Toward a possible next geomagnetic transition?

Abstract

Abstract. The geomagnetic field is subject to possible reversals or excursions of polarity during its temporal evolution. Considering that: (a) in the last 83 million yr the typical average time between one reversal and the next (the so-called chron) is around 400 000 yr, (b) the last reversal occurred around 780 000 yr ago, (c) more excursions (rapid changes in polarity) can occur within the same chron and (d) the geomagnetic field dipole is currently decreasing, a possible imminent geomagnetic reversal or excursion would not be completely unexpected. In that case, such a phenomenon would represent one of the very few natural hazards that are really global. The South Atlantic Anomaly (SAA) is a great depression of the geomagnetic field strength at the Earth's surface, caused by a reverse magnetic flux in the terrestrial outer core. In analogy with critical point phenomena characterized by some cumulative quantity, we fit the surface extent of this anomaly over the last 400 yr with power law or logarithmic functions in reverse time, also decorated by logperiodic oscillations, whose final singularity (a critical point t c ) reveals a great change in the near future (2034 ± 3 yr), when the SAA area reaches almost a hemisphere. An interesting aspect that has recently been found is the possible direct connection between the SAA and the global mean sea level (GSL). That the GSL is somehow connected with SAA is also confirmed by the similar result when an analogous critical-like fit is performed over GSL: the corresponding critical point (2033 ± 11 yr) agrees, within the estimated errors, with the value found for the SAA. From this result, we point out the intriguing conjecture that t c would be the time of no return, after which the geomagnetic field could fall into an irreversible process of a global geomagnetic transition that could be a reversal or excursion of polarity.

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

“…The SAA region was delimited by the intensity contour line of 32,000 nT. Our results (see Figure 4A) agree with those of De Santis et al (2013) showing how the SAA area extent has been continuously growing. However, our results reveal more details (see Figure 4B): the SAA area extent is increasing with periods of accelerations (1840-1875 and 1900-1960) and decelerations (1975-1900 and FIGURE 4 | SAA extent area (A) and its first time derivative (B) given by the GUFM1 (red points) and IGRF-12 (blue points) models.…”

“…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).…”

“…Finally, in terms of statistics, the average occurrence of geomagnetic transition for the last 83 Ma is 400 kyear (De Santis et al, 2013). They calculated the average value only for the last 83 Ma to avoid the Cretaceous Normal Superchron (from 83 to 121 Ma) where the geomagnetic field kept the same normal polarity for 38 Myear.…”

“…In fact, the decreasing of the intensity values of the SAA is far from a regional effect, and the depressed values of the SAA cover a large area in the South Atlantic Ocean and adjacent areas (Figure 1). In addition, very recent studies (De Santis et al, 2013) indicate that the area extent of the SAA follows a log-periodic acceleration that resembles the behavior of a critical system moving toward a critical transition. This behavior of the geomagnetic field seems present since there are available historical or instrumental measurements of the geomagnetic field.…”

“…Using this approximation they characterized the SAA by an "equivalent monopole" which is moving close to the CMB with a mean drift of 10-20 km/year in an anticyclonic rotation centered at 55 • S latitude and 0 • E longitude. De Santis et al (2013) defined the SAA at the Earth's surface as the region limited by the intensity isoline of 32,000 nT and they calculated the area extent using the GUFM1 model. Results from that work indicate that the area extent of the SAA has been continuously growing since there are available historical or instrumental geomagnetic data (see Figure 2).…”