SPRITES PRODUCED BY QUASI-ELECTROSTATIC HEATING AND IONIZATION IN THELOWER IONOSPHERE

Quasi-electrostatic (QE) fields that temporarily exist at high altitud es following the sudden removal (e.g.; by a lightning discharge) of th undercloud charge at low altitudes lead to ambient electron heating (u p to similar to 5 eV average energy), ionization of neutrals, and exci tation of optical emissions in the mesosphere/lower ionosphere. Model calculations predict the possibility of significant (several orders of magnitude) modification of the lower ionospheric conductivity in the form of depletions of electron density due to dissociative attachment to O-2 molecules and/or in the form of enhancements of electron densit y due to breakdown ionization. Results indicate that the optical emiss ion intensities of the 1st positive band of N-2 corresponding to fast (similar to 1 ms) removal of 100-300 C of thundercloud charge from 10 km altitude are in good agreement with observations of the upper part (''head'' and ''hair'' [Sentman et al., 1995]) of the sprites. The typ ical region of brightest optical emission has horizontal and vertical dimensions similar to 10 km, centered at altitudes 70 km and is interp reted as the head of the sprite. The model also shows the formation of low intensity glow (''hair'') above this region due to the excitation of optical emissions at altitudes similar to 85 km during similar to 500 mu s at the initial stage of the lightning discharge. Comparison o f the optical emission intensities of the 1st and 2nd positive bands o f N-2, Meinel and 1st negative bands of N-2(+), and 1st negative band of O-2(+) demonstrates that the Ist positive band of N-2 is the domina ting optical emission in the altitude range around similar to 70 km, w hich accounts for the observed red color of sprites, in excellent agre ement with recent spectroscopic observations of sprites. Results indic ate that the optical emission levels are predominantly defined by the lightning discharge duration and the conductivity properties of the at mosphere/lower ionosphere (i.e., relaxation time of electric field in the conducting medium). The model demonstrates that for low ambient co nductivities the lightning discharge duration can be significantly ext ended with no loss in production of optical emissions. The peak intens ity of optical emissions is determined primarily by the value of the r emoved thundercloud charge and its altitude. The preexisting inhomogen eities in the mesospheric conductivity and the neutral density may con tribute to the formation of a vertically striated fine structure of sp rites and explain why sprites often repeatedly occur in the same place in the sky as well as their clustering. Comparison of the model resul ts for different types of lightning discharges indicates that positive cloud to ground discharges lead to the largest electric fields and op tical emissions at ionospheric altitudes since they are associated wit h the removal of larger amounts of charge from higher altitudes.