The effects of lightning on the ionosphere/magnetosphere: whistlers and ionospheric Alfvén resonator

This chapter will present possible effects of atmospheric lightning on the upper atmosphere such as the ionosphere and magnetosphere composed of ionized plasmas. Though there exist several phenomena on the effect of lightning discharges onto the ionosphere/magnetosphere, we introduce only two major attractive topics: (1) lightning-induced whistlers in the ionosphere/magnetosphere, and (2) ionospheric Alfvén resonator (IAR) in an altitude region between the lowest ionosphere and lower magnetosphere, where one likely candidate of its source is lightning discharges. The former is quite a well-known phenomenon, and whistlers are bursts of ELF/VLF waves produced by lightning discharges. Some part of VLF/ELF lightning energy penetrates through the ionosphere, propagates along the magnetic field line in the magnetosphere, and penetrates again through the ionosphere in the opposite hemisphere, followed by reception on the ground as a whistler. We show initially the phenomena of ground-based whistlers, their brief theoretical explanation, and their use in the diagnostics of ionospheric/magnetospheric electron density. Also, earlier satellite observations of nonducted whistlers are presented. Further, we will describe recent satellite observations of short-fractional hop whistlers and VLF/ELF electromagnetic waves, with special reference to their use in the study of global lightning activity. On the other hand, the latter phenomenon, IAR in the ULF/ELF band is a rather new subject as compared with whistler studies, and so we pay more emphasis on IAR in this chapter. IARs exhibit an interesting feature of fingerprint resonance structures on the dynamic spectra. This IAR is apparently considered to be a kind of resonance of Alfvén waves in a region between the lowest ionosphere and lower magnetosphere, whose resonance frequencies (f = 1-10 Hz) are lower than the well-known Schumann resonances. We present our own statistical results on morphological characteristics of IARs (spectral resonance structures) at middle latitudes as your basis to understand the resonance structure of IARs. Then we will review the physical mechanisms, in other words, the energy source of IAR signatures seems to depend on latitude. Though there are considerable uncertainties in the physical modeling, we will suggest a plausible hypothesis as the origin of IARs at middle and low latitudes; the link to nearby lightning discharges. Lastly, a summary will follow.