global map of lightning activity

Fig 2: Zoomed view - parts of South Africa are very lightning active

https://en.wikipedia.org/wiki/Lightning

It follows that 75% of lightning discharges are of the cloud-to-cloud (CC) or in-cloud (IC) variety.

The lightning flash forms a highly electrically conductive plasma channel. The core temperature of the plasma during the flash may exceed 50,000 K, causing it to brilliantly radiate with a blue-white color. Once the electric current stops flowing, the channel cools and dissipates over tens or hundreds of milliseconds, often disappearing as fragmented patches of glowing gas. The nearly instantaneous heating during the flash causes the air to explosively expand, producing a powerful shock wave heard as thunder. This travels at about 1.6km per 5 seconds. The average duration of a stroke of lightning is about 30 microseconds. An average thunderstorm is up to 16 km wide and travels at 40 km/h.

High-energy radiation

Lightning produces both X-ray and gamma ray radiation - the mechanism is not fully understood. This undoubtedly contributes to the development of the highly ionised plasma cone around and in the atmosphere above the discharge path, which is so effective in reflecting radio waves.

Red Sprites

Sprites are massive but weak luminous flashes that appear directly above an active thunderstorm. They occur at the same time as CG or IC lightning discharges. Sprites can extend to altitudes of about 95 km and are most often red. The sprites are rarely seen singly. They usually occur in clusters of two or more. The colour indicates ionisation at the location of the sprite.

Fig 3: Red sprites

http://sprite.gi.alaska.edu/html/sprites.htm

Blue Jets

Blue jets appear above thunderstorms. They are narrow cones which are ejected from the electrically active core regions of a thunderstorm and also indicate ionisation at the location. Blue jets are typically emitted at speeds of approximately 100 km/s (Mach 300). They then fan out and disappear at altitudes of 40-50 km.

Fig 4: A Blue Jet

http://umbra.nascom.nasa.gov/spd/images/Blue_Jet_FULL.GIF

Lightning Flash as a Propagation Mechanism

Since lightning flash ionisation around a thunderhead may commonly reach altitudes of 20000m (20km or over 60000 ft), it is obvious that distance enhancement will be available in a point-to-point VHF or UHF communications link. This distance is enhanced further due to the altitude of up to 90-odd km reached by the sprite and jet zones mentioned above.

But what is the nature of the available ionisation and why is it usable?

Using a rotatable high gain UHF antenna system with preamp pointing to Aliwal North TV (distance about 550 km) and a UHF receiver in USB mode, the trace in Fig 5 was obtained. The trace shows much spread and amplitude enhancement over the tropospheric scatter path due to the very favourable weather conditions prevailing, amounting to some 30 dB. Additionally lightning bursts ride on this signal level producing pulse waveforms lasting typically 0.1 to 0.6 seconds and providing another 20 to 30 dB of signal recovery on top of the already available 30 dB on the enhanced tropospheric scatter path. These pulses display as the horizontal streaks across the TV carrier frequency and from the timer an average of 15 major discharges per minute can be counted. Many lesser discharges, possibly two to three times as many, could be heard during the one minute interval.

Each lightning burst lasts about 30 microseconds and during that period the electromagnetic pulse effect (EMP) completely disables any chance of communication due to the overpowering RF noise accumulation.

But the ionised zone still exists after this and readily reflects high radio frequencies over the remainder of the 0.1 to 0.6 seconds of the ionised effect.

The audio impression when listening is a sharp click from the discharge's EMP, followed by a Doppler-shifted zinging audio tone or tones, very reminiscent of a meteor scatter echo and possessing the same rise and fall wave shape, Fig 6b.

From this it follows that lighting flash is not suitable for slow communication such as voice, but the ionised zones last more than long enough to communicate using fast data modes such as FSK441 and JTMS offered in the WSJT suite of PC soundcard-based programs.

Path Prediction

Predicting a path is simply a matter of examining weather maps (Fig7) for the most likely thunder cloud formations and directions and the near real time lightning discharge map in Fig 8, and aiming the antenna in that direction. Note the correlation between the thunder clouds in Fig 7 and the actual discharges in Fig 8.

The longest distance will be obtained when the partner station is on the other side of the thundercloud formation, total distance 700 to 800 km looks very feasible, while more nearby stations may be worked on lightning flash backscatter when both stations point to the same thundercloud.

Fig 5: Waterfall recording of an active 791 MHz UHF propagation path over 550 km. The horizontal pulses superimposed on the carrier are lightning discharges

Fig 6a: Amplitude versus 1 minute Time response of the UHF path

Fig 6b: Zoomed view of a major pulse

Fig 7: Eumetsat real light image of the path discussed (approximately along the red line)

http://oiswww.eumetsat.org/IPPS/html/MSG/RGB/NATURALCOLOR/SOUTHERNAFRICA/index.htm

Fig 8: Map indicating the active lightning strike zones

http://webflash.ess.washington.edu/EurAfricaL_plot_weather_map.jpg