Masayuki Kikuchi

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.

Telescope of extreme ultraviolet (TEX) onboard SELENE: science from the Moon

The Upper Atmosphere and Plasma Imager (UPI) is to be launched in 2007 and sent to the Moon. From the lunar orbit, two telescopes are to be directed towards the Earth. The Moon has no atmosphere, which results in there being no active emission near the spacecraft; consequently, we will have a high-quality image of the near-Earth environment. As the Moon orbits the Earth once a month, the Earth will also be observed from many different directions. This is called a “science from the Moon”. The two telescopes are mounted on a two-axis gimbal system, the Telescope of Extreme ultraviolet (TEX) and Telescope of Visible light (TVIS). TEX detects the O II (83.4 nm) and He II (30.4 nm) emissions scattered by ionized oxygen and helium, respectively. The targets of extreme-ultraviolet (EUV) imaging are the polar ionosphere, the polar wind, and the plasmasphere and inner magnetosphere. The maximum spatial and time resolutions are 0.09 Re and 1 min, respectively.

Synoptic observations of auroras along the postnoon oval: a survey with all-sky TV observations at Zhongshan, Antarctica

Abstract All-sky TV data obtained at Zhongshan, Antarctica, have been used to survey auroral displays along the postnoon auroral oval. The auroral occurrence peak around 15 MLT, which was previously shown by satellite observations, is confirmed to exist in ground observations as well. The so called ‘midday gap’ of discrete aurora, however, is not confirmed by ground observations. This survey reveals that the noon region appears to involve another auroral occurrence peak. The noon auroral peak observed from the ground is dominated by an aurora termed as dayside corona in the present study. A dayside corona is usually weak and changing rapidly in its appearance, luminosity and locale. The electron precipitation causing the dayside corona might be too soft, have reduced flux and/or have too rapid a motion of its rayed structures which would result in less luminosity, all of which could account for the ‘midday gap’ in satellite observations. We thus argue for a new synoptic picture of auroral displays along the postnoon oval, in which beside the 15 MLT peak, the noon region is filled with the dayside corona rather than a ‘gap’ in discrete aurora.

Enhancement of optical aurora triggered by the solar wind negative pressure impulse (SI

A geomagnetic negative sudden impulse (SI−) occurred on 3 August 1997 in association with a sudden decrease of solar wind dynamic pressure. The discrete auroras observed by an all-sky TV imager at Zhongshan in Antarctica (magnetic latitude is ∼74.5°S) suddenly enhanced their luminosity and moved poleward at the time of the SI−. Following the primary enhancement, three cycles of quasi-periodic luminosity pulsations were observed with periods of about 10–14 min and their average position drifted equatorward. The luminosity pulsations showed one-to-one correspondence with the magnetic pulsations, with the characteristic features of a field-line resonance. The low-altitude DMSP-F13 satellite was almost directly over Zhongshan and observed accelerated electron precipitation with an inverted ‘V structure and an upward field-aligned current. These signatures suggest that the solar wind negative pressure impulse triggered a field-line resonance of the geomagnetic field. The resonance caused the upward and downward field-aligned current sheets, and the field-aligned electron acceleration.

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 ∼10u2009ms, 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.

Plasmaspheric EUV images seen from lunar orbit: Initial results of the extreme ultraviolet telescope on board the Kaguya spacecraft

[1]xa0The Telescope of Extreme Ultraviolet (TEX) aboard Japans lunar orbiter Kaguya has succeeded in imaging of the plasmaspheric helium ions by detecting resonantly scattered emission at 30.4 nm. After the initial instrumental check was completed, TEX has been operated routinely, and EUV images from TEX have become available from the perspective of the lunar orbit. The view afforded by the Kaguya orbit encompasses the plasma (He+) distribution in a single exposure, enabling us to examine for the first time the globally averaged properties of the terrestrial plasmasphere from the “side” (meridian) perspective. In this paper we report the inward motion of the nightside plasmapause on 2 May 2008 as seen from this remote meridian view of the Earth. The southward turning of the IMF initiated the inward motion of the plasmapause, and the nightside plasmasphere shrunk at a rate of 0.2 Re/h. Simultaneous solar wind velocity measurements provide a possible explanation for the total radial displacement of the plasmasphere observed in the EUV images.

The Upper Atmosphere and Plasma Imager/the Telescope of Visible Light (UPI/TVIS) onboard the Kaguya spacecraft

The Upper Atmosphere and Plasma Imager (UPI) was placed in a lunar orbit in order to study both the Moon and Earth. The UPI consists of two telescopes: a Telescope of Extreme Ultraviolet (TEX) and a Telescope of Visible Light (TVIS), which are both mounted on a two-axis gimbals system. The TVIS is equipped with fast catadioptric optics and a high-sensitivity CCD to image swift aurora and dark airglow in the terrestrial upper atmosphere. TVIS has a field-of-view equivalent to the Earth’s disk as seen from the Moon. The spatial resolution is about 30 km × 70 km on the Earth’s surface at auroral latitudes. The observation wavelengths can be changed by selecting different bandpass filters. Using the images of the northern and southern auroral ovals taken by TVIS, the intensities and shapes of the conjugate auroras will be quantitatively compared. Using the airglow imaging, medium- and large-scale ionospheric disturbances will be studied. In this paper, the instrumental design and performance of TVIS are presented.

Horizontal distributions of sprites derived from the JEM-GLIMS nadir observations

Global Lightning and Sprite Measurements on Japanese Experiment Module (JEM-GLIMS) started the nadir observations of lightning discharges and transient luminous events (TLEs) from the International Space Station (ISS) since November 2012. In the nadir observations, JEM-GLIMS optical instruments have to simultaneously detect incomparably intense lightning emissions and weak TLE emissions. To distinguish TLEs, especially sprite events, from lightning events, combined data analytical methods are adopted: (1) a subtraction of the wideband camera image from the narrowband camera image, (2) a calculation of the intensity ratio between different photometer channels, and (3) an estimation of the polarization and charge moment changes for the TLE-producing lightning discharges. We succeeded in identifying numbers of sprite events using the combined analytical methods, and here we report three sprite events detected by JEM-GLIMS as a case study. In the subtracted images, sprite emissions are located over the area of the sprite-producing lightning emissions. However, these sprites and sprite-producing lightning discharges did not occur at the nadir point of the ISS. For this reason, the geometry conversion of the sprite and sprite-producing lightning emissions as observed from the point just over the sprite-producing lightning discharges is performed. In the geometry-converted images, the locations of the sprite emissions are clearly displaced by 8–20xa0km from the peak positions of the sprite-producing lightning emissions. Thus, the first quantitative spatial distributions of sprites and sprite-producing lightning discharges from the JEM-GLIMS nadir observations are revealed.

Direction-of-Arrival Estimation of VHF Signals Recorded on the International Space Station and Simultaneous Observations of Optical Lightning

We report an initial investigation of the new location method of a very high frequency (VHF) radiation source, using signals recorded at the International Space Station. A VHF interferometer (VITF) has two VHF sensors. Locating lightning with VHF bands is useful to locate the position of the charge distribution in the thunderstorm. The location method of a radio source proposed used two direction-of-arrival estimation techniques. One is the interferometric technique, and another is based on the ionospheric propagation delay measurement of received signals. The combination of the two techniques provides two angular positions of the radiation source. When an altitude of a radiation source is assumed, we can determine two possible positions. One of the two positions was associated with the radiation source, while the other was not. In this paper, we compared the position of lightning and sprite imager (LSI) data, which are simultaneously captured during a lightning emission, with the locating position near the emission. The data set of the VITF within 100 ms of the optical lightning emission captured with the LSI was used. The temporally simultaneous event seems to be associated with the same lightning event. The estimated radiation positions were spatially in close agreement with the optical lightning positions captured with LSI, under nighttime ionosphere conditions. From statistical analysis, the spatial difference of the standard deviation changed from 15.3 to 30.8 km depending on the installation direction of the VHF sensors. The usefulness and limitations of the method are also discussed.

Ground Observations of Post-Noon Aurora: a Case Study

The characteristics of the post-noon aurora observed at Antarctic Zhongshan station on June 12, 1999, were discussed and analyzed. In the condition of the magnetic activity is not large (Kp≈1), for post-noon 630.0 nm emissions, the total fluxes of soft precipitating particles were increasing from 10∶50 UT to 13∶35 UT and were decreasing from 13∶35 UT to 18∶00 UT in almost monotonous way. Away from noon, the 557.7 nm emissions increased gradually from 10∶50 UT to 17∶10 UT. The behaviors of the precipitating particles for exciting 630.0 nm aurora and 557.7 nm aurora were quite different. The peak intensity of 630.0 nm and 557.7 nm emissions appeared at about 13∶35 UT and 15∶40 UT respectively, the time difference of two peaks is about 2 h. The energy of precipitating electrons remained fairly steady until 15∶00 UT when it rose dramatically.