Precession Of Equinoxes Research Articles

Experimental study of the flows in a non-axisymmetric ellipsoid under precession

Precession driven flows are of great interest for both, industrial and geophysical applications. While cylindrical, spherical and spheroidal geometries have been investigated in great detail, the numerically and theoretically more challenging case of a non-axisymmetric cavity has received less attention. We report experimental results on the flows in a precessing triaxial ellipsoid, with a focus on the base flow of uniform vorticity, which we show to be in good agreement with existing theoretical models. As predicted, the uniform vorticity component exhibits two branches of solutions leading to a hysteresis cycle as a function of the Poincaré number. The first branch is observed at low forcing and characterized by large amplitude of the total fluid rotation and a moderate tilt angle of the fluid rotation axis. In contrast, the second branch displays only a moderate fluid rotation and a large tilt angle of the fluid rotation axis, which tends to align with the precession axis. In addition, we observe the occurrence of parametric instabilities early in the first branch, which saturate in the second branch, where we observe the same order of the kinetic energy in the base flow and instabilities.

Thermoelectric precession in turbulent magnetoconvection

We present laboratory measurements of the interaction between thermoelectric currents and turbulent magnetoconvection. In a cylindrical volume of liquid gallium heated from below and cooled from above and subject to a vertical magnetic field, it is found that the large-scale circulation (LSC) can undergo a slow axial precession. Our experiments demonstrate that this LSC precession occurs only when electrically conducting boundary conditions are employed, and that the precession direction reverses when the axial magnetic field direction is flipped. A thermoelectric magnetoconvection (TEMC) model is developed that successfully predicts the zeroth-order magnetoprecession dynamics. Our TEMC magnetoprecession model hinges on thermoelectric current loops at the top and bottom boundaries, which create Lorentz forces that generate horizontal torques on the overturning large-scale circulatory flow. The thermoelectric torques in our model act to drive a precessional motion of the LSC. This model yields precession frequency predictions that are in good agreement with the experimental observations. We postulate that thermoelectric effects in convective flows, long argued to be relevant in liquid metal heat transfer and mixing processes, may also have applications in planetary interior magnetohydrodynamics.

Effect of the Gravity and Magnetic Field to Find Regular Precessions of a Satellite-gyrostat with Principal Axes on a Circular Orbit

We consider the motion of a magnetized satellite-gyrostat in a circular orbit due to the combined influence of uniform gravity and magnetic fields. Based on the Lagrangian equations, the necessary conditions for the existence of regular precessions are determined in which the axis of precession is perpendicular to the orbital plane. All possible regular precessions and permanent rotations are determined and classified. We show the usage of Lagrange equations taking Eulerian angles as generalized coordinates for determining the regular precessions is more effective and accurate than utilization of Euler-Poisson equations.

Spin state and moment of inertia of Venus

Fundamental properties of the planet Venus, such as its internal mass distribution and variations in length of day, have remained unknown. We used Earth-based observations of radar speckles tied to the rotation of Venus obtained in 2006–2020 to measure its spin axis orientation, spin precession rate, moment of inertia and length-of-day variations. Venus is tilted by 2.6392 ± 0.0008 deg (1σ) with respect to its orbital plane. The spin axis precesses at a rate of 44.58 ± 3.3 arcsec per year (1σ), which gives a normalized moment of inertia of 0.337 ± 0.024 and yields a rough estimate of the size of the core. The average sidereal day on Venus in the 2006–2020 interval is 243.0226 ± 0.0013 Earth days (1σ). The spin period of the solid planet exhibits variations of 61 ppm (~20 min) with a possible diurnal or semidiurnal forcing. The length-of-day variations imply that changes in atmospheric angular momentum of at least ~4% are transferred to the solid planet.

Non-marine tetrapod extinctions solve extinction periodicity mystery

ABSTRACT Based on their compiled data set of ten extinction episodes (four of which had no known extinction rates) published earlier in this journal, Rampino and co-authors claimed a 27.5-My period in non-marine-tetrapod extinctions. I reassessed that claim using the Gauss–Vaníček spectral analysis (GVSA), which revealed spectra of very low fidelity (mostly <<1) dominated by the Earth’s axial precession, without 99%-significant periods, but with hundreds of 95%-significant periods unrelated to the extinctions and the claimed period. Therefore, the data are physically nonsensical as far as any underlining cyclicity is concerned. The analysis did not reveal the claimed period in any band, at either 99% or 95% significance, so the claimed period is a ghost due to intermediary astronomical forcing of highly gapped data sampled arbitrarily and processed with inapt techniques. Thanks to the GVSA’s absolute accuracy, and insensitivity of non-marine data to the ocean-tidal component, I present remarkable proof that very long periods such as ~9 My (~27 My), ~11 My (~22 My), and ~33 My (~66 My), previously claimed in extinction data sets, have a common astronomical origin. They primarily arise due to the Earth’s axial precession. This circumstance then makes the enforcing, i.e., removing the axial-precession period — as the most enduring noise constituent — necessary in most if not all paleostudies.

Orbital Forcing Strongly Influences Seasonal Temperature Trends During the Last Millennium

AbstractInsolation changes caused by the axial precession induce millennial trends in last millennium temperature, varying with season and latitude. A characteristic seasonal trend pattern can be detected in both insolation and modeled surface temperature response. In the extratropical Northern Hemisphere, the maximum insolation trend occurs around April/May, while the minimum trend occurs between July and September. The temperature trend lags behind insolation trend by around a month. Hence orbital forcing potentially affects long‐term trends in proxy data, which are often sensitive to a distinct seasonal window. We find that tree‐ring reconstructions based on early growing season dominated records show different millennial trends from those for late summer dominated proxies. The differential response is similar to that seen in pseudo proxy reconstructions when considering proxy seasonality. This suggests that orbital forcing has influenced long‐term trends in climate proxies. It is therefore vital to use seasonally homogeneous data for reconstructing multicentennial variability.

Capturing complex right ventricular wall motion abnormalities in arrhythmogenic right ventricular cardiomyopathy by combining longitudinal and radial myocardial dynamics in feature-tracking MRI

Abstract Funding Acknowledgements Type of funding sources: Public Institution(s). Main funding source(s): INSERM Liliane Bettencourt doctoral grant Background Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC) is ass­­ociated with complex spatial and temporal right ventricular (RV) wall motion abnormalities. While cardiac magnetic resonance (CMR) is the gold-standard imaging technique, its diagnosic performance remains suboptimal and additional CMR biomarkers reflecting ARVC pathophysiology are needed. Purpose To evaluate the performance of a CMR feature-tracking (FT)-derived parameter combining both longitudinal and radial RV deformation and motion for the characterization of RV wall motion abnormalities in ARVC. Methods Thirty-nine patients with definite or borderline ARVC (median age 45 years, interquartile range 31–51, 56% males) were compared to 20 healthy controls with comparable age, sex and weight distributions. All subjects had 1.5T CMR including short axis and 4-chamber views steady-state free precession acquisitions. A custom FT software adapted to RV wall segmentation and tracking was used to assess RV wall deformation and motion in the 3 space directions resulting in: 1) global longitudinal strain (GLS) estimated on the 4 chamber view from the RV free wall, 2) basal circumferential strain (BCS) and radial motion fraction (BRMF) estimated as an average of short-axis slices comprised in the RV third basal portion. To capture the complex RV motion in ARCV, a longitudinal to radial strain loop (LRSL) was displayed and its area was calculated. Results The ARVC group comprised 28 (72%) patients with definite and 11 (28%) with borderline diagnosis . As compared to controls, LVEF and RVEF were significantly lower in ARVC patients (61(interquartile range (IQR) 52-71) vs. 71%(IQR 55-88) , p = 0.03 and 47%(IQR 16-63) vs. 57%(IQR 49-63) , p = 0.02, respectively), LVEF remaining within normal range limits. While there was no significant difference in RV GLS between ARVC patients and controls (median -17.7%(IQR -24–15) vs. -17.5%(IQR -20.1–15.2), p = 0.67) , BCS and BRMF were significantly lower in ARVC patients vs. controls [-7.5%(IQR -12.3–8.4.) vs. -9.8%(IQR -13.8–8.6.), p = 0.004 and -12.2(IQR -14.4–8.7.) vs. -14.9%(IQR -16.6–13.2) p = 0.0007, respectively] . The LRSL area was significantly and markedly lower in ARVC patients vs. controls [70.6 (IQR 16.3-63.1) vs. 144.1 (IQR 110.4-251.3), p = 0.0002] . LRSL area outperformed RVEF, BCS and BRS in separating ARVC from controls (area under receiving operator characteristics curve 0.82 vs. 0.78, 0.73 and 0.78, respectively). Conclusion In ARVC, a FT-derived parameter combining longitudinal and radial RV wall deformation and motion provided better discrimination of ARVC patients from controls than conventional FT measurements. Its implementation in clinical practice may bolster CMR performance to characterize ARVC wall motion abnormalities. Abstract Figure

New Long-Term Climate Oscillations

The astronomical theory of climate change is based on the solution of differential equations describing Earth’s orbital and rotational motions. The equations are used to calculate the change in insolation over the Earth’s surface. As a result of the author’s solution of the orbital problem, the periods and amplitudes of Earth-orbit variations and their evolution have been refined. Unlike previous studies, the equations of Earth’s rotational motion are solved completely. The Earth’s rotational axis precesses relative to a direction different from the direction of the orbit’s axial precession, and oscillates with periods of half a month, half a year and 18.6 years. Also, its oscillations occur with irregular periods of several tens of thousands of years and more. All these motions lead to oscillations of the obliquity in the range of 14.7° to 32.1°, which prove to be 7 - 8 times larger than obtained by a previous theory. In the same proportion, the Earth’s insolation oscillations increase in amplitude, with insolation extremes occurring in other epochs than those in the previous theory. The amplitudes and the onset times of the extremes correlate with known paleoclimate changes. Thirteen insolation periods of paleoclimate variation over an interval of 200 thousand years are identified. From the insolation evolution calculated over a time interval of 1 million years, 6 climate gradations from very cold to very warm are identified.

Efficient spin excitation via ultrafast damping-like torques in antiferromagnets

Damping effects form the core of many emerging concepts for high-speed spintronic applications. Important characteristics such as device switching times and magnetic domain-wall velocities depend critically on the damping rate. While the implications of spin damping for relaxation processes are intensively studied, damping effects during impulsive spin excitations are assumed to be negligible because of the shortness of the excitation process. Herein we show that, unlike in ferromagnets, ultrafast damping plays a crucial role in antiferromagnets because of their strongly elliptical spin precession. In time-resolved measurements, we find that ultrafast damping results in an immediate spin canting along the short precession axis. The interplay between antiferromagnetic exchange and magnetic anisotropy amplifies this canting by several orders of magnitude towards large-amplitude modulations of the antiferromagnetic order parameter. This leverage effect discloses a highly efficient route towards the ultrafast manipulation of magnetism in antiferromagnetic spintronics.

Electrical magnetochiral effect and kinetic magnetoelectric effect induced by chiral exchange field in helical magnetics

The features of spin and charge transport in conductive helimagnets, which are due to the action of an inhomogeneous exchange magnetic field on the spin of conduction electrons, are theoretically studied. The interaction between the spin of moving particles and an inhomogeneous external magnetic field was first recorded in the famous Stern-Gerlach experiment that investigated the quantum nature of spin. In the present paper, we have demonstrated that two physical effects---the electrical magnetochiral effect (EMChE) and the kinetic magnetoelectric effect (KMEE)---can be explained through the interaction of the spins of itinerant electrons in chiral helimagnets with spatially inhomogeneous effective magnetic field of exchange origin. All parameters of the EMChE and KMEE are presented in terms of both the characteristic frequencies of spin relaxation of conduction electrons in a helimagnet and the frequencies of their Larmor precession in external and internal exchange fields. It has been shown that the effective frequency of conduction-electron spin relaxation in a helimagnet contains three components: (i) the rate of spin-lattice relaxation caused by spin-orbit scattering of conduction electrons by defects of the crystal lattice, (ii) the rate of change in the average spin of electrons due to their ``diffusion'' escape from a region with a given direction of the average spin to a region with a different direction of spin density, and (iii) the contribution of the Larmor precession of the spins of electrons moving in the helimagnet's exchange field that assigns the precession axis altering its direction in space. The peculiarities of the EMChE and KMEE that substantially depend on the ratio of the above-listed spin relaxation rates and the angular frequencies of electron precession are described. The numerical estimates performed show that the mechanism of generating EMChE provides the effect magnitude sufficient to be experimentally detected in metallic helimagnets. The frequency regions of spin relaxation and spin precession are determined to observe a giant electrical magnetochiral effect and resonant behavior of the chiral magnetoresistance. We have called the appropriate effect ``magnetochiral kinetic resonance'' (MChKR). The physical nature of MChKR is elucidated. The latter arises due to the coincidence of the Larmor precession frequency of an electron in the effective field and the phase change frequency of the helicoidal exchange field acting on the electron moving along the helicoid's axis with a speed equal to that of the electron flow. We have demonstrated how the experimental studies of the KMEE can be used to directly determine the chirality of helimagnets.

Nonminimal Spin-Field Interaction of the Classical Electron and Quantization of Spin

I shortly describe semi-classical models of spinning electron and list a number of theoretical issues where these models turn out to be useful, see arXiv:1710.07135 for details. Then I discuss the possibility to extend the range of applicability of these models by introducing an interaction, that forces the spin to align up or down relative to its precession axis.

Загадка звёздного каталога Коперника (часть вторая) Система счёта времени и определение тройного движения Земли

Objecting to those who consider Copernicus’s work as repeating the achievements of Antiquity, we pay attention to his determination of uniform time necessary for applying any mathematical theory in particular, the one describing the observed uneven movements through a composition of circular and uniform rotations. Using the Egyptian year of 365 solar days, Copernicus analyzed the observations for the period from the first Olympiad up to his own observations. Generalizing the results of observations of two millennia and reducing them to the unique time system enabled his explaining the observed precession of equinoxes, and changes in the angle of inclination of the Earth’s equator to ecliptic by the lag of the Earth’s center rotation from that of its axis, simultaneously to exclude empty spheres from the medieval mechanical models of the Universe. Analyzing the observations of the period 4 times longer than that of Ptolemy, Copernicus managed to obtain the mean value of precession and the period of the Earth axis rotation practically coinciding with modern determinations. The proof of “the triple motion of the Earth” was necessary for affirmation of heliocentric system of the world.

Current-driven transverse domain wall oscillations in perpendicular spin-valve structures

Spin-transfer-driven oscillations of a transverse domain wall confined to a perpendicular spin-valve structure are investigated using a one-dimensional model. The stack consists of a polarizer, nonmagnetic spacer, soft free layer, and pinned magnetic layer. It is found that the domain-wall oscillation frequency is a nonmonotonic, highly asymmetrical function of applied electric current, showing a strong dependence on the current direction and the relative strengths of the interfacial and bulk spin-transfer torques. Micromagnetic analysis reveals that the surprising and atypical oscillator response is due to an interplay between the interfacial spin-transfer torque, the bulk spin-transfer torque, the exchange torque, and the damping torque. The underlying physical and material responses are examined, including the important role of the domain-wall twist. The competitions between the involved torques under different operating conditions suggest that the oscillator could serve as a model system to investigate magnetic and spintronic phenomena at the nanoscale. The observed current-dependent twisting of the free-layer magnetization about the axis of precession may further be found interesting for investigations of the interaction between spin-polarized current and chiral spin structures.

Atomic Resolution Imaging of Light Elements in a Crystalline Environment using Dynamic Hollow-Cone Illumination Transmission Electron Microscopy.

Multiple electron scattering and the nonintuitive nature of image formation with coherent radiation complicate the interpretation of conventional transmission electron microscopy images. Precession of the illuminating beam in transmission electron microscopy (TEM) can lead to more robust and interpretable images with some penalty to image contrast, a technique known as dynamic hollow-cone illumination TEM. We demonstrate direct and robust imaging of light and heavy atoms in a crystalline environment with this technique. This method is similar to the annular bright-field technique in scanning transmission electron microscopy, via the principle of reciprocity. Dynamic hollow-cone illumination TEM is challenging in practice due to sensitivity to the misalignment of the precession axis, microscope objective aperture, and crystal zone axis.

Retrograde-rotating Exoplanets Experience Obliquity Excitations in an Eccentricity-enabled Resonance

Abstract Previous studies have shown that planets that rotate retrograde (backward with respect to their orbital motion) generally experience less severe obliquity variations than those that rotate prograde (the same direction as their orbital motion). Here, we examine retrograde-rotating planets on eccentric orbits and find a previously unknown secular spin–orbit resonance that can drive significant obliquity variations. This resonance occurs when the frequency of the planet’s rotation axis precession becomes commensurate with an orbital eigenfrequency of the planetary system. The planet’s eccentricity enables a participating orbital frequency through an interaction in which the apsidal precession of the planet’s orbit causes a cyclic nutation of the planet’s orbital angular momentum vector. The resulting orbital frequency follows the relationship , where and are the rates of the planet’s changing longitude of periapsis and longitude of ascending node, respectively. We test this mechanism by simulating cases of a simple Earth–Jupiter system and confirm the predicted resonance. Over the course of 100 Myr, the test Earths with rotation axis precession rates near the predicted resonant frequency experienced pronounced obliquity variations of the order of 10°–30°. These variations can be significant and suggest that while retrograde rotation is a stabilizing influence most of the time, retrograde rotators can experience large obliquity variations if they are on eccentric orbits and enter this spin–orbit resonance.

Mixing and Segregation of Granular Materials in a Precessing Cylindrical Container

It is difficult to mix different kinds of granular materials. The so-called Brazil nut effect is a well-known phenomenon showing the segregation of granular materials with different density or size. Since the motion of granular materials depends on a lot of parameters such as the diameter, density, elastic coefficient, frictional coefficient, and so on, the physics of granular materials is complicated. In this study, we numerically simulate the mixing of two kinds of spherical particles with different diameters confined in a precessing cylindrical container. We change the angular velocity of the spinning container and the angle between the spin and precession axis to evaluate how they affect the mixing of the particles.

Nalanda

Nalanda, in eastern India's Bihar state, was once the most important Buddhist monastery in the world. The six excavated temples there are oriented approximately to the cardinal directions, but their east–west axes are tilted more than 4° south of east. Other key Buddhist temples within the same cultural milieu (Bodhgaya, Vikramasila, Somapura, and Samye) are similarly oriented a few degrees south of east. As M. B. Rajani and Viraj Kumar argue in Nalanda: A Tale in the Twist, this demands an explanation, because a method to orient structures to within 0.5° of the cardinal directions was known in the region for several centuries prior to the construction of these temples. Rajani and Kumar identify a temporal pattern in the orientation of Nalanda's temples; hypothesize that each temple was aligned with the rise of a specific star (Spica or Beta Librae), with the temporal pattern being a consequence of axial precession; propose a simple, unified explanation for the few exceptions to the pattern; and use this hypothesis to propose an approximate construction date for each temple based on its orientation.

Hannay angles in magnetic dynamics

We consider, within the framework developed by Hannay for classical integrable systems (Hannay, 1985), the geometric phases that occur in semi-classical magnetic dynamics. Such geometric phases are generically referred to as Hannay angles, and, in the context of magnetic dynamics, may arise as a result of both adiabatically-varying ellipticity and axis of magnetization precession. We elucidate both effects and their interplay for single-domain magnetic dynamics within a simple model with time-dependent anisotropies and external field. Subsequently, we consider spin waves and rederive, from our classical approach, some known results on what is commonly referred to as the magnon Berry phase. As an aside, these results are used to give an interpretation for geometric phases that occur in superfluids. Finally, we develop a Green’s function formalism for elliptical magnons. Within this formalism, we consider magnon transport in a mesoscopic ring and show how it is influenced by interference effects that are tuned by the Hannay angle that results from a varying ellipticity. Our results may inform the field of magnonics that seeks to utilize spin waves in applications.

ẢNH HƯỞNG LIÊN HỆ CHÉO ĐẾN ĐỘ DỰ TRỮ ỔN ĐỊNH ĐẾ BỘ ĐỊNH PHƯƠNG THẲNG ĐỨNG VÀ GIẢI PHÁP KHẮC PHỤC

The paper investigates the effect of cross-couplings in gyrovertical platform and proposes a method to decouple. The gyroplatform uses two gyroscopes whose axial precession is perpendicular to the respective axis of platform and has an angular momentum vector in opposite directions. This layout will reduce the influence of the rotation of gyro motors. However, between the gyroplatform channels there appears cross-coupling that reduces the stability of system stabilization. The simulation results show effect of cross-couplings on the stability and method to overcome these problems.

Excitation of Planetary Obliquities through Planet–Disk Interactions

Abstract The tilt of a planet’s spin axis off its orbital axis (“obliquity”) is a basic physical characteristic that plays a central role in determining the planet’s global circulation and energy redistribution. Moreover, recent studies have also highlighted the importance of obliquities in sculpting not only the physical features of exoplanets but also their orbital architectures. It is therefore of key importance to identify and characterize the dominant processes of excitation of nonzero axial tilts. Here we highlight a simple mechanism that operates early on and is likely fundamental for many extrasolar planets and perhaps even solar system planets. While planets are still forming in the protoplanetary disk, the gravitational potential of the disk induces nodal recession of the orbits. The frequency of this recession decreases as the disk dissipates, and when it crosses the frequency of a planet’s spin axis precession, large planetary obliquities may be excited through capture into a secular spin–orbit resonance. We study the conditions for encountering this resonance and calculate the resulting obliquity excitation over a wide range of parameter space. Planets with semimajor axes in the range 0.3 au ≲ a ≲ 2 au are the most readily affected, but large-a planets can also be impacted. We present a case study of Uranus and Neptune, and show that this mechanism likely cannot help explain their high obliquities. While it could have played a role if finely tuned and envisioned to operate in isolation, large-scale obliquity excitation was likely inhibited by gravitational planet–planet perturbations.