1. Introduction

This work describes optical measurements of high altitude discharges which represent electrodynamic coupling between thunderstorms and the lower nighttime ionosphere of Earth. Over the last decade such processes have for the first time been studied extensively both experimentally and theoretically. These discharges occur high above the troposphere but overlie thunderstorm systems and are known to cause electron heating, modification of the electron density, and the production of optical emissions as a result of strong electric fields following lightning activity in the troposphere. Two distinct discharge mechanisms lead to detectable optical emissions in the lower ionosphere and correspond to two distinct optical phenomena, `elves' and `sprites.' In `elves,' an electromagnetic pulse (EMP) launched by a cloud-to-ground (CG) lightning stroke impinges on the lower ionosphere, is partially absorbed, and causes optical emissions between 80 and 95 km altitudes over a horizontal region several hundred km wide and over a period of $\sim$1 ms. Sprites consist of longer laster (up to $\sim$100 ms) emissions which may be highly structured and can extend from 40 to 85 km altitudes and over $\sim$10 km horizontally. Sprites occur in response to the intense electric fields developed in the high altitude, thin, conducting atmosphere following a major redistribution of electric charge in the troposphere -- typically a positive cloud-to-ground lightning return stroke, which moves positive charge from a cloud to the Earth's surface.

Sprites and elves have sparked scientific interest in part as a result of their effect on the propagation of very low frequency (VLF) radio waves travelling between the ground and the ionosphere, their contribution to the natural radio frequency spectrum, their role in the global electric circuit, and their demonstration of a novel mechanism of coupling between atmospheric regions.

In this chapter some quantitative properties of cloud-to-ground lightning are outlined (Section 1.2) and an overview of the physical factors governing the existence of high altitude discharges is given in Section 1.3. We begin by mentioning some theoretical foundations.