Transient Luminous Events as Lightning Effects in the Lower Ionosphere: Recent Progresses by ISUAL Measurements on FORMOSAT-2 satellite
T. Adachi1, Y. Takahashi2, R. R. Hsu3, H. T. Su3, A. B. Chen3, S. B. Mende4, and H. U. Frey4
- RISH, Kyoto University, Japan
- Department of Geophysics, Tohoku University, Japan
- Physics Department, National Cheng Kung University, Taiwan
- Space Sciences Laboratory, University of California at Berkeley, USA
In this talk, studies on the electrical effects of transient luminous events in the lower ionosphere are summarized. The main focuses are on the spectroscopic studies of sprites and elves measured with ISUAL payload on FORMOSAT-2 satellite.
Recent discoveries of transient luminous events (TLEs: sprites, elves, blue jets and gigantic jets) visualized new aspects of the electromagnetic coupling between the troposphere and the lower ionosphere. Past experimental and theoretical studies clarified the generation mechanisms and electrodynamical processes of TLEs and their effects in the lower ionosphere. The associated subionospheric modifications were identified primarily by VLF remote sensing [Inan et al., 1995; Dowden et al., 1996; Haldoupis et al., 2004]. Numerical studies explained some parts of the experimental results [Moore et al., 2003] and expected long-time electron density modifications in the lower ionosphere by successive lightning discharges [Rodger et al., 2001].
Since 2004, the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) on the Taiwanese FORMOSAT-2 satellite has been measuring TLEs from space. It consists of an imager with a selectable six-color filter wheel, a six-color spectrophotometer and a dual-color array photometer. The main scientific purpose of the ISUAL is to clarify the global distributions of TLEs and to investigate their spatiotemporal and spectral properties. Chen et al.  reported that sprites and elves occur all over the world, which suggests lightning effects in the lower ionosphere would be global phenomena. In order to clarify electron density modification in each sprite and elve event, several studies analyzed the ISUAL spectral data [Kuo et al. 2005; Mende et al., 2006; Adachi et al., 2006]. Adachi et al.  analyzed array photometer data and estimated spatiotemporal-resolved electric field intensity in sprites. The obtained results showed a distinct transition at an altitude of ~75 km, which corresponds to the morphological transition between the upper-diffuse region and the lower-streamer region of sprites. The magnitudes of electric fields in the diffuse region were 0.6–0.8 Ek where Ek is the conventional breakdown field, supporting theoretical expectation that diffuse emissions could be produced without significant ionization process. On the other hand, those in the streamer region were 1–2 Ek, which represents significant enhancement of the electron density. Mende et al.  also presented evidence of electron density enhancements in elves based on spectrophotometer measurements. The electron density enhancement in an elve event was estimated to be 210 electrons cm-3 over a region of 165 km at an altitude of 90 km, comparable to the ambient electron density in the nighttime D-region ionosphere. More recently, Mika et al.  carried out comprehensive observations using VLF receivers, ground-based camera, and ISUAL imager. The obtained data showed direct relation between subionospheric VLF perturbations and elve events.
By summarizing the recent progresses, current problems and future studies about lightning effects in the lower ionosphere are further discussed