講演情報
[PPS01-P06]Evaluation of Jovian lightning studies based on the developed Planetary Lightning Detector and light curve model
*大野 辰遼1、高橋 幸弘1 (1.北海道大学・大学院理学院・宇宙理学専攻)
キーワード：
木星、雷、地上望遠鏡、光度曲線シミュレーションモデル
Planetary lightning observations provide the data for atmospheric research, contributing insights into convection and atmospheric composition. The light curves and time constants of lightning discharges on Jupiter remain poorly understood. On Jupiter, spacecraft such as Galileo have detected deep lightning discharges of approximately 109 J at altitudes corresponding to 5 bar atmospheric pressure. However, the long exposure times, measured in seconds, make it unclear whether these observations represent single large-scale discharges or a sequence of lightning events similar in scale to the 107 J lightning detected at higher altitudes by Juno.
This study aims to detect lightning discharges on Jupiter using light curve observations to gain new insights into the lightning light curve. To achieve this, a new photometric observation instrument (PLD) was developed with a time resolution of 2 msec. The PLD simultaneously observed light curves at two wavelengths, using waveform correlation to discriminate between daytime background light, noise, and lightning flashes. A new three-dimensional light-scattering model was developed that incorporates distributions of cloud particle size and composition. This model reproduced light curves by including, for the first time, the time-delay process of photon scattering within cloud structures, based on vertical cloud distribution estimated from numerical simulations of Jupiter’s cumulonimbus clouds. The results were compared with Earth lightning observation data from the JEM-GLIMS to validate the model.
The PLD observed Jupiter for approximately 880 minutes during 2021 and 2022. Multiple events were detected in the lightning observation wavelength light curve, with peak values exceeding 5.2σ. These event waveforms exhibited a FWHM of approximately 3.5 msec and a peak value near 109 J. The observed FWHM was smaller than the maximum FWHM in the model’s cumulonimbus structure light curve, and longer than the maximum FWHM in the typical Jupiter cloud structure. If a discharge time of approximately 1 msec had been assumed, the time constant of the scattered light would have increased. Thus, lightning with an energy of 109 J can be interpreted as flashes generated deep within early cumulonimbus clouds, as light leakage from regions of thin optical thickness within developing cumulonimbus clouds, or from clouds surrounding the cumulonimbus. The estimated occurrence frequency of 109 J-class lightning events was lower than in previous studies. Since Earth-based observations of Jupiter are limited to its dayside, a high detection threshold was necessary to prevent signals from being obscured by noise. When the event detection threshold was set to 4.5σ, the detection frequency matched that reported in previous studies. The statistically low number of waveform detections observed with the PLD suggests that Jupiter’s 109 J lightning events may produce prolonged light curves. Even with high total energy, if the peak luminosity falls below the PLD’s sensitivity, the time constant is estimated to extend to 13 msec or longer. This interpretation is consistent with previous radio observations, which detected approximately 10 lightning discharges within 16 msec, suggesting that lightning with energies exceeding 109 J may consist of multiple overlapping discharges.
This study aims to detect lightning discharges on Jupiter using light curve observations to gain new insights into the lightning light curve. To achieve this, a new photometric observation instrument (PLD) was developed with a time resolution of 2 msec. The PLD simultaneously observed light curves at two wavelengths, using waveform correlation to discriminate between daytime background light, noise, and lightning flashes. A new three-dimensional light-scattering model was developed that incorporates distributions of cloud particle size and composition. This model reproduced light curves by including, for the first time, the time-delay process of photon scattering within cloud structures, based on vertical cloud distribution estimated from numerical simulations of Jupiter’s cumulonimbus clouds. The results were compared with Earth lightning observation data from the JEM-GLIMS to validate the model.
The PLD observed Jupiter for approximately 880 minutes during 2021 and 2022. Multiple events were detected in the lightning observation wavelength light curve, with peak values exceeding 5.2σ. These event waveforms exhibited a FWHM of approximately 3.5 msec and a peak value near 109 J. The observed FWHM was smaller than the maximum FWHM in the model’s cumulonimbus structure light curve, and longer than the maximum FWHM in the typical Jupiter cloud structure. If a discharge time of approximately 1 msec had been assumed, the time constant of the scattered light would have increased. Thus, lightning with an energy of 109 J can be interpreted as flashes generated deep within early cumulonimbus clouds, as light leakage from regions of thin optical thickness within developing cumulonimbus clouds, or from clouds surrounding the cumulonimbus. The estimated occurrence frequency of 109 J-class lightning events was lower than in previous studies. Since Earth-based observations of Jupiter are limited to its dayside, a high detection threshold was necessary to prevent signals from being obscured by noise. When the event detection threshold was set to 4.5σ, the detection frequency matched that reported in previous studies. The statistically low number of waveform detections observed with the PLD suggests that Jupiter’s 109 J lightning events may produce prolonged light curves. Even with high total energy, if the peak luminosity falls below the PLD’s sensitivity, the time constant is estimated to extend to 13 msec or longer. This interpretation is consistent with previous radio observations, which detected approximately 10 lightning discharges within 16 msec, suggesting that lightning with energies exceeding 109 J may consist of multiple overlapping discharges.