Thursday March 9, 2017

The class after the midterm and the last class before Spring Break seemed like a good place have a look at rocket triggered lightning.  We'll look at the technique and view some videos of triggered lightning (from the International Center for Lightning Research and Testing near Gainesville, Florida).  Some results from a difficult experiment that sought to determine the conditions needed to successfully trigger lightning will also be examined.  I hope also to look at least one example fairly recent research results coming from the ICLRT site in northern Florida.

Being able to trigger lightning during a thunderstorm means you are able to cause lightning to strike at a known location (usually) and known time.  This makes possible a wide range of experiments that would otherwise be difficult to conduct.  Lightning return stroke current can be directly measured in triggered lightning and can be used to validate return stroke current models which attempt to infer lightning current from remote measurements of the E and B field.  This is a topic we'll look at in more detail later in the course.  High speed optical, photographic, and spectroscopic measurements can be made close to the discharge.  You can cause the lightning to strike structures,and  electrical transmission and distribution lines and to see how they and protective equipment respond.  Triggered lightning can also be used as ground truth for the National Lightning Detection Network (NLDN).

One of the first (if not the first) successful attempts to trigger lightning was done aboard a ship positioned a couple of kilometers or so offshore near St. Petersburg, Florida.  Seventeen discharges were triggered (out of 23 attempts) in August, 1966 (see Newman et al., 1967 for further details, a full citation and link to the reference can be found at the end of today's notes).   Beginning in 1973, French researchers began to successfully trigger light at the St. Privat d'Allier station in the Massif Central region in France.  Some of the early experiments were described in Fieux et al., 1975.  A second short article appeared the next year warning of an unexpected hazard associated with triggered lightning (see Fieux and Hubert, 1976).  Triggered lightning doesn't always follow the wire path all the way to the ground.  A summary of 8 years of research results can be found (in English and French) in Groupe de Recherches de Saint Privat-d'Allier, 1982.  Since these earlier experiments, triggered lightning experiments have been conducted at the Langmuir laboratory in New Mexico, at the Kennedy Space Center in Florida, and in China.

The "classical" triggering technique is illustrated below (adapted from Rakov et al., 1998)


The original French experiments used a fairly sizable rocket originally meant to carry and detonate an explosive charge in a thunderstorm as a means of preventing hail formation.  The rockets were roughly 3 ft. in length and carried a spool of thin steel wire wrapped with Kevlar.  The wire must be able to unspool rapidly without breaking or pulling on the rocket enough to cause it to veer to one side or another during launch.  The rocket launch command is sent from a safe and shielded nearby location using either pneumatics or fiber optics to ensure there is no electrical connection between the launch tower and the launch building.  Ideally the rocket needs to be launched as the E field at the ground is peaking but before a natural discharge occurs (which would cause the E field to collapse and result in an unsuccessful trigger).  This requires some practice and experience.

In a successful trigger attempt an upward positively charged leader will begin to develop off the top of the wire.  Development of the upward leader quickly vaporizes the wire.  Once the leader has propagated into the cloud an initial continuous current (ICC) of 100s of Amperes amplitude and 100s of milliseconds duration develops.  Shortly after cessation of the ICC the first of usually several downward dart or dart-stepped leader/subsequent return strokes occurs.  Measurements indicate that the leaders and subsequent return strokes in triggered lightning are essentially identical to those that occur in natural lightning.

Here's a schematic of the sequence of events in one of these classically triggered lightning events.


The top sketch shows a streaking camera or high speed video view of the discharge.  The low amplitude, long duration ICC produces a large field change on the Slow E field record.  The return stroke field changes are smaller because they usually transport less total charge to ground.   Two or more current sensors would probably be required to record all of the currents that occur during a triggered discharge.  A relatively sensitive channel would be needed to record the ICC and a lower sensitivity channel to prevent saturation during the higher peak current return strokes.  The bottom sketch shows the direction and polarity of the different discharge processes (orange is positive and green negatively charged)


Photograph of a triggered lightning discharge taken from a few 100 meters away.  The straight part of the channel is where the discharge followed the wire.  The lightning channel becomes much more jagged when traveling through air above the wire.  (source of this photo) A closeup of triggered lightning striking the launch tower.  The green portion of the image at left is produced by heating and vaporization of the copper wire used to trigger the discharge.   The brighter whiter strokes of lightning are seen at right.  They have been spread across the picture by wind.  Photo credit: Doug Jordan and Martin Uman International Center for Lightning Research and Testing

Still photographs of triggered lightning

Here is a link to a video with examples of lightning triggered at the ICLRT site.  The triggered lightning is found at the end of the video. 

Time segment
 comments
0 - 1:25 min
 natural lightning in the Tucson area
1:25 - 2:00 min
 natural lightning recorded
during experiments at the
Grand Canyon
2:00 - 2:38

2:15
2:24
2:27
 lightning recorded during a study of lightning strikes to wind turbines
probably a + CG discharge
another + CG discharge
upward lightning discharge initiated
by a wind turbine
2:38 - 3:47
 rocket triggered lightning in Florida


Because a triggered lightning flash does not contain a stepped leader and first return stroke modifications of the triggering technique have been tried.  One such attempt is shown below.


The idea here would be to observe and study the last step or two of the stepped leader and the attachment process and it neared the ground.  Bidirectional leaders are of interest because they develop off aircraft that are struck by lightning in flight.

In this next section we will discuss an experiment that sought to determine the conditions needed above the ground to trigger lightning.  The experiment was conducted at the ICLRT rocket triggered lightning site.  The experiment and results discussed below are from Willett et al. (1999).

The basic set up is illustrated below

The plan was to launch an electric field sounding rocket about 1 second before sending a launch command to one of the triggering rockets.  It would take about 2 seconds for the command to reach the triggering rocket (the firing signal was sent pneumatically) and another 2 seconds roughly before the triggering rocket reached triggering altitude.  The electric field as a function of altitude could then be determined just prior to a triggered lightning discharge.  Launches were attempted after the active periods in storms to try to avoid having a natural discharge occur during a triggering attempt.

The sounding rocket is very similar to one described and pictured in Marshall et al. (1995) which is reproduced below:


The rocket is just over 2 m tall.  A big part of the outer body of the rocket spins.  Six windows (three are shown above) alternately cover and expose 8 field mill stators mounted inside.  I've included the figure caption because the arrangement of the inner stators was not immediately clear to me.  The eight field measurements made by the rocket are enough to determine all 3 components (x, y, and z) of the ambient field.  Data from the rocket were transmitted to a telemetry station about 2 km from the triggering site (the receiving antenna would follow and track the sounding rocket).  Streak cameras and video cameras were also located at the telemetry site.  One major disappointment of the experiment was that no photographs of the upward leader were obtained with the streaking cameras.  No direct measurements of leader velocity were made.

The rocket was fired at an 80 degree elevation angle, would reach a peak altitude of just over 3500 m in about 24 seconds and fall to ground about 2 km away from the triggering site.  Total duration of the flight was just under 1 minute. 


Useable data was obtained for 10 of 15 attempts and lightning was triggered in 9 of these 10 cases.

An example of field measured as a function of altitude.  The field is remarkably constant at about 15 kV/m once above the 1.5 km thick space charge layer found near the ground.  The abrupt drop in field near the end of the record was caused by a triggered lightning discharge..

The wire from the triggering rocket was connected to a 207 mΩ shunt so that low amplitude currents flowing in the wire during the upward leader process that initiates a triggered lightning flash could be measured.  Currents in the downward dart leader and subsequent return stroke that follow in a successful triggering attempt attached to a grounded lightning rod and were not measured.


This is an example of currents recorded in the triggering wire at the beginning of a successful triggered discharge.  The current waveform recorder was triggered by the 1st precursor.  This is the first sign of a discharge forming at the upper end of the wire.  The triggering rocket was 112 m high at the time of the 1st precursor.


The first precursor shown on a much faster time scale.  Most precursors consisted of just a single pulse like this.  The oscillations are caused by the current reflecting repeatedly off the top and bottom ends of the wire. 

At just over 600 ms into the discharge an unsuccessful upward leader was initiated.  The triggering rocket was 260 m high at this point.  This "failed leader is shown on a faster time scale below.


This failed leader lasted about 1.8 ms during which time it propagated about 35 m at a velocity of 1.9 x 104m/s.

Ultimately at about 900 ms into the discharge, when the triggering rocket was at 307 m altitude, a successful leader was initiated and ultimately led to a triggered discharge.  This leader is shown below.  Note the continuous current that develops once the leader is underway.



The table below summarizes the results from the experiment, conditions at the time of  initiation of a successful trigger.
Launch
 height (m)
of the triggering rocket
 electric field (kV/m)
Potential (MV)
2
 447
 12.4
 -5.0
6
 307
 14.5
 -4.7
7
 295
 19.4
 -4.1
8
 279
 15.0
 -3.6
9
 312
 13.4
 -3.7
12
 336
 15.9
 -4.6
13
 230
 18.5
 -3.6
14
 324
 17.3
 -4.2
15
 299
 16.4
 -4.1


Potential above is the voltage at the triggering height relative to the ground (the integral of electric field with altitude)

By comparison there was one case where lightning wasn't triggered.  The E field in that case ranged from 2.5 to 4.5 kV/m and the potential at 500 m was -1.4 MV.

Finally (at least for the March 9 class) we'll look at some fairly recent research results from the ICLRT site that include simultaneous measurements of lightning current and high-speed video records of triggered lightning discharges.


This first figure shows the current recording leading up to the beginning of an positive upward leader (see Biagi et al., (2009)).  As with the example from Willett (1999) we first see current pulses from precursors, corona discharge streamers that are unable to initiate a sustained upward propagating leader channel.  A was the first precursor seen, precursor C marks the beginning of the positive upward leader and the start of the ICC.  On the high speed video (5400 frames/second = 185.2 μs/frame), precursor A has a length of 1.5 m and was found 128 m above ground level (AGL).  Precursor B was 8 m long and was found 158 AGL.  Precursors A, B, and C are shown on faster time scales in the cutouts; all three had amplitudes of roughly 50 Amperes.  This is roughly comparable to what was shown on the Willett (1999) current record (current on that record saturated at 20 Amps).  The wire was completely vaporized by Pt. D (vaporization of the wire makes it visible on the high-speed video).  Upward propagation of the positive leader was photographed for about 2 ms (11 consecutive 185 μs frames) and had a constant upward speed of 5.6 x 104   m/s.

The last item from my quick perusal of recent triggered lightning research in Florida is shown below.

This shows the last frame before a subsequent return stroke obtained with a 50,000 frames/second video camera (20 μs/frame).  The image is shown both in positive (white channel on black at left) and negative.  This photograph is also from Biagi et al. (2009).

What we see is the bottom 25 m of a downward moving dart-stepped leader extending down from the top of the image.  A dart-stepped leader is intermediate between a stepped leader and a dart leader.  The steps are shorter (perhaps 10 m long) and occur more frequently (every 10 μs  or so) than in a stepped leader (50 m steps every 50 μs  or so).  A 16 m tall upward connecting discharge extends upward from the bottom of the image.  The leader tip is the bright bottom end of the highly conducting leader arc channel.  Extending downward beyond the tip are dimmer streamer zone containing higher resistance corona discharge filaments. 

Earlier in the semester I gave you my understanding of what happens during a leader step.  I explained that periodically the current feeding the expanding "fan" of corona discharge filaments would grow to the point that heating of the filament channels would cause them to become highly conducting.  The leader arc would extend downward from the leader tip.  That might not be the case at all.

In laboratory studies of long arcs researchers have observed (and photographed) that a short segment (or segments) of arc channel form in the streamer zone out ahead of and separate from the arc channel of the leader.  This is referred to a space stem.  At some point a bidirectional leader grows outward from the two ends of the space stem.  The upward moving space stem leader connects with the bottom tip of the main leader leader.  The high potential of the main leader channel transfers rapidly downward to the tip of the downward moving space stem leader and causes a sudden increase in corona discharge.  The sudden demand for charge causes a wave of luminosity to propagate up the space stem channel to the bottom of the leader channel.

The short bright segment (labeled space stem) in the photograph above suggests that the same kind of process may occurring in lightning stepped and dart stepped leaders.

References:
M.M. Newman, J.R. Stahmann, J.D. Robb, E.A. Lewis, S.G. Martin, and S.V. Zinn, "Triggered Lightning Strokes at Very Close Range," J. Geophys. Res., 72, 4761-4764, 1967.

R. Fieux, C. Gary, and P. Hubert, "Artificially triggered lightning above land," Nature, 257, 212-214, 1975.

R. Fieux and P. Hubert, "Triggered lightning hazards," Nature, 260, 188, 1976.

Groupe de Recherces de Saint-Privat-d'Allier, "Eight years of lightning experiments at Saint-Privat-d'Allier," Revue Generale de l'Electricite, 9/82, 561-582, 1982.

V.A. Rakov, M.A. Uman, K.J. Rambo, M.I. Fernandez, R.J. Fisher, G.H. Schnetzer, R. Thottappillil, A. Eybert-Berard, J.P. Berlandis, P.Lalande, A. Bonamy, P. Laroche, and A. Bondiou-Clergerie, "New insights into lightning processes gained from triggered-lightning experiments in Florida and Alabama," J. Geophys. Res., 103, 14117-14130, 1998.

Marshall, T.J., W. Rison, W.D. Rust, M. Stolzenburg, J.C. Willett, and W.P. Winn, "Rocket and balloon observations of electric field in two thunderstorms," J. Geophys. Res., 100, 20815-20828, 1995.

Willett, J.C., D.A. Davis, and P. Laroche, "An experimental study of positive leaders initiating rocket-triggered lightning," Atmospheric Research, 51, 189-219, 1999.

C.J. Biagi, D. M. Jordan, M.A. Uman, J.D. Hill, W.H. Beasley and J. Howard, "High-speed video observations of rocket-and-wire initiated lightning," Geophys. Res. Lett, 36, L15801, doi:10.1029/2009GL038525, 2009.