Virtual Presentation at CGU 2021

Again this year, the Canadian Geophysical Union (CGU) held their annual meeting as a virtual event. Together with my co-author Keith Aldridge, some of our recent research was presented. To capture and share an account of this, you will find here:

• A plain-language summary of our research in the broader context
• Our submitted response to the call for abstracts
• A video of the presentation given

The Plain-Language Version

The following plain-language summary includes highlights from the abstract (submitted May 10, 2021) as well as the presentation itself (July 15, 2021):

• Earth’s magnetic field is critical to life – it protects the biosphere and the atmosphere
• The geologic record provides evidence for geomagnetic field dating back some 4 billion years
• Earth’s magnetic field is extremely dynamic – the most profound examples are the reversals in its polarity … again, well documented in the geologic record
• The geodynamo is the means through which Earth’s magnetic field is generated – it requires a stirring of the liquid metal alloy that comprises Earth’s outer core … as, by stirring an electrically conducting fluid, currents are created … and these currents create a magnetic field
• The means for stirring Earth’s outer core remains a topic of scientific curiosity and debate
• Cooling of the planet is a likely contributor as it could result in thermal convection in the outer core – as the planet’s thermal history is not uniquely constrained, the contribution of this mechanism over geologic timescales of some 4.5 billion years is somewhat uncertain
• Solidification of Earth’s inner core, from the liquid metal alloy outer core, introduces buoyancy forces that seem likely to contribute to stirring over the past 1 or so billion years – so-called compositional convection has only been available for less than a quarter of our planet’s history
• This research places emphasis on an alternative means for contributing to the geodynamo based on mechanical turbulence
• White water serves as an excellent analog for flows generated by mechanical turbulence – in the white-water case, water in a river flows in a complex way owing to the presence of rocks, boulders, etc., on the river bed … these objects disrupt an otherwise laminar (or sheet-like) flow
• Tides on Earth’s surface (e.g., in places like the Bay of Fundy) are the consequence of the combined gravitational attraction of the Sun and Moon
• These same forces that distort our oceans on a daily basis can also distort the boundary that contains Earth’s outer core (aka. the Core-Mantle Boundary or CMB) – some 2900 km below our feet … Earth’s liquid core is sometimes referred to as Earth’s third ocean
• When tidally distorted, a contained and rotating fluid can become unstable – i.e., by elliptically distorting its boundary, mechanical turbulence is generated
• In Earth’s case then, distortions of the CMB could be responsible for generating mechanical turbulence in the outer core … and in so doing, stir this liquid to ultimately contribute towards a magnetic field
• Even though the distortion of the CMB is expected to the be small, it is postulated that is a feasible mechanism
• From theory and laboratory experiments it is known that the instabilities generated by this mechanism can be substantial – this is a nonlinear effect, so the outcome is an amplified effect
• To validate tidally generated elliptic instability as a viable means for contributing towards the geodynamo this research places emphasis on the analysis of observations
• The most interesting observations, to ‘test’ this hypothesis, are those that capture the reversals of the geodynamo
• Relative paleointensity data well captures reversals of the geomagnetic field through sedimentary and volcanic rocks – examples of this are provided in the accompanying presentation and video
• A method pioneered by Keith Aldridge almost two decades ago allows properties of the geomagnetic field to be characterized – i.e., the growth and decay of the field can be estimated around the time of a reversal
• Aldridge and his collaborators have found a consistency between these observational estimates for growths and decays and expectations from theory and experiment (representative experimental results are provided in the accompanying presentation and video) – a consistency that is encouraging in making the case for a geodynamo driven by an elliptic instability
• Tidal strain (i.e., the response of the CMB to the stresses of luni-solar gravitational attraction) can be estimated from these growth and decay rates
• Aldridge & McMillan in 2017 demonstrated that peaks in tidal strain correlate well with reversals of the geomagnetic field … notably the amplitude of their estimates (for tidal strain) from observations are comparable to those expected from theory
• Beryllium (Be) isotope ratio data has become available in recent years as an independent means for capturing the variations in the geomagnetic field … thus, the methods described above could be applied to this ‘new’ data set
• One way to make sense of all these results is to plot decay rates against event duration (e.g., how long it takes a reversal to happen) – an example of this is provided in the accompanying presentation and video
• A striking conclusion results from this decay rate vs. event-duration plot: simulations of thermochemical convection do not account for the observations
• Further research is clearly required – and this includes a heightened effort with respect to mechanically generated turbulence via an elliptic instability

The Scientific Version

The abstract we submitted on May 10, 2021 reads as follows:

Growth and Decay Asymmetry in the Axial Dipole Field as a Criterion for Modeling the Geodynamo

L. I. Lumb & K. D. Aldridge, York University

According to observations that proxy for its Axial Dipole Moment (ADM), the geomagnetic field is asymmetric about its local maximum value. Initially identified some 18 years ago in the context of reversals, the ADM is observed to decay more slowly in intensity, than the rate at which it recovers. More recently, and based upon geographically diverse data, this asymmetry in growths and decays has been demonstrated to be a robust feature of the observations. In fact, statistical measures (e.g., skewness) reveal the existence of the asymmetry to be a feature that is persistent even when the geodynamo is not undergoing reversals or excursions. Thus, asymmetric growths and decays have emerged as another criteria with the potential to connect laboratory or numerical models and simulations with observations. Moreover, as there is a natural tendency to associate this imbalance with processes affecting fluid flow in Earth’s outer core, there exists the potential to derive insight into the mechanism responsible for the geodynamo. After briefly reviewing attempts to rationalize the asymmetry through diffusive versus advective effects, attention here focuses on instability in the liquid core generated by luni-solar tides. This mechanically induced instability is inherently asymmetric: growths are predicted to exceed decays on theoretical grounds. Subsequently, this theoretical result has been validated by a number of physical experiments. When this model for elliptical instability is applied to paleointensity data, it is evident that reversals coincide closely in time with maximum values for the tidal strain rate. Owing to its affinity for this ‘new criteria’ that places emphasis on asymmetric growths and decays in the geomagnetic field’s ADM, tidally induced instability merits consideration in any discussion of mechanisms for the geodynamo.

A video capture of the corresponding presentation follows below:

The slides we used can be found below:

Feel free to reach out to either of us if you have any questions to ask, feedback to share, etc.