Origin Of The Pancake-Shaped And The Donut-Shaped Elves

Among the known species of upper atmospheric transient luminous events, elve is the most abundant in the ISUAL recorded events. The correlation between elves and their causative lightning is an interesting problem needed to be addressed. We selected 760 elves with distinct morphology from the 4000-plus elves recorded by ISUAL since June 2004. From these elves, the occurrence ratio between the pancake-shape and the donut- shape elves is deduced and the value is around unity. This ratio was unexpected since the known occurrence ratio between the cloud discharges (IC) and the cloud-to ground flashes (CG) is known to be around three to one. To explore the root cause(s) behind this discrepancy, a 2-dimensional EMP model with horizontal lightning components are studied numerically. The results show that the initiating current for the pancake elves from ICs is substantially higher than that for the donut elves from CGs. The morphological origin of these two distinct types of elves will also be discussed.