Impulsive Coupling Between the Atmosphere and Ionosphere/Magnetosphere

This review covers various aspects of the impulsive coupling in the ULF frequency range between atmospheric discharge processes and upper ionosphere. Characteristic feature of the upper ionosphere is the occurrence of the ionospheric Alfven resonator (IAR) and MHD waveguide, which can trap the electromagnetic wave energy in the range from fractions of Hz to few Hz. Induction magnetometer observations at mid-latitude stations are considered as an example of a transient ULF response to the regional and global lightning activity. For many events, besides the main impulse produced by a lightning discharge, a secondary impulse delayed about 1 sec was observed. These secondary echo-impulses are probably caused by the partial reflection of wave energy of the initial lightning pulse from the upper IAR boundary in the topside ionosphere. The multi-band spectral resonant structure (SRS) can be formed owing to the occurrence of paired pulses in analyzed time series. The statistical superposed epoch method indeed has revealed a dominance of two-pulse structure in the magnetic field background during the periods of the SRS occurrence. The numerical modeling shows that during the lightning discharge a coupled wave system comprising IAR and MHD waveguide is excited. In the lightning proximity (about few hundred km) the amplitudes of radial component is 1–2 orders less than those of the azimuthal component, and only the lowest IAR harmonics are revealed in the radial magnetic component. At distances ～10³?km the spectral power densities of both components are comparable, and the SRS is more pronounced. The problems and further prospects of the study of the impulsive magnetosphere–ionosphere–atmosphere coupling via transient processes during thunderstorms are discussed.