[1] Computer simulations on transient luminous emissions in the mesosphere and lower ionosphere have been performed for realistic lightning modelings with fast-varying current surges (M components) superimposed on the lightning continuing current (CC). The algorithm used here is an electromagnetic (EM) code, which enables us to estimate self-consistently the reduced electric field, electron density, conductivity, and luminosity as a function of space and time by solving the Maxwell equations. It is found that M components in the CC with small amplitudes, but with a fast-varying EM effect, can initiate or enhance the occurrence of sprites. Even for a return stroke (RS) without CC, subsequent high-frequency current variations (like M components) are found to lead to dramatic changes in the sprite occurrence. The physics underlying these changes is studied by means of, e.g., temporal and spatial variations of luminosity, electron density, and conductivity. As the conclusion, the RS is a fundamental agency for spites, but high-frequency variations as EM effects exhibit an additional essential influence on sprite occurrence. These computational results are used to offer some useful ideas concerning the unsolved problems of sprites and halos, including polarity asymmetry, long-delay characteristics, and morphological shapes of sprites.

[1] We demonstrate that the timescale of charge removal by lightning is an essential parameter for the criterion of sprites along with the charge moment. Sprites can be initiated under small charge moment changes of several hundreds of Coulomb kilometers if the discharge time is sufficiently short as hundreds of microseconds, whereas they cannot be done even with 1000 C km if the discharge time is in the order of milliseconds. We generalize the criterion as an analytical expression of charge moment and discharge time and discuss its sensitivity to the ambient plasma conductivity based on the result of the classical electrostatic model simulation. Our result can explain almost the critical values of charge moment for sprite initiation recently derived from analysis of lightning-induced electromagnetic pulses. We deduce from our criterion that charge moment changes necessary for halo are smaller than those for streamers because the former can be initiated without significant ionization. Considering the role of charge moment and discharge time, we discuss further the condition for initiation of various classes such as a halo and a cluster of streamers with or without halo.Citation: Hiraki, Y., and H. Fukunishi (2006), Theoretical criterion of charge moment change by lightning for initiation of sprites,

[1] We analyze optical data of twenty sprites and three halos observed by the array photometer which is a scientific instrument of the ISUAL payload on the FORMOSAT-2 satellite. The altitude distribution of electric field is derived from the ratio of blue to red emission intensity by assuming all emissions are due to the electron impact excitation of nitrogen molecules. We find a clear transition at $75 km altitude from the upper-diffuse to lower-streamer region. Estimated electric field intensities in the diffuse region are 0.5-0.7 E k , which support the theoretical expectation that their optical emissions could be produced without significant ionization. On the other hand, those in the streamer region are 1 -2 E k which is a few times less than predicted fields in the streamer head. We suggest that this discrepancy is due to the long-lasting components such as the lower portions of the upward branches and bead structures. Citation: Adachi, T

The fundamental electrodynamical coupling processes between lightning and sprites are investigated. By combining the observed spectral data with the Monte Carlo swarm experiments, reduced electric fields and electron energies in sprite streamers and halos are estimated. The obtained fields inside sprite halos (70–97 Td with an analysis error of ±5 Td) are lower than the conventional breakdown field, Ek ∼ 128 Td, indicating a significant reduction of electrons associated with halos while those in sprite streamers (98–380 Td with an error of ±50 Td) are higher than Ek, suggesting that a significant ionization process drives their formation and development. A combined analysis of photometric and electromagnetic data makes it possible to estimate temporal evolutions of lightning charge moment. It is found that lightning discharges with a short time scale (∼1 ms) and a moderate amount of charge moment (∼400 C km) produce discernible halos. On the other hand, lightning discharges with a large amount of charge moment (∼1300 C km) produce streamers regardless of their time scale. The results obtained are comprehensively interpreted with both the conventional breakdown field necessary for the formation of streamers and the electric field necessary for the production of optical emissions of halo which is sensitive to the time scale of the thundercloud field due to the significant reduction of electrons.

Abstract. We simulate auroral electron precipitation into the Jovian atmosphere in which electron multi-directional scattering and energy degradation processes are treated exactly with a Monte Carlo technique. We make a parameterization of the calculated ionization rate of the neutral gas by electron impact in a similar way as used for the Earth's aurora. Our method allows the altitude distribution of the ionization rate to be obtained as a function of an arbitrary initial energy spectrum in the range of 1-200 keV. It also includes incident angle dependence and an arbitrary density distribution of molecular hydrogen. We show that there is little dependence of the estimated ionospheric conductance on atomic species such as H and He. We compare our results with those of recent studies with different electron transport schemes by adapting our parameterization to their atmospheric conditions. We discuss the intrinsic problem of their simplified assumption. The ionospheric conductance, which is important for Jupiter's magnetosphere-ionosphere coupling system, is estimated to vary by a factor depending on the electron energy spectrum based on recent observation and modeling. We discuss this difference through the relation with fieldaligned current and electron spectrum.