fluctuations of Earth’s outer core – Towards a better detection

with ground-based magnetic observations

Asari, S. (1), Wardinski, I. (2)

Kakioka Magnetic Observatory, Ishioka, Japan,

University of Nantes, Nantes, France


Time evolution of

Earth’s fluid outer core may be viewed as a superposition of (1)

rapid perturbations on (2) a slowly growing basic state. Among (1) are

various modes of the hydromagnetic wave. Their characteristic

morphologies and frequencies have been theoretically and numerically

predicted for simple models of the quasi-steady state (2). Detecting

tiny signals due to (1) is important for not just investigating the

rapid core dynamics itself but also constraining (2) from

observations. Precise magnetic measurements by recent satellite

missions after 1999 have made it possible to reliably extract

interannual fluctuations of the core magnetic field (~2 nT/yr at Earth

surface). These imply a presence of the 6-year torsional Alfven waves,

which is supported as well by the corresponding oscillation in the

length-of-day (LOD) observation extending back to the early 1960s.

Here we investigate detectability of the interannual core fluctuations

without using satellite magnetic observations. A temporally continuous

magnetic model C3FM2.x is developed alone from ground-based monthly

mean data available for 1957.0-2015.4. C3FM2.x is then inverted for a

core flow model, in order to evaluate its rapid fluctuations after

1999 by comparing them with those of a flow model based on CHAOS-6.x5,

the latest satellite magnetic model. According to temporal behaviors

of the core angular momentum (CAM), C3FM2.x is of as much use as the

satellite model for inferring at least the phase of the 6-year

variation. The 6-year CAM variations calculated from the two flow

models are both coherent with the observed LOD oscillation. They are

even consistent with an anomalous LOD periodicity after 2010. C3FM2.x

is expected to provide comparable performance prior to the recent

satellite-era, in view of its dataset distributed roughly evenly in

time. For a further optimization of the ground-based model,

nevertheless, it is absolutely necessary to maximally eliminate

interannual signals from the magnetospheric currents, which we think

still contaminate C3FM2.x. A careful strategy for dataset cleaning

needs to be applied, for instance, by using the Dst index or its

alternative indices recently proposed.