Masa-yuki Yamamoto

Structures in sporadic‐E observed with an impedance probe during the SEEK Campaign: Comparisons with neutral‐wind and radar‐echo observations

In order to clarify the origin of the so-called quasi periodic echoes (QPE) that have been often detected by radar observations in the presence of sporadic-E (Es) layers in the nighttime midlatitude ionosphere, two sounding rockets were launched during the SEEK (Sporadic-E Experiment over Kyushu) campaign. Each rocket carried a swept-frequency impedance probe to measure the E-region electron-density (Ne) profile. Using the four Ne profiles obtained during the two rocket flights together with a neutral-wind profile obtained from a trimethyl aluminum (TMA) chemical release experiment on one of the rockets and QPE obtained with a ground-based radar, we consider the role of wind shear in the formation of the observed Es layers, and the question of whether QPE are associated with Es layers that are modulated in altitude. The Ne profiles of Es structures that were obtained in the presence of QPE were characterized by the highly concentrated thin layers. The formation of such a thin layer by a neutral-wind shear process was confirmed in comparison with the TMA measurements. The peak Ne values of the Es layers ranged from 2.2 to 9.3 × 104 el/cm³ near 100-km altitude. These primary Es layers were accompanied by significant secondary structures that were located about 12 to 20 km above the main Es layers and had peak Ne that ranged from 5.2 × 10³ to 1.3×104 el/cm³. The average altitude profiles of QPE approximately covered the range where the Es-layer peaks appeared. Our principal finding is that the observed Es structures tended to resemble horizontally stratified layers rather than structures with deep altitude modulation like previous QPE model, although the rocket measurements were separated from those by radar by 90 to 145 km.

Correlations between ion density and temperature in the topside ionosphere measured by ROCSAT‐1

From the ROCSAT-1 satellite plasma data at an altitude of 600 km, the correlation between ion temperature (Ti) and density (Ni) was investigated. The data were obtained in a magnetic dip latitude (MLAT) of less than ±40° in 2000–2004. Positive and negative correlations between Ni and Ti were observed around the magnetic dip equator, while weak positive correlations were observed in |MLAT| > 25° during daytime (10:00–16:00 local time). These variations were found in all longitudes, seasons, solar flux (F10.7) levels, and magnetic disturbance levels, although the minimum value of Ti clearly increased with increasing solar flux levels. The results suggest that the solar flux dependence of Ti arises from the solar flux dependence on neutral temperature (Tn). Since Ti is determined by heating through Coulomb collision with electrons and cooling through elastic collision with neutral species, the ratio of ion density to neutral density is an important factor. The ratio reaches its maximum value around the magnetic dip equator and decreases with increasing MLAT. The correlation between Ni and Ti in the topside ionosphere can be explained by electron temperature (Te) and Tn as well as the ratio because Ti follows Te variation when the ratio is high, while it follows Tn when the ratio is low.

Seismic Wave Interactions Between the Atmosphere - Ocean - Cryosphere System and the Geosphere in Polar Regions

At the time of the International Geophysical Year (IGY; 1957-1958), it was generally understood by a majority of seismologists that no extreme earthquakes occurred in polar regions, particularly around Antarctica. Despite the Antarctic being classified as an aseismic region, several significant earthquakes do occur both on the continent and in the surrounding oceans. Since IGY, an increasing number of seismic stations have been installed in the polar regions, and operate as part of the global network. The density of both permanent stations and temporary deployments has improved over time, and has recently permitted detailed studies of local seismicity (Kaminuma, 2000; Reading, 2002; 2006; Kanao et al., 2006).

Infrasound/seismic observation of the Hayabusa reentry: Observations and preliminary results

The Hayabusa, the world’s first sample-return minor body explorer, returned to the Earth, and reentered the Earth’s atmosphere on June 13, 2010. Multi-site ground observations of the Hayabusa reentry were carried out in the Woomera Prohibited Area (WPA), Australia. The ground observations were configured with optical imaging with still and video recordings, spectroscopies, and shockwave detection with infrasound and seismic sensors. At three main stations, we installed small aperture infrasound/seismic arrays, as well as three single component seismic sub stations. The infrasound and seismic sensors clearly recorded sonic-boom-type shockwaves from the Hayabusa Sample Return Capsule (H-SRC) and the disrupted fragments of the Hayabusa Spacecraft (H-S/C) itself. Positive overpressure values of shockwaves (corresponding to the H-SRC) recorded at the three main stations were 1.3 Pa, 1.0 Pa, and 0.7 Pa with slant distances of 36.9 km, 54.9 km, and 67.8 km, respectively. Incident vectors of the shockwave from the H-SRC at all three arrays are estimated by an F-K spectrum and agree well with those predicted. Particle motions of ground motions excited by the shockwave show characteristics of a typical Rayleigh wave.

Optical Tracking and Spectroscopic Measurement of Hayabusa Capsule Reentry Fireball

The asteroid explorer HAYABUSA finally returned to the earth on June 13rd 2010 and the sample return capsule experienced a super-orbital atmospheric reentry. To recover the sample return capsule and to conduct optical measurements, the Japan Aerospace Exploration Agency organized a ground observation team and conducted optical tracking of the sample return capsule, spectroscopy of the fireball as well as the fireball trail, and measurement of infrasounds and shock waves generated by the fireball. In this article, an overview of the ground observation is presented, and the preliminary results derived from observations are reported.

Characteristic atmosphere-ocean-solid earth interactions in the Antarctic coastal and marine environment inferred from seismic and infrasound recording at Syowa Station, East Antarctica

Abstract Several characteristic waves detected by seismographs in Antarctic stations have been recognized as originating from the physical interaction between the solid earth and the atmosphere–ocean–cryosphere system surrounding the Antarctic and may be used as a proxy for characterizing ocean wave climate. A Chaparral-type infrasound sensor was installed at Syowa Station (SYO; 39.6E, 69.0S), East Antarctica, in April 2008 during the International Polar Year (IPY2007–2008). Matching data are also available for this time period from the existing broadband seismic recorder located close by. Continuous infrasound data for 2008–2009 include background signals (microbaroms) with a broad peak in the wave period between the values of 4 and 10 s. Signals with the same period are recorded by the broadband seismograph at SYO (microseisms). This period band is identified as double-frequency microseisms/baroms (DFM). The DFM have relatively lower amplitudes during winter. We suggest that this is due to the sea-ice extent around the coast causing a decreased ocean loading effect. In contrast, the single frequency microseisms/baroms with a peak in period between 12 and 30 s are observed under storm conditions, particularly in winter. On the infrasound data, stationary signals are identified with harmonic overtones at a few Hertz to lowermost human audible band, which we suggest is due to local effects such as sea-ice cracking and vibration. Microseism measurements are a useful proxy for characterizing ocean wave climate, complementing other oceanographic and geophysical data. At SYO, continuous monitoring by both broadband seismograph and infrasound contributes to the Federation of Digital Seismographic Networks, the Comprehensive Nuclear-Test-Ban Treaty in the high southern latitudes and the Pan-Antarctic Observations System under the Scientific Committee on Antarctic Research.

Poster Abstract: Multiple Door Opening/Closing Detection System Using Infrasound Sensor

Our research group is currently focused on research and development of Internet of Things-based robust emergency and disaster prevention systems to transmit tsunami information retrieved from infrasound sensors. To constantly test the effective sensor operation, it is necessary generate infrasound. Therefore, we considered the infrasound generated when opening and closing doors in this study, and used one sensor to detect the state variation of multiple doors. We constructed and verified the detection system for various doors.

Multiple door opening/closing detection system using infrasound sensor: poster abstract

Our research group is currently focused on research and development of Internet of Things-based robust emergency and disaster prevention systems to transmit tsunami information retrieved from infrasound sensors. To constantly test the effective sensor operation, it is necessary generate infrasound. Therefore, we considered the infrasound generated when opening and closing doors in this study, and used one sensor to detect the state variation of multiple doors. We constructed and verified the detection system for various doors.

Study of medium-scale traveling ionospheric disturbances (MSTID) with sounding rockets and ground observations

Medium-scale traveling ionospheric disturbance (MSTID) is an interesting phenomenon in the F-region. The MSTID is frequent in summer nighttime over Japan, showing wave structures with wavelengths of 100-200 km, periodicity of about 1 hour, and propagation toward the southwest. The phenomena are observed by the total electron content (TEC) from GEONET, Japanese dense network of GPS receivers, and 630 nm airglow imagers as horizontal pattern. It was also measured as Spread-F events of ionograms or as field-aligned echoes of the MU radar. MSTID was, in the past, explained by Perkins instability while its low growth rate was a problem [1]. Recently 3D simulation study by Yokoyama et al. [2] hypothesized a generation mechanism of the MSTID, which stands on electromagnetic E/F-region coupling of the ionosphere. The hypothesis is that the MSTID first grows with polarization electric fields from sporadic-E, then show spatial structures resembling to the Perkins instability. We recently conducted an observation campaign to check this hypothesis. We launched JASA ISAS sounding rockets S-310-42 and S-520-27 at 23:00 JST and 23:57JST on July 20, 2013 while an MSTID event was monitored in real-time by the GPS-TEC from GEONET. We found 1-5mV/m northeastward/eastward electric fields during the flight. Variation of electric fileds was associated with horizontal distribution of plasma density. Wind velocity was measured by the TME and Lithium releases from S-310-42 and S-520-27 rockets, respectively, showing southward wind near the sporadic-E layer heights. These results are consistent to the expected generation mechanism shown above. In the presentation we will discuss electric-field results and its relationship with plasma density variability together with preliminary results from the neutral-wind observations.