Mitsuteru Sato

Gigantic jets between a thundercloud and the ionosphere

Transient luminous events in the atmosphere, such as lighting-induced sprites and upwardly discharging blue jets, were discovered recently in the region between thunderclouds and the ionosphere. In the conventional picture, the main components of Earths global electric circuit include thunderstorms, the conducting ionosphere, the downward fair-weather currents and the conducting Earth. Thunderstorms serve as one of the generators that drive current upward from cloud tops to the ionosphere, where the electric potential is hundreds of kilovolts higher than Earths surface. It has not been clear, however, whether all the important components of the global circuit have even been identified. Here we report observations of five gigantic jets that establish a direct link between a thundercloud (altitude ∼16 km) and the ionosphere at 90 km elevation. Extremely-low-frequency radio waves in four events were detected, while no cloud-to-ground lightning was observed to trigger these events. Our result indicates that the extremely-low-frequency waves were generated by negative cloud-to-ionosphere discharges, which would reduce the electrical potential between ionosphere and ground. Therefore, the conventional picture of the global electric circuit needs to be modified to include the contributions of gigantic jets and possibly sprites.

Validation of sprite-inducing cloud-to-ground lightning based on ELF observations at Syowa station in Antarctica

Abstract Waveform monitoring of ELF radio signals in the frequency range of 1– 400 Hz have been carried out on a routine basis at Syowa station (69.0°S, 39.6°E in geographic coordinates), Antarctica since February, 2000. The main purpose of these observations is to monitor global lightning activity and to locate lightning-induced sprites and elves. The ELF observation system consisting of two search coil sensors (geomagnetic north–south (H) and east–west (D) sensors) was installed at a remote unmanned observatory in West Ongul Island located 5 km southwest from Syowa station. As a back up system, the same system was installed near Syowa station in East Ongul Island. Signals from these sensors were digitally sampled at 1000 Hz with a GPS time code. On July 4, 2000 during the STEPS (Severe Thunderstorm Electrification and Precipitation Studies) 2000 campaign carried out over the Great Plains in the US, 57 sprite events were observed from Yucca Ridge Field Station (40.7°N, 104.9°W), Colorado, and 53 out of these sprite events had one-to-one correspondence to transient Schumann resonances (SR) detected at Syowa station. The waveforms of these SR are characterized by sharp initial pulses and following damped oscillations. The great circles representing the propagation paths are determined from the Lissojous plots of the H and D magnetic field data of the transient SR. It has been demonstrated that the minimum distance between the great circles and the locations of causative cloud-to-ground (CG) discharges is ∼240 km on average. It is thus concluded that the method to determine the propagation paths from Lissajous plots is extremely accurate when we use the Syowa ELF waveform data. Consequently, it would be possible to globally triangulate sprite-inducing CG locations by setting up at least one more observation site with the same system.

Global distribution of intense lightning discharges and their seasonal variations

In order to study the temporal and regional variation of lightning occurrences and their relation to sprite activity and climate variability, we have analysed the 1?100?Hz ELF magnetic field waveform data obtained at the Syowa station in Antarctica, Onagawa in Japan and Esrange in Sweden for a one year period from September 2003 to August 2004. We have selected totally 1.7 ? 105 events of transient Schumann resonances from the ELF magnetic field data whose amplitude exceeds 40?pT at all stations. Then, the lightning locations are estimated by a triangulation method with an estimation error of 0.5?Mm. It is found that in the summer season (from June to August) the lightning occurrence rates are higher in the northern hemisphere than in the southern hemisphere with large enhancements in North America, South-East Asia and the northern part of Africa. On the other hand, in the winter season (from December to February) these rates are higher in the southern hemisphere with large enhancements in South America, Australia and the southern part of Africa. These features are consistent with the results of global lightning measurements from space conducted by the Optical Transient Detector and the Tropical Rainfall Measuring Mission satellite. Then, we have also calculated the charge moment value (Q ? dl) of lightning discharges using ELF magnetic field waveform data and have estimated the distribution function of charge moments for positive cloud-to-ground (+CG) discharges and for negative cloud-to-ground (?CG) discharges. It is found that the shape of the distribution function for both +CG and ?CG discharges is almost the same for all seasons and that the distribution function of ?CG discharges has a steeper slope at high Q ? dl than that of +CG discharges.

Overview and early results of the Global Lightning and Sprite Measurements mission

Global Lightning and Sprite Measurements on Japanese Experiment Module (JEM-GLIMS) is a space mission to conduct the nadir observations of lightning discharges and transient luminous events (TLEs). The main objectives of this mission are to identify the horizontal distribution of TLEs and to solve the occurrence conditions determining the spatial distribution. JEM-GLIMS was successfully launched and started continuous nadir observations in 2012. The global distribution of the detected lightning events shows that most of the events occurred over continental regions in the local summer hemisphere. In some events, strong far-ultraviolet emissions have been simultaneously detected with N2 1P and 2P emissions by the spectrophotometers, which strongly suggest the occurrence of TLEs. Especially, in some of these events, no significant optical emission was measured by the narrowband filter camera, which suggests the occurrence of elves, not sprites. The VLF receiver also succeeded in detecting lightning whistlers, which show clear falling-tone frequency dispersion. Based on the optical data, the time delay from the detected lightning emission to the whistlers was identified as ∼10 ms, which can be reasonably explained by the wave propagation with the group velocity of whistlers. The VHF interferometer conducted the spaceborne interferometric observations and succeeded in detecting VHF pulses. We observed that the VHF pulses are likely to be excited by the lightning discharge possibly related with in-cloud discharges and measured with the JEM-GLIMS optical instruments. Thus, JEM-GLIMS provides the first full set of optical and electromagnetic data of lightning and TLEs obtained by nadir observations from space.

MSI: a visible multispectral imager for 1.6-m telescope of Hokkaido University

We have built a visible multi-spectral imager (MSI) for the 1.6-m Pirka telescope of the Hokkaido University in Hokkaido, Japan. The instrument is equipped with two liquid crystal tunable filters and a 512 × 512 pixel EMCCD camera. One of the major purposes of this instrument is to obtain multi-spectral images (series of narrow-band images at many different wavelengths) of the solar planets rapidly. These tunable filters are a Lyot filter with liquid crystal variable retarders and thus can tune the transmitting wavelength rapidly without moving parts. Their spectral ranges are 400–720 nm and 650–1100 nm and the bandwidth is typically 10 nm on both filters. The EMCCD camera can obtain images at a frame rate of about 32 Hz, which also enables us to improve the spatial resolution with the shift-and-add or the Lucky imaging techniques. The field of view is 3.3 × 3.3 arcmin with a pixel scale of 0.39 arcsec pixel−1. The instrument also has UBV RI-band broad-band filters and several narrow-band filters. MSI is mounted at the f/12 Cassegrain focus of the telescope. It had the first light on February 2011, and then have been used for several astronomical and planetary science programs as a major facility instrument at this telescope. We describe the design, construction, integration, and performance of this multi-spectral imager.

Photonuclear reactions triggered by lightning discharge

Lightning and thundercloud are the most dramatic natural particle accelerators on the Earth. Relativistic electrons accelerated by electric fields therein emit bremsstrahlung gamma rays, which have been detected at ground observations, by airborne detectors, and as terrestrial gamma-ray flashes (TGFs) from space. The energy of the gamma rays is sufficiently high to potentially invoke atmospheric photonuclear reactions 14N(gamma, n)13N, which would produce neutrons and eventually positrons via beta-plus decay of generated unstable radioactive isotopes, especially 13N. However, no clear observational evidence for the reaction has been reported to date. Here we report the first detection of neutron and positron signals from lightning with a ground observation. During a thunderstorm on 6 February 2017 in Japan, a TGF-like intense flash (within 1 ms) was detected at our monitoring sites 0.5-1.7 km away from the lightning. The subsequent initial burst quickly subsided with an exponential decay constant of 40-60 ms, followed by a prolonged line emission at about 0.511 megaelectronvolt (MeV), lasting for a minute. The observed decay timescale and spectral cutoff at about 10 MeV of the initial emission are well explained with de-excitation gamma rays from the nuclei excited by neutron capture. The centre energy of the prolonged line emission corresponds to the electron-positron annihilation, and hence is the conclusive indication of positrons produced after the lightning. Our detection of neutrons and positrons is unequivocal evidence that natural lightning triggers photonuclear reactions. No other natural event on the Earth is known to trigger photonuclear reactions. This discovery places lightning as only the second known natural channel on the Earth after the atmospheric cosmic-ray interaction, in which isotopes, such as 13C, 14C, and 15N, are produced.Lightning and thunderclouds are natural particle accelerators. Avalanches of relativistic runaway electrons, which develop in electric fields within thunderclouds, emit bremsstrahlung γ-rays. These γ-rays have been detected by ground-based observatories, by airborne detectors and as terrestrial γ-ray flashes from space. The energy of the γ-rays is sufficiently high that they can trigger atmospheric photonuclear reactions that produce neutrons and eventually positrons via β+ decay of the unstable radioactive isotopes, most notably 13N, which is generated via 14N + γ → 13N + n, where γ denotes a photon and n a neutron. However, this reaction has hitherto not been observed conclusively, despite increasing observational evidence of neutrons and positrons that are presumably derived from such reactions. Here we report ground-based observations of neutron and positron signals after lightning. During a thunderstorm on 6 February 2017 in Japan, a γ-ray flash with a duration of less than one millisecond was detected at our monitoring sites 0.5–1.7 kilometres away from the lightning. The subsequent γ-ray afterglow subsided quickly, with an exponential decay constant of 40–60 milliseconds, and was followed by prolonged line emission at about 0.511 megaelectronvolts, which lasted for a minute. The observed decay timescale and spectral cutoff at about 10 megaelectronvolts of the γ-ray afterglow are well explained by de-excitation γ-rays from nuclei excited by neutron capture. The centre energy of the prolonged line emission corresponds to electron–positron annihilation, providing conclusive evidence of positrons being produced after the lightning.

An NMR study of the new strong magnetism in the quasicrystalline Al-Pd-Mn-B system

The authors have measured 27Al, 55Mn and 11B NMR spectra of the quasicrystalline Al-Pd-Mn-B system with the new strong magnetism. The zero-field spectra observed around 220 MHz at 4.2 K provide evidence that ferromagnetic Mn atoms exist in these icosahedral quasicrystals. Further, Mn atoms with no internal magnetic field are also detected in these icosahedral quasicrystals from spectra obtained in applied magnetic fields. From the microscopic point of view, by means of NMR, it is concluded that the icosahedral phase of this system consists of a magnetically heterogeneous atom distribution, although the X-ray diffraction pattern and transmission electron microscopy exhibit the formation of a single icosahedral phase in the Al-Pd-Mn-B alloy.

Magnetic impulse events and related Pc1 bursts observed by the Automatic Geophysical Observatories network in Antarctica

Magnetic field data obtained by fluxgate and search coil magnetometers installed at four Automatic Geophysical Observatories (AGO Pl, P2, P3, and P4) and at South Pole and McMurdo in Antarctica and at Iqaluit on Baffin Island are used to study the relationship between magnetic impulse events (MIEs) and Pc1 bursts at high geomagnetic latitudes. The spatial scale sizes of MIEs are determined in the investigation of the sources of these events in the magnetosphere. Four MIE events with simultaneous Pc1 bursts are studied. Contour plots of MIE amplitudes show the scale sizes of the examined MIEs to be 5° to 7° in the magnetic latitude direction and 40° to 60° in the magnetic longitude direction. Temporal changes of the ionospheric currents calculated from the fluxgate H- and D-component data indicate that the “convection vortices” associated with the MIEs traveled westward and decayed rapidly. An important finding is that MIE-related Pc1 bursts were observed at higher-latitude (Pl, P4 near 80°) and lower-latitude (P2, P3 near 70°) AGO stations and South Pole Station at 74° with different spectral structures, although the Pc1 spectral power was always a maximum at South Pole Station. These results imply that Pc1 bursts are excited not only in the dayside outer magnetosphere but also in the low-latitude boundary layer region.

GLOBAL MEASUREMENT OF LIGHTNING-ASSOCIATED TRANSIENT LUMINOUS EVENTS (TLEs) FROM SPACE

In this study we present possibility of continuous measurements of lightning-associated transient luminous events (sprites, elves and blue jets) from the ISS altitude using the EUSO telescope. From global lightning data we estimated possible detection rates of lightning and TLEs. We also estimated photon numbers and optical spectra of TLEs in the near-ultraviolet region (300 – 400 nm) where the fluorescence emission caused by CRs exists. These results imply that EUSO has enough capabilities to monitor not only super-GZK CRs but also global lightning and TLEs.

Polarimetric Study of Near-Earth Asteroid (1566) Icarus

We conducted a polarimetric observation of the fast-rotating near-Earth asteroid (1566) Icarus at large phase (Sun-asteroid-observers) angles