Undersampling in action and at scale: application to the COVID-19 pandemic

Stochastic Environmental Research and Risk Assessment - It is the purpose of this short communication to analyze the possible caveats in the statistical interpretation of collected data,...     

Analytical expressions for the borehole seismic signatures excited by an explosive source buried in an elastic half‐space

Borehole guided waves that are excited by explosive sources outside of the borehole are important for interpreting borehole seismic surveys and for rock property inversion workflows. Borehole seismograms are typically modelled using numerical methods of wave propagation. In order to benchmark such numerical algorithms and partially to interpret the results of modelling, an analytical methodology is presented here to compute synthetic seismograms. The specific setup is a wavefield emanating from a monopole point source embedded within a homogeneous elastic medium that interacts with a fluid‐filled borehole and a free surface. The methodology assumes that the wavelength of the seismic signal is much larger than the borehole radius. In this paper, it is supposed that there is no poroelastic coupling between the formation and the borehole. The total wavefield solution consists of P, PP, and PS body waves; the surface Rayleigh wave; and the low‐frequency guided Stoneley wave (often referred as the tube wave) within the borehole. In its turn, the tube wave consists of the partial responses generated by the incident P‐wave and the reflected PP and PS body waves at the borehole mouth and by the Rayleigh wave, as well as the Stoneley wave eigenmode. The Mach tube wave, which is a conic tube wave, additionally appears in the Mach cone in a slow formation with the tube‐wave velocity greater than the shear one. The conditions of appearance of the Mach wave in a slow formation are formulated. It is shown that the amplitude of the Mach tube wave strongly depends on Poisson's ratio of the slow surrounding formation. The amplitude of the Mach tube wave exponentially decreases when the source depth grows for weakly compressible elastic media with Poisson's ratio close to 0.5 (i.e., saturated clays and saturated clay soils). Asymptotic expressions are also provided to compute the wavefield amplitudes for different combinations of source depth and source‐well offset. These expressions allow an approximate solution of the wavefield to be computed much faster (within several seconds) than directly computing the implicit integrals arising from the analytical formulation.     

Eruptive Instability of the Magnetic-Flux Rope: Gravitational Force and Mass-Unloading

We investigate the coronal imaging capabilities of the Solar UltraViolet Imager (SUVI) on board the Geostationary Operational Environmental Satellite-R series spacecraft. Nominally Sun-pointed, SUVI provides solar images in six extreme ultraviolet (EUV) wavelengths. On-orbit data indicated that SUVI had sufficient dynamic range and sensitivity to image the corona to the largest heights above the Sun to date while simultaneously imaging the Sun. We undertook a campaign to investigate the existence of the EUV signal well beyond the nominal Sun-centered imaging area of the solar EUV imagers. We off-pointed the SUVI line of sight by almost one imaging area around the Sun. We present the details of the campaign we conducted when the solar cycle was at near the minimum and some results that confirm that EUV emission is present to beyond three solar radii.

     

Assessment of time-series of troposphere zenith delays derived from the Global Data Assimilation System numerical weather model

Troposphere zenith path delays derived from the Global Data Assimilation System (GDAS) numerical weather model (NWM) are compared with those of the International GNSS Service (IGS) solutions over a 1.5-year period at 18 globally distributed IGS stations. Meteorological parameters can be interpolated from the NWM model at any location and at any time after December 2004. The meteorological parameters extracted from the NWM model agree with in situ direct measurements at some IGS stations within 1 mbar for pressure, 3° for temperature and 13% for relative humidity. The hydrostatic and wet components of the zenith path delay (ZPD) are computed using the meteorological parameters extracted from the NWM model. The total ZPDs derived from the GDAS NWM agree with the IGS ZPD solutions at 3.0 cm RMS level with biases of up to 4.5 cm, which can be attributed to the wet ZPDs estimates from the NWM model, considering the less accurate interpolated relative humidity parameter. Based on this study, it is suggested that the availability and the precision of the GDAS NWM ZPD should be sufficient for nearly all GPS navigation solutions.

Estimation of ionospheric radiowave absorption based on one vertical sounding ionogram

A method for estimating absorption value on the basis on one ionogram of a vertical sounding dynasonde was proposed. The main idea is that averaging the amplitudes of the signals reflected from the ionosphere is replaced over time by frequency averaging of the amplitude factor introduced by the authors. The amplitude factor was determined. The regularities of daily variations in the amplitude factor are discussed.     

Forecasting Water Levels in Krasnodar Krai Rivers with the Use of Machine Learning

Water Resources - The potential of construction of machine learning models was considered as applied to water level forecasting in mountain river reaches in Krasnodar Krai based on observation data...     

The instrumentally recorded fall of the Križevci meteorite,Croatia, February 4, 2011

The Kri?evci H6 meteorite was recovered on the basis of fireball data obtained by the cameras of the Croatian Meteor Network. The fireball, which occurred on February 4, 2011, 23:20:40 UT, was also observed by meteor cameras in Slovenia and by the Autonomous Fireball Observatory in Martinsberg, Austria, which belongs to the European Fireball Network. Here, we present detailed data on fireball trajectory, velocity, deceleration, light curve, and orbit. We also modeled the atmospheric fragmentation of the meteoroid on the basis of the light curve and deceleration. The initial mass of the meteoroid was between 25–100 kg, most probably about 50 kg. Severe fragmentation occurred at heights of approximately 60 and 31 km, under dynamic pressures of 0.1 and 3 MPa, respectively. The peak absolute magnitude of ?13.7 was reached during the second severe fragmentation event. The recovered 291 g meteorite was probably the only fragment with a terminal mass exceeding 100 g. The orbit had a low inclination of 0.6 degrees, perihelion distance 0.74 AU, and semimajor axis 1.54 AU. Kri?evci can be ranked among the 10 best documented meteorite falls.     

The thermo-tectonic history of the Song-Kul plateau, Kyrgyz Tien Shan: Constraints by apatite and titanite thermochronometry and zircon U/Pb dating

The Song-Kul Basin sits on a plateau at the Northern and Middle Kyrgyz Tien Shan junction. It is a lacustrine basin, occupied by Lake Song-Kul and predominantly developed on igneous basement. This basement was targeted for a multi-method chronological study to identify the different magmatic episodes responsible for basement formation and to constrain the timing of the development of its present-day morphology. Zircon U/Pb dating by LA-ICP-MS revealed four different magmatic episodes: a Late Cambrian (~ 500 Ma) island arc system, a Late Ordovician (~ 450 Ma) subduction related intrusion, an Early Permian (~ 290 Ma) collisional stage, and a Middle to Late Permian (~ 260 Ma) post-collisional magmatic pulse. Middle to Late Triassic (~ 200–230 Ma) titanite fission-track ages and Late Triassic – Early Jurassic (~ 180–210 Ma) apatite fission-track ages and thermal history modeling indicate the Song-Kul basement was already emplaced in the shallow crust at that time. An exhumed fossil apatite fission-track partial annealing zone is recognized in the bordering Song-Kul mountain ranges. The area experienced only minor post-Early Mesozoic denudation. The igneous basement was slowly brought to apatite (U–Th)/He retention temperatures in the Late Cretaceous–Palaeogene. Miocene to present reactivation of the Tien Shan does not manifestly affect this part of the orogen.     

Small-scale inviscid accretion discs around black holes

Gas falling quasi-spherically on to a black hole forms an inner accretion disc if its specific angular momentum l exceeds l ∗∼ r _g c , where r _g is the Schwarzschild radius. The standard disc model assumes l ≫ l ∗. We argue that, in many black hole sources, accretion flows have angular momenta just above the threshold for disc formation, l ≳ l ∗, and assess the accretion mechanism in this regime. In a range l ∗< l < l _cr, a small-scale disc forms in which gas spirals fast into the black hole without any help from horizontal viscous stresses. Such an 'inviscid' disc, however, interacts inelastically with the feeding infall. The disc–infall interaction determines the dynamics and luminosity of the accretion flow. The inviscid disc radius can be as large as 14 r _g, and the energy release peaks at 2 r _g. The disc emits a Comptonized X-ray spectrum with a break at ∼100 keV. This accretion regime is likely to take place in wind-fed X-ray binaries and is also possible in active galactic nuclei.     

A Russian record of a Middle Ordovician meteorite shower: Extraterrestrial chromite at Lynna River,St. Petersburg region

Abstract– Numerous fossil meteorites and high concentrations of sediment‐dispersed extraterrestrial chromite (EC) grains with ordinary chondritic composition have previously been documented from 467 ± 1.6 Ma Middle Ordovician (Darriwilian) strata. These finds probably reflect a temporarily enhanced influx of L‐chondritic matter, following the disruption of the L‐chondrite parent body in the asteroid belt 470 ± 6 Ma. In this study, a Volkhovian‐Kundan limestone/marl succession at Lynna River, northwestern Russia, has been searched for EC grains (>63 μm). Eight samples, forming two separate sample sets, were collected. Five samples from strata around the Asaphus expansus–A. raniceps trilobite Zone boundary, in the lower‐middle Kundan, yielded a total of 496 EC grains in 65.5 kg of rock (average 7.6 EC grains kg^?1, but up to 10.2 grains kg^?1). These are extremely high concentrations, three orders of magnitude higher than “background” levels in similar condensed sediment from other periods. EC grains are typically about 50 times more abundant than terrestrial chrome spinel in the samples and about as common as terrestrial ilmenite. Three stratigraphically lower lying samples, close to the A. lepidurus–A. expansus trilobite Zone boundary, at the Volkhov‐Kunda boundary, yielded only two EC grains in 38.2 kg of rock (0.05 grains kg^?1). The lack of commonly occurring EC grains in the lower interval probably reflects that these strata formed before the disruption of the L‐chondrite parent body. The great similarity in EC chemical composition between this and other comparable studies indicates that all or most EC grains in these Russian mid‐Ordovician strata share a common source––the L‐chondrite parent body.