Abstract. The importance of space weather and its forecasting is growing as interest in studying geoeffective processes in the Sun -solar wind -magnetosphere -ionosphere coupled system is increasing. In this paper higher order statistical moments of interplanetary magnetic field and geomagnetic SYM-H index fluctuations are compared. The proper description of fluctuations in the solar wind can elucidate important aspects of the geoeffectivity of upstream turbulence and contribute to our understanding of space weather. Our results indicate that quasi-stationary intervals during both quiet and stormy periods have to be investigated in order to find correlations between upstream and geomagnetic conditions. We found that the fourth statistical moment (kurtosis), which was not considered in previous studies, appears to be a new geoeffective parameter. Intermittency of the magnetic turbulence in the solar wind can influence the efficiency of the solar wind -magnetosphere coupling through affecting magnetic reconnection at the Earth's magnetopause.
Linear magnetoconvection in a model of a non-uniformly stratified horizontal rotating fluid layer with a toroidal magnetic field is investigated for no-slip and finitely electrically conductive boundaries and with very thin stably stratified upper sublayer. The basic parabolic temperature profile is determined by the temperature difference between the boundaries and by the homogeneous heat source distribution in the layer. This results in a density pattern, in which a stably stratified upper sublayer is present. The developed diffusive perturbations (modes) are strongly affected by the complicated coupling of viscous, thermal and magnetic diffusive processes. The calculations were performed for various values of Roberts number (q ≪ 1 and q = O(1)). The mean electromotive force produced by the developed hydromagnetic instabilities is investigated to find the modes, which can be appropriate for creating the α-effect. It was found that the azimuthal part of the EMF is dominant for westward modes when the Elsasser number Λ ∼ < O(1).K e y w o r d s : non-uniform stratification of the Earth's core, linear magnetoconvection, diffusive hydromagnetic instabilities, mean electromotive force Stud. Geophys. Geod., 50 (2006), 645-661 645 c 2006 StudiaGeo s.r.o., Prague J.Šimkanin et al.
S U M M A R YThis paper investigates the dependence of hydromagnetic dynamos on the thermal and viscous diffusion processes and stratification. We analyse cases when thermal and viscous diffusion processes equally affect the dynamo action and when thermal diffusion processes dominate over viscous ones. The spherical shell is considered to be stratified either uniformly or nonuniformly. In the case of non-uniform stratification, the upper thin subshell is stably stratified and the lower subshell is unstably stratified, while for uniform stratification the whole shell is unstably stratified. In all the investigated cases, the generated magnetic field is dipolar. If thermal diffusion processes dominate over viscous ones (low Prandtl numbers), neither transition to hemispherical dynamos nor weaker magnetic fields (which are less dipole dominated) take place because our magnetic Prandtl number is large enough, although the inertia becomes important. This indicates that magnetic Prandtl numbers could govern a measure of inertia for low Prandtl numbers. Dependences of dynamos on the type of stratification (uniform and non-uniform) are very weak in all the investigated cases. Consequently, there is no reason to continue in a dynamo modelling in the non-uniformly stratified shells for low Ekman numbers.
Abstract. The importance of space weather and its forecasting is growing as interest in studying geoeffective processes in the Sun -solar wind -magnetosphere -ionosphere coupled system is increasing. This paper introduces the proper selection criteria for solar wind magnetic turbulence events during duskward electric field and southward B z driven geomagnetic storms. Two measures for the strength of solar wind fluctuations were investigated: the standard deviations of magnetic field components and a proxy for the so-called Shebalin anisotropy angles. These measures were compared to the strength of geomagnetic storms obtained from a SYM-H index time series. We found a weak correlation between standard deviation of interplanetary magnetic field GSM component B z and SYM-H index, and a strong correlation between Shebalin anisotropy angle and the SYM-H index, which can be the result of an increase of probability of magnetic reconnection in fluctuating magnetic fields.
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