Non-dipole features of the geomagnetic field on the 100 kyr timescale

Sedimentary paleomagnetic records provide valuable insights into the behavior of Earth’s magnetic field on millennial to multi-millennial timescales. Directional scatter produced by paleosecular variation (PSV) tends to be elongated along the north–south axis of the magnetic meridian, and the magnitude of this effect depends on site latitude. In contrast, inclination shallowing generates an elongation that is oriented east–west, perpendicular to the magnetic meridian. One of the major issues in the PSV studies is whether such archives have undergone inclination flattening, caused by sediment compaction, which distorts the primary direction of remanence. Moreover, it is necessary to distinguish the non-dipole component of the geomagnetic field with inclination shallowing in the recording signal.

To address this, we applied the recently developed SVEI method (based on the THG24 model) to examine 82 lacustrine and marine records spanning the past 100 kyr for inclination flattening, and found evidence in only one case. However, the traditional E/I approach, based on the TK03.GAD, suggests flattening at 27 mid-latitude sites. When correction inclinations were utilized to construct the model, the comparison result reveals that octupole terms were most affected, underscoring the sensitivity of these higher-order components to inclination flattening. The THG24 model extends beyond the geocentric axial dipole (GAD) representation, employed in TK03.GAD, by incorporating additional axial quadrupole and octupole contributions to the geomagnetic field. This implies that mid-latitude “flattening” signals are non‑dipole contributions rather than by compaction, highlighting that the non-dipole features remain a significant component of the geomagnetic field over the past 100 kyr.