Broadband radio mapping and imaging of lightning processes

Though thunderstorms and lightning are commonplace on Earth, it is still unclear how lightning initiates, propagates, and how it is involved in generating intense bursts of gamma-rays that can be detected by spacecraft. Lightning is a hot, highly-ionized plasma channel, capable of carrying up to hundreds of kiloamperes electric current, and extending many kilometers in length for hundreds of milliseconds at a time. Despite its immensity, lightning can be difficult to observe, as it primarily initiates and propagates deep within thunderclouds, optically obscured by thousands of cubic kilometers of cloud water and ice. Broadband radio interferometry has been developed to study lightning at radio frequencies, offering us a way to “see” inside the clouds. The technique, which is still in its infancy for lightning research, allows for lightning radio emissions to be mapped and/or imaged with extremely fine time resolution. In this dissertation, a newly-developed three-element, broadband VHF (~14-88 MHz), 16-bit radio interferometer (INTF) is used to investigate extremely transient thunderstorm electrical phenomena involved in lightning initiation, propagation, and high-energy photon production. The investigations demonstrate the novel science that can be done with the INTF system, and reveal previously unforeseen dynamics of lightning formation. Specifically, we image and map the VHF emissions of narrow bipolar events (NBEs), initial breakdown pulses (IBPs), and an energetic in-cloud pulse (EIP) with sub-microsecond resolution. NBEs have long been of interest to the lightning community because they are the most powerful natural emitters of high-frequency and very-high-frequency radio waves on Earth. Moreover, NBEs are readily identifiable by their narrow (~10 µs wide), bipolar sferics (~3 kHz-3 MHz radio emissions). NBEs are not lightning, but appear to be a precursor to lightning, occurring either in complete isolation, or at the beginning of a lightning flash. IBPs, in contrast, never occur in isolation, but rather are the hallmark of lightning channel formation. IBPs typically occur in long trains of sferic pulses, and indicate the imminence of lightning during the first milliseconds after lightning initiation. An IBP is also identified by its sferic, having a bipolar waveform some tens of microseconds wide, the initial pulse of which is superimposed by ~1 µs-wide subpulses. Lastly, EIPs are high-peak-current (\u3e200 kA) events that are involved in the generation of terrestrial gamma-ray flashes (TGFs), which are intense bursts of gamma-rays that radiate out the tops of thunderclouds and are detected in space. EIPs have a signature high-amplitude, ~50 µs-wide sferic, which is time-aligned with satellite-borne gamma-ray detections. EIPs can thus serve as a proxy for TGFs, offering a way to investigate TGFs using ground-based radio sensors, without necessarily needing satellite data. The physical natures of NBEs, IBPs, and EIPs have been active areas of research over the last decade. For over half a century, the role that IBPs play in initial hot channel formation has been under debate. More recently, intense investigation has been focused on exactly how NBEs are involved in lightning initiation. Just in the last few years, EIPs were discovered, offering a new way to investigate the role that lightning plays in TGF generation. By investigating NBEs with the INTF, we discovered a newly-identified form of streamer-based breakdown, termed fast negative breakdown, that does not fit with our current understanding of lightning initiation. Streamers are cold filamentary plasma channels, and based on conventional dielectric theory, it was hypothesized that lightning should be initiated by positive streamers, which carry electric current in their propagation direction. However, fast negative breakdown carries electric current opposite its propagation direction, propagating ~500 m through virgin air with an unusually fast speed of ~10^7 m/s. Aside from breakdown polarity, fast negative breakdown is in many ways similar to recently reported fast positive breakdown that generates the majority of NBEs, and that is expected from conventional dielectric theory. We additionally show that similarly fast breakdown is involved in the production of both IBPs and EIPs. Using the INTF, we show that the IBP process is dominated by a fast-propagating ∼10^7 m/s streamer-based negative breakdown that propagates the channel about ~100 m into virgin air, similar to the fast negative breakdown associated with NBEs. We show that the streamer-based channel extension leads to a sustained electric current, indicating the existence of a hot conductive lightning channel. Fast-propagating ~10^7-10^8 m/s breakdown of both polarities is also a prominent feature during the EIP, but occurs over a larger (\u3e1-km altitude) volume than during NBEs or IBPs. We show that repeated downward- and upward-propagating fast positive and negative breakdown are somehow coupled to the generation of relativistic electrons and associated ionization. We conclude that the electric current that produces the EIP sferic is generated by a newly discovered type of self-sustaining discharge termed a relativistic feedback discharge (RFD), which involves multiple generations of relativistic electron avalanches and back-scattered positrons and X-rays. Our study further demonstrates that TGFs can be produced by RFDs. The INTF was developed by New Mexico Tech, and deployed and operated at Kennedy Space Center (KSC) in Florida during summer 2016 to obtain the data used herein

State of the Art: Small Spacecraft Technology

This report provides an overview of the current state-of-the-art of small spacecraft technology, with particular emphasis placed on the state-of-the-art of CubeSat-related technology. It was first commissioned by NASAs Small Spacecraft Technology Program (SSTP) in mid-2013 in response to the rapid growth in interest in using small spacecraft for many types of missions in Earth orbit and beyond, and was revised in mid-2015 and 2018. This work was funded by the Space Technology Mission Directorate (STMD). For the sake of this assessment, small spacecraft are defined to be spacecraft with a mass less than 180 kg. This report provides a summary of the state-of-the-art for each of the following small spacecraft technology domains: Complete Spacecraft, Power, Propulsion, Guidance Navigation and Control, Structures, Materials and Mechanisms, Thermal Control, Command and Data Handling, Communications, Integration, Launch and Deployment, Ground Data Systems and Operations, and Passive Deorbit Devices

Modulating Retro-Reflector CubeSat Payload operating at 1070 nm for Asymmetric Free-Space Optical Communications

Preceding papers submitted to this conference introduced the concept of modulating retroreflectors (MRR) for free space optical communications. The major advantage of MRRs over conventional laser communication systems is that they require significantly less pointing accuracy on the spacecraft; typical values are between a few degrees to 10s of degrees. However, this advantage is bought at the price of an increased optical power from the ground station on the order of several kilowatts. Lasers capable of producing these relatively high continuous optical powers are commercially available, but only in a limited subset of wavelengths, typically ranging from 1.0 to 1.1 microns. Consequently, to take advantage of these commercially available lasers, MRRs operating in a corresponding wavelength, at sufficiently high data rates, with an adequate aperture are required. Modulators based on multiple quantum wells fulfill the latter two requirements but had not been demonstrated in the specified wavelength range. In the first part of this paper we describe design, production and testing of a multiple quantum well MRR that operates at a wavelength of 1070 nm. This wavelength corresponds to that of the popular Ytterbium doped fiber lasers which are extensively used in laser machining. The second part of the paper describes the design and testing of driver electronics in a 1U form factor, allowing the MRR to operate in a cubesat. The driver is versatile and can therefore act either as a communication subsystem or an independent payload that could be utilized for a technical demonstration of the MRR technology in space

Implications of Multiple Corona Bursts in Lightning Processes for Radio Frequency Interferometer Observations

Recent observations from LOFAR indicate that multiple, spatially distributed corona bursts can occur in lightning processes with a timescale of 10 microseconds. The close proximity of the corona bursts in space and time poses a great observation challenge for short-baseline (typically ≤100 m) radio interferometers. This paper reports simulations to show the interferometry results that would be obtained with such an interferometer. In particular, spatially-separated corona bursts at fixed locations may be seen as a fast (>107 m/s) propagating source with large power variation if the resolution of the instrument is greater than the spatial separation of the bursts. The implications and suggestions for lightning interferometry studies are discussed in the paper