Browsing by Subject "Lightning"

The high power, broadband very low frequency (VLF, 3--30 kHz) and extremely low frequency (ELF, 3--3000 Hz) electromagnetic waves generated by lightning discharges and propagating in the Earth-ionosphere waveguide can be used to measure the average electron density profile of the lower ionosphere (D region) across the wave propagation path due to several reflections by the upper boundary (lower ionosphere) of the waveguide. This capability makes it possible to frequently and even continuously monitor the D region electron density profile variations over geographically large regions, which are measurements that are essentially impossible by other means. These guided waves, usually called atmospherics (or sferics for short), are recorded by our sensors located near Duke University. The purpose of this work is to develop and implement algorithms to derive the variations of D region electron density profile which is modeled by two parameters (one is height and another is sharpness), by comparing the recorded sferic spectra to a series of model simulated sferic spectra from using a finite difference time domain (FDTD) code.

In order to understand the time scales, magnitudes and sources for the midlatitude nighttime D region variations, we analyzed the sferic data of July and August 2005, and extracted both the height and sharpness of the D region electron density profile. The heights show large temporal variations of several kilometers on some nights and the relatively stable behavior on others. Statistical calculations indicate that the hourly average heights during the two months range between 82.0 km and 87.2 km with a mean value of 84.9 km and a standard deviation of 1.1 km. We also observed spatial variations of height as large as 2.0 km over 5 degrees latitudes on some nights, and no spatial variation on others. In addition, the measured height variations exhibited close correlations with local lightning occurrence rate on some nights but no correlation with local lightning or displaced lightning on others. The nighttime profile sharpness during 2.5 hours in two different nights was calculated, and the results were compared to the equivalent sharpness derived from International Reference Ionosphere (IRI) models. Both the absolute values and variation trends in IRI models are different from those in broadband measurements.

Based on sferic data similar to those for nighttime, we also measured the daytime D region electron density profile variations in July and August 2005 near Duke University. As expected, the solar radiation is the dominant but not the only determinant source for the daytime D region profile height temporal variations. The observed quiet time heights showed close correlations with solar zenith angle changes but unexpected spatial variations not linked to the solar zenith angle were also observed on some days, with 15% of days exhibiting regional differences larger than 0.5 km. During the solar flare, the induced height change was approximately proportional to the logarithm of the X-ray fluxes. During the rising and decaying phases of the solar flare, the height changes correlated more consistently with the short (wavelength 0.5-4 Å), rather than the long (wavelength 1-8 Å) X-ray flux changes. The daytime profile sharpness during morning, noontime and afternoon periods in three different days and for the solar zenith angle range 20 to 75 degrees was calculated. These broadband measured results were compared to narrowband VLF measurements, IRI models and Faraday rotation base IRI models (called FIRI). The estimated sharpness from all these sources was more consistent when the solar zenith angle was small than when it was large.

By applying the nighttime and daytime measurement techniques, we also derived the D region variations during sunrise and sunset periods. The measurements showed that both the electron density profile height and sharpness decrease during the sunrise period while increase during the sunset period.

Lightning is one of the most familiar, impressive, but catastrophic natural phenomena that occur commonly on Earth. It produces perhaps the loudest sound, the most broadband radio emission, and the brightest light in the atmosphere. However, lightning remains relatively poorly understood since it is so transient (usually < 1 second) and so unpredictable that hinders direct measurements inside thunderstorms. For these reasons, radio remote sensing has been widely used for lightning studies. With recent advances in instrumentation and remote sensing technique, some basic problems like how lightning initiates inside the thundercloud begin to be addressed, and new challenging scientific problems are being discovered, such as the Terrestrial Gamma-ray Flashes (TGFs, energy > 20 MeV) associated with lightning, photonuclear reactions triggered by lightning, and needle-like plasma structures on the positive lightning leader, connecting lightning as part of atmospheric physics to high-energy physics, plasma physics, etc.

This dissertation aims to address fundamental questions like how lightning initiates and propagates, and how are TGFs related to lightning processes, by applying state-of-the-art radio remote sensing and imaging techniques. We measure and analyze electromagnetic signals produced by lightning from the vicinity to more than a thousand miles away, at radio frequencies from VLF, LF, to VHF and UHF. First, we investigated LF/VLF lightning sferics at the time of TGFs and found a statistically consistent connection between a slow LF pulse (~80 $\mu s$ duration) and TGFs, suggesting that the radio pulse is produced directly by the TGF production process. Second, in light of the slow pulse-TGF connection, we discovered a new type of downward CG-TGF with a reverse positron beam detected by Fermi GBM on the orbit, which could constitute 5--10 % of the previously known TGF population. Third, we employed supervised and unsupervised machine learning approaches to classify energetic lightning radio pulses for unprecedented ground detection of TGFs as well as understanding lightning sferics and ionospheric effects. In the meanwhile, we developed a short-baseline VHF interferometer with 200 MHz bandwidth to image lightning channels in high spatiotemporal resolution, shedding new insights into needles and lightning leader dynamics. Last but not least, we demonstrated and applied a new approach to indirectly measuring electric fields in the discharge region during lightning initiation and positive leader propagation using VHF-UHF radio spectrum, enabling an entirely new and useful capability for probing the ambient condition during lightning discharge processes. Implications of the estimated electric fields for lightning physics and high-energy physics are discussed.

Electromagnetic waves in the very low frequency (VLF, 3-30 kHz) and extremely low frequency (ELF, 3-3000 Hz) bands propagate extremely well in the cavity between the earth and the ionosphere with low attenuation. Because of this, radio waves emitted in this frequency range can be measured at extremely large distances (thousands of kilometers) from the sources of such emissions. Two main sources of signals at these frequencies are lightning events and VLF transmitters designed for communicating with submarines and other naval vessels. Measurement of the signals from both of these sources can be used to discover information about the source, in the case of lightning, or to measure the factors affecting propagation and other signal properties, as with VLF transmitter signals. This document provides a summary of the work undertaken to measure both of these signal sources and to outline goals and briefly outlines some objectives for future work.

A brief background on the atmospheric ELF and VLF environments is given in chapter 1, including a description of the conditions that allow for excellent propagation. A brief introduction to the lightning processes, as well as classification and measurement techniques is included as background information. Details describing current VLF transmitters examined in this work and basics of minimum-shift keying are also described.

Chapter 2 describes the design process and operating characteristics of a sensor designed for measuring magnetic fields in the ELF and VLF frequency ranges of interest in this work. This sensor system is robust and suitable for long-term deployment in thunderstorm environments. Chapter 3 details a method of measuring faint average signals generated by some lightning processes at large distances. Such an averaging process allows for the extraction of extremely small-magnitude processes that are otherwise not visible and enables the comparison of lightning on a larger scale. Averaged waveforms for four separate thunderstorms are compared and post-first stroke flash parameters are analyzed. Chapter 4 applies the averaging procedure to a specific type of lightning known as narrow bipolar events (NBEs). NBEs play an important role in the initiation of other types of lightning but not all NBEs initiate other lightning. This work divides positive NBEs according to whether they initiate other lightning events and examines the differences between them, helping to investigate the processes and conditions that give rise to lightning. Chapter 5 describes a method of unambiguously determining the position of a receiver through the measurement of terrestrial MSK-encoded VLF transmitters. Such a system has many advantages over other methods of navigation and simulated and field-tested capabilities and limitations are discussed, as well as factors affecting system accuracy. Finally, proposals and suggestions for future work are given in chapter 6.

Universal software radio peripheral (USRP) was utilized to receive the radiation produced by lightning flashes in VHF and UHF bands, with the bandwidth ranging from 2MHz to 8MHz. The software radio was programmed to record this radiation by integrating GPS clock and absolute timing. Moreover, two USRP N210 were employed to simultaneously record data at VHF and UHF bands with different programmable gain settings. This data was compared with the data from National Lightning Detection Network (available as location, type and peak current of lightning) and the magnetic sensor operating at LF (30 to 300 kHz). The output of USRP is the antenna displacement current ∂E/∂t (uncalibrated) and of LF magnetic sensor is the induced voltage ∂B/∂t. From comparison, the following results were obtained. K processes or regular pulse bursts in both cloud and cloud to ground discharges were clearly visible at UHF-VHF-LF. These processes were even visible at VHF with 0 dB gain, if superimposed on high magnitude slow (electric field change) processes such as J process probably. Distant Narrow bipolar pulses were observed with significant magnitude at VHF. Initial breakdown in cloud discharge was strong at LF and VHF but not significant at UHF. Instead the short pulses, probably stepped leaders, with 1 to 2.5 µs of time duration produced high magnitudes at UHF (while LF pulses remained small yet visible). Moreover, in few cloud discharges some processes occurring during final stage produced strong VHF-UHF radiation.