The investigators will develop a full 3-dimensional time-domain model of the lightning-ionosphere interaction, including nonlinear effects and propagation into the magnetosphere. The model will enable accurate measurements of the effects of lightning on the lower ionosphere (70-110 km), in terms of electron density depletions and enhancements, as well as estimates of the fractional energy transmitted through the ionosphere into the magnetosphere. The model calculations can be calibrated against parameters such as the background ionospheric density, lightning peak current and duration, time of day, and type of lightning (cloud-to-ground versus in-cloud). Using this information, a global estimate of the density enhancement due to lightning can be made from existing satellite and ground-based worldwide lightning data. Comparison with analytical models and experimental data will also be made. The coupling of lightning energy to the ionosphere and magnetosphere has remained an unsolved problem for decades. The study will quantify the coupling process through both electrostatic and electromagnetic energy, both on a per-stroke basis and on a global basis, through comparison with worldwide lightning distribution statistics. The model results will provide calibration of lightning energy to the ionosphere and magnetosphere as a function of lightning peak current, duration, geographic location, time of day, and ionospheric conditions. This study will contribute greatly to understanding of the energy coupling between the lower atmosphere and the D-region ionosphere due to lightning, as well as coupling of energy into the inner magnetosphere. Lightning-generated whistler-mode waves are responsible for precipitation of energetic particles from the radiation belts, in the form of Lightning-induced Electron Precipitation (LEP) events. The model will be instrumental in support of the upcoming ASIM (European Space Agency) and TARANIS (CNES, France) satellite missions, which will monitor TLE activity globally beginning in 2012. The model will be disseminated to interested users and will readily run on a reasonable desktop computer; as such, scientists studying TLEs can determine the electric field intensities and ionospheric disturbances associated with their optical observations. The same comparisons will be enabled with ground-based observations of Transient Luminous Events.
In this program, we have developed a numerical model of the electromagnetic pulse (EMP) which is emitted by a lightning discharge. This lightning EMP radiates in all directions, and can be detected as a radio pulse thousands of miles away. In addition, the EMP propagates up to the Earth’s lower ionosphere, and some of the energy is transmitted through the ionosphere into space. The interaction with the ionosphere includes heating of ionospheric electrons, the creation of new ionization, and the excitation of optical emissions. These optical emissions have come to be known as "elves" — brief flashes of lightning at 90 km altitude directly above a lightning discharge. The numerical model developed here is a complete model of the lightning EMP, from the generation of radio waves by the lightning return stroke; their propagation through a 3D space that includes realistic ground, atmosphere, ionosphere, and magnetic field; and the interaction with the ionosphere, including heating, ionization, and optical emissions. We also simulate what these "elves" would look like from a camera on the ground or on a satellite, to directly compare with data. Through a series of simulations, we have been able to explain a number of outstanding questions about elves and sprites, another form of optical emission that occurs at high altitudes. We have also found a number of interesting effects that were not expected: for example, the heating of the ionophere by the lightning EMP directly affects the propagation of that same EMP through the ionosphere, sometimes by a factor of 10. This could not have been predicted by any other type of model. We also found that the magnitude and direction of the Earth’s magnetic field has a profound effect on this heating. Most recently, this model has been used to compare with elve observations from the ground. We have been able to use these ground observations together with the model to measure the source parameters of the lightning discharges that caused the elves. These parameters - the current rise time, return stroke speed, etc - cannot be directly measured, so the indirect measurement through elves is extremely valuable. We have also been able to predict the occurrence of elves as a function of the lightning source parameters, and have extrapolated that prediction to calculate the total number of elves around the globe per year, and the total heating of the ionosphere by lightning. We have found that the effect of lightning on the lower ionosphere through heating is the deposition of tens of terajoules (TJ) of energy per year. For comparison, a single lightning discharge contains a few gigajoules (GJ) of energy, where 1 TJ = 1000 GJ; but most of that energy is dissipated as heat in the lower atmosphere.
