Abstract
Abstract. The ionospheric and magnetospheric current systems are responsible of the daily magnetic field changes. Recently, the Natural Orthogonal Components (NOC) technique has been applied to model the physical system responsible of the daily variation of the geomagnetic field, efficiently and accurately (Xu and Kamide, 2004). Indeed, this approach guarantees that the number of parameters used to represent the physical process is small as much as possible, and consequently process control for such system becomes apparent. We focus our present study on the analysis of the hourly means of the magnetic elements H, D and Z recorded at L'Aquila observatory in Italy from 1993 to 2004. We apply to this dataset the NOC technique to reconstruct the 3-dimensional structures of the different ionospheric and magnetospheric current systems which contribute to the geomagnetic daily variations. To support our interpretation in terms of the different ionospheric and magnetospheric current systems, the spectral and statistical features of the time-dependent amplitudes associated to the set of natural orthogonal components are analyzed and compared to those of a set of descriptors of the magnetospheric dynamics and solar wind changes.
Highlights
The geomagnetic field varies on a huge range of time scales: from milliseconds to millions of years
We investigate the crosstalk among the different principal components, obtained via the Natural Orthogonal Components (NOC) decomposition, by estimating an information theory quantity: the mutual information
We can assume that most of the variability observed in the geomagnetic field daily variation is explained in terms of a very small number of natural orthogonal components (k < 5)
Summary
The geomagnetic field varies on a huge range of time scales: from milliseconds to millions of years. Among the possible short-term geomagnetic variations the smoothest and most regular is that observed on magnetically quiet days, and it is known as “solar quiet daily variation” This variation mainly arises from the ionospheric current system flowing in the so-called dynamo region. This current system, which can be quite well approximated by a 2-D current flowing in the ionospheric E-region between 90 and 130 km (Chapman, 1929; Richmond et al, 1976), is driven by different processes This current is related with the expansion and contraction of the atmosphere as the Sun rises and falls daily through the year, with the global scale horizontal upper-atmosphere winds, with the lunar tidal forces upon the region, and with variations of the sun electromagnetic emissions responsible for extra fotoionization of the region. The solar daily variation is a function of latitude, local time, season and solar activity level (Campbell, 2003)
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