GNGTS 2013 - Atti del 32° Convegno Nazionale

Regional modeling of the geomagnetic field in Europe using satellite and ground data: geological applications E. Qamili 1 , F.J. Pavón-Carrasco 1 , A. De Santis 1,2 , M. Fedi 3 , M. Milano 3 1 Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy 2 Università G. D’Annunzio, Chieti, Italy 3 Università Federico II, Napoli, Italy Introduction. The geomagnetic field varies in time and space. In order to describe these spatial and temporal variations, systematic surveys are necessary. Before the satellite era, continuous records at magnetic observatories and repeat station measurements were the principal resources of the geomagnetic field studies. But the Earth’s surface is not uniformly covered by these ground magnetic measurements (especially over the seas). If we want to model properly the geomagnetic field, more data are necessary to cover the area under study and these data should be also distributed at different heights, in order to better sample the entire space of interest. For this reason, satellites play an outstanding role for geomagnetic field measurements and modeling. Different satellite missions (i.e. Magsat, Ørsted, Champ, SAC-C etc.) have produced high quality geomagnetic data with global coverage and high spatial resolution (Matzka et al. , 2010). Taking advantages of the high quality of the satellite data, in this work, we want to see if it is possible to model both main and anomaly Earth’s magnetic fields using an appropriate satellite dataset, with a particular objective in mind: to construct a reliable European model of the geomagnetic anomaly field. To do this, we have here selected a set of 6 years (1999-2005) of data (vector and total intensity) from the Ørsted, CHAMP and SAC-C satellites, which is the same dataset used for deriving the CHAOS-4 model (Olsen et al. , 2010). In order to properly model the above-mentioned contributions of the geomagnetic field, appropriate selection criteria should be applied to these satellite data. Data selection based on geomagnetic indices and local time minimizes the influence of the external fields. When considering the data selection procedure, it is necessary to take into account also the characteristics of the region under study. Considering what said above, we considered data that satisfy the following stringent criteria: 1. at the European latitudes it is required that the Dst index (that measures the strength of the magnetospheric ring-current) does not change by more than 2 nT/hr, 2. at non-polar latitudes is required that the geomagnetic activity index should be Kp ≤ 2o, 3. data sampling interval applied is 60 sec, 4. only data from dark regions (sun 10° below horizon) were considered; 5. for the polar region, data are selected satisfying E m < 0.8mV/m. Here we have tried to model the main and crustal fields by means of the Spherical Cap Harmonic Analysis (SCHA) in space. The crustal field is time invariant so it does not need to be modeled in time, while the changes in time of the main field are modeled by means of the classical penalized cubic B-splines, covering the indicated time interval. We will see below that these techniques provide optimal representation of the internal field over the area of investigation. Methodology. Modeling the global geomagnetic field at the Earth’s surface and above is usually approached using the well-known Spherical Harmonic Analysis (SHA). However, global techniques do not provide in general higher resolution when the study is carried out in a restricted part of our planet, and then, a regional approach is usually the most plausible mathematical technique. A useful contribution for such a regional analysis is given by the Spherical Cap Harmonic Analysis (SCHA) introduced for the first time by Haines (1985). The SCHA is a powerful analytical technique for modeling the Laplacian potential and the corresponding field components over a spherical cap. The solution of Laplace’s equation in spherical coordinated ( r,θ, ) for the magnetic potential V due to internal and external sources over a spherical cap can be written as an expansion of non-integer degree spherical harmonics (Haines, 1985): 164 GNGTS 2013 S essione 3.2