The blue luminous events observed by ISUAL payload on board FORMOSAT-2 satellite

We report the blue luminous events observed by the Imager of Sprite/Upper Atmospheric Lightning (ISUAL) payload on board the FORMOSAT-2 satellite. The ISUAL 427.8 nm filtered Imager campaign was conducted near Australia in the summer of the Southern Hemisphere during February-March 2008. On 18 February 2008, the 427.8 nm filtered Imager recorded a series of blue luminous events. One blue starter and nine smaller blue starters were recorded in 2 min and 34 s in a localized region with the radius < 4 km over the cloud top. The average time interval between subsequent blue luminous events was ∼ 17 s. The occurrence rate of blue luminous events was 3.5 events per minute, slightly lower than the occurrence rate of pixies (4.2 events per minute) but higher than the occurrence rate of gnomes and blue jets in the previous observations. The recorded first blue starter lasted up to 2-3 frame times (60-90 ms) and extended its altitude about 8 ± 0.3 km with a width of ∼2-4 km over the cloud top. After the first blue starter, subsequent nine smaller blue starters had the decreased heights of ∼2-4 km, and their optical duration was shorter and is down to 1 ms. But their major emissions were 2PN₂ and 1NN₂⁺, without lightning OI 777.4 nm emission. The ISUAL recorded blue smaller starters had the spatial average brightness of 130 kR for the 427.8 nm filtered Imager with exposure time (29 ms) and 1.2 MR for the spectrophotometer (337 nm). Using the spectrophotometer, the emission time of blue starters was 1 ms. It is estimated that the 1NN₂⁺ emission was ∼ 22 MR and the 2PN₂ emission was ∼ 132 MR. We can estimate the degree of ionization was 10^-11-10^-12 in these blue luminous events using the 427.8 nm filtered Imager measured 1NN₂⁺ (0,1) emission.