Characteristic Infrasound Events Associated with Sea-Ice Discharges in the Lützow-Holm Bay of Antarctica: April 2016

Infrasound waves detected in Antarctica contain information on the physical interaction among the surface environment at the margin of the continent and surrounding ocean. Time-space variation of source location for infrasound excitation during mid-April 2016 was investigated by using a combination of two arrays deployed along the coast of the Lützow-Holm Bay (LHB), East Antarctica. The infrasound array observations detected temporal variations in distance from the sources and propagation direction. A few tens of infrasound events were identified during 10 days of the period, and many of them located in the northward direction from the array stations were inside the LHB and offshore in the Southern Indian Ocean. Many of the events had predominant frequency content of few Hz, which were higher than microbaroms generated from the ocean. By comparing with MODIS satellite image at the same period, these sources were considered to be the ice-related phenomenon associated with the discharge of fast sea ice from the LHB

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

International audienceSeveral 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

Calibration of CRL all-sky imagers using an integrating sphere

As part of an international collaboration with the Geophysical Institute of the University of Alaska, we have developed two all-sky imagers (CRL-ASIs). A sensitivity calibration of the CRL-ASIs was performed using an integrating sphere belonging to the National Institute of Polar Research (NIPR). The two-dimensional sensitivities of the CRL-ASIs produced symmetrical distributions. Using this sensitivity data, we converted airglow/aurora images into two-dimensional distributions of absolute intensity. The sensitivity of the CRL-ASIs was measured for 13 wavelengths between 427.8 nm and 866.5 nm, and the relationship between the sensitivity and the wavelength was investigated for both imagers. The peak sensitivity occurred at about 550 nm

Performance Test of Infrasound Sensor in Low-temperature Environment ─ Potential for Application in Antarctic Observation ─

For infrasound monitoring in Antarctica, there is a need for infrasound sensors with low power consumption and high resistance to low-temperature environments. A new-type infrasound sensor (TYPE7744N/5002A) manufactured by ACO Co., Ltd. (Japan) with the cooperation of the Earthquake Research Institute, the University of Tokyo, achieves less than half the power consumption of existing models. To evaluate the applicability of the new sensor to Antarctic observation, we conducted a low-temperature (－30℃) test for four types of sensors, including the new one. We compared the results to those from a room temperature (21℃) test and examined changes in amplitude-phase characteristics based on a reference sensor (Model60Vx2, Chaparral Physics), proven in use in polar regions. There were no problems in the operation of the new sensor during the 30 days of the test. Spectral power ratio to the reference sensor changed up to 19% compared to the room temperature test, suggesting that the sensitivity fluctuates with temperature. Phase characteristics were not significantly affected by low temperatures. Future trials are desired to evaluate the long-term stability of the new sensor, e.g., by conducting experimental overwintering observations at Syowa Station

Measurement and characterization of infrasound waves from the March 25, 2023 thunderstorm at the near equatorial

Thunderstorm activity on March 25, 2023 provided a unique opportunity to study the mechanism of lightning events on changes in air pressure. In particular, this event made it possible to study changes in air pressure during thunderstorms using various instruments. This paper presented comprehensive results of infrasound, satellite data, weather radar and weather measurements at the ground during the storm. Observations of lightning events were confirmed using observational data from the International Space Station's Lightning Imaging Sensor (ISS LIS). This work estimated three spectral percentile values on infrasonic sensor data, time series interpolation of standard meteorology profiles, weather radar reflectivity and total radiant energy of lightning from ISS LIS observations during the day and night periods. As a result, during the investigation, it was seen that the recorded infrasound signal in the 0.6–0.8 Hertz (Hz) range was contaminated by background environmental noise, but in the 1–3 Hz band range it was consistent with the appearance of storms that produce high energy blows. Infrasound detection and electromagnetic lightning detection show good correlation up to a distance of 100 km from the infrasonic station. During a thunderstorm, the ISS LIS flight directly above the observation site detected more than 2,000 lightning events. In addition, the application of lightning detection from several independent instruments can provide a complete picture of the observed event

Identification of the infrasound signals emitted by explosive eruption of Mt. Shinmoedake by three-dimensional ray tracing

Mt. Shinmoedake, a part of the Mt. Kirishima cluster of volcanoes in Kyushu, Japan, erupted on 10 March 2018. Our infrasound sensor network located at a distance of more than 200 km from the source detected signals emitted by an explosive eruption of Mt. Shinmoedake. The arrival time of the signals is divided into three time intervals. To reveal how the observed infrasound signals propagated from the source to the sensors, we carry out three-dimensional ray tracing on the basis of the Hamilton equations including the vertical profiles of the temperature and wind around the ray path. We present formulas for calculating travel time and distance of infrasound from a source to an observation site and its turning altitude in the atmosphere. We have identified four kinds of signals, namely, the waves propagated in the troposphere undergoing multiple refraction and those refracting from the stratosphere, the mesosphere, and the lower thermosphere. Brief discussion is devoted to some of the unidentified signals

Antarctica - A Key To Global Change

The most exciting initiative in the polar region was the International Polar Year (IPY) in 2007-2008, conducted as the 50th anniversary of the International Geophysical Year (1957-1958). The initiative greatly enhanced the exchange of ideas across nations and scientific disciplines to unveil the status and changes of planet Earth. This sort of interdisciplinary exchange helps us to understand and address grand challenges, such as rapid environmental change and its impact on society. In this regard, this book aims to compile the achievements of projects related to the IPY and post-IPY era, focusing especially on surface environmental variations associated with climate change, such as global warming