New progress of lightning discharge observation
Of the many familiar luminous phenomena, lightning still has many unsolved mysteries. Lightning discharge is estimated to occur 40 to 100 times a second on the Earth. Nonetheless, it was only recently discovered that the main cause of the charge-separation mechanism in lightning was friction of ice crystals and graupel. It is still a mystery why the actual dielectric breakdown voltage is smaller by one order of magnitude than theoretical prediction. Discharge events occurring in dynamic air currents in cumulonimbus cloud impede researchers’ direct observation because they arise in severe, limited places. The scale of the phenomenon hampers researchers’ attempts at simulation such as in-room or numerical experiments. Limited observation stations on the ground make it difficult to assess global lightning activities.
In the past 15 years, however, the situation has greatly changed. First, a new discharge phenomenon called a sprite, an emission in the middle to upper atmosphere (i.e., altitudes from 50 to 90km), was unintentionally discovered by a high-sensitive camera on the ground in 1989 (Fig. 1). This prompted researchers specializing in the upper atmosphere and ionosphere field as well as researchers in the atmospheric electricity field to participate in lightning discharge research in the early 1990s. Electrical phenomena in the troposphere and ionosphere/magnetosphere, which had previously been studied separately, were combined into a single research field. The MicroLab 1 satellite carrying an Orbital Transient Detector (OTD), a measurement instrument used exclusively for lightning discharge emission, was launched in 1995 and the TRMM satellite carrying an LIS (Lightning Imaging Sensor) was launched in 1997. We were surprised by the lightning discharge maps provided by these instruments because they clearly showed dynamic seasonal variations. In 2004, an ISUAL (Imager of Sprites and Upper Atmospheric Lightning) designed to observe Transient Luminous Events (TLEs) such as sprites was launched onboard Taiwan’s FORMOSAT-2 satellite and started observations.
On the other hand, lightning conduction experiments on the ground began to provide us with new findings. It is notable, in particular, that the experiments succeeded in identifying X-rays and gamma rays induced by lightning discharge. This phenomenon, which was verified in both artificial and natural lightning, could lead to the elucidation of dielectric breakdown mechanism. Gamma rays associated with thundercloud activities were also confirmed by the cosmic gamma ray sensor BATSE (Burst and Transient Source Experiment) onboard the Compton Gamma Ray Observatory (CGRO) in 1994. This phenomenon was initially thought to be linked directly to the sprite phenomenon. Because of the abundant data obtained by the RHESSI satellite in 2004, however, we were forced to change our models drastically.
In addition, since lightning-positioning systems using radio waves are being constructed around the world, local lightning-discharge activities are assessed in more detail. In the U.S., such local data are assimilated with numerical prediction models in an attempt to improve greatly forecast accuracy for severe weather conditions such as storms. It is now a worldwide trend for next-generation meteorological satellites to carry lightning-discharge optical detectors as standard instrumentation.
A number of technological innovations in observation and new discoveries boosted by these innovations are dramatically revising the conventional picture of the lightning-related phenomena and their position in science. Atmospheric electricity science is now merging with ionosphere/magnetosphere physics, meteorology and gamma-ray physics. We are entering into a new phase in both fundamental and applied research.
Significance of lightning-discharge observation
It was mentioned above that lightning discharge data are effective for short-term weather forecasts. In addition, some researchers believe that global electric currents in the atmosphere (i.e., global circuit) affect long-term climate change. The idea of the global circuit has existed for a long time, but never went beyond an hypothesis. Today’s progress in lightning-discharge observation and the discovery of TLEs could prompt a reconstruction of the global-circuit model. The electric current in the atmosphere is a factor to determine distribution of ions and electrons. Ions affect the behavior of aerosol in the lower atmosphere and, as a result, it is believed that they can change weather and climate. There is high possibility that the drastic rise in temperature and ionization caused in lightning channels boost chemical reactions in the atmosphere to eventually change atmospheric composition. Though still controversial, there is a supposition that 20% to 30% of NOx is produced by lightning discharge. The electric currents in thunderclouds and aurorae (magnetosphere and ionosphere) are the electrical generator in the global circuit and both have large temporal and seasonal variations. In order to understand anthropogenic impact on ozone or NOx, understanding and quantitative evaluation of lightning-discharge activities and aurora events are also essential.
The author et al. developed and manufactured the array photometer, part of ISUAL onboard the Taiwan’s satellite, and are now analyzing the data sent from it extensively. Optical observation data from space, which are free from absorption and scattering by the earth’s atmosphere, aerosol and clouds, provide us with quantitative information on global distribution of TLE occurrences, and electron energy and consumption energy in the TLE region. Until now, we have obtained a great deal of new information, such as the complexity of variations depending on seasons and regions (whether land or sea), energy consumption, and that TLE occur more frequently than forecast. In the future, the influence on atmospheric composition in the lower atmosphere, etc., must be clarified too.
The study of earth’s gamma rays is now the hottest subject in the lightning / TLE research community, which is excited by the consecutive discoveries of new phenomena. In the next five years, the France-led small-satellite project TARANIS and the Denmark-led ESA/ISS mission ASIM will attempt to elucidate the mysteries of earth’s gamma rays and lightning discharge. On the other hand, Japanese universitie’s satellite project is now moving in advance of these European projects.