We statistically analyzed lightning whistlers detected from the analog waveform data below 15 kHz observed by the VLF instruments onboard Akebono. We examined the large amount of data obtained at Uchinoura Space Center in Japan for 22 years from 1989 to 2010. The lightning whistlers were mainly observed inside the L shell region below 2. Seasonal dependence of the occurrence frequency of lightning whistlers has two peaks around July to August and December to January. As lightning is most active in summer, in general, these two peaks correspond to summer in the Northern and Southern Hemispheres, respectively. Diurnal variation of the occurrence frequency showed that lightning whistlers begin to increase in the early evening and remain at a high‐occurrence level through the night with a peak around 21 in magnetic local time (MLT). This peak shifts toward nightside compared with lightning activity, which begins to rise around noon and peaks in the late afternoon. This trend is supposed to be caused by attenuation of VLF wave in the ionosphere in the daytime. Comparison study with the ground‐based observation revealed consistent results, except that the peak of the ground‐based observation appeared after midnight while our measurements obtained by Akebono was around 21 in MLT. This difference is explained qualitatively in terms that lightning whistlers measured at the ground station passed through the ionosphere twice above both source region and the ground station. These facts provide an important clue to evaluate quantitatively the absorption effect of lightning whistler in the ionosphere.

AKEBONO SpacecraftAKEBONO (EXOS-D) has been continuously operated for more than 25 years since 1989 to observe particles and plasma waves in the auroral region and the plasmasphere of the Earth. It covers altitude region from 300 km to about 10,000 km with an inclination of 75 degree. Analyses of large amounts of data obtained by AKEBONO enable us to study comprehensively how the magnetosphere varies temporally and spatially.The WBA (Wide Band Analyzer) is one of subsystems of the VLF instruments onboard AKEBONO. It measures 1 component of electric or magnetic analogue waveform in the frequency band of 50 Hz -15 kHz. Typical waves such as chorus, hiss and whistler were frequently observed by the WBA. Data obtained by the WBA for more than 25 years are originally recorded as analogue waveform format in the magnetic audio tapes. Data conversion from analogue to digital is now being carried out and the converted data are stored in our computer storage as digital WAVE format. Lightning WhistlerLightning whistler is one of typical waves frequently observed by the WBA. It is generated by lightning discharge and propagates generally along the geomagnetic field line from northern to southern hemisphere and vice-versa. The dispersive spectrum property of lighting whistler depends on the electron density profile along the propagation path and the path length. Therefore, lightning whistler has potential to monitor electron density variations in the ionosphere as well as the plasmasphere and lightning flash activity on the Earth extensively. Statistical Analysis of Lightning Whistler Automatic Detection SystemAn automatic detection system to find lightning whistlers from spectrograms of the WBA data was developed. Arrival time t of a lightning whistler at frequency f is approximately represented by the following equation:where D represents dispersion of the lightning whistler, and t 0 and f 0 represent arrival time and frequency of head of the lightning whistler, respectively. The system detects straight lines in the 1 ⁄ diagram using the relationship represented by the equation (1). It records observed time, frequency band and dispersion of each detected lightning whistler. Statistical AnalysisIn the present study, statistical analyses about occurrence frequency of lightning whistlers were performed using magnetic component of the WBA data for 22 years from 1989 to 2010 tracked from Uchinoura Space Center (USC), Japan, and for 9 years from 1999 to 2007 tracked from Esrange (ESR) Space Center, Sweden. The data obtained at ESR covered higher geomagnetic latitude region while the data obtained at USC mainly covered the mid-latitude and the equatorial region. Spatial DistributionIt was found that lightning whistlers were mainly observed only inside of the plasmasphere, therefore in the following statistics, the data obtained only in the L-shell region less than 3 were analyzed. Most of lightning whistlers detected in the USC data were observed below L = 2 but those in the ESR data were also detected in the outer L-shell region ...