The European Infrasound Bulletin highlights infrasound activity produced mostly by anthropogenic sources, recorded all over Europe and collected in the course of the ARISE and ARISE2 projects (Atmospheric dynamics Research InfraStructure in Europe). Data includes high-frequency (> 0.7 Hz) infrasound detections at 24 European infrasound arrays from nine different national institutions complemented with infrasound stations of the International Monitoring System for the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Data were acquired during 16 years of operation (from 2000 to 2015) and processed to identify and locate ∼ 48,000 infrasound events within Europe. The source locations of these events were derived by combining at least two corresponding station detections per event. Comparisons with ground-truth sources, e.g., Scandinavian mining activity, are provided as well as comparisons with the CTBT Late Event Bulletin (LEB). Relocation is performed using ray-tracing methods to estimate celerity and back-azimuth corrections for source location based on meteorological wind and temperature values for each event derived from European Centre for Medium-range Weather Forecast (ECMWF) data. This study focuses on the analysis of repeating, man-made infrasound events (e.g., mining blasts and supersonic flights) and on the seasonal, weekly and diurnal variation of the infrasonic activity of sources in Europe. Drawing comparisons to previous studies shows that improvements in terms of detection, association and location are made within this study due to increasing the station density and thus the number of events and determined source regions. This improves the capability of the infrasound station network in Europe to more comprehensively estimate the activity of anthropogenic infrasound sources in Europe.

The ARISE (Atmospheric dynamics InfraStructure in Europe) project funded by the European Union combines the International infrasound monitoring system developed for the CTBT (Comprehensive nuclear-Test-Ban Treaty) verification, the NDACC (Network for the Detection of Atmospheric Composition Changes) lidar network, European observatories at mid latitudes (OHP observatory), tropics (OPAR observatory), high latitudes (ALOMAR), the European infrasound stations and satellites for the study of the dynamics of the atmosphere from ground to thermosphere. The ARISE network is unique by its coverage from equatorial to polar regions and the involved scales both in time (to tens of years) and space (one to thousands of kilometers). One of the major objective of ARISE is the monitoring of extreme events including lightning, earthquakes, tornadoes, and volcanoes. A review of the ARISE results will be presented. Volcano monitoring by the infrasound technology presents a strong interest for aviation safety in case of eruption of distant non instrumented volcanoes in broad coverage in complement to satellite observations, which can be limited by the cloud cover. One of the ARISE objectives is to provide eruption notifications including confidence index representative on propagation conditions and additional analysis results. Such studies interest the VACCs (including VAAC Toulouse) and the IAVWOPSG.

On 15 February 2013, a large Earth-impacting fireball disintegrated over the Ural Mountains. This extraordinary event is, together with the 1908 Tunguska fireball, among the most energetic events ever instrumentally recorded. It generated infrasound returns, after circling the globe, at distances up to similar to 85,000 km, and was detected at 20 infrasonic stations of the global International Monitoring System (IMS). For the first time since the establishment of the IMS infrasound network, multiple arrivals involving waves that traveled twice round the globe have been clearly identified. A preliminary estimate of the explosive energy using empirical period-yield scaling relations gives a value of 460 kt of TNT equivalent. In the context of the future verification of the Comprehensive Nuclear-Test-Ban Treaty, this event provides a prominent milestone for studying in detail infrasound propagation around the globe for almost 3 days as well as for calibrating the performance of the IMS network.

Volcanic eruptions are valuable calibrating sources of infrasonic waves worldwide detected by the International Monitoring System (IMS) of the Comprehensive Nuclear Test-Ban-Treaty Organization (CTBTO) and other experimental stations. In this study, we assess the detection capability of the European infrasound network to remotely detect the eruptive activity of Mount Etna. This well-instrumented volcano offers a unique opportunity to validate attenuation models using multi-year near-and far-field recordings. The seasonal trend in the number of detections of Etna at the IS48 IMS station (Tunisia) is correlated to fine temporal fluctuations of the stratospheric waveguide structure. This observed trend correlates well with the variation of the effective sound speed ratio which is a proxy for the combined effects of refraction due to sound speed gradients and advection due to along-path wind on infrasound propagation. Modeling results are consistent with the observed detection capability of the existing regional network. In summer, during the downwind season, a minimum detectable amplitude of ~10 Pa at a reference distance of 1 km from the source is predicted. In winter, when upwind propagation prevails, detection thresholds increase up to ~100 Pa. However, when adding four experimental arrays to the IMS network, the corresponding thresholds decrease down to ~20 Pa in winter. The simulation results provide here a realistic description of long- to mid-range infrasound propagation and allow predicting fine temporal fluctuations in the European infrasound network performance with potential application for civil aviation safety.

Expert knowledge suggests that the performance of automated infrasound event association and source location algorithms could be greatly improved by the ability to continually update station travel-time curves to properly account for the hourly, daily, and seasonal changes of the atmospheric state. With the goal of reducing false alarm rates and improving network detection capability we endeavor to develop, validate, and integrate this capability into infrasound processing operations at the International Data Centre of the Comprehensive Nuclear Test-Ban Treaty Organization. Numerous studies have demonstrated that incorporation of hybrid ground-to-space (G2S) enviromental specifications in numerical calculations of infrasound signal travel time and azimuth deviation yields significantly improved results over that of climatological atmospheric specifications, specifically for tropospheric and stratospheric modes. A robust infrastructure currently exists to generate hybrid G2S vector spherical harmonic coefficients, based on existing operational and emperical models on a real-time basis (every 3- to 6-hours) (D rob et al., 2003 ). Thus the next requirement in this endeavor is to refine numerical procedures to calculate infrasound propagation characteristics for robust automatic infrasound arrival identification and network detection, location, and characterization algorithms. We present results from a new code that integrates the local (range-independent) τp ray equations to provide travel time, range, turning point, and azimuth deviation for any location on the globe given a G2S vector spherical harmonic coefficient set. The code employs an accurate numerical technique capable of handling square-root singularities. We investigate the seasonal variability of propagation characteristics over a five-year time series for two different stations within the International Monitoring System with the aim of understanding the capabilities of current working knowledge of the atmosphere and infrasound propagation models. The statistical behaviors or occurrence frequency of various propagation configurations are discussed. Representative examples of some of these propagation configuration states are also shown.

The Comprehensive Nuclear Test Ban Treaty Organization (CTBTO) is tasked with monitoring compliance with the CTBT. In order to fulfill this mission, the CTBTO is building the International Monitoring System (IMS), which consists of 337 seismic, hydroacoustic, infrasound, and radionuclide monitoring facilities. Data from the IMS are collected, processed and reviewed by the International Data Centre (IDC). Event listings are formed and subsequently communicated to authorized users designated by States Signatories, which are ultimately responsible of characterizing the CTBT relevance of each event. To be better prepared for this some States Signatories have established National Data Centers (NDCs).

The first atmospheric event built only from infrasound arrivals was reported in the Reviewed Event Bulletin (REB) of the International Data Centre (IDC) of the Comprehensive Nuclear Test Ban Treaty Organization (CTBTO) in 2003. In the last decade, 42 infrasound stations from the International Monitoring System (IMS) have been installed and are transmitting data to the IDC. The growing amount of infrasound data and detections produced by the automatic system challenged the station and network processing at the IDC, which required the Organization to redesign the way infrasound data are processed.

Inverse modelling for the phenomenon of atmospher ic dispersion is often hindered by a limited number of measurements which makes the problem severely underdetermined. Superim osed are inaccuracies in the meteorological fields which drive the atmospheric transport models, simplifications in the models the ms lves and the errors intrinsic to the measurement procedures. Consequently, obtaining informative and reliable results of inverse modelli ng requires additional information which needs to b e provided to an inversion algorithm. Mathematical techniques could be employed in order to constrain the underdetermined inverse problem, o r, in case of a simple source characterised by a small number of parameters, the estimation could be limited to those parameters. In the context of monitoring compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTB T) yet other pieces of information are taken advant age of. Large regions indicating a possible presence of a source of radionuclides can be overlaid with an accurate location of the phenom ena emitting seismic, infrasound or hydroacoustic signals. Ultimately, under the hypoth esis that the source of radioactivity coincides in pace and time with one of the detected phenomena, the atmospheric transport modelling (ATM ) aids to discriminate between those which could ha ve been and could not have been at the origin of a detected release.

Atmospheric infrasound signals are observed across a wide frequency range (~0.01-20 Hz, Campus and Christie, 2010). We may consider three general frequency bands of interest: • Above 0.5 Hz: impulse signals of natural or man-made origin, which may propagate over distances of several hundred kilometers. • Between 0.1 and 0.5 Hz: microbaroms and isolated large remote events such as explosions, meteorites and volcanoes. • Below 0.1 Hz: large-scale atmospheric disturbances such as mountain associated waves and auroral infrasound.

The so-called National Data Centres (NDCs) to the Provisional Technical Secretariat of the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty (CTBT) Organization are in charge to provide for the final judgement on the CTBT relevance of explosion events encountered in the PTS International Monitoring System (IMS). The latter is a 321 stations network set-up by the PTS (to date completion level: 80%) in order to globally monitor for occurrence of CTBT relevant seismo-acoustic and radionuclide signals. In doing so, NDCs learn about any seismo-acoustic or radionuclide event by active retrieval or subscription to corresponding event lists and products provided by the International Data Centre (IDC) to the PTS.