•emissions (+spectra) •interaction of Electrons with Neutrals •electron Energization Fokker Planck Code •propagation of Em Fields including Self Absorption •source Is Lightning •horizontal Dendritic

Red Sprites: Lightning as a Fractal Antenna J. A. Valdivia 1, G. Milikh 2, and K. Papadopoulos 1;2 Dept. of Physics1, Dept. of Astronomy2, Univ. of Maryland, College Park, MD 20742 Abstract A new and improved model of the red sprites is presented. Emphasis is placed in accounting for the puzzling observation of small scale structure in the emissions. The model relies in a horizontal lightning discharge that includes the fact that the discharge has an intricate fractal structure. It is shown that the model accounts for the observed structure, while reducing the typical charge required to approximately 100 C. Introduction Observations of optical emissions at altitudes between 60 to 90 km associated with thunderstorms have been the focus of many recent ground and aircraft campaigns [Boek et al., 1992; Vaughan et al., 1992; Winckler et al., 1993; Sentman et al., 1993; Lyons, 1994]. The emissions, termed red sprites, have been the subject of several papers. Among the most puzzling aspects of the observations is the presence of ne structure in the emissions [Winckler et al., 1996]. Vertical striations with horizontal size of 1 km or smaller, often limited by the instrumental resolution, are apparent in the red sprite emissions. There is yet no theoretical explanation for the presence of such striated emissions. It is the purpose of this letter to provide the rst theoretical model that accounts for the presence of ne structure. It will be shown that the model not only accounts for the structure, but simultaneously reduces the required charge for the lightning discharge driving the red sprites, which have a total optical emission of 10-50 kR, to levels more consistent with observational requirements. The model presented here is based on electron energization in the upper atmosphere caused by a transient electromagnetic pulse or series of pulses induced by low altitude lightning. However, previous considerations of the lightning induced electric elds were based on horizontal [Milikh et al., 1995] or vertical [Pasko et al., 1995, Rowland et al., 1995] dipole electric models, which produce highly homogeneous and smooth electron heating in the lower ionosphere resulting in the absence of internal structure in the optical emissions. The presented model retains the internal spatio-temporal structure of the lightning discharge.