The Imager for Sprites and Upper Atmospheric Lightning (ISUAL) was the first specifically dedicated instrument to observe lightning-induced transient luminous events (TLE): sprites, elves, halos, and gigantic jets from space. The Imager is an intensified CCD system operating in the visible wavelength region with a filter wheel to select from six positions with filters. The Imager has a 5 degrees x 20 degrees (vertical times horizontal) field of view. The spectrophotometer (SP) is populated with six photometers with individual filters for emissions from the far ultraviolet to the near infrared. An array photometer with two channels operating in the blue and red provides altitude profiles of the emission over 16 altitude bins each. The Associated Electronics Package (AEP) controls instrument functions and interfaces with the spacecraft. ISUAL was launched 21 May 2004 into a Sun-synchronous 890 km orbit on the Formosat-2 satellite and has successfully been collecting data ever since. ISUAL is running on the nightside of the orbit and is pointed to the east of the orbit down toward the limb. The instrument runs continuously and writes data to a circular buffer. Whenever the SP detects a sudden signal increase above a preset threshold, a trigger signal is generated that commands the system to keep the data for about 400 ms starting from similar to 50 ms before the trigger. Over its lifetime of similar to 11 years the system recorded thousands of TLE and also successfully observed aurora and airglow.

An ultraviolet imaging spectrometer (UVS) on board the PLANET-B (NOZOMI) spacecraft has been developed. The UVS instrument consists of a grating spectrometer (UVS-G), an absorption cell photometer (UVS-P) and an electronics unit (UVS-E). The UVS-G features a flat-field type spectrometer measuring emissions in the FUV and MUV range between 110 nm and 310 nm with a spectral resolution of 2–3 nm. The UVS-P is a photometer separately detecting hydrogen (H) and deuterium (D) Lyman α emissions by the absorption cell technique. They take images using the spin and orbital motion of the spacecraft. The major scientific objectives of the UVS experiment at Mars and the characteristics of the UVS are described. The MUV spectra of geocoronal and interplanetary Lyman α emissions and lunar images taken at wavelength of hydrogen Lyman α and the background at 170 nm are presented as representative examples of the UVS observations during the Earth orbiting phase and the Mars transfer phase.

It is important to investigate the small‐scale dynamics of thermospheric neutrals and ionospheric plasmas at high latitudes since there are local and temporal variations in energy input and transfer processes associated with auroral activities. To clarify these ionosphere‐thermosphere interactions in the auroral E region, we examined neutral winds and plasma motions obtained by coordinated Fabry‐Perot imager and VHF radar observations at Syowa Station, Antarctica. From case studies on 11 and 12 September 1996, we found a good correlation between temporal variation (period < 1 hr) of E region neutral winds measured by the Fabry‐Perot imager and the Doppler velocities of plasmas obtained from the VHF radar echoes. On the other hand, the absolute mean values of the neutral winds were around zero whereas the plasma Doppler velocities were 350–400 m/s, showing a significant discrepancy. Considering that the neutral‐ion and ion‐neutral momentum transfer collision frequencies in the E region are ∼10−6 Hz and ∼102 Hz, respectively, these good correlations are probably due to ion motions driven by neutral drag force, suggesting a strong coupling between neutrals and ions in the E region. The observed correlation and discrepancy between the radar Doppler velocity and the neutral wind velocity are consistent with the linear theory of E region irregularities that shows the radar Doppler velocity is the sum of two components; one is the E × B drift speed, and another is proportional to the neutral wind velocity.

The fundamental electrodynamical coupling processes between lightning and sprites are investigated. By combining the observed spectral data with the Monte Carlo swarm experiments, reduced electric fields and electron energies in sprite streamers and halos are estimated. The obtained fields inside sprite halos (70-97 Td with an analysis error of +/- 5 Td) are lower than the conventional breakdown field, E-k similar to 128 Td, indicating a significant reduction of electrons associated with halos while those in sprite streamers (98-380 Td with an error of +/- 50 Td) are higher than Ek, suggesting that a significant ionization process drives their formation and development. A combined analysis of photometric and electromagnetic data makes it possible to estimate temporal evolutions of lightning charge moment. It is found that lightning discharges with a short time scale (similar to 1 ms) and a moderate amount of charge moment (similar to 400 C km) produce discernible halos. On the other hand, lightning discharges with a large amount of charge moment (similar to 1300 C km) produce streamers regardless of their time scale. The results obtained are comprehensively interpreted with both the conventional breakdown field necessary for the formation of streamers and the electric field necessary for the production of optical emissions of halo which is sensitive to the time scale of the thundercloud field due to the significant reduction of electrons.

We estimate the concentration changes, caused by streamer discharge in sprites, of ozone and related minor species as odd nitrogen (NO x ) and hydrogen (HO x ) families in the upper stratosphere and mesosphere. The streamer has an intense electric field and high electron density at its head, where a large number of chemically-radical ions and atoms are produced through electron impact on neutral molecules. After its propagation, densities of minor species can be perturbed through ion-neutral chemical reactions initiated by the relaxation of these radical products. We evaluate the production rates of ions and atoms using an electron kinetics model and by assuming that the electric field and electron density are in the head region. We calculate the density variations mainly for NO x , O x , and HO x species using a one-dimensional model of the neutral and ion composition of the middle atmosphere, including the effect of the sprite streamer. Results at the nighttime condition show that the densities of NO, O 3 , H, and OH increase suddenly through reactions triggered by the first atomic nitrogen and oxygen product, and electrons just after streamer initiation. It is shown that NO and NO 2 still remain for 1 h by a certain order of increase with their source-sink balance, predominantly around 60 km; for other species, increases in O 3 , OH, HO 2 , and H 2 O 2 still remain in the range of 40–70 km. From this affirmative result of long-time behavior previously not presented, we emphasize that sprites would have the power to impact local chemistry at night. We also discuss the consistency with previous theoretical and observational studies, along with future suggestions.

Field‐aligned currents (FACs) play an important role in the magnetosphere‐ionosphere coupling system for transporting electromagnetic energy. Although large‐scale region 1 (R1) and region 2 (R2) FACs have been statistically investigated for a few decades, there have been relatively fewer investigations of the fundamental characteristics of mesoscale (ranging from about 10 to 100 km) FACs and the relationship between them and large‐scale FACs. Here we report some fundamental characteristics of mesoscale FACs on the basis of a statistical analysis using long‐term Akebono satellite data from March 1989 to December 1996. We found that the current density of mesoscale FACs with a spatial scale of 10–80 km at ionospheric altitude is on average the largest in the dayside region of 75°–82° in corrected geomagnetic latitude and 0600–1500 in corrected geomagnetic local time. The current density is especially enhanced under the sunlit condition in summer. The intensities and distributions of mesoscale FACs change with interplanetary magnetic field conditions. We also found altitude dependence of mesoscale FACs for the first time. Current densities of mesoscale FACs gradually increase with the altitude. Mesoscale FACs tend to distribute in the R1 current region, and the current density in the morning to noon sector (downward R1 FAC region) is stronger than that in the noon to evening sector (upward R1 FAC region). Further, the seasonal variations of mesoscale FACs correspond to those of large‐scale FACs on the dayside but areopposite to those of large‐scale FACs on the nightside.

The Imager for Sprites and Upper Atmospheric Lightning (ISUAL) on the FORMOSAT‐2 spacecraft observes Transient Luminous Events (TLE) like sprites, elves, and halos from space. We analyzed halos that were observed in Central America close enough to ELF/VLF receivers that allowed for the determination of the polarity of the parent lightning. All halos were created by negative cloud to ground lightning (−CG) strokes that occurred almost exclusively over the open water. Only three out of the 31 events happened over land. We conclude that the Central American region seems to be special with respect to the large proportion of −CG created halos. Such a behavior is very different from the occurrence of sprites that are mostly created by positive cloud to ground lightning.

From simultaneous observations of the European incoherent scatter Svalbard radar (ESR) and the Cooperative UK Twin Located Auroral Sounding System (CUTLASS) Finland radar on 9 March 1999, we have derived the height distributions of the thermospheric heating rate at the F region height in association with electromagnetic energy inputs into the dayside polar cap/cusp region. The ESR and CUTLASS radar observations provide the ionospheric parameters with fine time-resolutions of a few minutes. Although the geomagnetic activity was rather moderate (Kp=3+~4), the electric field obtained from the ESR data sometimes shows values exceeding 40 mV/m. The estimated passive energy deposition rates are also larger than 150 W/kg in the upper thermosphere over the ESR site during the period of the enhanced electric field. In addition, enhancements of the Pedersen conductivity also contribute to heating the upper thermosphere, while there is only a small contribution for thermospheric heating from the direct particle heating due to soft particle precipitation in the dayside polar cap/cusp region. In the same period, the CUTLASS observations of the ion drift show the signature of poleward moving pulsed ionospheric flows with a recurrence rate of about 10–20 min. The estimated electromagnetic energy deposition rate shows the existence of the strong heat source in the dayside polar cap/cusp region of the upper thermosphere in association with the dayside magnetospheric phenomena of reconnections and flux transfer events.

The ISUAL experiment on the FORMOSAT‐2 satellite has confirmed the existence of ionization and Lyman‐Birge‐Hopfield (LBH) band emissions in elves. In this paper, an in‐depth study of the ISUAL recorded elves was carried out. Numerical simulation results of elves based on an electromagnetic finite difference time domain (FDTD) model of the emissions between 185–800 nm and of their spatial‐temporal evolution are presented. To account for the effect of atmospheric attenuation, three major attenuation mechanisms: O2, O3, and molecular Rayleigh scattering are considered. Validations of the electromagnetic FDTD model were conducted in three ways: by comparing the calculated and observed photon fluxes in the ISUAL spectrophotometric channels, by directly comparing the simulated and observed morphologies of elves, and by comparing the computed photon counts of the ISUAL Imager based on the derived peak currents for two elve‐associated NLDN (National Lightning Detection Network) cloud‐to‐ground discharges (CGs) with those recorded by the ISUAL Imager. In all three ways, very good agreement was achieved.

We investigate the vertical and latitudinal structure of the migrating diurnal tide in a low dust condition (dust optical depth of 0.3 at 0.67 μm) in the Martian atmosphere by using a general circulation model (GCM) and a linear response model (LRM). The migrating diurnal tide simulated in our Mars GCM well represents general characteristics of the migrating diurnal tide which have been reported in previous observational and GCM studies. The GCM simulation shows that the vertical wavelength of the migrating diurnal tide in the low latitude region at equinox is ∼45 km which is larger than that of the major propagating mode predicted from the classical tidal theory (∼25–35 km). The Hough function decomposition and the numerical experiments using the LRM reveal that the large vertical wavelength of the migrating diurnal tide is caused by the effects of the zonal mean vorticity . It is suggested that the vertical wavelength of the migrating diurnal tide increases through the changes of the effects of the planetary rotation in the presence of non‐zero zonal mean vorticity . Such a strong dependence of the vertical wavelength of the migrating diurnal tide on is not observed in the Earth's atmosphere. The Martian radius, about the half of the Earth's radius, would be one of the important factors to cause more effective in the Martian atmosphere than that in the Earth's atmosphere.

The Imager of Sprites and Upper Atmospheric Lightning (ISUAL) is a scienti fic payload on Taiwan's FORMOSAT-2 (previously known as ROCSAT-2) that provides new observations of transient luminous events (TLEs) from space. The ISUAL project is an international collaboration between the National Cheng Kung University, Taiwan, Tohoku University, Japan and the instrument development team from the University of California, Berkeley. The project was supported by the National Space Program Office in Taiwan. The ISUAL payload includes a visible wavelength intensified CCD imager, a boresighted six wavelength spectrophotometer, and a two channel Array Photometer (AP) with 16 vertically spaced horizontally wide sensitive regions. The imager is equipped with 5 selectable filters on a filter wheel and a 6th open position. The spectrophotometer contains six filter photometer channels, their bandpasses ranging from the far ultraviolet to the near infrared regions. The two channel AP is fitted with broadband blue and red filters.

We analyze optical data of twenty sprites and three halos observed by the array photometer which is a scientific instrument of the ISUAL payload on the FORMOSAT‐2 satellite. The altitude distribution of electric field is derived from the ratio of blue to red emission intensity by assuming all emissions are due to the electron impact excitation of nitrogen molecules. We find a clear transition at ∼75 km altitude from the upper‐diffuse to lower‐streamer region. Estimated electric field intensities in the diffuse region are 0.5–0.7 Ek, which support the theoretical expectation that their optical emissions could be produced without significant ionization. On the other hand, those in the streamer region are 1–2 Ek which is a few times less than predicted fields in the streamer head. We suggest that this discrepancy is due to the long‐lasting components such as the lower portions of the upward branches and bead structures.