and radio observations

[1] The lightning current waveforms observed simultane-ously with high-speed video records of a column and a carrot sprite event are incorporated in a plasma fluid model to provide quantitative explanation of these two distinct mor-phological classes of transient luminous events. We calculate the strength of the lightning-induced electric field at sprite altitudes using a time integral of the ionization frequencyR t 0 i(E/N)dt. For the studied two events, modeling results indicate that these integral values never exceed 18 in the lower ionosphere, which is the minimum value required for the initiation of streamers from single seed electrons accord-ing to the Raether-Meek criterion. It is therefore suggested that the presence of electron inhomogeneities is a necessary condition for the initiation of sprite streamers. It is further demonstrated using streamer modeling that a minimum value of the integral 10 is necessary to initiate upward negative streamers from inhomogeneities, corresponding to a minimum charge moment change of 500 C km under typ-ical nighttime conditions. If the integral values in the entire upper atmosphere are smaller than 10, only column sprites can be produced, dominated by downward positive stream