| dc.description.abstract | The ISUAL (Imager of Sprites and Upper Atmosphere Lightnings) is a scientific payload of FORMOSAT-2 satellite. The ISUAL project not only conducts the survey of the global occurring rate of sprites, but also investigates the optical emissions associated with sprites. In past studies, Adachi et al. (2006, 2008) reported that the sprite emission ratio of N2 2P (Second positive band) to N2 1P (First positive band) obtained from the ISUAL AP (Array Photometer) data can be used to derive the reduced electric field in the peak of streamers of recorded sprites. Adachi et al. (2006, 2008) also found that the reduced electric field is 0.24~1.24 E_k in the upper-diffuse region and 0.81~3.16 E_k in the lower-structured region of sprites. In this work, the sprite emission ratio can be one of the critical parameters to reveal the driving electric field in the plasma chemistry in sprites.
We used 28 selected sprite events from July 4th, 2004 to January 22nd, 2008 and January 18th, 2009 to January 24th, 2013 with ULF (Ultra Low Frequency) data from ISUAL array photometer (AP) on FORMOSAT-2 satellite to analyze the sprite emission ratio using the sum, temporal filter and peak three different methods. Compared the sprite emission ratios calculated from photon yield, the results show that the difference in the sprite emission ratio between the sum, temporal filter and peak methods are smaller at higher altitudes and larger at lower altitudes. This has been pointed out by Adachi et al. (2006, 2008) that the difference between different methods is a result of the errors in altitude estimation and the exponential increase of the quenching term k_q [M]. Our results suggest that with altitude decrease, a one-term exponential parameter ?ae?^hx should let quencher density [M] become lower than the original [M] in central Africa.
In this work, we also compared the sprite emission ratios with plasma chemistry model. The result shows that the sprite emission rates 5.6×?10?^16 photon/s for N2 2P and 4.86×?10?^16 photon/s for N2 1P at the reduced electric field of 5 E_k. The estimated brightness of N2 2P and N2 1P calculated by the model are 9.33×?10?^3 Rayleigh and 8.1×?10?^3 Rayleigh, respectively (1 Rayleigh = 106 photon/cm2-column/s). Compared with the sprite emission ratio estimated from the model (1.15), the average sprite emission ratio calculated using the temporal filter method at 70 km height (1.17) has the most similar value among three methods (the sum method: 1.09; the peak method: 1.93).
In addition, our data also show the speeds of the downward and upward moving streamers were calculated using time-series signals of ISUAL AP, and it gives a range of ~107 to ~108 m/s. Among the 28 selected sprite events, 15 events have ULF data. Within these 15 sprite events, most of the impulse charge moment changes (iCMCs) are higher than the threshold of producing sprites (>200 C-km) and one of the sprite events shows a relatively stronger peak current of 133 kA. In the future, we can modify the quencher [M], add the model results at lower (~40 km) or higher (~90 km) altitudes and increase the samples of sprite events to understand of the relations of the sprite emission ratio between the three methods and the model results. | en_US |