Narrow Bipolar Pulses Research Articles

Current propagation model for a narrow bipolar pulse

Narrow bipolar pulses (NBPs) are a class of high‐altitude, high‐energy discharges that occur during some thunderstorms. We use a modified transmission line model (called MTLEI) with a current that increases exponentially along the propagation channel to test mechanisms that might produce NBPs. Model outputs were compared to measured E data from a single NBP collected at near and far field locations. We were unable to fit the measured data using the fast current propagation speeds appropriate for a runaway breakdown/extensive air shower mechanism. Instead, by using currents that travel relatively slowly (6 × 107 m/s), the MTLEI model fit the data reasonably well. This result is compatible with a mechanism that uses runaway breakdown to produce charge carriers along with a moving electric field to drive the main NBP current. Using this model for the measured NBP, we estimate a charge moment of 0.6 C·km.

Amplification and nonlinear modification of runaway breakdown

The astonishing natural phenomenon of narrow bipolar pulses (NBP) is an isolated discharges in thunderstorms at tropopause heights (10–20 km) that generates enormously powerful radio emission but lasts only a few microseconds. The theory of this phenomenon based on runaway breakdown is elaborated. At such high heights extensive atmospheric shower (EAS) become well developed only if the energetic cosmic ray particle momentum is directed close to the horizon. For these conditions the runaway breakdown–extensive atmospheric shower (RB–EAS) discharge amplified strongly what lead to the effective diminishing of thundercloud electric field and results in nonlinear saturation of the discharge current. A reasonable agreement of the theory with NBP observations is demonstrated.

Radio frequency emissions from a runaway electron avalanche model compared with intense, transient signals from thunderstorms

We present a one‐dimensional model of a runaway electron avalanche in a thunderstorm electric field. Previous simulations have calculated the ionization rates and energy distribution functions for runaway electrons, for various atmospheric values of E/p, through the solution of the modified relativistic Boltzmann equation. We use the field‐ and pressure‐dependent ionization rates in a hydrodynamic macroscopic treatment. The runaway electron avalanche modeled here includes the production of runaway and low‐energy electrons, electric field relaxation, electron attachment, and runaway electron loss. The model ambient electric field is established from two disks of charge with a sinusoidally spatially varying charge density of 9 nC/m3 peak amplitude. The peak ambient electric field from this configuration is 538 kV/m at 5 km. The numerically calculated radio frequency radiation exhibits relativistic effects. We hypothesize that runaway electron avalanches are sources of intense HF/VHF impulses radiated from within electrified clouds. The results from this case study are compared with ground‐based and FORTE satellite observations of HF and VHF radiation observed during the rise portion of narrow bipolar pulses (NBP). Given the specified rates and ambient environment, the radiation electric field HF and VHF spectra covering 3–25, 26–48, and 60–66 MHz are in agreement with observations for limited angular ranges. The modeled peak radiation electric field in the time domain is just below one standard deviation from the observed mean for NBPs.

Gamma ray flashes due to plasma processes in the atmosphere: Role of whistler waves

A proposed mechanism for the generation of gamma ray flashes in the atmosphere, observed on Compton Gamma Ray Observatory satellite, is a runaway discharge mediated by whistler waves. In this model the relativistic electrons produced by a cosmic ray shower during a thunderstorm lead to a runaway discharge, which is maintained by the interaction with whistler waves and the relativistic electrons propagate up in ducts produced by the self‐focusing of these waves. The paper presents a detailed study of the instability which develops in the lower stratosphere when a beam of hot magnetized electrons interacts with whistler waves. It was shown that the growth rate of such instability depends on the number density of hot electrons and on their collision rate, and it peaks at about 25 km. The instability develops under the conditions for runaway breakdown, similar to those leading to the generation of strong narrow bipolar pulses, which indicates a possible correlation between these two phenomena.

New type discharge generated in thunderclouds by joint action of runaway breakdown and extensive atmospheric shower

The kinetic theory of electric discharge in thunderclouds generated by runaway breakdown (RB) and extensive atmospheric shower (EAS) is developed. A giant growth of relativistic and thermal electrons, gamma and radio emission is predicted. The theory is compared with the recent results of EAS-radio experiments at Tien Shan mountains and with observations of lightning initiation process and narrow bipolar radio pulses. A reasonable agreement between the theory and observations is established. A new methods in cosmic ray physics is proposed: RB detection of extra high energy particles ɛ p > 10 18 eV .

The Los Alamos Sferic Array: A research tool for lightning investigations

Since 1998 the Los Alamos Sferic Array (LASA) has recorded electric field change signals from lightning in support of radio frequency (RF) and optical observations by the Fast On‐orbit Recording of Transient Events (FORTE) satellite. By “sferic” (a colloquial abbreviation for “atmospheric”), we refer to a remote measurement of the transient electric field produced by a lightning flash. LASA consisted of five stations in New Mexico in 1998 and was expanded to 11 stations in New Mexico, Texas, Florida, and Nebraska in 1999. During the 2 years of operation described in this paper, the remote stations acquired triggered 8‐ or 16‐ms duration, 12‐bit waveforms and GPS‐based sferic time tags 24 hours per day year‐round. Source locations were determined daily using differential time of arrival techniques, and the waveforms from all geolocated events were transferred to Los Alamos National Laboratory (LANL), where they have been archived for further analysis, including event classification and characterization. We evaluated LASA location accuracy by comparing temporally coincident (occurring within 100 μs) LASA and National Lightning Detection Network (NLDN) event locations. Approximately one half of the locations agreed to within 2 km, with better agreement for events that occurred within the confines of LASA subarrays in New Mexico and Florida. Of the ∼900,000 events located by the sferic array in 1998 and 1999, nearly 13,000 produced distinctive narrow bipolar field change pulses resembling those previously identified as intracloud discharges.

Electromagnetic pulse environment of cloud-to-ground lightning for EMC studies

The characteristics of lightning electromagnetic pulse (LEMP) from individual processes in lightning such as return strokes, preliminary breakdown pulses, pulses associated with the leader process, K-changes and M-changes, the isolated narrow bipolar pulses, and the pulse bursts have been known. However, there is a need for a combined characterization of the total LEMP environment created during the entire duration of lightning. An attempt is made to provide such a description that gives the distribution of the LEMP characteristics, which is important in its ability to interfere with electronic systems, such as peak amplitude, peak time derivative, pulse duration, number of pulses, and time interval between pulses, during the entire duration of a cloud-to-ground lightning. Separate electromagnetic environment characterizations for negative cloud-to-ground lightning and positive cloud-to-ground lightning are proposed, including all the significant LEMP sources. Based on the LEMP characterization, models for negative and positive cloud-to-ground lightning flashes, that could be used in electromagnetic compatibility (EMC) studies are proposed. The electromagnetic environment models for cloud-to-ground lightning can be coupled to the locations of lightning given by the lightning location system to give comprehensive information about the electromagnetic environment at a desired geographical location.

Determining the source of strong LF/VLF TIPP events: Implications for association with NPBPs and NNBPs

Transionospheric pulse pairs (TIPPs) as detected by the Blackbeard VHF radio instrument onboard the ALEXIS satellite have been shown to correlate with pulses detected by stations in the National Lightning Detection Network (NLDN). The short peak‐to‐zero time of these NLDN‐detected pulses (<10 μs) are indicative of interior cloud processes, as opposed to the longer pulses associated with cloud‐to‐ground discharges. TIPPs are most probably generated by the same discharge responsible for narrow bipolar pulses (NBPs), which have been detected on the ground but are also believed to be generated entirely inside the cloud. Here we report on five TIPPs detected by Blackbeard that are correlated with cloud‐generated pulses detected by multiple stations in the NLDN during powerful storms. Previously, a maximum of two stations at a time showed such a correlation. Given the greater area of detection, these pulses radiate more powerfully in the LF/VLF frequencies than those previously detected as TIPPs by Blackbeard. We are able to place the sources of the TIPPs in Hurricane Fausto off the coast of Mexico, and in a remnant of the same storm eight days later over Texas. The heights of the sources are higher than those previously determined for TIPPs. The higher altitudes and the greater power of the TIPP‐correlating signals may be related to the intensity of the storms. Two of these five TIPPs were also correlated with the ground detection of HF signals, and the TIPPs and HF signals were determined to originate from the same source region. These HF signals are similar to those previously recorded with the field change signature of narrow positive bipolar pulses (NPBPs), but we find that both polarities are detected, four out of the five being negative. This indicates that TIPPs correlate with narrow negative bipolar pulses (NNBPs) as well as NPBPs. The detection of NNBPs, which has previously been much rarer than NPBPs, may also be related to the intensity of the storms and the resultant higher‐altitude sources.

A class of unusual lightning electric field waveforms with very strong high‐frequency radiation

The first wideband dE/dt recordings have been obtained for the narrow bipolar pulses previously identified by Le Vine (1980) as “sources of the strongest RF radiation from lightning.” These dE/dt waveforms are dramatically different from those of other known lightning processes. A burst of high‐frequency “noise” is superimposed on the slower bipolar pattern one might expect from the relatively smooth E waveforms. For 18 such pulses from an isolated thunderstorm cell at known range, the mean peak E and dE/dt, range‐normalized to 100 km, were 8.0±5.3 V/m and 20±15 V/m/μs, respectively. Spectral analysis indicates that the sources of these pulses radiate much more strongly than first‐return strokes at frequencies from 10 MHz to at least 50 MHz. Absolutely calibrated power and energy spectra are presented which are reliable from 200 KHz to perhaps 20 MHz. At 18 MHz the narrow pulses appear to contain nearly 16 dB more spectral energy than first return‐stroke waveforms from the same range. Supporting evidence shows that they generally occur as isolated pulses in intracloud flashes but are not associated with K changes or other known phenomena. They can occur in either polarity.