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Abstract: Sudden impulses of the magnetospheric and geomagnetic field are caused by sudden increases in the dynamic pressure of the solar wind, generally associated with the Earth’s arrival of interplanetary shock waves or discontinuities. We compared the observed field jumps, at geosynchronous orbit and at ground, with those predicted by a theoretical model (T04, Tsyganenko and Sitnov, 2005) for transitions between two steady states representations of the magnetosphere under different solar wind conditions. The geosynchronous response (mostly along the northward component, B z) in the dayside hemisphere is basically consistent with the magnetic field jump expected for changes of the magnetopause current alone. A similar conclusion holds for the positive changes observed in the nightside hemisphere; by contrast, the competing contributions of several current systems (from the magnetopause, cross-tail current, ring current, field aligned currents) determine in this time sector a large variety of small amplitude and negative responses that cannot be univocally interpreted. A different situation emerges in ground observations, at low latitudes. Here the observed responses along the northward component, H, in the dawn sector are consistent with changes of the magnetopause current; by contrast, in the entire postnoon hemisphere, they are greater than expected. The H variations are also typically accompanied by a significant change of the eastward component, D. These aspects reveal additional ionospheric contributions which determine a negative change of the D component through the entire day and a remarkable enhancement (≈50%) of the H component in the postnoon sector. The comparison between observations and model representations in several cases allows to discriminate between the roles of the current systems which compete in determining the field variations. Occasionally, SI manifestations are accompanied by trains of ULF fluctuations (f≈1–100mHz); we present a case in which the experimental observations suggest evidence for a cavity/waveguide oscillation at f≈3.2mHz. [Copyright &y& Elsevier]

Abstract: This study seeks to establish a new system characteristic describing dayside convective flows in the coupled magnetosphere–ionosphere: the low-pass filter function through which interplanetary magnetic field (IMF) fluctuations are processed as they are communicated from the magnetopause to the high-latitude ionosphere near local noon. In doing so, this study confirms that variations in the ionospheric flows at high-latitudes near local noon are well correlated with variations in the IMF orientation and magnitude on short timescales. We construct the filter function by comparing time series of the ionospheric equivalent flows at a fixed location at magnetic local noon and 80° latitude with time series of the IMF. The coherence spectra of these two parameters—averaged over 330h of comparison—indicate that there is a low-pass cutoff in the ionospheric response to IMF driving at a periods shorter than 20min (frequencies higher than 0.8mHz). When there is sufficient power in the IMF fluctuations, this cutoff is relatively sharp—the coherence drops by roughly a factor of three between the periods 32 and 21min (0.5 and 0.8mHz). The results also show that on average the coherence between the east–west component of the equivalent flows and IMF By tends to be less than the coherence between the north–south component of the equivalent flows and IMF Bz . [Copyright &y& Elsevier]

Abstract: In the first place, this work will demonstrate that there are at least several cases for which the magnetosphere–ionosphere system “does not behave as a low-pass filter”, or better it transmits high frequencies. In one selected case study, a monochromatic solar wind pressure wave in the Pc5 band directly excites the same frequency of ground pulsations. The other two case studies are composed of repetitive solar wind pressure pulses, like quasi-periodic solar wind pressure waves, which are associated to ionospheric currents in a one-to-one correspondence. Our most interesting case shows that the magnetosphere sometimes has the ability to respond in such small timescales as 2.7min. The same case provides strong evidence that the successive solar wind pressure pulses excite a global mode wave in the magnetosphere. We further discuss these events under the perspective established by the three recent papers of Sarafopoulos (2004a. Distinct solar wind pressure pulses producing convection twin-vortex systems in the ionosphere. Annales Geophysicae 22, 2201–2211; 2004b. Repetitive X-line Hall current structures over the dawnside ionosphere induced by successive exo-magnetosphere pressure pulses. Annales Geophysicae 22, 4153–4163; 2005. Pseudo-field line resonances in ground Pc5 pulsation events. Annales Geophysicae 23, 593–608) that the solar wind pressure pulses produce pseudo-field-line resonances on the ground station data; and this result certainly do not support the cavity mode model. Previously, many researchers have suggested that the magnetosphere acts as low-pass filter with a threshold separating the driven from the loading–unloading processes that are based on the inherent system dynamics. Essentially, their works are based on the AE (or AL) index. Our suggestion here is that the methodology whereby the AE index was introduced may eliminate the power density in high frequencies. The AE index may fail to carry the high-frequency part of input information. Neighboring ground stations with a phase-shift in their recorded waveforms may produce a no fluctuating AE index. The research efforts using AE may result in an error estimate of the cutoff frequency for the magnetosphere considered as a low-pass filter. Additionally, these works are not able to capture high-frequency couplings between the solar wind and the magnetosphere because they use low-resolution measurements. The presented high-frequency couplings emphasize the complementary character of this work. We shall not search special mechanisms or conditions permitting or preventing the direct entry of Pc5 frequency activity on Earth and any statistical approach is beyond the scope of this paper. [Copyright &y& Elsevier]