CEDAR: Modification of Ionosphere by Lightning ElectroMagnetic Pulse (LEMP): Dependence on Lightning Type, Stroke Order, and Other Factors

Atmospheric lightning is a very familiar phenomenon seen during thunderstorms. However, it is not well-known that lightning strokes seen in the lower atmosphere could also travel upward as an electromagnetic pulse, modifying the properties of the upper atmosphere. The study of lightnings is especially useful for the understanding of the ionosphere, which is the layer of atmosphere above 50 miles altitude that has abundance of charged particles such as ions and electrons.

Cumulative effects of lightnings could potentially modify the ionosphere, which in turn affect radio communication and global navigation.

This project will carry out a cross-disciplinary study of the modification of ionosphere by Lightning Electro Magnetic Pulse (LEMP). The atmospheric conductivity profile that largely determines the apparent ionospheric reflection height is expected to be modified as a result of ionization caused by LEMP. This modification will be detected via comparison of conductivity profiles inferred from electromagnetic field signatures (including skywaves) of individual lightning strokes occurring within the same flash.

There is some evidence of cumulative effect of multiple strokes in lowering the apparent ionospheric reflection height. For the first time, ionospheric reflection heights estimated using in-cloud lightning pulses (elevated sources) and return-stroke pulses in cloud-to-ground flashes (CGs) (near-ground-level sources) of different polarity will be compared.

This project will attempt to answer the following new scientific questions (SQs) that are relevant to the primary objective of the CEDAR Program, with emphasis on perturbations (LEMPs) that propagate upward from the lower atmosphere: 1. How is the atmospheric conductivity profile changing on the time scale of individual lightning flash?

2. What are the limits of applicability of the Hetzian dipole model that is widely used in studying lightning interaction with the ionosphere?

3. Why is the apparent ionospheric reflection height for positive lightning larger than for negative lightning in the same thunderstorm? 4. How important are lightning processes other than negative return strokes in CGs in disturbing the lower ionosphere?

Wideband LEMPs will be recorded at the well-established facilities (Lightning Observatory in Gainesville and Golf Course Station) in Florida, at distances ranging from 50 to 500 km, in different seasons, over land and over sea, under both daytime and nighttime conditions. Data acquired will be used to address SQs 1, 3 and 4. A Finite-Difference Time-Domain (FDTD) model of LEMP propagation in the Earth-ionosphere waveguide in the spherical coordinate system will be used to address SQs 1 to 3.

An optimization procedure to find best-fit parameters of equivalent exponential atmospheric conductivity profile from measured LEMP waveforms (including skywaves) produced by individual strokes of a flash will be used for SQ 1. The model will employ realistic lightning sources and cover an essentially full frequency bandwidth of interest (up to hundreds of kilohertz).

Research results will be used in the popular University of Florida senior undergraduate/graduate course “Lightning”. The project researchers will remain committed to high-school science education and to enhancement of public awareness of lightning hazards. The studies will continue to foster international collaboration on FDTD modeling.

The results of this project will have implications for other disciplines, including radio communication, ionospheric remote sensing, assessment of methods for artificial modification of the ionosphere, ionospheric chemistry, and ionospheric plasma physics.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.