Rocket Triggered Lightning Program (RTLP) Results

"Rocket-triggered lightning" is initiated by a small rocket towing a grounded wire aloft under a thunderstorm (St. Privat D'Allier Group, 1985). The rocket-and-wire technique for triggering lightning was pioneered by Newman et al. (1958, 1967). The key to its success is likely an observation by Brook et al. (1961) that the sufficiently rapid introduction of a grounded conductor into a high-field region might actually initiate the discharge.

The KSC Rocket-Triggered-Lightning Program (RTLP) provided an invaluable source of data on, and a better understanding of, lightning processes during a key period in the evolution of such knowledge. The primary triggering technique used in this program was so-called "classical triggering," in which a small rocket lifts a grounded wire aloft below an active thunderstorm, and produces a lightning discharge that is usually guided by the wire to the launch platform and any instrumentation connected thereto. The primary categories of results from the RTLP that are relevant for space operations are (1) data in support of test standards, (2) exposure of hardware to real lightning strikes, (3) test and calibration of lightning-detection systems, (4) better

understanding of the triggering conditions, (5) better understanding of the "attachment process," and (6) data to constrain theoretical models. Each of these areas is treated in turn below. Wherever possible, preference is given to references arising directly from the RTLP.

5.4.3.1 Data in Support of Test Standards

The best and most comprehensive dataset on the peak currents, peak current derivatives, and current rise times produced by subsequent return strokes was compiled by the RTLP, primarily during 1985, 1987, and 1988 (Leteinturier et al., 1990, 1991). The largest peak currents produced by strikes to ground-based objects are generally found in first return strokes (e.g., Rakov and Uman, 2003, Table 4.4), which do not occur in rocket- triggered flashes, but the subsequent return strokes in triggered and natural lighting are believed to be similar (e.g., Le Vine et al., 1989). Further, the peak electric-field derivatives and the rise times of the fast-rising portions in natural first and subsequent stroke waveforms are very similar (Bailey et al., 1988; Willett et al., 1990; Krider et al., 1992; Willett and Krider, 2000). Finally, Willett et al. (1989a) found approximately linear relationships for peak derivative and rise time between field change and current in rocket-triggered return strokes. (These similarities themselves were first documented with data recorded during the RTLP.) Thus it is reasonable to conclude that the RTLP data on lightning current rise times and derivatives are directly relevant for lightning test standards, even for first strokes, although the peak currents, charge transfers, and action integrals are not.

Peak current derivatives and current rise times are especially relevant to the coupling of damaging signals into electromechanical and electronic systems. For example, the induced EMF in a conducting loop is directly proportional to the time derivative of the magnetic flux passing through that loop. The magnetic field

Page 61 of 234

lightning channel) that is producing the magnetic field. Further, the peak current that is induced in an inductive circuit is proportional (at least over short times) to the rate of rise (di/dt) of this nearby current. If the circuit of interest is protected by a Faraday shield, the penetration of the transient magnetic field through that shield, hence the induced current in the circuit, also increases with the duration of that transient, or the duration (or rise time) of the fast-rising source current. The knowledge that there are very large current transients with very short current rise times in lightning return strokes was relatively new to the community prior to the RTLP and was based primarily on recent remote measurements of the electromagnetic fields produced by strokes in natural lightning (e.g., Uman and Krider, 1982). As a result of the RTLP, however, the current derivatives and rise time in triggered subsequent return strokes were well known from direct measurements. Test standards given, for example, by Plumer (1992) fully reflected these observed peak current derivatives and current rise times.

5.4.3.2 Exposure of Hardware to Real Lightning Strikes

The kinds of hardware that have been tested by exposure to rocket-triggered lightning at the RTLP include protection systems for structures, radomes, power lines, and even nuclear devices. Here we list a few examples of such tests that were conducted at the RTLP. During 1985 Lawrence Livermore and Sandia National

Laboratories tested a prototype protective canister for nuclear devices against rocket-triggered-lightning strikes (Melander et al., 1988)]. Rubinstein et al. (1991, 1994) describe measurements of induced voltages on an overhead power-distribution line by rocket-triggered discharges striking 20 m from one end of the line during 1986. Testing of electric power equipment was conducted during 1987 and 1988 by Power Technology Incorporated for the Electric Power Research Institute. In 1990 Fisher and Schnetzer (1991) measured actual damage to metal samples that were directly exposed to rocket-triggered lightning, as a calibration for laboratory testing of burn-through.

5.4.3.3 Test and Calibration of Lightning-Detection Systems

There are three characteristics of lightning-detection systems that might be tested or calibrated against rocket- triggered lightning at the RTLP. The most obvious is location accuracy, since the location of the triggered discharges is precisely known. The primary detection systems at the time (the National Lightning Detection System (e.g., Orville, 2008) -- called the NLDN -- and a medium-range version of it that was deployed around KSC (e.g., Krider, 1988) -- called the LLP system, later CGLSS) depended on radio direction finding (DF) and triangulation from multiple DFs to locate strikes. DFs are notorious for "site errors" -- errors in reported direction due to natural or man-made electromagnetic inhomogeneities of their surroundings. Statistical techniques have been developed for computing site-error corrections for DFs based on the self consistency of results from a multi-station network over many strikes distributed throughout its domain (e.g., Hiscox et al., 1984), but there remains a need for ground-truth verification. Examples of such verification using flashes triggered by the RTLP include Maier and Jafferis (1985) and Maier (1991).

In addition to location accuracy, there are the two related questions of amplitude accuracy and detection efficiency. In general, it was shown by Willett et al. (1988, 1989a) that there is an approximately linear relationship between peak electric-field change (proportional to the peak magnetic-field change that is measured by DFs in the "far field") and the peak current, as predicted by the simple transmission-line model. Stroke magnitude (in terms of inferred peak current) has been calibrated against peak currents directly measured in the RTLP (Orville, 1991; Idone et al., 1993). To our knowledge, detection efficiency has never been checked relative to RTLP data.

5.4.3.4 Understanding of the Triggering Conditions

Arguably the most important impact of the RTLP on spaceflight operations has come from a better

understanding of the triggering process and the triggering conditions. This is a large subject that will only be outlined here, but the key contributions provided by the RTLP will be highlighted.

Page 62 of 234

A basic understanding of the physics of air breakdown in long gaps had already been obtained from

experiments on long laboratory sparks, especially during the 1970s (e.g., Les Renardières Group, 1977, 1981). Unfortunately, the longer the spark, the lower the marginal ambient field -- that is, the increase in applied potential per unit increase in gap length (for "critical" time to voltage crest and 50% breakdown probability) -- required to sustain its propagation. The longest sparks then available in the laboratory implied a marginal field of about 55 kV/m for gap lengths of 27 m (Pigini et al., 1979). This was generally believed to be much larger than the average fields in which lightning discharges could propagate. For example, Pierce (1971) had suggested that lightning could be triggered in ambient fields of several kilovolts per meter if the ambient potential spanned by the triggering conductor (building, rocket, aircraft, etc.) reached a few megavolts. There seemed to be a considerable gap between lightning and sparks that could be produced in the laboratory, hence there was a clear need for the rocket-triggering experiments.

Figure 5.4.3-1 shows a histogram of all the successful and unsuccessful trigging attempts versus the electric field measured at the ground over the eight-year lifetime of the RTLP (including 1983, when the program was begun at a site south of Melbourne, FL) (Jafferis, 1995). It is evident that foul-weather (negative) surface fields of 3 kV/m or greater over land provide a high probability of success. The fact that lightning was triggered in fields that were greater than 1 kV/m certainly justifies a 1 kV/m surface-field criterion in the current LLCC. Under a thunderstorm, the ambient field a few hundred meters aloft is considerably larger than that at the surface under conditions like those when triggering was successful (e.g., Standler and Winn, 1979). This was demonstrated at the RTLP triggering site near the Mosquito Lagoon by Soula and Chauzy (1991), who used several electric-field sensors suspended by a captive balloon along an insulating tether at altitudes ranging from 80 to 800 m. Using the same instrumentation, Chauzy et al. (1991) and Soula and Chauzy (1991) showed that lightning could be triggered in fields aloft of 50 - 65 kV/m using grounded and ungrounded triggering wires of 200 - 300 m length. Since only four triggering attempts, all of which were successful, were made while the balloon sensors were operating, however, it must be assumed that triggering can occur in lower ambient fields. Indeed it was later shown that lightning could be triggered in ambient fields as low as 10 - 20 kV/m (Willett et al., 1999).

In the foul-weather fields generally encountered at KSC, triggered discharges were initiated by positive "leaders" (highly ionized, conducting, filamentary channels extending into virgin air) propagating upward from the tips of the triggering rockets (Rakov and Uman, 2003, Section 7.2.1). Important new information about such leaders was obtained from the RTLP. With optical and near-ultraviolet (UV) observations Idone (1992) showed that between the rocket and cloud base, they propagate at speeds of up to almost 106 m/s. This propagation speed was often found to increase with altitude, sometimes abruptly in association with a marked decrease in channel tortuosity [Idone and Orville, 1988]. Even more interestingly, Idone [1992] showed that this positive-leader propagation is not smooth and steady, instead comprising a relatively regular sequence of optical "pulses" separated by roughly 20 µs of time and spanning several meters of height. The structure of individual pulses was revealed by the UV streak photographs to consist of "a bright, thin stem of typically 3-5 m in length which is surrounded in the direction of propagation by a diffuse, hemispherical corona brush that extends outward about 5-10 m."

Page 63 of 234

Figure 5.4.3-1 Triggering success vs. surface field during the RTLP

Detailed electrical recordings were also made during the initiation and ascent of positive leaders in the RTLP (e.g., Laroche et al., 1989, Fig. 5). Much of this electrical phenomenology has been summarized in a review paper by Willett (1992). Synchronized records of channel-base current and close electric-field change show that brief current pulses with amplitudes of a few tens of Amperes and repetition intervals of several tens of microseconds are superimposed on a gradually increasing steady current of only a few Amperes. The current pulses coincide with steps in the electric field record, implying discrete pulses of charge deposition,

presumably near the tip of the extending leader. Because of their similar time history, these electrical events are believed to correspond to the optical pulses mentioned above, although no synchronized optical and electrical records are available to date.

Positive-leader onset is typically preceded hundreds of milliseconds by a series of brief, irregularly spaced, current pulses, sometimes called "precursors," with amplitudes of a few Amperes and repetition intervals on the order of 30 ms (e.g., Barret, 1986). These precursors, in turn, have been shown by Laroche et al. (1988) to

Page 64 of 234

consist of individual current and electric-field pulses like those in the leader onset itself, or groups of a few such pulses with similar repetition intervals, but no optical records of these events are available. This observation, however, has led to the hypothesis that the precursors are essentially attempted leaders that are unable sustain their propagation. If this hypothesis is correct, it implies that air breakdown, and even leader initiation, generally begin long before the rocket has ascended to sufficient altitude to produce self-sustaining leader propagation. This tentative conclusion points to leader "viability" (the ability to propagate indefinitely in the ambient field) as the most important criterion for lightning triggering by long, thin conductors.

In the "altitude-triggering" technique, the ascending rocket first unspools a predetermined length (typically a few hundred meters) of insulating line attached to the ground, followed by conducting wire for the remainder of its flight. (Sometimes an initial length of conducting wire, typically 50 m, is unspooled before the insulating section of line to "encourage" the triggered lightning to attach to ground-based measurement instrumentation.) This new technique, which was intended to better simulate triggering by a flying aircraft or spacecraft, was first exploited to advantage at the RTLP (e.g., Laroche et al., 1988, 1989, 1991). In a field of foul weather polarity it was found that the triggered discharge still begins with an upward-propagating positive leader from the tip of the rocket, apparently identical to those in classical triggering, followed a few milliseconds later by a downward-propagating negative leader from the lower end of the conducting wire. (To our knowledge this technique has yet to be successfully attempted in elevated fields of fair-weather polarity.)

Since the same sequence of events has been inferred to begin most strikes to instrumented aircraft (Boulay et al., 1988; Mazur, 1989), there is good reason to conclude that the positive-leader-viability conditions (as deduced from classical rocket-triggering experiments) are important in determining the triggering conditions to flying aircraft and spacecraft. In this way, results from the RTLP are still playing a vital role in the

development of the LLCC.

5.4.3.5 Understanding of the Attachment Process

The term, "attachment process," refers to the details of the junction between a natural-lightning discharge and an object on the earth (e.g., Rakov and Uman, 2003, Section 4.5). For negative cloud-to-ground lightning this process comprises the initiation of an upward positive leader from the grounded object, its connection to the downward negative ("stepped") leader from the thundercloud that caused it, and the ultimate development of an upward-propagating return stroke. A better understanding of this process is obviously key to the lightning protection of grounded objects.

The attachment process can be simulated, albeit at relatively low intensity, by a variation on altitude triggering in which a short length of grounded, conducting wire is unspooled from the ascending rocket prior to the insulating segment. As described in Section 5.4.3.4 above, this leads to a downward, negative leader from the lower end of the triggering wire, which in turn causes the emission of a positive leader from the grounded segment. The first successful experiment of this type was conducted at the RTLP in 1989 (Laroche et al., 1991).

The attachment process for negative, subsequent return strokes was also investigated during the RTLP by Idone (1990) and by Willett et al. (1989a), who showed optically and electrically that there are probably non- negligible upward-connecting discharges that meet downward dart or dart-stepped leaders.

5.4.3.6 Constraints on Theoretical Models

The most important RTLP data for constraint of theoretical modeling have already been mentioned in Section 5.4.3.4 above. Having concluded that the positive leader is the controlling phenomenon, we can also conclude that those leaders that initiate rocket-triggered discharges are significantly different from those studied to date in the laboratory. Thus there is a need for further theoretical and laboratory-experimental work to develop adequate models of these leaders. Such models could then be used (in concert with other, yet to be developed, models of the effects of rocket-exhaust plumes) for quantitative prediction of the ambient fields necessary for

Page 65 of 234

spacecraft triggering of lightning. To date one detailed, self-consistent, physical model of the positive leader has been developed (Bondiou et al., 1994), and an unrelated, semi-empirical model of the triggering conditions has been proposed (Bazelyan and Raizer, 2000, Section 4.1.1). Nevertheless, the theoretical problem is far from solved, and more sophisticated rocket-triggering experiments will undoubtedly be needed to complete this process.

Thottappillil and Uman (1993) have demonstrated the value of RTLP measurements for testing of various return-stroke models. Other observations made as part of the RTLP that have already been, or will be, useful in lightning theory include direct measurements of the time evolution of return stroke (luminous) diameter by Idone (1992); the first wide-band measurements by Willett et al. (1989b) of the "narrow bipolar pulses" that had been discovered by Le Vine (1980); deductions about the behavior of return-stroke currents above ground by Willett et al. (2008); and the first fairly comprehensive measurements of close leader/return-stroke fields by Hubert and Hubert (1986) and by Rubinstein et al. (1992, 1995).