Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userThe consistency of interdecadal changes in the Earths rotation variations
58
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
To accurately restore interdecadal oscillations from the length of day variation ({Delta}LOD) and the polar motion (PM), we propose a normal time-frequency transform (NTFT) combing with curve fitting scheme. Compared with the NTFT, the combined NTFT with a boundary extreme point mirror-image-symmetric extension (BEPME) process, and singular spectrum analysis (SSA) in some simulated tests, the superiority and reliability of this new scheme have been confirmed; and we further verified the validity of it in a mature case analysis from the Earths free oscillation modes 0S0 and 1S0. After then, we use it to restore the ~5.9yr oscillation (referred to as SYO) and ~8.5yr oscillation (referred to as EYO) from the {Delta}LOD and the PM records. Our results reconfirm that the SYO and EYO in the {Delta}LOD (and PM) have no stable damping trend (which is different from results in some previous studies), and for the first time, we find that the SYOs (and EYOs) contained in the {Delta}LOD and the PM show very good consistency. Such consistency demonstrates that the SYOs/EYOs in the {Delta}LOD and the PM must come from the same source. As the external excitation sources of the Earth rotation contain no such oscillations, we suggest that core motions are possible sources.
rate research
Read More
It has long been assumed the Earths solid inner core started to grow when molten iron cooled to its melting point. However, the nucleation mechanism, which is a necessary step of crystallization, has not been well understood. Recent studies found it requires an unrealistic degree of undercooling to nucleate the stable hexagonal close-packed (hcp) phase of iron, which can never be reached under the actual Earths core conditions. This contradiction leads to the inner core nucleation paradox [1]. Here, using a persistent-embryo method and molecular dynamics simulations, we demonstrate that the metastable body-centered cubic (bcc) phase of iron has a much higher nucleation rate than the hcp phase under inner-core conditions. Thus, the bcc nucleation is likely to be the first step of inner core formation instead of direct nucleation of the hcp phase. This mechanism reduces the required undercooling of iron nucleation, which provides a key factor to solve the inner-core nucleation paradox. The two-step nucleation scenario of the inner core also opens a new avenue for understanding the structure and anisotropy of the present inner core.
We used the quantitative theory of solubility of karst rocks of Shopov et. al, (1989, 1991a) in dependence of the temperature and other thermodynamic parameters to make reconstructions of past carbonate denudation rates. This theory produced equations assessing the carbonate denudation rates in dependence on the temperature or on the precipitation. We estimated the averaged denudation rate in the region to 14 mm/kyr or 38 t/km2 per year. We used this estimate as starting point and substituted our proxy records of the annual temperature and the annual precipitation in the equations of dependence of karst denudation rate on precipitation and temperature. This way we reconstructed variations of the annual karst denudation rate for the last 280 years in dependence on the annual precipitation and for the last 1250 years in dependence on the temperature. Both reconstructions produce quite reasonable estimate of the variations of carbonate denudation, which is within observed variation of 8- 20 mm/kyr (86% variation). Precipitation dependence of carbonate denudation produces 79 % variation in the denudation rate in result of the reconstructed variation of 300 mm/yr from the driest to the wettest year during the last 280 years. Temperature dependence of carbonate denudation due to temperature dependence of solubility of the carbonate dioxide produce only 9.3% variation in the denudation rate in result of the reconstructed variation of 4.7 deg. C during the last 1250 years, so it is negligible in respect of the precipitation dependence.
Traditional methods of reporting changes in student responses have focused on class-wide averages. Such models hide information about the switches in responses by individual students over the course of a semester. We extend unpublished work by Steven Kanim on escalator diagrams which show changes in student responses from correct to incorrect (and vice versa) while representing pre- and post-instruction results on questions. Our extension consists of consistency plots in which we represent three forms of data: method of solution and correctness of solution both before and after instruction. Our data are from an intermediate mechanics class, and come from (nearly) identical midterm and final examination questions.
Plate motions are governed by equilibrium between basal and edge forces. Great earthquakes may induce differential static stress changes across tectonic plates, enabling a new equilibrium state. Here we consider the torque balance for idealized circular plates and find a simple scalar relationship for changes in relative plate speed as a function of its size, upper mantle viscosity, and coseismic stress changes. Applied to Japan, the 2011 $mathrm{M}_{mathrm{W}}=9.0$ Tohoku earthquake generated coseismic stresses of $10^2-10^5$~Pa that could have induced changes in motion of small (radius $sim100$~km) crustal blocks within Honshu. Analysis of time-dependent GPS velocities, with corrections for earthquake cycle effects, reveals that plate speeds may have changed by up to $sim3$ mm/yr between $sim3.75$-year epochs bracketing this earthquake, consistent with an upper mantle viscosity of $sim 5times10^{18}$Pa$cdot$s, suggesting that great earthquakes may modulate motions of proximal crustal blocks at frequencies as high as $10^-8$~Hz.
The crystal structure of iron in the Earths inner core remains debated. Most recent experiments suggest a hexagonal-close-packed (hcp) phase. In simulations, it has been generally agreed that the hcp-Fe is stable at inner core pressures and relatively low temperatures. At high temperatures, however, several studies suggest a body-centered-cubic (bcc) phase at the inner core condition. We have examined the crystal structure of iron at high pressures over 2 million atmospheres (>200GPa) and at high temperatures over 5000 kelvin in a laser-heated diamond cell using microstructure analysis combined with $textit{in-situ}$ x-ray diffraction. Experimental evidence shows a bcc-Fe appearing at core pressures and high temperatures, with an hcp-bcc transition line in pressure-temperature space from about 95$pm$2GPa and 2986$pm$79K to at least 222$pm$6GPa and 4192$pm$104K. The trend of the stability field implies a stable bcc-Fe at the Earths inner core condition, with implications including a strong candidate for explaining the seismic anisotropy of the Earths inner core.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
