Electric field transition between the diffuse and streamer regions of sprites estimated from ISUAL/array photometer measurements

We analyze optical data of twenty sprites and three halos observed by the array photometer which is a scientific instrument of the ISUAL payload on the FORMOSAT-2 satellite. The altitude distribution of electric field is derived from the ratio of blue to red emission intensity by assuming all emissions are due to the electron impact excitation of nitrogen molecules. We find a clear transition at ∼75 km altitude from the upper-diffuse to lower-streamer region. Estimated electric field intensities in the diffuse region are 0.5-0.7 E_k, which support the theoretical expectation that their optical emissions could be produced without significant ionization. On the other hand, those in the streamer region are 1-2 E_k which is a few times less than predicted fields in the streamer head. We suggest that this discrepancy is due to the long-lasting components such as the lower portions of the upward branches and bead structures.