Introduction

A serendipitous video observation of what was then called a ``cloud-to-stratosphere discharge'' (Franz et al.(1990)) provided the first recorded image of optical emissions in the middle- and upper-atmosphere associated with thunderstorms, and launched a rapidly evolving field of both observational (Rodger(1999),Sentman(1998)) and theoretical efforts (reviewed in Rowland(1998)). Since then a number of phenomena have been discovered, which are now called sprites, blue jets and blue starters, and elves. The common sources of energy driving these processes in the stratosphere and mesosphere are tropospheric thunderstorms and lightning. The electric field generated by these storms heats ambient electrons which result in electron impact excitation of the ambient leading to the observed optical emissions. Thus, these emissions are evidence of rapid transmissions of electromagnetic energy from the troposphere, across the stratopause, to altitudes as high as the lower thermosphere/ionosphere. In this paper, we discuss optical and temporal characteristics of sprites, blue jets and starters, and elves. Our primary focus will be on the most recent observations to provide insight into the energetics of these events. Modeling efforts will also be discussed with regard to energy implications. Although there are numerous issues remaining to be resolved prior to including sprites in our global understanding of the middle- and upper-atmosphere, this paper provides atmospheric scientists information necessary to consider possible mesospheric effects of these newly discovered phenomenon.

The optical energy (between 395-700 nm) radiated by a sprite has been estimated to be 12-60 kJ per event (Sentman et al.(1995b)). The radiated optical energy represents only a small fraction of the total energy deposited in the middle- and upper-atmosphere. This energy was calculated by convolving spectral observations with the camera response. Based on these values of radiated energy and a number of assumptions about the kinetic processes which affect the emissions in this wavelength range, estimates of the total energy deposition into and (vibrational and electronic states), range from ~ 250 MJ to ~1 GJ per event (see section 3.1 for details of this optical based energy estimate). For comparison, recent calculations of the available energy in a sprite, made using electrostatic field magnitude considerations finds 1-10 MJ. One of the major goals of current research is to reduce the broad range of these estimates by way of more accurate observation.

A second measure of the energetics is the determination of the distribution of energy required to produce the observed emissions, that is, what is the energy associated with the processes occurring in the middle atmosphere which cause the optical emissions? We present evidence of ionized emissions from N₂⁺ which have an energy threshold of 18.6 eV assuming excitation is occurring directly from the ground electronic state of N₂.

Molecular nitrogen (N₂) is the major constituent (~80%) of the atmosphere in the region (20-90 km, or ~10 pressure scale heights) where blue jets, blue starters, sprites, and elves occur. N₂ is a spectroscopically rich molecule with many excited electronic states which produce significant emission. These provide a good diagnostic for studying the energetic processes occurring in sprites, blue jets, elves, in a similar manner as has been done with airglow and aurora. The primary optical emission of sprites is from the molecular nitrogen first positive group (N₂(1PG)) (Hampton et al.(1996),Mende et al.(1995)) although we will discuss other N₂ and N₂⁺ emissions which have been recently observed.

The optical emissions observed in sprites, blue jets, and elves are caused by collisions of energetic (heated) electrons with neutrals. These emissions are similar to those observed in the aurora (Vallance Jones(1974)) and are discussed in detail in the following sections. A fundamental difference between auroral and sprite observations involves the altitude where these emissions are produced. At lower altitudes, where sprites are produced, quenching plays an important role in understanding the observed emissions. The energies of the various electronic states are used in conjunction with the observed emissions to determine the energetics of processes associated with sprites, blue jets/blue starters, and elves. State energies are often characterized in terms of monoenergetic electrons as can be measured in the laboratory while physical processes in nature generally have a more complex energy distribution. Modeling and analysis efforts typically describe an electron distribution similar to a Boltzmann distribution with characteristic energy of ~1 eV but modified to also have an additional `high' energy tail (a Druvysteyn distribution has been suggested by (Green et al.(1996))).