Features of propagation of compressional long-period oscillations penetrating from the interplanetary medium in the magnetosphere—ionosphere system

We have studied properties of Pi3 pulsations with a period of ~30 min in the magnetosphere—ionosphere system, using satellite and ground-based observations. According to the data from ground-based magnetic stations in the pre-noon sector of the magnetosphere, propagation of pulsations was revealed in azimuth from the day side to the night side at a velocity 3–9 km/s in the band of corrected geomagnetic latitudes Φʹ=76–79°. Along the meridian, the signal propagated poleward at a velocity 0.5–5 km/s. Analysis of signal spectra at stations located along different meridians shows three maxima: one latitude-independent maximum at a frequency of 0.55 mHz, and two latitude-dependent maxima at frequencies of 0.82 and 0.96 mHz respectively, at higher and lower latitudes. The first maximum corresponds to ULF waves penetrating from the solar wind; the other two, to magnetospheric field line resonances. The equivalent current system (ECS) during the pulsation recording was obtained by two methods: the method of spherical elementary current systems and the magnetogram inversion technique. Analysis of ECS derived by both methods has demonstrated that they match each other. The ECS during pulsations in the pre-noon sector is a large vortex consisting of smaller vortices that propagate in the ionosphere along the “sea-land” boundary line, i.e. meridional poleward propagation at velocities close to the average pulsation propagation velocities prevailed. According to the map of field-aligned current distribution in the ionosphere, the width of the maximum of the westward electrojet lies at the latitude of the ECS maximum (in the south of the large vortex) on the boundary between the regions of inflowing and outflowing field-aligned currents (regions 1 and 2), where field line resonances are observed. The obtained ECS corresponded to the DP2 current system with a predominant westward electrojet in the pre-noon and night sectors. Satellite data analysis has shown the following. In the solar wind, ULF waves in the Pi3 pulsation range propagated at a velocity of 186.4 km/s, which is significantly lower than the velocity of the average being as high as 550 km/s. This velocity is explained by the fact that the waves propagate toward the Sun and are carried by the solar wind to Earth. In the magnetosphere, pulsations with a predominant compression component propagated from the night side to the day side at a velocity 90–110 km/s; from the delays in the onset of maxima of energetic electron differential fluxes, velocities 20–40 km/s were identified. Pulsations in this event were caused by both external (oscillations in the solar wind) and internal sources (magnetospheric resonator, which could be excited, among other things, by a substorm). The dynamics of the “fine structure” of a large vortex - small vortices, in the magnetosphere as a whole coincides in propagation velocity and direction with geomagnetic pulsations.