Time evolution of the South Atlantic Magnetic Anomaly

Abstract

The South Atlantic Magnetic Anomaly (SAMA) is one of the most outstanding anomalies of the geomagnetic field. The SAMA secular variation was obtained and compared to the evolution of other anomalies using spherical harmonic field models for the period. An analysis of data from four South American observatories shows how this large scale anomaly affected their measurements. Since SAMA is a low total field anomaly, the field was separated into its non-dipolar, quadrupolar and octupolar parts. The time evolution of the non-dipole/total, quadrupolar/total and octupolar/total field ratios yielded increasingly high values for the South Atlantic since 1750. The SAMA evolution is compared to the evolution of other large scale surface geomagnetic features like the North and the South Pole and the Siberia High, and this comparison shows the intensity equilibrium between these anomalies in both hemispheres. The analysis of non-dipole fields in historical period suggests that SAMA is governed by (i) quadrupolar field for drift, and (ii) quadrupolar and octupolar fields for intensity and area of influence. Furthermore, our study reinforces the possibility that SAMA may be related to reverse fluxes in the outer core under the South Atlantic region.

Latitudinal variation rate of geomagnetic cutoff rigidity in the active Chilean convergent margin

“…In the same line, Florindo and Alfonsi (1995), thought that seismic events could be linked to abrupt topographic changes at the CMB, which generated magnetic variations through the mantle. Another possible explanation of this apparent link between magnetic field, cutoff rigidity and geological systems arises from the instabilities in the CMB that are able to produce secular variations in the magnetic field on the Earth surface, and that corresponds to non-dipolar evolution of the geodynamo (Constable, 2007), since the topography of the CMB is significant in subduction zones, where the subducted slabs can generate downwelling or sinkholes in the deeper areas of the mantle and upwelling or outcrops in areas of divergence (Heirtzler, 2002;Koper, 2003;Hartmann and Pacca, 2009;Lassak et al, 2010;Calkins et al, 2012;Koelemeijer et al, 2012;Bayanjargal, 2013;Tarduno et al, 2015;Pavón-Carrasco and De Santis, 2016;Terra-Nova et al, 2016). However, the latest research on magnetic field and seismicity seems to come from the socalled lithosphere-atmosphere-ionosphere coupling (LAIC) (Hayakawa et al, 2015;De Santis et al, 2015Contoyiannis et al, 2016;Potirakis et al, 2016a, b;Oikonomou et al, 2017).…”

The South Atlantic Anomaly throughout the solar cycle

“…There, the field intensity reaches less than 60% of the field strength at comparable latitudes. The loca-tion of the minimum has moved from Southern Africa to South America over the last 300 yr (Mandea et al, 2007;Hartmann and Pacca, 2009). The total dipole strength has diminished by 9% from 1840 to 2015, although the field decrease occurs non-uniformly over the globe.…”

The South Atlantic Anomaly: The Key for a Possible Geomagnetic Reversal

“…In other words, the decay of the dipolar field increases the extent area of the SAA and decreases the averaged total intensity field at global scale. On the other hand, according to other studies (Hartmann and Pacca, 2009;De Santis et al, 2013), the behavior of the SAA during the last centuries is related to the higher harmonic degrees n = 2 and 3, i.e., the quadrupole and octupole fields. This is an important issue because these non-dipolar contributions play an important role during the geomagnetic reversals that are characterized by high ratios between the non-dipolar over dipolar contribution (e.g., Valet et al, 1999).…”

“…The minimum intensity curve is characterized by a continuous decrease with a mean SV of −30 nT/year. On the other hand, as indicated by Hartmann and Pacca (2009), the movement of the SAA is directly linked with the westward drift of the geomagnetic field due to the non-dipolar field evolution. In fact, the velocity of the intensity minimum for the last decades agrees pretty well with the rate of the present westward drift, i.e., ∼0.18 • /year (Dumberry and Finlay, 2007).…”

“…The use of global models to analyse the behavior of the SAA is not innovative and some studies have been already carried out using the GUFM1 model, such as the work of Hartmann and Pacca (2009). They applied the GUFM1 model along with observatory data from four geomagnetic observatories located in South America (Argentina and Brazil).…”