This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). The proposal seeks funding to investigate quantitatively the lightning-induced electron precipitation (LEP) using the ELF/VLF (~5 Hz?32 kHz) observations on the Antarctic continent. A coordinated effort will fully characterize individual lightning strokes occurring in the northern hemisphere while simultaneously observing the associated ELF/VLF waveforms in the Antarctic. In addition to advancing knowledge of the ELF/VLF wave studies in general, these observations will be used to differentiate between lightning energy that couples to the ionosphere/magnetosphere and that which couples to the Earth-ionosphere waveguide. Simultaneous observations of LEP will be conducted in the northern and southern hemispheres in order to: (1) quantitatively relate the upward-coupling lightning energy and LEP production, and (2) evaluate the conditions under which LEP occurs in both hemispheres. Rocket-triggered lightning in Florida allow for the direct measurement of lightning currents and will contribute to both the lightning- and LEP-related research efforts. The Antarctic research stations are ideally situated to perform both the long-distance ELF/VLF lightning observations and the VLF scattering observations for LEP detection in the conjugate region, necessary for the success of this program. Continued reliance on graduate and undergraduate students provides a broader impact to this proposed research and firmly grounds this effort in its educational mission.
NSF Award ID 0944639: Annual Project Report R. C. Moore, University of Florida Final Report: Project Outcomes This is the final report on our project "Collaborative Research: Antarctic ELF/VLF Observations of Lightning and Lightning-Induced Electron Precipitation." This program is jointly conducted with Dr. Antony Fraser-Smith of Stanford University and supports the valuable long-duration ELF/VLF observations at Arrival Heights and VLF observations at Palmer Station. Our efforts aim to quantitatively investigate lightning and lightning-induced electron precipitation (LEP) using ELF/VLF (~10 Hz - 32 kHz) observations on the Antarctic continent. Observations are used to discern between lightning components that promote the upward-coupling of lightning energy and those that excite the Earth-ionosphere waveguide. These results are directly applied to interpret observations of LEP in both the northern and southern hemispheres. Furthermore, this program addresses a novel concept by critically evaluating whether rocket-triggered lightning may be detected in the Antarctic, allowing for controlled broadband ELF/VLF propagation studies, and whether it may produce LEP in the conjugate hemisphere. Over the 3.5-year period of this program, the University of Florida (UF) has produced 5 refereed journal publications, 1 refereed conference proceeding publication, 5 invited conference oral presentations, 10 oral conference presentations, and 19 conference poster presentations. This project provided the primary support for 3 UF Ph.D. students. I summarize our most important project outcomes below. Operational: Field Deployments The UF team has deployed to Antarctica four times over the course of this program. During these deployments, we have maintained and successfully upgraded three VLF receivers (at Palmer Station, Arrival Heights, and South Pole) and one ELF receiver (at Arrival Heights) in the southern hemisphere. We have conducted extensive noise surveys (at all three stations) and determined that the wind turbines and UPSs at Arrival Heights and the power grid at South Pole produce a significant level of ELF/VLF hum. We have successfully operated each of the receivers over the course of the program, recording broadband (100 kHz) data nearly continuously at all sites. Rocket-Triggered Lightning UF has successfully triggered lightning (in the northern hemisphere) over 30 times over the course of this program. We report 10 rocket-triggered lightning flashes that have generated ELF sferics observed at distances greater than 4,000 km from the lightning flash, including in Antarctica. Based on our observations, we conclude: 1) The spectrum of rocket-triggered lightning (as measured at the lightning channel base) is highly variable. Peak currents measured at the base of the lightning channel (with 5 MHz bandwidth) are not proportional to peak currents measured with lower bandwidths. This result implies that lightning detection networks may not accurately represent the peak currents of lightning that produce electron precipitation events. 2) We have successfully detected the ELF sferic emanated by rocket-triggered lightning at multiple receiver sites around the globe, including at Arrival Heights, Antarctica (~13,000 km distant). 3) We have successfully implemented a broadband ELF propagation model that is now in use to determine how the lightning channel best excites the Earth-ionosphere waveguide as well as the Schumann resonances. The comparison between this model and Antarctic observations will provide the Ph.D. thesis topic for UF graduate student Neal Dupree. 4) The ELF sferic emanated by rocket-triggered lightning is proportional to the current moment of the lightning current waveform. Lightning Mapping Array observations and Antarctic ELF observations together indicate that the lightning channel may lengthen between return strokes, with the result that the same peak current can produce a larger current moment during subsequent return strokes. Lightning-induced Electron Precipitation Over 200 lightning-related ionospheric disturbances were detected using the sensitive VLF remote sensing technique. All events were associated with natural lightning, rather than rocket-triggered lightning. On this topic, our program contributed the following results: 1) A new method for VLF remote sensing of ionospheric disturbances was developed and serves as the Ph.D. thesis topic for UF graduate student Michael Mitchell. The method allows for amplitude and phase observations at multiple frequencies to be performed. Propagation within the Earth-ionosphere waveguide is highly frequency dependent, and we believe this new technique will significantly advance the modeling of VLF scattering by ionospheric disturbances. 2) UF, together with colleagues at the University of Colorado Denver, developed an LEP model that is driven by the currents measured at the base of the triggered lightning channel. The model predicts that the energy distribution of precipitating electrons is significantly altered by the spectrum of the causative lightning waveform. 3) UF reported simultaneous observations of LEP events in both the northern and southern hemisphere in GRL using observations at Palmer Station. Simultaneous observations of the lightning-generated whistler waves are currently being analyzed in order to experimentally quantify the relative timing of the events.
