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Formation of filamentary gas discharge forms, commonly referred to as streamers, is one of the conditions required for initiation and subsequent propagation of lightning leaders. It is quantitatively demonstrated that streamers can be initiated under thunderstorm conditions when two precipitation particles cause an enhancement of the electric field by passing in close vicinity of each other. Conditions for avalanche to streamer transition are documented using a model of two spherical hydrometeor particles placed in uniform ambient field. The results are presented in scaled form using similarity relations for gas discharges and can be applied for a wide range of thunderstorm conditions, including different air pressures, electric fields and particle dimensions.

This work analyzes different current source and conductivity parameterizations and their influence on the diurnal variation of the global electric circuit (GEC). The diurnal variations of the current source parameterizations obtained using electric field and conductivity measurements from plane overflights combined with global Tropical Rainfall Measuring Mission satellite data give generally good agreement with measured diurnal variation of the electric field at Vostok, Antarctica, where reference experimental measurements are performed. An approach employing 85 GHz passive microwave observations to infer currents within the GEC is compared and shows the best agreement in amplitude and phase with experimental measurements. To study the conductivity influence, GEC models solving the continuity equation in 3‐D are used to calculate atmospheric resistance using yearly averaged conductivity obtained from the global circulation model Community Earth System Model (CESM). Then, using current source parameterization combining mean currents and global counts of electrified clouds, if the exponential conductivity is substituted by the conductivity from CESM, the peak to peak diurnal variation of the ionospheric potential of the GEC decreases from 24% to 20%. The main reason for the change is the presence of clouds while effects of 222Rn ionization, aerosols, and topography are less pronounced. The simulated peak to peak diurnal variation of the electric field at Vostok is increased from 15% to 18% from the diurnal variation of the global current in the GEC if conductivity from CESM is used. This research was supported by NSF under grant AGS-1135446. CESM can be obtained through http://www.cesm.ucar.edu. RBF model is available at https:// bitbucket.org/vbayona/gec_rbffd. FVS model is fully described in cited references. Ali data representing figures, current sources, and conductivity profiles used for simulations are available at https://doi.org/10.18113/033958.

This paper contributes to the understanding of the influence of conductivity perturbations on the ionospheric potential in the Earth's global electric circuit (GEC). The conductivity perturbations appearing in the middle atmosphere produced by γ ray bursts from magnetars are studied first. The transient response of the ionospheric potential is modeled in this case, and timescales of interest are identified (0.01–10s). In this case modification of ionospheric potential is small. Additionally, the principal effects of topography and reduction of conductivity inside the thundercloud are studied. Both of these factors effectively increase the ionospheric potential for a classic source in the GEC represented by a current dipole leading to formation of two main charge centers of the thunderstorm. On the other hand, for GEC including topography and conductivity reduction in thunderclouds the contribution of sequence of negative cloud-to-ground lightning discharges to the ionospheric potential is decreased. Simulation results show a very good agreement with equivalent circuit models for conductivity perturbations with horizontal dimensions exceeding 20 km.

We have developed a time-dependent model of global electric circuit (GEC) in spherical coordinates. The model solves time-dependent charge continuity equation coupled with Poisson's equation. An implicit time stepping is used to avoid a strict dielectric relaxation time step condition, and boundary conditions for Poisson's equation are implemented to allow accurate description of time evolution of the ionospheric potential. The concept of impulse response of GEC is introduced that allows effective representation of complex time dynamics of various physical quantities in the circuit using model results obtained for instantaneous deposition of a point charge. The more complex problems are then reconstructed using convolution and linearity principles. For a point charge instantaneously deposited at a typical thundercloud altitude the impulse response of the charge density shows induction of the same value and polarity charge at the ionospheric boundary, while charge of the same value but opposite sign is moving down logarithmically with time and neutralizes the source point charge on time scale corresponding to the dielectric relaxation time at altitude of the source point charge. The ionospheric potential is modified immediately with input of the source point charge based on free space solution of Poisson's equation. Then the ionospheric potential relaxes. It is shown that during formation of two main charge centers of the thundercloud, typically represented by a current dipole, the ionospheric potential can be determined from the difference of time integrals of two ionospheric potential impulse responses corresponding to charge locations at the opposite ends of the current dipole. For latitude- and longitude-independent conductivity model, the total charge on the Earth is exactly zero at all times. During cloud-to-ground lightning discharge, the ionospheric potential changes instantaneously by a value proportional to the charge moment change produced by lightning and then relaxes to zero. For a typical charge moment change of 35Ckm and lightning frequency 10s−1, the ionospheric potential changes by 9.3kV; this value agrees well with the results presented by Rycroft et al. (2007) and Rycroft and Odzimek (2010).