Modeling the Long-term of the Geomagnetic Field

Abstract:

The temporal variability of the geomagnetic field over the past thousand years, known as the paleosecular variation, gives valuable information about the mechanism that maintains the geomagnetic field and helps to establish constraints in the dynamo theories. In this work, the paleosecular variation for the last 10 kyr is analyzed using two global paleomagnetic field reconstructions: SHA.DIF.14k and CALS10k.2. A frequency analysis is performed using the Empirical Mode Decomposition procedure over the first three Gauss coefficients allowing us to decompose their evolution into long- and short-term. This decomposition shows that the long-term of the geomagnetic field is carried by the axial dipole.

In order to model the last recorded increase (from 4500 BC to 100 BC) and decrease (from 100 BC to present) of the axial dipole, we propose a combination of a continuously decay, representing the diffusion term of the geomagnetic field, and a pulse that reinforces the strength of the field. Our approach considers the diffusion as a decreasing exponential with characteristic diffusion times of 11000 – 15000 yr, which is compatible with the diffusion times of the dipole field used in geodynamo theories. Meanwhile, the pulse is defined by a skewed normal function that reaches peak values of increase between 6 nT/yr and 8 nT/yr. These results are compared with the actual variation of the long-term of the axial dipole showing that the diffusion process takes nearly 50% of the increase provided by the pulse. Finally, we explore if this approach could represent the axial dipole variability for the last 30 kyr using the global paleoreconstruction GGF100k.