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[en] Complete text of publication follows. National Observatory of Athens (NOA) currently operates ENIGMA (HellENIc GeoMagnetic Array), an array of 4 ground-based magnetometer stations in the area of south-eastern Europe (central and southern Greece). The current stations are latitudinally equi-spaced between 30 deg and 33 deg corrected geomagnetic latitude. In the near future another station will be installed in Macedonia or Thrace, and there are plans for the installation of an additional station in Crete by the end of 2009. One of the primary research objectives assigned to ENIGMA is the study of geomagnetic field line resonances (FLRs). The latter is a well-established phenomenon taking place in the Earth's magnetosphere. It can be pictured as the formation of standing magnetohydrodynamic waves on magnetic field lines with fixed ends at the conjugate ionospheres. An interesting option in this field of research would be to compare ultra-low-frequency (ULF) wave observations in space made by ESA's Cluster mission and on the ground acquired by these mid-to-low-latitude ground-based observation sites of the Earth's magnetic field. Cluster has a high inclination orbit; insofar studies at high latitudes are more justified for direct interactions along the magnetic field lines. So, for a Cluster-ENIGMA study one has to expect some indirect, somehow related reactions with propagations perpendicular to the B-field. The Cluster-ENIGMA study can serve as a pilot-study for the upcoming Swarm mission of ESA. The Swarm constellation of spacecraft will allow, for the first time, the unique determination of the near-Earth field aligned currents, which connect various regions of the magnetosphere with the ionosphere and can be regarded as a complement to the Cluster mission.
[en] Complete text of publication follows. The complex system of the Earth's magnetosphere corresponds to an open spatially extended nonequilibrium (input - output) dynamical system. The non-extensive Tsallis entropy has been recently introduced (Balasis et al., 2008) as an appropriate information measure to investigate dynamical complexity in the magnetosphere. The method has been employed for analyzing Dst time series and gave promising results, detecting the complexity dissimilarity among different physiological and pathological magnetospheric states (i.e., pre-storm activity and intense magnetic storms, respectively). This paper explores the applicability and effectiveness of a variety of computable entropy measures (e.g. Block entropy, Kolmogorov entropy, T complexity and Approximate entropy) to the investigation of dynamical complexity in the magnetosphere. We show that as the magnetic storm approaches there is clear evidence of significant lower complexity in the magnetosphere. The observed higher degree of organization of the system agrees with that inferred previously (Balasis et al., 2006), from an independent linear fractal spectral analysis based on wavelet transforms. This convergence between nonlinear and linear analyses provides a more reliable detection of the transition from the quiet-time to the storm-time magnetosphere, thus showing evidence that the occurrence of an intense magnetic storm is imminent. More precisely, we claim that our results suggest an important principle: significant complexity decrease and accession of persistency in Dst time series can be confirmed as the magnetic storm approaches, which can be used as diagnostic tools for the magnetospheric injury (global instability). Overall, Approximate entropy and Tsallis entropy yield superior results for detecting dynamical complexity changes in the magnetosphere in comparison to the other entropy measures presented herein. Ultimately, the analysis tools developed in the course of this study for the treatment of Dst index can provide convenience for space weather applications.
[en] Complete text of publication follows. The ESA Standard Radiation Environment Monitor (SREM) is the second generation of instruments in a program established by ESA's European Research and Technology Centre (ESTEC) to provide minimum intrusive particle radiation detectors for space science and applications. SREM is a solid state particle detector consisting of three silicon diode detectors in a two-detectors-head configuration. All the pre-amplified detector pulses are scrutinized by a set of fifteen fast comparators. In order to determine proton and electron flux spectra without the need of pre-assuming their spectral form we have implemented a regularized unfolding method which is based on the Singular Value Decomposition (SVD) of SREM calibration matrix. The method includes proper schemes that treat the numerical issues arising by the electron-proton contamination, the energy range overlapping and the low number of SREM counters. First test studies show that this method can be successful for the unfolding of both monotonic and non-monotonic spectra of energetic particles using SREM data.