Global Observations of the 630-nm Nightglow and Patterns of Brightness Measured by ISUAL

This study investigates the distributions and occurrence mechanisms of the global local-midnight airglow brightness through FORMOSAT-2/ISUAL satellite imaging observations. We focus on the OI 630.0 nm nightglow emission at altitudes of ~250 km along equatorial space. The database used in this study included data from 2007 to 2008 under solar minimum conditions. The data were classified into four specified types in the statistical study. We found that the occurrence of equatorial brightness was often in the vicinity of the geographic equator and mostly at equinoxes with a tendency to move toward the summer hemisphere as the season changes. Conjugate brightness occurring simultaneously on both sides of the geomagnetic equator was observed predominantly in the northern winter. Furthermore, midnight brightness appeared to have lower luminosity from May to July. We suggest that the global midnight brightness associated with the locations and seasons was the result of several effects which include the influence of the thermospheric midnight temperature maximum (MTM), summer-to-winter neutral wind, and ionospheric anomalies

Galactic structure studies with BATC star counts

We report the first results of star counts carried out with the National Astronomical Observatories (NAOC) 60/90 cm Schmidt Telescope in 15 intermediate-band filters from 3000 to 10000 {\AA} in the BATC survey. We analyze a sample of over 1400 main sequence stars ($14\le$V$\le21$), which lie in the field with central coordinates R.A.=$09^h53^m13^s.30$ and DEC=47$^\circ49^{\prime}00^{\prime\prime}.0$ (J2000). The field of view is 0.95 deg$^{2}$, and the spatial scale was 1\arcsec.67. In our model, the distribution of stars perpendicular to the plane of the Galaxy is given by two exponential disks (thin disk plus thick disk) and a de Vaucouleurs halo. Based on star counts, we derive the scale heights of the thin disk to be $320^{+14}_{-15}$ pc and of the thick disk to be $640^{+30}_{-32}$ pc, respectively, with a local density of $7.0\pm1$ of the thin disk. We find that the observed counts support an axial ratio of $c/a\le0.6$ for a de Vaucouleurs $r^{1/4}$ law, implying a more flattened halo. We also derive the stellar luminosity function (SLF) for the thin disk, and it partly agrees with the Hipparcos luminosity function.Comment: 17pages,9 figure

Multicolor Photometric Observation of Lightning from Space: Comparison with Radio Measurements

This study evaluates the effectiveness of spectrophotometric measurements from space in revealing properties of lightning flash. The multicolor optical waveform data obtained by FORMOSAT-2/Imager of Sprites and Upper Atmospheric Lightning (ISUAL) were analyzed in relation to National Lightning Detection Network (NLDN), North Alabama Lightning Mapping Array (LMA). As of July 2011, we found six lightning events which were observed by ISUAL and North Alabama LMA. In two of these events, NLDN showed clear positive cloud-to-ground (CG) discharges with peak current of +139.9 kA and +41.6 kA and, around that time, LMA showed continuous intra-cloud (IC) leader activities at 4-6 km altitudes. ISUAL also observed consistent optical waveforms of the IC and CG components and, interestingly, it was found that the blue/red spectral ratio clearly decreased by a factor of 1.5-2.5 at the time of CG discharges. Other four lightning events in which NLDN did not detect any CG discharges were also investigated, but such a feature was not found in any of these cases. These results suggest that the optical color of CG component is more reddish than that of IC component and we explain this as a result of more effective Rayleigh scattering in blue light emissions coming from lower-altitude light source. This finding suggests that spectral measurements could be a new useful technique to characterize ICs and CGs from space. In this talk, we will also present a result from lightning statistical analysis of ISUAL spectrophotometric data and ULF magnetic data

Selected results from the ISUAL/FORMOSAT2 mission

The ISUAL (Imager of Sprites and Upper Atmospheric Lightning) is a scientific payload onboard the FORMOSAT2 satellite (FS2). This the first satellite project survey of global transient luminous events (TLEs) as one of the mission objectives. Since the ISUAL/FS2 launch in 2004, ISUAL has continuously monitored TLE occurrences over pre-midnight regions in the past ten plus years. As of December 2015, more than forty thousand TLEs, including the sub-species like elves, sprites, sprite-halos, blue jets and gigantic jets, have been recorded from this space platform. In the meantime, as the supporting facilities to the space-borne ISUAL experiment, ground optical imagery systems have been deployed to observe TLEs occurring near Taiwan with several radio wave detection ground stations also installed to register lightning- or TLE-related radio signals called sferics. From analyzing the observed events and the associated sferics, many TLE characteristics have been inferred. Among the most notable factors are the global distributions, the occurrence rates, and the charge injection and the energy impact to the upper atmosphere. In this article, the ISUAL payload, the auxiliary ground facilities and a few selected TLE survey results from the ISUAL mission are concisely presented

Sensitivity Degradation of ISUAL Instruments and Its Impact on Observations

In the past 6 years, the ISUAL payload onboard the Taiwanese FORMOSAT-2 satellite has successfully carried out the primary mission of performing a global survey of transient luminous events (TLE) and lightning activities. The observation data have been used to construct the first global distribution map of TLEs and to infer their occurrence rates. To register dim TLE emissions at night, ISUAL employs electron-multiplying devices like a photomultiplier, multi-anode array, and micro-channel plate to intensify the photons emitted by transient events. These devices, however, will degrade gradually with age. The degradation rate and the performance change were monitored through periodic calibration observations and are investigated carefully in order to achieve precise photometry and accurate statistics. The annual degradation rates up to 14% were identified for some of the sensor elements. This paper presents the variations of instrument efficiencies and overall detection capabilities, before and after applying a gain-adjustment to compensate for sensor degradation. The resulting stable trend of the ISUAL trigger and lightning rates suggests that the bias from instrument degradation and gain adjustments is minimal. This point is also supported by the near constant sprite and halo detection rates deduced from different statistical temporal intervals. Hence, the anomalies in the ISUAL elve detection rate for 2008 ~ 2009 likely are due to the El Niño and Southern Oscillation (ENSO) events. Forecast based on the ISUAL performance trend also indicates that the ISUAL sensors have sufficient sensitivity margins to operate efficiently until the end of 2013, or even beyond