Instability and Threshold of ULF Waves Triggered by Positive Cloud to Ground Lightning

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In this article we explore theoretically the role of lightning and atmospherical discharges in destabilizing Ultra Low Frequency (ULF) waves in the Ionosphere. The idea, which could seem quite exotic, takes its origin from the experimental observation that the occurrence of Ultra Low Frequency ULF ~ 6–50 Hz) waves is strongly related to the occurrence of positive polarity cloud-to-ground flashes (+CG) The interest in the subject, on the other hand, comes from the fact that the occurrence of ULF waves is not only related to the occurrence of positive polarity flashes but also to the occurrence of newly discovered transient luminous phenomena as sprites, blue jets and gigantic jets. Sprites, as well as blue jets and gigantic jets, are luminous events which have been detected in discharges between thunderclouds (~ 10 km from Earth's surface) and the lower Ionosphere (mesosphere ~90 km) These transient luminous phenomena strongly affect the atmospheric electricity, due to transfer of large amounts of charge between different regions of the atmosphere (a gigantic jet can remove as much as 0.02% of the total atmospheric charge), and suggest that some important components of the global electric circuit have still to be identified and incorporated in the theoretical framework. Another important aspect is that, during the charge transfer process (electrical discharge), sprites, blue jets and gigantic jets modify the chemistry of a large portion of the stratosphere and mesosphere, with poorly understood influences on global climate changes. In order to survey the total rate of occurrence and the implications of such phenomena on continental and global scale one needs a signature that can be easily detected by remote measurements and independent from in situ video observations. To understand whether the ULF waves can be used as sprites signatures, we explore the relation between positive polarity lightning and ULF waves by presenting a theory which explains the origin and threshold of the observed ULF waves. As a result, we obtain that very large positive polarity flashes give rise to ULF Dust Acoustic waves in the Ionosphere. We also find that the ULF waves may be destabilized, in absence of positive polarity flashes, by mean of very large Ionosphere to thunderclouds discharges, such as gigantic blue jets, which create large dipole electric fields, comparable with the ones produced by +CG lightning.