scholarly journals Are planetary dynamos driven by helical waves?

2018 ◽  
Vol 84 (3) ◽  
Author(s):  
P. A. Davidson ◽  
A. Ranjan
Keyword(s):  
Zero Order ◽  
Ekman Pumping ◽  
Inertial Waves ◽  
Statistical Bias ◽  
The Core ◽  
The North ◽  
Helicity Distribution ◽  
Planetary Cores ◽  
Carry Over ◽  
Planetary Dynamos

In most numerical simulations of the Earth’s core the dynamo is located outside the tangent cylinder and, in a zero-order sense, takes the form of a classical$\unicode[STIX]{x1D6FC}^{2}$dynamo. Such a dynamo usually requires a distribution of helicity,$h$, which is asymmetric about the equator and in the simulations it is observed that, outside the tangent cylinder, the helicity is predominantly negative in the north and positive in the south. If we are to extrapolate the results of these simulations to the planets, we must understand how this asymmetry in helicity is established and ask if the same mechanism is likely to operate in a planet. In some of the early numerical dynamos, which were too viscous by a factor of at least$10^{9}$, as measured by the Ekman number, the asymmetric helicity distribution was attributed to Ekman pumping. However, Ekman pumping plays much less of a role in more recent, and less viscous, numerical dynamos, and almost certainly plays no significant role in the core of a planet. So the question remains: what establishes the asymmetric helicity distribution in the simulations and is this mechanism likely to carry over to planetary cores? In this paper we review the evidence that planetary dynamos, and their numerical analogues, might be maintained by helical waves, especially inertial waves, excited in and around the equatorial regions. This cartoon arises from the observation that there tends to be a statistical bias in the buoyancy flux towards the equatorial regions, and so waves are preferentially excited there. Moreover, upward (downward) propagating inertial waves carry negative (positive) helicity, which leads naturally to a segregation in$h$.

scholarly journals On the spatial segregation of helicity by inertial waves in dynamo simulations and planetary cores

2018 ◽  
Vol 851 ◽  
pp. 268-287 ◽  
Author(s):  
P. A. Davidson ◽  
A. Ranjan
Keyword(s):  
Numerical Simulations ◽  
Strongly Correlated ◽  
Inertial Waves ◽  
The South ◽  
The North ◽  
Planetary Cores ◽  
Local Wave ◽  
Wave Flux ◽  
The Relationship

The distribution of kinetic helicity in a dipolar planetary dynamo is central to the success of that dynamo. Motivated by the helicity distributions observed in numerical simulations of the Earth’s dynamo, we consider the relationship between the kinetic helicity, $h=\boldsymbol{u}\boldsymbol{\cdot }\unicode[STIX]{x1D735}\times \boldsymbol{u}$, and the buoyancy field that acts as a source of helicity, where $\boldsymbol{u}$ is velocity. We show that, in the absence of a magnetic field, helicity evolves in accordance with the equation $\unicode[STIX]{x2202}h/\unicode[STIX]{x2202}t=-\unicode[STIX]{x1D735}\boldsymbol{\cdot }\boldsymbol{F}+S_{h}$, where the flux, $\boldsymbol{F}$, represents the transport of helicity by inertial waves, and the helicity source, $S_{h}$, involves the product of the buoyancy and the velocity fields. In the numerical simulations it is observed that the helicity outside the tangent cylinder is predominantly negative in the north and positive in the south, a feature which the authors had previously attributed to the transport of helicity by waves (Davidson & Ranjan, Geophys. J. Intl, vol. 202, 2015, pp. 1646–1662). It is also observed that there is a strong spatial correlation between the distribution of $h$ and of $S_{h}$, with $S_{h}$ also predominantly negative in the north and positive in the south. This correlation tentatively suggests that it is the in situ generation of helicity by buoyancy that establishes the distribution of $h$ outside the tangent cylinder, rather than the dispersal of helicity by waves, as had been previously argued by the authors. However, although $h$ and $S_{h}$ are strongly correlated, there is no such correlation between $\unicode[STIX]{x2202}h/\unicode[STIX]{x2202}t$ and $S_{h}$, as might be expected if the distribution of $h$ were established by an in situ generation mechanism. We explain these various observations by showing that inertial waves interact with the buoyancy field in such a way as to induce a source $S_{h}$ which has the same sign as the helicity in the local wave flux, and that the sign of $h$ is simply determined by the direction of that flux. We conclude that the observed distributions of $h$ and $S_{h}$ outside the tangent cylinder are consistent with the transport of helicity by waves.

scholarly journals North Atlantic warming over six decades drives decreases in krill abundance with no associated range shift

2021 ◽  
Vol 4 (1) ◽  
Author(s):  
Martin Edwards ◽  
Pierre Hélaouët ◽  
Eric Goberville ◽  
Alistair Lindley ◽  
Geraint A. Tarling ◽  
...  
Keyword(s):  
North Atlantic ◽  
Range Shift ◽  
The Core ◽  
Krill Abundance ◽  
The North ◽  
Living Space ◽  
Warming Climate ◽  
Two Temperatures ◽  
The North Atlantic

AbstractIn the North Atlantic, euphausiids (krill) form a major link between primary production and predators including commercially exploited fish. This basin is warming very rapidly, with species expected to shift northwards following their thermal tolerances. Here we show, however, that there has been a 50% decline in surface krill abundance over the last 60 years that occurred in situ, with no associated range shift. While we relate these changes to the warming climate, our study is the first to document an in situ squeeze on living space within this system. The warmer isotherms are shifting measurably northwards but cooler isotherms have remained relatively static, stalled by the subpolar fronts in the NW Atlantic. Consequently the two temperatures defining the core of krill distribution (7–13 °C) were 8° of latitude apart 60 years ago but are presently only 4° apart. Over the 60 year period the core latitudinal distribution of euphausiids has remained relatively stable so a ‘habitat squeeze’, with loss of 4° of latitude in living space, could explain the decline in krill. This highlights that, as the temperature warms, not all species can track isotherms and shift northward at the same rate with both losers and winners emerging under the ‘Atlantification’ of the sub-Arctic.

Dispersion Characteristic of the Symmetric Slab Waveguide with Chiral Negative Refractive Medium

2014 ◽  
Vol 571-572 ◽  
pp. 915-919
Author(s):  
Xiao Xue Xu ◽  
Zhong Yin Xiao ◽  
Quan Wen Ma ◽  
Xiao Long Ma ◽  
De Jun Liu ◽  
...  
Keyword(s):  
Maxwell Equations ◽  
Dispersion Characteristic ◽  
Constitutive Relations ◽  
Great Influence ◽  
Zero Order ◽  
Slab Waveguide ◽  
Order Mode ◽  
The Core ◽  
Left Handed ◽  
Refractive Medium

According to Maxwell equations and constitutive relations in chiral medium, the dispersion characteristic of the symmetric slab waveguide with chiral negative refractive medium is studied. The dispersion equations expressed in the normalized parameter are deduced in the slab waveguide. The numerical results show that the chiral parameter has great influence on the dispersion characteristic of slab waveguide. When the relative chiral parameter is about 0.0477, the zero order mode of left-handed polarized wave disappears. While it is greater than 1, the core becomes chiral negative refractive medium and the zero order mode disappears. In addition, compared with the general chiral waveguide, the tendency of dispersion curves in slab waveguide with chiral negative refractive medium is also analyzed.

Magneto-inertial waves and planetary rotation

2021 ◽  
Author(s):  
Jérémy Rekier ◽  
Santiago Triana ◽  
Véronique Dehant
Keyword(s):  
Magnetic Field ◽  
Polar Motion ◽  
Density Stratification ◽  
Length Of Day ◽  
Inertial Waves ◽  
Torsional Oscillations ◽  
The Core ◽  
The Earth ◽  
Planetary Rotation ◽  
The Magnetic Field

<p>Magnetic fields inside planetary objects can influence their rotation. This is true, in particular, of terrestrial objects with a metallic liquid core and a self-sustained dynamo such as the Earth, Mercury, Ganymede, etc. and also, to a lesser extent, of objects that don’t have a dynamo but are embedded in the magnetic field of their parent body like Jupiter’s moon, Io.<br>In these objects, angular momentum is transfered through the electromagnetic torques at the Core-Mantle Boundary (CMB) [1]. In the Earth, these have the potential to produce a strong modulation in the length of day at the decadal and interannual timescales [2]. They also affect the periods and amplitudes of nutation [3] and polar motion [4]. <br>The intensity of these torques depends primarily on the value of the electric conductivity at the base of the mantle, a close study and detailed modelling of their role in planetary rotation can thus teach us a lot about the physical processes taking place near the CMB.</p><p>In the study of the Earth’s length of day variations, the interplay between rotation and the internal magnetic field arrises from the excitation of torsional oscillations inside the Earth’s core [5]. These oscillations are traditionally modelled based on a series of assumptions such as that of Quasi-Geostrophicity (QG) of the flow inside the core [6]. On the other hand, the effect of the magnetic field on nutations and polar motion is traditionally treated as an additional coupling at the CMB [1]. In such model, the core flow is assumed to have a uniform vorticity and its pattern is kept unaffected by the magnetic field. </p><p>In the present work, we follow a different approach based on the study of magneto-inertial waves. When coupled to gravity through the effect of density stratification, these waves are known to play a crucial role in the oscillations of stars known as magneto-gravito-inertial modes [7]. The same kind of coupling inside the Earth’s core gives rise to the so-called MAC waves which are directly and conceptually related to the aforementioned torsional oscillations [8]. </p><p>We present our preliminary results on the computation of magneto-inertial waves in a freely rotating planetary model with a partially conducting mantle. We show how these waves can alter the frequencies of the free rotational modes identified as the Free Core Nutation (FCN) and Chandler Wobble (CW). We analyse how these results compare to those based on the QG hypothesis and how these are modified when viscosity and density stratification are taken into account. </p><p>[1] Dehant, V. et al. Geodesy and Geodynamics 8, 389–395 (2017). doi:10.1016/j.geog.2017.04.005<br>[2] Holme, R. et al. Nature 499, 202–204 (2013). doi:10.1038/nature12282<br>[3] Dumberry, M. et al. Geophys. J. Int. 191, 530–544 (2012). doi:10.1111/j.1365-246X.2012.05625.x<br>[4] Kuang, W. et al. Geod. Geodyn. 10, 356–362 (2019). doi:10.1016/j.geog.2019.06.003<br>[5] Jault, D. et al. Nature 333, 353–356 (1988). doi:10.1038/333353a0<br>[6] Gerick, F. et al. Geophys. Res. Lett. (2020). doi:10.1029/2020gl090803<br>[7] Mathis, S. et al. EAS Publications Series 62 323-362 (2013). doi: 10.1051/eas/1362010<br>[8] Buffett, B. et al. Geophys. J. Int. 204, 1789–1800 (2016). doi:10.1093/gji/ggv552</p>

Inefficient compaction in small planetary cores -- application to the Moon

2021 ◽  
Author(s):  
Marine Lasbleis
Keyword(s):  
Magnetic Field ◽  
Latent Heat ◽  
Energy Budget ◽  
Inner Core ◽  
Light Elements ◽  
The Moon ◽  
Small Bodies ◽  
Typical Size ◽  
The Core ◽  
Planetary Cores

<div> <p>Growth of the solid inner core is generally considered to power the Earth's present geodynamo. Cristallisation of a solid central inner core has also been proposed to drive the lunar dynamo and to generate a magnetic field in smaller bodies. In a previous work, we estimated the compaction of planetary cores for different scenarios of growth (with or without supercooling) and different sizes of the inner core. Our main results indicated that small inner cores are unlikely to compact efficiently the liquid trapped during the first steps of the growth.</p> <p>This is especially true for small bodies for which the typical size of the core is similar to the compaction length. The light elements are thus trapped during the cristallisation, reducing the release of latent heat and of light elements. We present here a model to include the effect of an inefficient compaction in the energy budget of a planetary core and investigate the implications for the dynamo evolution in small bodies. We apply this model for the evolution of the core of the Moon. </p> </div>

Radiogenic Isotope Signatures of Holocene Sediments from Kane Basin: Linkage with the Re-opening and Evolution of Nares Strait

2021 ◽  
Author(s):  
Lina Madaj ◽  
Friedrich Lucassen ◽  
Claude Hillaire-Marcel ◽  
Simone A. Kasemann
Keyword(s):  
Ellesmere Island ◽  
The Arctic ◽  
Baffin Bay ◽  
Low Salinity ◽  
The Core ◽  
The Past ◽  
Nares Strait ◽  
The North ◽  
Sediment Input ◽  
Detrital Sediment

<p>The re-opening of the Arctic Ocean-Baffin Bay gateway through Nares Strait, following the Last Glacial Maximum, has been partly documented, discussed and revised in the past decades. The Nares Strait opening has led to the inception of the modern fast circulation pattern carrying low-salinity Arctic water towards Baffin Bay and further towards the Labrador Sea. This low-salinity water impacts thermohaline conditions in the North Atlantic, thus the Atlantic Meridional Overturning Circulation. Available land-based and marine records set the complete opening between 9 and 7.5 ka BP [1-2], although the precise timing and intensification of the southward flowing currents is still open to debate. A recent study of a marine deglacial sedimentary record from Kane Basin, central Nares Strait, adds information about subsequent paleoceanographic conditions in this widened sector of the strait and proposed the complete opening at ~8.3 ka BP [3].</p><p>We present complementary radiogenic strontium, neodymium and lead isotope data of the siliciclastic detrital sediment fraction of this very record [3] further documenting the timing and pattern of Nares Strait opening from a sediment provenance approach. The data permit to distinguish detrital material from northern Greenland and Ellesmere Island, transported to the core location from both sides of Nares Strait. Throughout the Holocene, the evolution of contributions of these two sources hint to the timing of the ice break-up in Kennedy Channel, north of Kane Basin, which led to the complete opening of Nares Strait [3]. The newly established gateway of material transported to the core location from the north via Kennedy Channel is recorded by increased contribution of northern Ellesmere Island detrital sediment input. This shift from a Greenland (Inglefield Land) dominated sediment input to a northern Ellesmere Island dominated sediment input supports the hypothesis of the newly proposed timing of the complete opening of Nares Strait at 8.3 ka BP [3] and highlights a progressive trend towards modern-like conditions, reached at about 4 ka BP.</p><p>References:</p><p>[1] England (1999) Quaternary Science Reviews, 18(3), 421–456. [2] Jennings et al. (2011) Oceanography, 24(3), 26-41. [3] Georgiadis et al. (2018) Climate of the Past, 14 (12), 1991-2010.</p>

Property concepts in Tabaq: More than one road can lead to Rome

2020 ◽  
Vol 51 (1) ◽  
pp. 63-78
Author(s):  
Gertrud Schneider-Blum ◽  
Birgit Hellwig
Keyword(s):  
Small Class ◽  
North West ◽  
The Core ◽  
The North ◽  
Limited Role ◽  
Core Function ◽  
Word Classes ◽  
Semantic Types ◽  
Property Concepts

Abstract In this paper, we investigate the interplay between the morphosyntactic class of adjectives, the semantics of property concepts, and the function of noun modification in Tabaq, a Kordofan Nubian language (Nilo-Saharan phylum) spoken in the north-west of the Nuba Mountains in Sudan. Tabaq has a small class of adjectives containing few semantic types, and playing only a limited role with respect to the core function of adjectives: the modification of nouns. By contrast, a large number of descriptive modifiers is derived from two other word classes, verbs and nouns, and this paper describes the different ways of coding property concepts in Tabaq.

Catawba

Fit for War ◽  
2017 ◽  
Author(s):  
Mary Elizabeth Fitts
Keyword(s):  
North Carolina ◽  
Eighteenth Century ◽  
Seventeenth Century ◽  
American Indian ◽  
Slave Trade ◽  
Daily Life ◽  
Territorial Expansion ◽  
Charles Town ◽  
The Core ◽  
The North

Chapter 3 documents the emergence, composition, and political interactions of the Catawba Nation through the mid-eighteenth century. Between the Spanish incursions of the 1560s and the establishment of Charles Town in 1670, a group of Catawba Valley Mississippians known as Yssa rose to become the powerful Nation of Esaws that formed the core of the eighteenth-century Catawba Nation. In the late seventeenth century this polity was a destination for European traders as well as American Indian refugees fleeing hostilities associated with the Indian Slave trade and settler territorial expansion. While many of these refugees were from the Catawba River Valley, others—most notably the Charraw—were Piedmont Siouans who fled southward from the North Carolina-Virginia border. The incorporation of refugees had significant implications for Catawba politics and daily life, which are explored in subsequent chapters.

What Is Decolonial Love?

2018 ◽  
pp. 1-14
Author(s):  
Joseph Drexler-Dreis
Keyword(s):  
North Atlantic ◽  
Atlantic World ◽  
Critical Reflection ◽  
Positive Response ◽  
The Core ◽  
The North ◽  
European Experience ◽  
Core Concepts ◽  
The North Atlantic

The introduction establishes the decolonial perspective that prompts the questions to which the book responds. In light of the modern/colonial context of the North Atlantic world, the introduction raises two basic questions. First, can theology, as a mode of critical reflection that employs core concepts and images within lineages grounded in the European experience, contribute to the task of decolonization? Second, if a positive response to this question were offered, what would the content of that response look like? The introduction then proceeds to map out how the core image of decolonial love is developed through the book as a basis for responding to these questions.

Amarna: Urban Manufacture of Luxury Items

2017 ◽  
Author(s):  
Anna K. Hodgkinson
Keyword(s):  
Far East ◽  
Ancient Egypt ◽  
Capital City ◽  
North West ◽  
Far North ◽  
The Core ◽  
The North ◽  
The Far East ◽  
Royal Road ◽  
The City

Little is necessary in terms of an introduction, since Amarna is one of the best-known settlements of ancient Egypt. The city was founded by pharaoh Amenhotep IV, known from his fifth regal year as Akhenaten, on his move away from Thebes and Memphis to found a new religious and administrative capital city. Akhenaten reigned approximately between 1348 and 1331 BC, and his principal wife was Nefertiti. Akhenaten’s direct successor appears to have been a figure named Smenkhare (or Ankhkheperure) who was married to Akhenaten’s daughter Meritaten. Like Nefertiti, Smenkhare/Ankhkheperure held the throne name Nefernefruaten. For this reason it is uncertain whether this individual was Nefertiti, who may have reigned for some years after the death of Akhenaten, possibly even with a brief co-regency, or whether this was a son or younger brother of the latter. The rule of Smenkhare/Ankhkheperure was short, and he or she was eventually succeeded by Tutankhamun. The core city of Amarna was erected on a relatively flat desert plain surrounded by cliffs on the east bank of the Nile, in Middle Egypt, approximately 60km south of the modern city of Minia, surrounded by the villages et- Till to the north and el-Hagg Qandil to the south. The site was defined by at least sixteen boundary stelae, three of which actually stand on the western bank, past the edge of the modern cultivation. In total, the city measures 12.5km north–south on the east bank between stelae X and J, and c.8.2km west–east between the projected line between stelae X and J and stela S to the far east, which also indicates approximately the longitude of the royal tomb. The distance between stelae J and F, to the far south-west, measures c.20km, and between stelae X and A, to the far north-west 19.2km. The core city, which is the part of the settlement examined in this section, was erected along the Nile, on the east bank, and it is defined by the ‘Royal Road’, a major thoroughfare running through the entire core city north–south.

Sign in / Sign up

Export Citation Format

Share Document