CORRELATION PROPERTIES OF SECULAR VARIATION FORECAST

In this work, statistical and correlation properties of secular variation (SV), SV forecast (SVf) and SV forecast error (ErrSVf) are compared. Spatial structure of SV components was calculated from the IGRF11 coefficients. The SV forecast was reconstructed from the IGRF3-10 coefficients. We calculate statistical and correlation characteristics both for the globe and six separate regions. Four of these regions were selected at the high latitudes and one of them includes Russia and adjacent waters. Two other regions are located near the equator and one of them includes the so-called Brazilian Geomagnetic Anomaly. The maximum of the ErrSVf was obtained for Russia and adjoining near-equatorial region. Three correlation coefficients for the spatial distribution of SV and SVf were calculated. The first one is the correlation coefficient of SVi and SVi-1 for two consequent periods; the second one is the correlation coefficient of SVi and SVfi for the same period; and the final one is the correlation coefficient of SVfi and SVi-1. The latter correlation coefficient proved to be the maximum. The SVf and SV for the same period correlate well only when high correlation between SVi and SVi-1 is observed. It means that the SVf reproduces a spatial structure of SV of previous epoch independent of the forecast method used. The existence of a large and long-term anomaly produces a high correlation of sequential periods, and the correlation coefficient between the SV forecast and SV for the same period is greater than 0.9. The spatial distribution of the ErrSVf is characterized by an existence of independent processes occurring in the liquid core, and single anomalies of small scales. However, even if one can identify an individual flow in the liquid core, study of its parameters changes does not let us to improve the forecast accuracy, since the origin of these flows is not well determined. We propose that the topographic heterogeneity of the core-mantle boundary which presently is a subject of active study could play an important role in these processes. Introduction The variability of the spatial structure of the Earth's main magnetic field leads to a request for the qualitative forecast of geomagnetic secular variation (SV) from geological exploration, navigation and other geologic and geophysical tasks. Nowadays the 11th generation coefficients (IGRF11) are widely used in different geophysical studies as a mathematical description of the geomagnetic field spatial structure. This model represents the sets of spherical harmonic expansion coefficients of geomagnetic field components up to the order 13 with the 5 years interval. Each following generation of coefficients is developed by group of scientists taking into account all data from magnetic observatories, repeat stations and satellites. However for calculations between epochs the coefficients are interpolated linearly that, owing to the complexity of spatial structure of SV, can lead to errors. At the same time, with the acceptance of a new generation of coefficients for the current epoch a SV forecast for the next 5 years is developed. The methods of SV forecast are constantly improved, but authors of the IGRF model allow for the possibility of forecast error up to 20 nT/year. The systematic decrease of the geomagnetic field intensity and the resulting decrease of the protection against cosmic rays stimulate the interest to the long-term SV forecasting. As long as the increase of the forecast accuracy, especially of the declination (D), is an important problem it is the subject of many studies. Despite the significant progress, the question about the long-term SV forecasting still remains open. As the geomagnetic field is generated in the liquid core of the Earth, its changes in time have to be connected with processes taking place there. Nowadays the mechanism causing SV remains uncertain. Owing to the complexity of a problem of magnetohydrodynamic description of the geodynamo, only numerical solutions [Hulot et al., 2010, Kuang et al., 2009] with the assumptions, allowing to simplify the problem, exist. But these models cannot be used for the SV forecasting, because they describe a field on the Earth surface only at a qualitative level. Proceedings of the 10th Intl Conf. “Problems of Geocosmos” (Oct 6-10, 2014, St. Petersburg, Russia)