H. Fukunishi

Global survey of upper atmospheric transient luminous events on the ROCSAT-2 satellite

Abstract Upper atmospheric transient luminous events (TLEs; sprite, elves, blue jet, etc.) are recently discovered thunderstorm-induced phenomena. Imager of sprites/upper atmospheric lightning (ISUAL) is a scientific payload on the Taiwans ROCSAT-2 satellite that aims primarily to provide crucial observation data on these TLEs from space. The ISUAL payload includes an intensified CCD imager, a six-channel spectrophotometer, and two array photometers. All the instruments are mounted on a common platform and boresighted in the same direction. The imager is equipped with six selectable filters, which have bandpasses covering the visible spectrum. The spectrophotometer contains six photometers, and each photometer is fitted with a special bandpass filter ranging from ultraviolet to red regions. The two array photometers are identical in every aspect, except one is fitted with a blue band filter and another one is equipped with a red band filter. With this set of well-chosen instrument, this project seeks to determine the location and timing of upper atmospheric transient events above thunderclouds, to investigate their spatial, temporal and spectral properties, to obtain a global survey of them, and to perform a global study of aurora and airglow. ISUAL project is an international collaboration supported by the National Space Program Office in Taiwan, with additional contributions from the National Cheng Kung University, Taiwan, the Space Science Laboratory of the University of California at Berkeley, and Tohoku University, Japan. ROCSAT-2, the platform of ISUAL, is scheduled to launch around October 2003.

Interrelation between ELF transients and ionospheric disturbances in association with sprites and elves

Red sprites and elves were observed in Japan during the winter of 1998/99 in Hokuriku region by the group of Tohoku Univ. [Fukunishi et al., EOS, 80(46), F217, 1999]. We analyze quantitatively the results from coordinated measurement consisting of ELF transients, VLF subionopheric disturbances and lightning discharges associated with the optical events. We find the clear straightforward relationship between charge transfer of the parent discharge calculated from ELF (f < 15 Hz) and the ionospheric disturbances regardless of the types of optical events indicating significant atmosphere-mesosphere-ionosphere coupling. Sprites tend to associate with a large ionospheric disturbance (−13 ∼ + 4.6 dB) with a large charge transfer (52 ∼ 175 C), whereas a large lightning peak current (+223 ∼ + 470 kA) (or slow-tail amplitude) leading to the strong EMP is necessary to initiate elves, but with rather small ionospheric disturbances.

Breaking of small-scale gravity wave and transition to turbulence observed in OH airglow

We report on observations of small-scale gravity wave breaking into turbulence around the mesopause height using an OH airglow CCD camera and a meteor radar performed on December 23, 1995 in Japan. The small-scale gravity wave propagated against the background wind with a horizontal wavelength of ∼27 km, an apparent horizontal phase velocity of ∼7 ms−1, and an intrinsic period of ∼5.6 minutes. Wave breaking event began with an appearance of smaller-scale structures, which were aligned parallel to the phase fronts of the breaking wave and advanced into more random, turbulence-like features. The instability structures were present in the OH images for about 40 minutes.

Modeling elves observed by FORMOSAT‐2 satellite

Received 18 March 2007; revised 20 July 2007; accepted 31 July 2007; published 22 November 2007. [1] 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 ,O 3, 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.

Traveling ionospheric disturbances observed in the OI 630‐nm nightglow images over Japan by using a Multipoint Imager Network during the FRONT Campaign

Pilot observations using a network of five all-sky imagers (ASIs) were conducted during the new moon period of May 19–22, 1998 as part of the F-region Radio and Optical measurement of Nighttime TID (FRONT) campaign. The network observation enabled us to track propagation of medium-scale traveling ionospheric disturbances (TIDs) in the OI 630-nm nightglow over a distance of more than 2500 km. The TIDs were observed every night during the campaign period, but occurrence was limited from evening to midnight. They have horizontal wavelengths of 200–600 km, travel a horizontal distance of more than 1000 km, and last for more than three hours. In every case, the TIDs moved southwestward with a velocity of 83–137 m/s. Using dual-site TID images, the altitude of the TID structures in the 630-nm nightglow was calculated to be ∼260 km, which corresponds to the bottom side of the mid-latitude ionospheric F layer.

Latitudinal structures of nightside field‐aligned currents and their relationships to the plasma sheet regions

Using the magnetic field and low-energy particle data from the Akebono (EXOS D) satellite, the latitudinal structures of field-aligned currents and auroral particles in the nightside auroral oval region have been studied in detail for 12 northern hemisphere passes in the 20-04 MLT sector and at altitudes of 5000-10,000 km. It is found that the nightside field-aligned current system consists of the latitudinally narrow (∼1°) current system located at the poleward boundary of the auroral oval and the latitudinally wide current system which occupies the main portion of the oval. The former has been designated as “the boundary current system” and the latter as “the central current system.” Both current systems are suggested to be located in the closed field line region, since the polar rain is observed just poleward of the boundary current system. The boundary current system is suggested to be independent of the traditional region 1 current system associated with inverted V electron precipitation, since suprathermal electrons with field-aligned pitch angle distributions are found to be the main charge carrier of this current system. Another important finding is that energy-dispersed ion precipitation events, which are interpreted to be the result of the E × B drift of ions flowing earthward in the plasma sheet boundary layer, appear in this boundary current region. The central current system consists of the two regions, the inverted V electron precipitation region on the poleward side and the unstructured electron and ion precipitation region on the equatorward side. It is suggested therefore that this central current system corresponds to the traditional region 1 and region 2 current system in the nightside sector. It is further found that the diffuse band of plasma sheet kilovolt electrons is continuously observed form the polar cap boundary defined as the polar rain terminator to the equatorward boundary of the unstructured precipitation region through the inverted V electron precipitation region. Consequently, inverted V structures and diffuse plasma sheet electron precipitation always coexist. From these spatial relationships, it is strongly suggested that the boundary current system is mapped to the plasma sheet boundary layer, while the central current system is mapped to the main or central plasma sheet.

Electric fields and electron energies in sprites and temporal evolutions of lightning charge moment

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, Ek ∼ 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 (∼1 ms) and a moderate amount of charge moment (∼400 C km) produce discernible halos. On the other hand, lightning discharges with a large amount of charge moment (∼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.

Relationship between field‐aligned currents and inverted‐V parallel potential drops observed at midaltitudes

The inverted-V field-aligned acceleration region existing in the altitude range of several thousand kilometers plays an essential role for the magnetosphere-ionosphere coupling system. The adiabatic plasma theory predicts a linear relationship between field-aligned current density (J∥) and parallel potential drop (Φ∥), that is, J∥ = KΦ∥, where K is the field-aligned conductance. We examined this relationship using the charged particle and magnetic field data obtained from the Akebono (Exos D) satellite. The potential drop above the satellite was derived from the peak energy of downward electrons, while the potential drop below the satellite was derived from two different methods: the peak energy of upward ions and the energy-dependent widening of electron loss cone. On the other hand, field-aligned current densities in the inverted-V region were estimated from the Akebono magnetometer data. Using these potential drops and field-aligned current densities, we estimated the linear field-aligned conductance KJΦ. Further, we obtained the corrected field-aligned conductance KJΦC by applying the full Knights formula to the current-voltage relationship. We also independently estimated the field-aligned conductance KTN from the number density and the thermal temperature of magnetospheric source electrons which were obtained by fitting accelerated Maxwellian functions for precipitating electrons. The results are summarized as follows: (1) The latitudinal dependence of parallel potential drops is characterized by a narrow V-shaped structure with a width of 0.4°−1.0°. (2) Although the inverted-V potential region exactly corresponds to the upward field-aligned current region, the latitudinal dependence of upward current intensity is an inverted-U shape rather than an inverted-V shape. Thus it is suggested that the field-aligned conductance KJΦC changes with a V-shaped latitudinal dependence. In many cases, KJΦC values at the edge of the inverted-V region are about 5–10 times larger than those at the center. (3) By comparing KJΦC with KTN, KJΦC is found to be about 2–20 times larger than KTN. These results suggest that low-energy electrons such as trapped electrons, secondary and back-scattered electrons, and ionospheric electrons significantly contribute to upward field-aligned currents in the inverted-V region. It is therefore inferred that non adiabatic pitch angle scattering processes play an important role in the inverted-V region.

Pi1 magnetic pulsations in space and at high latitudes on the ground

Nightside, high-latitude (68°–80° magnetic) Pi1 waves, measured with a ground array of induction magnetometers, are studied and compared with magnetic field measurements made at synchronous orbit near the meridian of the ground measurements. The objectives of the study are to relate the ground signatures of Pi1B and PiC to the auroral substorm and its manifestation at synchronous orbit in an attempt to understand the origin of the Pi1 waves. Pi1 waves are measured in the equatorial plane by the GOES spacecraft and appear to be initiated by the dipolarization process of the nightside tail magnetic field at the onset of substorms. Across two meridional arrays of ground stations the earliest onset of Pi1B generally occurred at the lowest-latitude station and, in many instances, this burst was superimposed on a ground signature of a sudden onset of the westward electrojet. In one instance, where good coverage of ground optical data was available, this sudden onset Pi1B was time related to the overhead passage of a westward traveling auroral surge. The timing of maximum Pi1B across the array in both longitude and latitude agrees with the westward motion of the local auroral surge and the poleward motion of the aurora after the surge has arrived at a given site, suggesting a local ionospheric source for some of the Pi1 waves. However, across the entire array, extending about 12° in latitude and 20° in longitude, there often was nearly simultaneous Pi1B wave power at all sites which occurred before the maximum signal at a given site (and presumably local aurora), suggesting horizontal ducting of wave power from the onset of PiIB seen earlier at the auroral zone latitude. Prompt turn-on of PiC waves across the array also indicates ducting of these waves. The narrow bandwidth of the PiC waves themselves suggests a resonant cavity source for them which would indicate that some wave power enters the ionosphere from space (consistent with the GOES in situ data) as the trigger wave for this resonant source mechanism. In conclusion, this study finds evidence for local ionospheric currents, magnetospheric waves, and resonant cavity modes as sources for Pi1 ground waves.

Electric field fluctuations and charged particle precipitation in the cusp

In the cusp region, irregular fluctuations of the electric field are often observed by EXOS D at altitudes of several thousands of kilometers. Amplitudes of the fluctuations sometimes reach 100 mV/m and their spectra are broad. The electric to magnetic field ratios in the frequency range of 0.5-3 Hz agree well with the Alfven velocity at observation point. Hence the electric fluctuations are considered to be Alfven waves. The flux of precipitating ions at 500 eV to 10 keV and that of the electrons at 70-500 eV are enhanced in the cusp and they are well correlated to each other. On the other hand, correlation coefficients between the power spectral density (PSD) of the electric field at 1 Hz and the precipitating particle flux vary from case to case. When the latitude of the cusp is low and IMF is expected to be southward the coefficient is high. This suggests that the waves are generated in association with the injection of particles into the magnetosphere when reconnection occurs. On the other hand, when the latitude of the cusp is high and IMF is expected to be northward, the coefficient is low and the PSD of the electric field is smaller for the same flux of particles than when IMF is southward. In these cases the intensity of the electric fluctuations in the region of the particle injection is possibly not so great as that when reconnection occurs.