Mechanism of column and carrot sprites derived from optical and radio observations

The lightning current waveforms observed simultaneously 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 morphological 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 frequency ∫0tνi(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 according 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 typical 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 streamers. Key Points The mechanism of column and carrot sprites is derived Sprite morphology and lightning characteristics are closely related Good agreement between observations and simulations has been achieved ©2013. American Geophysical Union. All Rights Reserved.

Duke Scholars

Author Steven A. Cummer Electrical and Computer Engineering