Cases for which the Earth's magnetosphere does not act as a “low-pass filter”

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 2.7 min. 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.