NASA and Electromagnetic Pulse Research

The phrase "electromagnetic pulse” usually raises visions of a nuclear detonation, because that is the most frequent context in which it is used. While EMP effects upon aircraft certainly would feature in a thermonuclear event, the phenomenon is commonly experienced in and around lightning storms. Lightning can cause a variety of EMP radiations, including radio-frequency pulses. An EMP "fries” electrical circuits by passing a magnetic field past the equipment in one direc­tion, then reversing in an extremely short period—typically a few nano­seconds. Therefore, the magnetic field is generated and collapses within that ephemeral time, creating a focused EMP. It can destroy or render useless any electrical circuit within several feet of impact.

Any survey of lightning-related EMPs brings attention to the phenom­ena of "elves,” an acronym for Emissions of Light and Very low-frequency perturbations from Electromagnetic pulses. Elves are caused by lightning­generated EMPs, usually occurring above thunderstorms and in the ion­osphere, some 300,000 feet above Earth. First recorded on Space Shuttle Mission STS-41 in 1990, elves mostly appear as reddish, expanding flashes that can reach 250 miles in diameter, lasting about 1 millisecond.

EMP research is multifaceted, conducted in laboratories, on air­borne aircraft and rockets, and ultimately outside Earth’s atmosphere. Research into transient electric fields and high-altitude lightning above thunderstorms has been conducted by sounding rockets launched by Cornell University. In 2000, a Black Brant sounding rocket from White Sands was launched over a storm, attaining a height of nearly 980,000 feet. Onboard equipment, including electronic and magnetic instru­ments, provided the first direct observation of the parallel electric field within 62 miles horizontal from the lightning.[155]

By definition, NASA’s NF-106B flights in the 1980s involved EMP research. Among the overlapping goals of the project was quantifica­tion of lightning’s electromagnetic effects, and Langley’s Felix L. Pitts led the program intended to provide airborne data of lightning-strike traits. Bruce Fisher and two other NASA pilots (plus four Air Force pilots) conducted the flights. Fisher conducted analysis of the informa­tion he collected in addition to backseat researchers’ data. Those flying as flight-test engineers in the two-seat jet included Harold K. Carney, Jr., NASA’s lead technician for EMP measurements.

NASA Langley engineers built ultra-wide-bandwidth digital tran­sient recorders carried in a sealed enclosure in the Dart’s missile bay. To acquire the fast lightning transients, they adapted or devised electro­magnetic sensors based on those used for measurement of nuclear pulse radiation. To aid understanding of the lightning transients recorded on the jet, a team from Electromagnetic Applications, Inc., provided math­ematical modeling of the lightning strikes to the aircraft. Owing to the extra hazard of lightning strikes, the F-106 was fueled with JP-5, which is less volatile than the then-standard JP-4. Data compiled from dedi­cated EMP flights permitted statistical parameters to be established for lightning encounters. The F-106’s onboard sensors showed that lightning strikes to aircraft include bursts of pulses lasting shorter than previously thought, but they were more frequent. Additionally, the bursts are more numerous than better-known strikes involving cloud-to-Earth flashes.[156]

Rocket-borne sensors provided the first ionospheric observations of lightning-induced electromagnetic waves from ELF through the medium frequency (MF) bands. The payload consisted of a NASA double-probe electric field sensor borne into the upper atmosphere by a Black Brant sounding rocket that NASA launched over "an extremely active thunder­storm cell.” This mission, named Thunderstorm III, measured lightning EMPs up to 2 megahertz (MHz). Below 738,000 feet, a rising whistler wave was found with a nose-whistler wave shape with a propagating fre­quency near 80 kHz. The results confirmed speculation that the leading intense edge of the lightning EMP was borne on 50-125-kHz waves.[157]

Electromagnetic compatibility is essential to spacecraft performance. The requirement has long been recognized, as the insulating surfaces on early geosynchronous satellites were charged by geomagnetic sub­storms to a point where discharges occurred. The EMPs from such dis­charges coupled into electronic systems, potentially disrupting satellites. Laboratory tests on insulator charging indicated that discharges could be initiated at insulator edges, where voltage gradients could exist.[158]

Apart from observation and study, detecting electromagnetic pulses is a step toward avoidance. Most lightning detections systems include an antenna that senses atmospheric discharges and a processor to deter­mine whether the strobes are lightning or static charges, based upon their electromagnetic traits. Generally, ground-based weather surveillance is more accurate than an airborne system, owing to the greater number of sensors. For instance, ground-based systems employ numerous antennas hundreds of miles apart to detect a lightning stroke’s radio frequency (RF) pulses. When an RF flash occurs, electromagnetic pulses speed outward from the bolt to the ground at hyper speed. Because the antennas cover a large area of Earth’s surface, they are able to triangulate the bolt’s site of origin. Based upon known values, the RF data can determine with con­siderable accuracy the strength or severity of a lightning bolt.

Space-based lightning detection systems require satellites that, while more expensive than ground-based systems, provide instantaneous visual monitoring. Onboard cameras and sensors not only spot light­ning bolts but also record them for analysis. NASA launched its first lightning-detection satellite in 1995, and the Lightning Imaging Sensor, which analyzes lightning through rainfall, was launched 2 years later. From approximately 1993, low-Earth orbit (LEO) space vehicles car­ried increasingly sophisticated equipment requiring increased power levels. Previously, satellites used 28-volt DC power systems as a leg­acy of the commercial and military aircraft industry. At those voltage levels, plasma interactions in LEO were seldom a concern. But use of high-voltage solar arrays increased concerns with electromagnetic compatibility and the potential effects of EMPs. Consequently, space­craft design, testing, and performance assumed greater importance.

NASA researchers noted a pattern wherein insulating surfaces on geosynchronous satellites were charged by geomagnetic substorms, building up to electrical discharges. The resultant electromagnetic pulses can couple into satellite electronic systems, creating potentially disrup­tive results. Reducing power loss received a high priority, and laboratory tests on insulator charging showed that discharges could be initiated at insulator edges, where voltage gradients could exist. The benefits of such tests, coupled with greater empirical knowledge, afforded greater operating efficiency, partly because of greater EMP protection.[159]

Research into lightning EMPs remains a major focus. In 2008, Stanford’s Dr. Robert A. Marshall and his colleagues reported on time­modeling techniques to study lightning-induced effects upon VLF trans­mitter signals called "early VLF events.” Marshall explained:

This mechanism involves electron density changes due to electromagnetic pulses from successive in-cloud light­ning discharges associated with cloud-to-ground dis­charges (CGs), which are likely the source of continuing current and much of the charge moment change in CGs. Through time-domain modeling of the EMP we show that a sequence of pulses can produce appreciable density changes in the lower ionosphere, and that these changes are primarily electron losses through dissociative attach­ment to molecular oxygen. Modeling of the propagat­ing VLF transmitter signal through the disturbed region shows that perturbed regions created by successive hor­izontal EMPs create measurable amplitude changes.[160]

However, the researchers found that modeling optical signatures was difficult when observation was limited by line of sight, especially by ground-based observers. Observation was further complicated by clouds and distance, because elves and "sprites” (large-scale discharges over thunderclouds) were mostly seen at ranges of 185 to 500 statute miles. Consequently, the originating lightning usually was not visible. But empirical evidence shows that an EMP from lightning is extremely short-lived when compared to the propagation time across an elve’s radius. Observers therefore learned to recognize that the illuminated area at a given moment appears as a thin ring rather than as an actual disk.[161]

In addition to the effects of EMPs upon personnel directly engaged with aircraft or space vehicles, concern was voiced about researchers being exposed to simulated pulses. Facilities conducting EMP tests upon avionics and communications equipment were a logical area of investi­gation, but some EMP simulators had the potential to expose operators and the public to electromagnetic fields of varying intensities, includ­ing naturally generated lightning bolts. In 1988, the NASA Astrophysics Data System released a study of bioelectromagnetic effects upon humans. The study stated, "Evidence from the available database does not estab­lish that EMPs represent either an occupational or a public health haz­ard.” Both laboratory research and years of observations on staffs of EMP manufacturing and simulation facilities indicated "no acute or short-term health effects.” The study further noted that the occupational exposure guideline for EMPs is 100 kilovolts per meter, "which is far in excess of usual exposures with EMP simulators.”[162]

NASA’s studies of EMP effects benefited nonaerospace communities. The Lightning Detection and Ranging (LDAR) system that enhanced a safe work environment at Kennedy Space Center was extended to pri­vate industry. Cooperation with private enterprises enhances commercial applications not only in aviation but in corporate research, construction, and the electric utility industry. For example, while two-dimensional commercial systems are limited to cloud-to-ground lightning, NASA’s three-dimensional LDAR provides precise location and elevation of in­cloud and cloud-to-cloud pulses by measuring arrival times of EMPs.

Nuclear – and lightning-caused EMPs share common traits. Nuclear EMPs involve three components, including the "E2” segment, which is similar to lightning. Nuclear EMPs are faster than conventional cir­cuit breakers can handle. Most are intended to stop millisecond spikes caused by lightning flashes rather than microsecond spikes from a high – altitude nuclear explosion. The connection between ionizing radiation and lightning was readily demonstrated during the "Mike” nuclear test at Eniwetok Atoll in November 1952. The yield was 10.4 million tons, with gamma rays causing at least five lightning flashes in the ionized air around the fireball. The bolts descended almost vertically from the cloud above the fireball to the water. The observation demonstrated that, by causing atmospheric ionization, nuclear radiation can trigger a short­ing of the natural vertical electric gradient, resulting in a lightning bolt.[163]

Thus, research overlap between thermonuclear and lightning­generated EMPs is unavoidable. NASA’s workhorse F-106B, apart from NASA’s broader charter to conduct lightning-strike research, was employed in a joint NASA-USAF program to compare the electromag­netic effects of lightning and nuclear detonations. In 1984, Felix L. Pitts of NASA Langley proposed a cooperative venture, leading to the Air Force lending Langley an advanced, 10-channel recorder for measur­ing electromagnetic pulses.

Langley used the recorder on F-106 test flights, vastly expand­ing its capability to measure magnetic and electrical change rates, as well as currents and voltages on wires inside the Dart. In July 1993, an Air Force researcher flew in the rear seat to operate the advanced equipment, when 72 lightning strikes were obtained. In EMP tests at Kirtland Air Force Base, the F-106 was exposed to a nuclear electro­magnetic pulse simulator while mounted on a special test stand and during flybys. NASA’s Norman Crabill and Lightning Technologies’

J. A. Plumer participated in the Air Force Weapons Laboratory review of the acquired data.[164]

With helicopters becoming ever-more complex and with increasing dependence upon electronics, it was natural for researchers to extend the Agency’s interest in lightning to rotary wing craft. Drawing upon the Agency’s growing confidence in numerical computational analysis, Langley produced a numerical modeling technique to investigate the response of helicopters to both lightning and nuclear EMPs. Using a UH-60A Black Hawk as the focus, the study derived three-dimensional time domain finite-difference solutions to Maxwell’s equations, com­puting external currents, internal fields, and cable responses. Analysis indicated that the short-circuit current on internal cables was generally greater for lightning, while the open-circuit voltages were slightly higher for nuclear-generated EMPs. As anticipated, the lightning response was found to be highly dependent upon the rise time of the injected current. Data showed that coupling levels to cables in a helicopter are 20 to 30 decibels (dB) greater than in a fixed wing aircraft.[165]