Upward Discharges from the Electrified Thunderstorm

Abstract

The upward discharges from the electrified thunderstorms have only been recorded about two decades ago They are termed as jets because of their fountain-like cone shapes Jets are members of the transient luminous events (TLEs) which are large-scale transient optical phenomena occurring between the top of the thunderstorms and the lower ionosphere Characterized by their terminating altitudes the prominent family of jets includes blue starters blue jets (BJs) and gigantic jets (GJs) The ISUAL (Imager of Sprites and Upper Atmospheric Lightning) experiment on the FORMOSAT-2 satellite has continuously and globally surveyed TLEs from space since it was launched in May 2004 As of 30 June 2015 ISUAL has recorded nearly a hundred gigantic jets which are believed to establish a direct discharge channel between the thunderstorm top and the lower ionosphere Besides the known class of negative GJs which are called type I the data reveal that there are two additional kinds of gigantic jets The dynamic evolution of the type I GJs resembles that of the negative GJs observed from the ground Based on the electromagnetic data type I GJs are identified as negative cloud?to?ionosphere?discharge events (?CIs) The dynamic evolution of a type II GJ starts as a blue jet then slowly develops into a GJ The type II GJs are conjectured to be blue-jet-type discharges that initiate between the upper positive charge layer and the negative screen charge layer in a thunderstorm Photometric features indicate that type II GJs are composed of positive streamers A further analysis of the associated atmospherics (also called sferics) of one ISUAL recorded type II GJ indicates that this GJ is a positive upward discharge Type III GJs were preceded by lightning and a GJ subsequently occurred near this preceding lightning The discharge polarity of the type III GJs is expected to vary and depends on the charge imbalance left behind by the trigger lighting On 22 July 2007 thirty-eight jets occurring over a thunderstorm were recorded during a ground campaign One of them is a type II GJ which was the first GJ of this type that was recorded from the ground Based on the images recorded by a cluster of filter?equipped cameras the jets are found to have significant red emissions However the blue emissions from these jets were not discernible due to severe atmospheric scattering A modeling estimation of the emissions from a streamer reveals that the red emissions originated in the upper parts of blue starters and blue jets would be mainly from the first positive system of N2 (1PN2) Since no lightning?associated sferics from this thunderstorm during the jet?generating period were found correlation patterns between the jets and the lightning activity are analyzed from the illumination of clouds due to intra-cloud (ICs) and cloud-to-ground (CGs) lightning The correlation patterns exhibit considerable complexity but also indicate that the occurrence of jets can be affected by the preceding local CG lightning or nearby lightning (IC or CG) while in turn the jets might also influence the ensuing lightning activity ISUAL has recorded many mystic blue luminous events (BLEs) that emit clear middle ultraviolet to blue emissions (230?450 nm) but contain dim red emissions (653?754 nm) Most BLEs appear to be dot-like on the ISUAL images and a few BLEs develop upwardly and slowly into blue jets or type II GJs The associated sferics of the BLEs in the extremely low frequency to the very low frequency (ELF/VLF) band and the low frequency (LF) band exhibit similar features to those of the VLF/LF waveform for the narrow bipolar events (NBEs) The ISUAL BLE is conjectured to be the accompanied light emissions of the NBE-like event Both positive and negative discharge polarity-types for the BLEs have been found Based on the sferics data and the ISUAL optical data a NBE-like event is found to be related to a rapidly-flowing current of the initiation discharge in the thunderstorm while a blue jet or a type II GJ is suggested to be associated with the slowly-varying current of the ensuing discharge The spectral characteristics of BLEs resemble those of the emissions mostly from the non-thermal air plasmas produced by the locally-enhanced electric field rather than those from the hot air plasmas in local thermodynamic equilibrium (LTE)

Date of Award	2015 Sept 10
Original language	English
Supervisor	Rue-Ron Hsu (Supervisor)