Mestrado: Lóbulos de fluxo reverso no campo arqueomagnético

Archeomagnetic field models may provide important insights into the geodynamo workings. Here I investigate the existence and mobility of reversed flux patches (RFPs) on the archeomagnetic field. In addition, the statistical behavior of RFPs, their robustness and the relations between RFPs and dipole changes were explored. Topological algorithms are introduced to define, identify and track RPFs in archeomagnetic field models. The magnetic equator was used to define RFPs, instead of the geographic equator. Once each point on the CMB grid is associated with a magnetic hemisphere and a polarity (normal or reversed), the next step is to identify the peaks of RFPs. For the final identification step a threshold criterion of intensity was imposed to avoid including insignificantly weak extreme points. To track RFPs in time, an algorithm was coded to calculate the distance of each RFP to all RFPs in the next snapshot. Model CALS3k.4b of Korte and Constable (2011) was applied for the interval 990 BC until 1990 AD, and both CALS3k.4b and GUFM1 models were applied for the interval between 1840 until 1990 AD. For both models on the CMB a grid of 1 degree in longitude and latitude was applied on the CMB. Robustness tests were applied to the RFPs and results from models CALS3k.4b and GUFM1 were compared for the period 1840 to 1990 AD. In contrast to previous definitions, patches that reside on the geographic equator are adequately identified based on the new RFP definition presented here. Most RFPs exhibit a westward drift and 75% of them migrate towards higher latitudes. Undulations of the magnetic equator and RFPs oppose the axial dipole moment (ADM). Filtered models show a tracking behavior similar to non-filtered ones and new RFPs occasionally emerge. Results from CALS3k.4b and GUFM1 for the period1840 to 1990 AD show a very good agreement between the tracking of RFPs in both field models. Stronger filters yield even more coincident RFPs positions and motions between the two models. As with CALS3k.4b, most RFPs in GUFM1 and their filtered models exhibit westward drift and migrate towards higher latitudes. Advection and diffusion of RFPs have worked in unison to yield the decrease of the ADM at recent times. The absence of RFPs in the period 550-1440 AD is related to a low in intermediate degrees of the geomagnetic power spectrum. I thus hypothesize that the RFPs are strongly dependent on intermediate spherical harmonic degrees 4 and above. Filtered models and comparison with GUFM1 suggest that RFPs are prominent features of the geomagnetic field over the past few centuries.