Jeffrey J. Love

Paleomagnetic volcanic data and geometric regularity of reversals and excursions

Mostly on the basis of paleomagnetic sedimentary data, it has been suggested that maps of virtual geomagnetic poles (VGPs), corresponding to directions of the magnetic field at each site, tend to fall along American and Asian longitudes during reversals and excursions. Such geometric regularity in transitional fields may indicate that the core and mantle are dynamically coupled. However, studies of paleomagnetic lava data have thus far failed to show any pattern in transitional fields. In this paper we examine a paleomagnetic lava database covering reversals and excursions which have occurred over the last 20 Myr. Volcanic eruptions occur sporadically, thus we normalize the data to account for the fact that reversal and excursional events at the various sites are recorded by different numbers of intermediate directions, but we prefer not to use averaging methods of previous investigators, who discarded or combined directions when they appeared to be similar. We find that volcanic data give intermediate VGPs which tend to fall along American and Asian longitudes, roughly consistent with the sedimentary data. This result is not an apparent artifact arising from the nonuniform geographic distribution of volcanic sites. Provided the appropriate polarity is assigned to intermediate VGPs, we find that Icelandic VGPs tend to fall along Asian longitudes. Other patterns in the data, for example, latitudinal clustering of VGPs or distinguishing longitudinal preferences of excursions from reversals, are not resolved. However, it appears that in general, transitional fields are nondipolar.

Magnetic monitoring of Earth and space

With data provided by magnetic observatories, geophysicists can gain insights into our planet’s interior and nearby space environment without even leaving the ground.With data provided by magnetic observatories, geophysicists can gain insights into our planet’s interior and nearby space environment without even leaving the ground.

A DATABASE FOR THE MATUYAMA-BRUNHES MAGNETIC REVERSAL

Abstract We present a paleomagnetic database, designated MBD97, comprising sedimentary and lava data for the Matuyama-Brunhes reversal transition field. Using criteria regarding suitability of the rock samples, measurement methods and temporal resolution of the data, we consider 62 different studies of the Matuyama-Brunhes reversal; of these, only 11 satisfy our selection criteria: four lava sites, one loess site, three shallow-marine sedimentary sites, and three deep-ocean sedimentary sites. We discuss our reasons for accepting or disguarding each candidate data set. We investigate the distribution of transitional virtual geomagnetic poles (VGPs); they fall along American and Asian-Australian longitudes. The spatial and temporal complexity of the data indicate that the transitional field was almost certainly nonaxisymmetric and nondipolar. The directional transition of the Matuyama-Brunhes reversal took ∼ 2300–5000 years, during which time the surface intensity fell to ∼ 10–20% of its pre- and post-transitional value. We do not find any systematic bias due to site distribution or inclination shallowing, although the errors in the data do vary over the duration of the reversal. MBD97 is a small database, but it has a fairly good geographic distribution and is suitable for studies of the Matuyama-Brunhes transition, nonetheless, it is clear that further analysis of transitions will benefit from a continued program of data collection and analysis. The data in MBD97 are listed.

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.

An International Network of Magnetic Observatories

Since its formation in the late 1980s, the International Real-Time Magnetic Observatory Network (INTERMAGNET), a voluntary consortium of geophysical institutes from around the world, has promoted the operation of magnetic observatories according to modern standards [e.g., Rasson, 2007]. INTERMAGNET institutes have cooperatively developed infrastructure for data exchange and management as well as methods for data processing and checking. INTERMAGNET institutes have also helped to expand global geomagnetic monitoring capacity, most notably by assisting magnetic observatory institutes in economically developing countries by working directly with local geophysicists.

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.

Spring‐fall asymmetry of substorm strength, geomagnetic activity and solar wind: Implications for semiannual variation and solar hemispheric asymmetry

Received 12 January 2011; revised 14 February 2011; accepted 18 February 2011; published 24 March 2011. [1] We study the seasonal variation of substorms, geomagnetic activity and their solar wind drivers in 1993–2008. The number of substorms and substorm mean duration depict an annual variation with maxima in Winter and Summer, respectively, reflecting the annual change of the local ionosphere. In contradiction, substorm mean amplitude, substorm total efficiency and global geomagnetic activity show a dominant annual variation, with equinoctial maxima alternating between Spring in solar cycle 22 and Fall in cycle 23. Thelargestannualvariationswerefoundin1994and2003,in the declining phase of the two cycles when high‐speed streams dominate the solar wind. A similar, large annual variation is found in the solar wind driver of substorms and geomagnetic activity, which implies that the annual variation of substorm strength, substorm efficiency and geomagnetic activity is not due to ionospheric conditions but to a hemispherically asymmetric distribution of solar wind which varies from one cycle to another. Our results imply that the overall semiannual variation in global geomagnetic activity has been seriously overestimated, and is largely an artifact of the dominant annual variation with maxima alternating between Spring and Fall. The results also suggest an intimate connection between the asymmetry of solar magnetic fields and some of the largest geomagnetic disturbances, offering interesting new pathways for forecasting disturbances with a longer lead time to the future. Citation: Mursula, K., E. Tanskanen, and J. J. Love (2011), Spring‐fall asymmetry of substorm strength, geomagnetic activity and solar wind: Implications for semiannual variation and solar hemispheric asymmetry, Geophys. Res. Lett., 38, L06104, doi:10.1029/2011GL046751.

The USGS geomagnetism program and its role in space weather monitoring

Magnetic storms result from the dynamic interaction of the solar wind with the coupled magnetosphericionospheric system. Large storms represent a potential hazard for the activities and infrastructure of a modern, technologically based society [Baker et al., 2008]; they can cause the loss of radio communications, reduce the accuracy of global positioning systems, damage satellite electronics and affect satellite operations, increase pipeline corrosion, and induce voltage surges in electric power grids, causing blackouts. So while space weather starts with the Sun and is driven by the solar wind, it is on, or just above, the surface of the Earth that the practical effects of space weather are realized. Therefore, ground-based sensor networks, including magnetic observatories [Love, 2008], play an important role in space weather monitoring.

Magnetic Storms and Induction Hazards

Magnetic storms are potentially hazardous to the activities and technological infrastructure of modern civilization. This reality was dramatically demonstrated during the great magnetic storm of March 1989, when surface geoelectric fields, produced by the interaction of the time-varying geomagnetic field with the Earths electrically conducting interior, coupled onto the overlying Hydro-Quebec electric power grid in Canada. Protective relays were tripped, the grid collapsed, and about 9 million people were temporarily left without electricity [Bolduc, 2002].

Geomagnetically induced currents: science, engineering, and applications readiness

This paper is the primary deliverable of the very first NASA Living With a Star Institute Working Group, Geomagnetically Induced Currents (GIC) Working Group. The paper provides a broad overview of the current status and future challenges pertaining to the science, engineering, and applications of the GIC problem. Science is understood here as the basic space and Earth sciences research that allows improved understanding and physics-based modeling of the physical processes behind GIC. Engineering, in turn, is understood here as the “impact” aspect of GIC. Applications are understood as the models, tools, and activities that can provide actionable information to entities such as power systems operators for mitigating the effects of GIC and government agencies for managing any potential consequences from GIC impact to critical infrastructure. Applications can be considered the ultimate goal of our GIC work. In assessing the status of the field, we quantify the readiness of various applications in the mitigation context. We use the Applications Readiness Level (ARL) concept to carry out the quantification.