The history of geomagnetic secular variation hemispherical dichotomies

Monitoring the geomagnetic field and its secular variation (SV) is essential for understanding the Earth's internal dynamics. In particular, the SV provides an image of the geodynamo at the top of the core. However, the SV is not available for paleomagnetic field models. Here, we propose a new index for assessing the paleomagnetic SV. This new index is based on the well-established inverse linear relationship between the SV timescales and the degree of spherical harmonics. We demonstrate using the historical field where the SV is available that this index adequately captures the large-scale features of the true SV, in particular the SV Atlantic/Pacific and North/South dichotomies. The recovery of these SV hemispherical dichotomies by our proposed index does not deteriorate from truncated fields at spherical harmonics degree 14 to 5. Applied to a paleomagnetic field model for the past 100 kyr, we find a persistent SV dichotomy between the quiet Pacific and active Atlantic hemispheres, consistent with heterogeneous inner core freezing. In addition, according to our index, a persistent stronger SV prevails at the northern hemisphere, which is the case at the present day due to a fast westward jet at high latitudes of the northern hemisphere.