The Hum is a widespread phenomenon, reported in many parts of the world. It manifests itself in the form of a hum, rumble and pulsing, often felt as a sensation more than an audible sound. Starting in 2011, residents of Windsor, Ontario, Canada started reporting intermittent low frequency sound, widely referred to as the Hum (and dubbed the Windsor Hum). This report outlines the deployment of a short term geophysical field survey, performed during 2013 in Windsor, Ontario, Canada, aimed at monitoring the airwave signals associated with the Windsor Hum. The summary of the low frequency sound array deployment is presented and discussed.
Electron density is a key parameter to characterize Martian ionospheric structure and dynamics. Based on the ephemeris and auxiliary information derived from the Spacecraft, Planet, Instruments, C-matrix, and Events (SPICE) toolkit, we calculated the bending angle of signal path from the frequency residuals measured by the Mars Express Radio Science Experiment (MaRS) of the Mars Express (MEX) mission under the assumption of a spherically symmetric ionosphere. We stratified the ionosphere into layers and assumed a linear change of bending angle between layers, to derive profiles in radial distance of refractivity with the optimized parameters of upper integral limit of 4890 km and baseline correction boundary of 3690 km. Meanwhile, we also compared the retrieved electron density profiles between the frequency residuals of the single-frequency and differential Doppler of the dual-frequency. In total, ~640 electron density profiles of Martian ionosphere between April 2004 and April 2015 were retrieved successfully. There are 24 profiles identified manually that exhibit an additional sporadic layer occurrence below the normal two-layers. We also found that the peak altitude of this layer increases with the main peak altitude.
On November 13, 2016, an MW7.8 earthquake struck Kaikoura in South Island of New Zealand. By means of back-projection of array recordings, ASTFs-analysis of global seismic recordings, and joint inversion of global seismic data and co-seismic InSAR data, we investigated complexity of the earthquake source. The result shows that the 2016 MW7.8 Kaikoura earthquake ruptured about 100 s unilaterally from south to northeast (~N28°–33°E), producing a rupture area about 160 km long and about 50 km wide and releasing scalar moment 1.01×1021 Nm. In particular, the rupture area consisted of two slip asperities, with one close to the initial rupture point having a maximal slip value ~6.9 m while the other far away in the northeast having a maximal slip value ~9.3 m. The first asperity slipped for about 65 s and the second one started 40 s after the first one had initiated. The two slipped simultaneously for about 25 s. Furthermore, the first had a nearly thrust slip while the second had both thrust and strike slip. It is interesting that the rupture velocity was not constant, and the whole process may be divided into 5 stages in which the velocities were estimated to be 1.4 km/s, 0 km/s, 2.1 km/s, 0 km/s and 1.1 km/s, respectively. The high-frequency sources distributed nearly along the lower edge of the rupture area, the high-frequency radiating mainly occurred at launching of the asperities, and it seemed that no high-frequency energy was radiated when the rupturing was going to stop.
The X-discontinuity, which appears at the depth of approximately 300 km, is an important seismic interface with positive velocity contrasts in the upper mantle. Detecting its presence and topography can be useful to understand phase transformations of relevant mantle minerals under the high-temperature and high-pressure circumstance of the Earth's interior. In this study, we detect the X-discontinuity beneath the Ryukyu subduction zone using five intermediate-depth events recorded by the dense Alaska Regional Network (AK). The X-discontinuity is successfully revealed from the robust slant stacking of the secondary down-going and converting SdP phases. From the depth distribution of conversion points, we find that the X-discontinuity's depth ranges between 269 km and 313 km, with an average depth of 295 km. All the conversion points are located beneath the down-dipping side of the Philippine Sea slab. From energy comparisons in vespagrams for observed and synthetic seismograms, the strong converted energy is more likely from a thin high-velocity layer, and the S-wave velocity jumps across the X-discontinuity are up to 5% to 8% with an average of 6.0%. According to previous petrological and seismological studies, the X-discontinuity we detected can be interpreted as the phase transformation of coesite to stishovite in eclogitic materials within the oceanic crust.
Irregular surface flattening, which is based on a boundary conforming grid and the transformation between curvilinear and Cartesian coordinate systems, is a mathematical method that can elegantly handle irregular surfaces, but has been limited to obtaining first arrivals only. By combining a multistage scheme with the fast-sweeping method (FSM, the method to obtain first-arrival traveltime in curvilinear coordinates), the reflected waves from a crustal interface can be traced in a topographic model, in which the reflected wave-front is obtained by reinitializing traveltimes in the interface for upwind branches. A local triangulation is applied to make a connection between velocity and interface nodes. Then a joint inversion of first-arrival and reflection traveltimes for imaging seismic velocity structures in complex terrains is presented. Numerical examples all perform well with different seismic velocity models. The increasing topographic complexity and even use of a high curvature reflector in these models demonstrate the reliability, accuracy and robustness of the new working scheme; checkerboard testing illustrates the method’s high resolution. Noise tolerance testing indicates the method’s ability to yield practical traveltime tomography. Further development of the multistage scheme will allow other later arrivals to be traced and used in the traveltime inversion.
Magnetic reconnection is the most fundamental energy-transfer mechanism in the universe that converts magnetic energy into heat and kinetic energy of charged particles. For reconnection to occur, the frozen-in condition must break down in a localized region, commonly called the ‘diffusion region’. In Earth’s magnetosphere, ion diffusion regions have already been observed, while electron diffusion regions have not been detected due to their small scales (of the order of a few km) (
A map of the average atomic number of lunar rock and soil can be used to differentiate lithology and soil type on the lunar surface. This paper establishes a linear relationship between the average atomic number of lunar rock or soil and the flux of position annihilation radiation (0.512-MeV gamma-ray) from the lunar surface. The relationship is confirmed by Monte Carlo simulation with data from lunar rock or soil samples collected by Luna (Russia) and Apollo (USA) missions. A map of the average atomic number of the lunar rock and soil on the lunar surface has been derived from the Gamma-Ray Spectrometer data collected by Chang’e-1, an unmanned Chinese lunar-orbiting spacecraft. In the map, the higher average atomic numbers (ZA > 12.5), which are related to different types of basalt, are in the maria region; the highest ZA (13.2) readings are associated with Sinus Aestuum. The middle ZA (~12.1) regions, in the shape of irregular oval rings, are in West Oceanus Procellarum and Mare Frigoris, which seems to be consistent with the distribution of potassium, rare earth elements, and phosphorus as a unique feature on the lunar surface. The lower average atomic numbers (ZA < 11.5) are found to be correlated with the anorthosite on the far side of the Moon.
Atmospheric composition is an important indicator of habitability and life. The presence or absence of a large exomoon around an Earth-size exoplanet could have important consequences for planet climate stability. Thus the detection of exomoons and retrieval of information regarding atmospheric composition of Earth-size exoplanets are important goals of future exoplanet observations. Here a data analysis method is developed to achieve both goals simultaneously, based on reflection spectra of exoplanet-exomoon systems. We show that the existence of exomoons, the size of exomoons, and the concentrations of some atomic and molecular species in the atmospheres of their hosting Earth-like exoplanets can be retrieved with high levels of reliability. In addition, the method can provide well-constrained fractions of basic surface types on the targets because of the characteristic spectral features of atmospheric species and surface types in the analyzed spectral range.