In order to fully characterize the effect of lightning on ionospheric density structures, this project will deploy co-located ionosphere and lightning RF arrays at HF (1-30 MHz) for D- and E-region imaging, and VLF (0-500 kHz) and VHF (120-150 MHz) radars for lightning studies. The observational program will be augmented by computations from the Ionospheric Data Assimilation Four-Dimensional (IDA4D) model. The model will perform tomographic reconstruction of the ionosphere for heights extending from the E region to above using the observations obtained as well as other opportunistic E region data sources, most notably from the Los Alamos Portable Pulser (LAPP) with the Cibola Flight Experiment (CFE) satellite. The goal of these activities is to elucidate coupling processes between the troposphere and ionosphere, in particular the interaction between lightning and E region ionization enhancements known as sporadic-E. Current thinking suggests that there are two primary mechanisms that transport energy from the troposphere to the lower ionosphere: the first is mechanical wave activity and the second is electrical effects associated with lightning, including electromagnetic pulses and relativistic electrons. The mechanical coupling of waves may increase the peak plasma densities of sporadic-E layers before propagating into the F-region. Discharges from lightning couple electromagnetically and may increase the peak plasma densities by creating more long-lived metal ions from the ambient population of meteoric metal atoms found at these altitudes. However, very little is known about the electromagnetic coupling between lightning and the ionosphere, even though observations of Transient Luminous Events (TLE) indicate that interactions between thunderstorms and the middle and upper atmospheres do occur. The goal is to make significant progress in answering the following questions on ionosphere/troposphere coupling: (1) What is the coupling mechanism between lightning emissions and the development of sporadic E layers? (2) What is the relationship between lightning emissions and the variations of conductances in the ionosphere? (3) To what degree can we use measured RF values of the lightning emissions and characterization of the ionosphere to simultaneously model and analyze the physical characteristics of lightning emissions and the detailed structuring of the ionospheric response?
The work performed under the TALIS project was designed to provide a better understanding of how thunderstorms affect the ionosphere. Two specific ionospheric anomalies were of interest, Traveling Ionospheric Waves (TIDs) and Sporadic-E. TIDs are waves which travel though the ionosphere (like ocean waves though water) and cause variations in the electron density as the move. Sporadic-E is caused by dense clouds of higher electron density in the ionosphere and does not have a regular variation like a wave has. Both these anomalies cause trouble with HF and VHF communications and are of particular interest to ocean-going vessels and other users that depend on long-range and satellite communications using these frequencies. A better understanding of these events could be used to provide more accurate forecasts of communication outages so that users could either shift frequencies or transmit important messages and data before the outage occurs. During the award period of this work, measurements of the ionosphere were made in coordination with lightning, in particular a lightning mapping array or LMA. The LMA provides detailed 3-D pictures of lightning within the cloud and would help us understand if a particular lightning type or geometry had a greater effect on the ionosphere than other lightning types. Initial results show that local thunderstorms produce TIDs with lower amplitude but higher frequency (shorter wavelength) than TIDs observed when no local storm was present. These may have been produced by storms outside our observation area or by other atmospheric effects. Data during winter periods when no storms were present in the US are currently being analyzed in order to determine which of these cases are responsible for the production of the high-amplitude TIDs.
