Steven J. Gibbons

Kinematic dynamo action in a sphere II. Symmetry selection

The magnetic fields of the planets are generated by dynamo action in their electrically conducting interiors. The Earth possesses an axial dipole magnetic field but other planets have other configurations: Uranus has an equatorial dipole for example. In a previous paper we explored a two–parameter class of flows, comprising convection rolls, differential rotation (D) and meridional circulation (M), for dynamo generation of steady fields with axial dipole symmetry by solving the kinematic dynamo equations. In this paper we explore generation of the remaining three allowed symmetries: axial quadrupole, equatorial dipole and equatorial quadrupole. The results have implications for the fully nonlinear dynamical dynamo because the flows qualitatively resemble those driven by thermal convection in a rotating sphere, and the symmetries define separable solutions of the nonlinear equations. Axial dipole solutions are generally preferred (they have lower critical magnetic Reynolds number) for D > 0, corresponding to westward surface drift. Axial quadrupoles are preferred for D < 0, and equatorial dipoles for convection with little D or M. No equatorial quadrupole solutions have been found. Symmetry selection can be understood if one assumes that the flow concentrates flux in certain places without reference to sign. Fields with dipole symmetry must change sign across the Equator; if flux is concentrated at the Equator, as tends to be the case for D < 0, they have a small length–scale and consequent high dissipation, making them harder to generate than axial quadrupoles. If flux is concentrated nearer the poles (D > 0), axial dipoles are preferred. The equatorial dipole must change sign between east and west hemispheres, and is not favoured by any elongation of the flux in longitude (caused by D) or polar concentrations (caused by M): they are preferred for small D and M. Polar and equatorial concentrations can be related to dynamo waves and the sign of Parkers dynamo number. For the three–dimensional flow considered here, the sign of the dynamo number is related to the sense of spiralling of the convection rolls, which must be the same as the surface drift.

Kinematic dynamo action in a sphere. I. Effects of differential rotation and meridional circulation on solutions with axial dipole symmetry

A sphere containing electrically conducting fluid can generate a magnetic field by dynamo action, provided the flow is sufficiently complicated and vigorous. The dynamo mechanism is thought to sustain magnetic fields in planets and stars. The kinematic dynamo problem tests steady flows for magnetic instability, but rather few dynamos have been found so far because of severe numerical difficulties. Dynamo action might, therefore, be quite unusual, at least for large–scale steady flows. We address this question by testing a two–parameter class of flows for dynamo generation of magnetic fields containing an axial dipole. The class of flows includes two completely different types of known dynamos, one dominated by differential rotation (D) and one with none. We find that 36% of the flows in seven distinct zones in parameter space act as dynamos, while the remaining 64% either fail to generate this type of magnetic field or generate fields that are too small in scale to be resolved by our numerical method. The two previously known dynamo types lie in the same zone, and it is therefore possible to change the flow continuously from one to the other without losing dynamo action. Differential rotation is found to promote large–scale axisymmetric toroidal magnetic fields, while meridional circulation (M) promotes large–scale axisymmetric poloidal fields concentrated at high latitudes near the axis. Magnetic fields resembling that of the Earth are generated by D > 0, corresponding to westward flow at the surface, and M of either sign but not zero. Very few oscillatory solutions are found.

Strong-motion observations of the M 7.8 Gorkha, Nepal, earthquake sequence and development of the N-shake strong-motion network

We present and describe strong-motion data observations from the 2015 M 7.8 Gorkha, Nepal, earthquake sequence collected using existing and new Quake-Catcher Network (QCN) and U.S. Geological Survey NetQuakes sensors located in the Kathmandu Valley. A comparison of QCN data with waveforms recorded by a conventional strong-motion (NetQuakes) instrument validates the QCN data. We present preliminary analysis of spectral accelerations, and peak ground acceleration and velocity for earthquakes up to M 7.3 from the QCN stations, as well as preliminary analysis of the mainshock recording from the NetQuakes station. We show that mainshock peak accelerations were lower than expected and conclude the Kathmandu Valley experienced a pervasively nonlinear response during the mainshock. Phase picks from the QCN and NetQuakes data are also used to improve aftershock locations. This study confirms the utility of QCN instruments to contribute to ground-motion investigations and aftershock response in regions where conventional instrumentation and open-access seismic data are limited. Initial pilot installations of QCN instruments in 2014 are now being expanded to create the Nepal–Shaking Hazard Assessment for Kathmandu and its Environment (N-SHAKE) network. Online Material: Figures of Pg arrivals, earthquake locations, epicenter change vectors, and travel-time misfit vector residuals, and tables of QCN and NetQuake stations and relocated hypocenter timing, location, and magnitude.

Seismic Monitoring of the North Korea Nuclear Test Site Using a Multichannel Correlation Detector

North Korea announced a second nuclear test on 25 May 2009, the first having taken place on October 9, 2006. Both tests were detected by the global seismic network of the Comprehensive nuclear Test-Ban-Treaty Organisation. We apply a correlation detector using a 10-s signal template from the 2006 test on the MJAR array in Japan to: 1) assess the potential for automatically detecting subsequent explosions at or near the test site; and 2) monitor the associated false alarm rate. The 2009 signal is detected clearly with no false alarms in a three-year period. By detecting scaled-down copies of the explosion signals submerged into background noise, we argue that a significantly smaller explosion at the site would have been detected automatically, with a low false alarm rate. The performance of the correlator on MJAR is not diminished by the signal incoherence that makes conventional array processing problematic at this array. We demonstrate that false alarm elimination by f-k analysis of single channel detection statistic traces is crucial for maintaining a low detection threshold. Correlation detectors are to be advocated as a routine complement to the existing pipeline detectors, both for reducing the detection threshold for sites of interest and providing automatic classification of signals from repeating sources.

Low Pacific Secular Variation

The historical record shows that secular variation at Hawaii is limited to a few degrees in the last 400 years, whereas in the Atlantic hemisphere it often exceeds 30°. Paleomagnetic measurements from Hawaii show virtually no change in declination during the last 5 kyr and only a slow, millenium-scale inclination change of less than 20°. The usual directional scatter analysis of paleomagnetic data cannot discriminate between the two time scales. The disparity of time scales and difference in activity suggest different physical mechanisms for secular variation in the two hemispheres. This could arise from thermal core-mantle interaction. Seismic models of the lower mantle give a pattern of lateral variations that is nearly symmetric about the Pacific rim: two slow regions centered beneath the Pacific and Atlantic separated by a fast ring below the Pacific rim. These seismic anomalies are thought to be caused mostly by temperature variations in the bottom 200 km of the mantle. It is difficult to see how such a symmetric pattern could lead to long-term hemispheric differences. We show here a convection solution in a rapidly rotating sphere with heat flux on the outer boundary determined from a seismic model. The convection is suppressed beneath the Pacific but the usual drifting convection (Busse) rolls remain beneath the Atlantic. This mode of periodic convection arises because the Pacific hot region extends east-west and is much larger than a single convection roll, whereas the Atlantic hot region is elongated north-south and is about the same east-west size as a convection roll. This could explain the absence of normal, century-long secular variation in the Pacific: hot mantle suppresses short wavelength phenomena at the century time scale but not the longer wavelengths at the millenium time scale.

Joint seismic-infrasonic processing of recordings from a repeating source of atmospheric explosions

A database has been established of seismic and infrasonic recordings from more than 100 well-constrained surface explosions, conducted by the Finnish military to destroy old ammunition. The recorded seismic signals are essentially identical and indicate that the variation in source location and magnitude is negligible. In contrast, the infrasonic arrivals on both seismic and infrasound sensors exhibit significant variation both with regard to the number of detected phases, phase travel times, and phase amplitudes, which would be attributable to atmospheric factors. This data set provides an excellent database for studies in sound propagation, infrasound array detection, and direction estimation.

Convection in rotating spherical fluid shells with inhomogeneous heat flux at the outer boundary

The Earths core is subject to a laterally varying heat flux at the outer boundary, which may account for correlations between the geomagnetic field and lower mantle structure. Studies of nonmagnetic, rotating convection in a spherical shell with fixed temperature boundary conditions have revealed flows resonating with, or locked to, the boundary anomalies when the length scales of the convection are close to those of the boundary condition. Here we study a similar system but for fixed heat flux upper boundary conditions, as in the Earths core. We first map out the onset of thermal instability in a rotating shell of aspect ratio 0.4 for uniform outer boundary cooling with both rigid and stress-free boundaries. A preference for large scale (azimuthal wavenumber m = 1) flows, not observed for the uniform temperature case, persists to Ekman numbers down to almost 10− 4. The preference for large scales is greatest for rigid boundaries and high (≥ 1) Prandtl numbers. Hemispheric asymmetry appears in the weakly nonlinear regime, with small scale columnar convection coexisting with larger scale flows. We next study the effect of heterogeneous cooling of strength ε proportional to the Y 2² spherical harmonic, which resembles the assumed heat flow at the core-mantle boundary. We illustrate the results with two sets of parameters, (1) when the most unstable mode in the uniform case is m = 2, the same as the heterogeneous boundary condition, and (2) when it is m = 1. We follow the numerical solutions from steady flows dominated by boundary heating, through periodic flows drifting at non-uniform rates, to chaotic flows. In case (1) both thermal convection and boundary-driven flow are dominated by the same azimuthal wavenumber m = 2 as the boundary condition; they give way to periodic flows of the same symmetry (even m) at low boundary heterogeneity. At higher ε the symmetry is broken as modes with odd m are excited. In case (2) at low ε the m = 1 and m = 2 modes compete, while higher ε imposes an m = 2 symmetry. Boundary effects depend strongly on the most unstable wavenumber at onset of convection with uniform boundary cooling: these simple linear results are a good guide to the probable behaviour of more complex, nonlinear regimes and have already been used to find suitable parameter ranges in a geodynamo calculation.

Probabilistic infrasound propagation using realistic atmospheric perturbations

This study demonstrates probabilistic infrasound propagation modeling using realistic perturbations. The ensembles of perturbed analyses, provided by the European Centre for Medium-Range Weather Forecasts (ECMWF), include error variances of both model and assimilated observations. Ensemble spread profiles indicate a yearly mean effective sound speed variation of up to 8 ms?1 in the stratosphere, exceeding occasionally 25 ms?1 for a single ensemble set. It is shown that errors in point estimates of effective sound speed are dominated by variations in wind strength and direction. One year of large mining explosions in the Aitik mine, northern Sweden, observed at infrasound array IS37 in northern Norway are simulated using 3-D ray tracing. Probabilistic propagation modeling using the ensembles demonstrates that small-scale fluctuations are not always necessary to improve the match between predictions and observations.

Three-Dimensional Dynamo Waves In a Sphere

The term dynamo wave was introduced by Parker to describe oscillatory solutions of the mean field induction equation containing differential rotation and an f -effect from helicity. In this article, we examine dynamo waves generated by a class of steady, three-dimensional flows in a sphere without using the mean-field approximation. The flows are defined by two parameters: D the differential rotation, and M the meridional circulation. Contrary to expectations based on the mean-field equations, most 3-dimensional solutions are stationary. Dynamo waves occupy a very narrow band in the parameter space of flows. They behave like solutions to the mean-field equations in three important ways: the magnetic Reynolds number approaches the asymptotic value, meridional circulation produces steady solutions, and radial and azimuthal components of flux tend to occupy the same region of the sphere. The last observation explains why steady solutions, in which radial and azimuthal flux occupy different parts of the sphere, dominate in 3-dimensions: the third dimension facilitates the separation of the two components. This kinematic result may apply to dynamical solutions in which any change in flow that tends to concentrate radial and azimuthal field in the same place leads to oscillations or reversal. This is a possible mechanism by which the Earths magnetic field reverses.

Toward an Improved Representation of Middle Atmospheric Dynamics Thanks to the ARISE Project

This paper reviews recent progress toward understanding the dynamics of the middle atmosphere in the framework of the Atmospheric Dynamics Research InfraStructure in Europe (ARISE) initiative. The middle atmosphere, integrating the stratosphere and mesosphere, is a crucial region which influences tropospheric weather and climate. Enhancing the understanding of middle atmosphere dynamics requires improved measurement of the propagation and breaking of planetary and gravity waves originating in the lowest levels of the atmosphere. Inter-comparison studies have shown large discrepancies between observations and models, especially during unresolved disturbances such as sudden stratospheric warmings for which model accuracy is poorer due to a lack of observational constraints. Correctly predicting the variability of the middle atmosphere can lead to improvements in tropospheric weather forecasts on timescales of weeks to season. The ARISE project integrates different station networks providing observations from ground to the lower thermosphere, including the infrasound system developed for the Comprehensive Nuclear-Test-Ban Treaty verification, the Lidar Network for the Detection of Atmospheric Composition Change, complementary meteor radars, wind radiometers, ionospheric sounders and satellites. This paper presents several examples which show how multi-instrument observations can provide a better description of the vertical dynamics structure of the middle atmosphere, especially during large disturbances such as gravity waves activity and stratospheric warming events. The paper then demonstrates the interest of ARISE data in data assimilation for weather forecasting and re-analyzes the determination of dynamics evolution with climate change and the monitoring of atmospheric extreme events which have an atmospheric signature, such as thunderstorms or volcanic eruptions.